Polyploidy in Plants

JENNIFER A. TATE , ... PAMELA South. SOLTIS , in The Evolution of the Genome, 2005

THE FREQUENCY OF ALLOPOLYPLOIDY VERSUS AUTOPOLYPLOIDY

Allopolyploidy has long been considered much more common than autopolyploidy. In fact, autopolyploids were traditionally idea to be very rare in natural populations of plants. Stebbins (1950) suggested that Galax aphylla (now Galax urceolata, Diapensiaceae) was one unambiguous example of autopolyploidy. He as well offered Sedum ternatum and Due south. pulchellum (Crassulaceae) as examples, with Fritillaria camschatcensis (Liliaceae) representing "a probable autotriploid." Stebbins (1950, p. 318) went on to talk over several examples of what he termed "intervarietal autopolyploids." A multifariousness is a rank below that of subspecies and is usually used for geographical races and well-marked ecotypes within morphologically variable species. Stebbins stated, in reference to intervarietal polyploids, that "this sort may be found to be non uncommon when more polyploids are analyzed with this possibility in mind." Intervarietal autopolyploids sensu Stebbins (1950) included Biscutella laevigatum (Brassicaceae), Dactylis glomerata (Poaceae), Allium schoenoprasum (Alliaceae), Polygonatum commutatum (Ruscaceae), Cuthbertia graminea (Commelinaceae), Eriogonum fasciculatum (Polygonaceae), and "some of the various polyploids of Vaccinium" (Ericaceae). Grant (1981) also suggested that autopolyploids were extremely rare in nature. Grant's list of "clear-cutting" autopolyploids included Galax aphylla, Biscutella laevigatum, Dactylis glomerata, and Solanum tuberosum (Solanaceae). He too noted several "likely" autopolyploids: Vaccinium uliginosum (Ericaceae), Eragrostis pallescens (Poaceae), and Galium mollugo and G. verum (Rubiaceae).

The perceived extreme rarity of natural autopolyploids was attributed to concerns about chromosome pairing. Geneticists such as Stebbins maintained that in an autotetraploid, with every chromosome represented four times, normal chromosome pairing at meiosis would be hard, and multivalent formation would atomic number 82 to reduced fertility (Fig. 7.two). This prompted harsh statements such every bit "autopolyploidy is not a help merely a hindrance" in natural populations (Stebbins, 1971, p. 126). Every bit noted before, nevertheless, chromosome pairing in even-numbered polyploids may be stable, such that it does non negate their ability to reproduce.

The inability to notice autopolyploidy probably also influenced early on assessments of its natural rarity. Although early investigators realized that an autotetraploid should showroom tetrasomic inheritance (e.g., Muller, 1914; Haldane, 1930), there was no simple means for determining the inheritance patterns of nigh genes until the concluding decades of the 1900s. Prior to that fourth dimension, researchers were required to make crosses and follow individual traits (presumed to be under uncomplicated genetic control) for several generations. The utilize of enzyme electrophoresis revolutionized plant and animal genetics by allowing multiple molecular forms of individual enzymes ("isozymes") to be detected. Indeed, isozymes have played a critical function in many areas of biology since their discovery by Hunter and Markert (1957), and for several decades they were the most widely used link between the organism and its genes. Allozymes, which are multiple forms of an enzyme encoded by different alleles at a locus, permit an individual to be genotyped. Because allozymic variation is typically present at multiple loci inside a species, these loci stand for a large pool of potential data for studies of inheritance. By examining the segregation patterns of allozymes, it became possible to determine if a tetraploid plant exhibited disomic or tetrasomic inheritance. Specifically, two plants possessing different allozymes (i.eastward., alleles) at a factor locus could be crossed (producing a heterozygous Fone plant), and seeds from this plant could be grown to maturity and either self-pollinated or cantankerous-pollinated amid each other to produce F2 plants. The array of allozymic genotypes in numerous F2 plants could then be scored to make up one's mind whether a i:two:1 ratio of homozygotes to heterozygotes to homozygotes was nowadays (disomic inheritance), or if the segregation ratios matched those expected for an autotetraploid with tetrasomic inheritance (Fig. 7.3). Another implication of tetrasomic and college-level polysomic inheritance in autopolyploids is a concomitant increase in levels of genetic variation in populations of autopolyploids compared to diploids (Moody et al., 1993; Soltis and Soltis, 1993).

Using the allozyme electrophoresis technique, studies of natural populations from diverse establish groups revealed a number of previously unrecognized autopolyploids, including Tolmiea menziesii (Saxifragaceae) (Soltis and Soltis, 1989b), Heuchera micrantha (Saxifragaceae) (Soltis et al., 1989), Heuchera grossulariifolia (Saxifragaceae) (Wolf et al., 1989), Turnera ulmifolia (Turneraceae) (Shore, 1991), and Allium nevii (Alliaceae) (Rieseberg and Doyle, 1989). Although autopolyploidy is certainly not as common as allopolyploidy, it has probably been an of import evolutionary force in plants whose prevalence had been greatly underestimated in the by. In a full general sense, some genera and species with chromosome counts existing in prominent polyploid series may represent autopolyploids. Of course, autopolyploidy may exist more prevalent in some plant groups than others; for example, several autopolyploids have been documented in the Saxifragaceae, whereas no unambiguous allotetraploids have withal been found in this group (Soltis, 2004).

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Polyploidy in Animals

T. RYAN GREGORY , BARBARA K. MABLE , in The Development of the Genome, 2005

VERTEBRATE POLYPLOIDS: A SUMMARY

As the examples provided here evidence, the ancient genome duplication of the vertebrate lineage has been followed by many more recent polyploidization events (including in the teleosts at large) (run into Affiliate half-dozen). Although recent polyploidy is rare in mammals and birds, this is clearly not true of groups similar fishes and amphibians. In reptiles, too, there are numerous known instances. In some cases, one or the other of car- or allopolyploidy may appear to predominate, but overall both mechanisms are more often than not found in each of the major taxa. Every bit in plants (see Chapter seven), at that place are signs of repeated polyploid formation in certain lineages, and there may in fact be complex networks of hybridization among some related species. Polyploidy is institute in association with all of the major reproductive systems in vertebrates, including bisexuality, thyletoky, gynogenesis, and hybridogenesis. In some cases, polyploidy is a characteristic shared by an entire family, whereas in others the polyploids are constitute in cryptic clan with a diploid species, or even but stand for a minor form within a mostly diploid species. There is clearly substantial diversity in the preponderance, mechanisms of formation, and biological and evolutionary implications of polyploidy among the vertebrates.

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Ingather Improvement | Mutation Techniques

Thou. Maluszynski , ... J. Maluszynska , in Encyclopedia of Applied Establish Sciences, 2003

Numerical chromosome mutations

The bones number of chromosomes characteristic for a genome is described by ten while n is the gametic number of chromosomes and iinorthward is the zygotic (somatic) chromosome number (in diploids 2due north=twoten ). Numerical chromosome variation is divided into two categories: euploidy and aneuploidy. Euploidy is further subdivided into autopolyploidy and allopolyploidy. A euploid is a prison cell or an private that carries an exact multiple of the bones chromosome number x. The triploid (3x), tetraploid (four10), and pentaploid (5x) are autopolyploids because one basic genome is multiplied. Autopolyploids can be obtained afterwards treatment with chemicals such as colchicine or oryzalin, which affects spindle formation and induces chromosome nondisjunction during the mitosis. The formation of unreduced gametes may too be a source of autopolyploid plants. Allopolyploid forms comprise two or more genomes derived from unlike species and they arise as a issue of interspecific hybridization and the formation of unreduced gametes.

An aneuploid is a cell or an individual with 1 or more whole chromosomes added or missing in the diploid or polyploid set (eastward.g., 2x+one; 4x+i; 2x−1). They tin announced every bit a result of irregular chromosome segregation at meiosis or as the offspring of translocation and inversion heterozygotes. Aneuploids are normally unbalanced and have reduced fertility.

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Crop IMPROVEMENT | Plant Breeding, Principles

N.West. Simmonds , A.East. Arthur , in Encyclopedia of Applied Establish Sciences, 2003

Polypoidy

Many plants have 2 sets of chromosomes and are called diploid. Others take four or more sets; these are called polyploids and are permanent hybrids with respect to homoeologous chromosomes. The nearly obvious kind of polyploid, an autopolypoid, which has replicated identical sets of chromosomes, is much less common considering of sterility acquired by pairing irregularities at meiosis. Many cultivated potatoes are probably virtually to this status, only other examples of autopolyploidy are virtually all experimental. Some of the wheats, rutabaga ( Brassica napobrassica; swede), tobacco (Nicotiana tabacum), and, C. arabica are plainly allopolyploids and, though derived from outbred diploids, are highly tolerant of inbreeding. Thus, the master merit of allopolyploidy is probably the maintenance of hybridity. It is no accident that the few examples of attempts to produce new species experimentally (e.grand., Raphanobrassica, Triticosecale) are allopolyploids, while the very numerous autopolyploids that have been studied, though often lush and vigorous, have been agriculturally useless; they sometimes have big grains, but just a few of them (due east.g., in barley). At that place was a considerable vogue for autopolyploids in the 1940s, only the mode soon died out.

Then far we have considered increased chromosome numbers. Withal, lower numbers are sometimes also possible: thus, haploids (also called monoploids) are possible and have their uses, principally equally a quick source of homozygotes after chromosome doubling to produce doubled haploids. The equivalent forms of autopolyploids are polyhaploids, east.g., dihaploids (i.e., diploids) out of tetraploid potatoes; these, besides, accept their uses, principally in formal genetic analysis. Making haploids and polyhaploids usually depends upon some genetic tricks, such as specific foreign pollination, often followed by in vitro civilization of very immature plants. The road to homozygosity may be greatly shortened through the production of doubled haploids if fluent haploid culture is possible; some proficient corn, barley, and oilseed rape (Brassica napus) inbred lines have been fabricated in this manner.

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Speciation, Process of

Jeffrey L. Feder , ... Peter J. Meyers , in Reference Module in Life Sciences, 2021

Hybrid, Polyploid, and Homoploid Speciation

Hybridization and polyploidization are two processes that can besides facilitate speciation (Arnold, 2006; Mallet, 2007). In hybrid speciation, interbreeding between 2 differentiated populations generates new variation that serves as the basis for creating new forms reproductively isolated from parent populations. In polyploid speciation, changes in the number of whole sets of chromosomes can cause problems in evolution or in meiosis in hybrids with parental forms, generating RI. Polyploid speciation appears to be more than common in plants (Jiao et al., 2011) than in animals (Mable, 2004).

Hybrid speciation can occur by either allopolyploid or homoploid mechanisms. In allopolyploid speciation, hybridization between different taxa is a trigger for polyploidization (Stebbins, 1950; Soltis and Soltis, 2000 ). The new hybrid polyploid, having a dissimilar chromosomal constitution, is immediately reproductively isolated from parental populations. Subsequent ecological adaptation of the allopolyploid population to novel environmental conditions is oft required for the new species to persist and not be displaced by parental forms. It is also possible, however, for polyploidy to occur without hybridization (conspecifically) in a process termed autopolyploidy (when the polyploid event involves individuals belonging to the same population or species) and generate RI leading to speciation ( Ramsey and Schemske, 1998).

Homoploid hybrid speciation does not involve ploidy changes. Instead, hybridization produces novel combinations of genes that can accommodate hybrids to new habitats or biotic conditions that crusade them to be reproductively isolated from parental populations (Mallet, 2007; Abbott et al., 2010). Divergent ecological adaptation is thought to be crucial in guild for hybrid populations to avoid being swamped by cistron flow from parental species or being competitively excluded (Coyne and Orr, 2004). A instance study in tephritid fruit flies showed that hybridization of Rhagoletis mendax (that infests blueberries) with Rhagoletis zephyria (that lives on snowberries) formed a new hybrid population that attacks Lonicera (honeysuckle) (Schwarz et al., 2005). As a upshot, the hybrid fly population can persist on honeysuckle, sympatric with the parental populations infesting dissimilar host plants. Many Heliconius butterflies may have been formed by hybridization (Mavarèz et al., 2006), involving changes in just a few genes affecting mimicry (Heliconius Genome Consortium, 2012). Certain Helianthus sunflowers also announced to have originated by hybridization and inhabit novel, often marginal, habitats to which they are divergently adapted (Gross et al., 2003). In dissimilarity to the butterflies, many genomic components from both parental taxa are present in hybrid sunflower species.

Hybrid and polyploid speciation are now accepted equally occurring in nature. Questions remain apropos how often homoploid hybrid speciation occurs, although recent studies suggest that information technology may be more common than previously thought (Gompert et al., 2006; Mallet, 2007; Abbott et al., 2013; Servedio et al., 2013; Nice et al., 2013; Schumer et al., 2014).

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Comparative and Evolutionary Genomics

Takeshi Kawashima , in Encyclopedia of Bioinformatics and Computational Biological science, 2019

Whole Genome Duplication

Whole genome duplication (WGD) is an evolutionary outcome in which the whole factor set of an organism has been duplicated, probably by chromosomal polyploidy and its fixation. WGD is postulated as one of the major sources of evolutionary innovations.

Although polyploidy is a common phenomenon in eukaryotes, evolutionary fixed WGDs are exceedingly rare. In metazoa, known WGD events are almost limited to vertebrate lineages includes famous ii round WGD (2R) which is occurred on the common ancestor of vertebrates (Ohno, 1970; Van de Peer et al., 2009, 2010). In invertebrates, it is likewise suggested that some lineages have experienced WGD although those traces are not and then clearly like vertebrates (Yoshida et al., 2011; Kenny et al., 2016). In plants and fungi, WGD occurs relatively more frequently than in animals (Van de Peer et al., 2009) merely there is still some bias.

Each blazon of duplicated genes created by WGD has named based on their forming mechanisms, respectively (Fig. three(B)). Cistron pairs produced by WGD are a special subset of paralogs (Wolfe, 2000). Because of their evolutionary significance as described above, they are sometimes distinguished by terms such as "ohnologs," or its synonym "syntenic paralogs" (Wolfe, 2004; Schnable et al., 2012). A similar word is "homoeologs" (or "homeologs" as another spelling) defined equally homologous genes resulting from allopolyploidy (Glover et al., 2016). The term "ohnolog" is derived from the name of Susumu Ohno who first proposed the hypothesis that factor duplication plays a key role in development (Ohno, 1970; Wolfe, 2000 ). Allopolyploid is a polyploid individual which composed of a chromosome set equanimous of two (or more than) chromosome gear up. It formed from the hybridization of two (or more) carve up only closely related species. The difference between the definitions of ohnolog and homoeolog is that the former refers to paralog made past autopolyploidy and the latter refers to paralog made by allopolyploidy. However, if polyploidization arose very long ago, it can not distinguish which mechanism truly occurred for creating the paralogus gene-sets. In such cases where autopolyploidy or allopolyploidy can non be distinguished, the term "paleolog" is sometimes used conveniently. Some other like term is "syntelogs" representing orthologous genes shared across species (or isolates for viruses) with a common syntenic genome location ( Hatcher et al., 2014).

The evolutionary procedure of WGD is ane of the central themes in evolutionary genomics. In general, duplicated genes tend to be fixed simply when those are to take on different functions but otherwise to be lost immediately after the duplication issue. Nevertheless, WGD is an event that a large number of duplication has occurred at once, it was quite difficult to investigate the process of gene losses until the era of genomics.

Because the function of an ohnolog pairs is presumably redundant each other just after WGD, it is theoretically expected that ane of the duplicated genes to exist lost gradually. Interestingly, the rate of gene losses may differ depending on the types of genes. For example in metazoa, it is known that ohnolog genes which relevant to embryonic development tend to exist conserved as paralog gene-set (Holland et al., 1994).

To distinguish a true pair of ohnolog or homoeolog from other mutual paralogs is technically difficult. Various methods accept been devised to excerpt the set of ohnolog that were idea to be made by WGD. Several models have been proposed why the conservation charge per unit of developmental genes afterward WGD is loftier (for example, Holland et al., 1994). A future explanation volition be awaited equally to why the same trend is shown for prison cell cycle regulators.

To extract the ohnolog created past 2R in vertebrate evolution, Dehal and Boore utilized a genome of an invertebrate which is branched earlier the 2R. Using a graph-based method for this purpose, they have adult a style to extract homologous genes of which number encoded in the genome of invertebrates and vertebrates at a ratio of approximately ane:2 to 1:4, respectively (Dehal and Boore, 2005).

Nishida and his colleagues developed a method to determine ohnolog gene-pair and examined the rate of gene loss afterwards the WGD which occurred the common ancestor of teleost fish (teleost genome duplication: TGD) (Inoue et al., 2015). As a effect of the analysis for the obtained ohnolog factor ready, they institute that the charge per unit of loss of ohnolog is roughly approximated past a double-exponential bend (2-phase model). According to their "ii phase model" of gene loss after TGD, duplicated genes are rapidly lost immediately later on the WGD event, then slowly lose at a roughly constant charge per unit.

It is estimated that the rapid rate of gene loss in the first phase probably depends on the deletion of contiguous clusters of genes or big chromosomal segments ("cake loss"), but the details accept non been clarified notwithstanding.

It may exist fifty-fifty more than hard to accurately place the set of paralogs in an allopolyploid (homoeolog). The african clawed frog Xenopus laevis is proposed to exist an allotetraploid animal that arose via the interspecific hybridization of diploid progenitors followed by subsequent genome doubling. In the sequence-decoding and reconstruction of the whole-genome of this frog, researchers noticed that the possibility that the subgenome derived from the two progenitor frogs each retained unique transposable elements in their genome and that those elements had been fossilized in each genome (Session et al., 2016). They have actually constitute three of such transposable chemical element and succeeded in separating reconstructed Dna sequences into ii homoeologous subgenomes using those fossilized transposable elements equally markers.

Homoeologs in the X. laevis were extracted from those separated subgenomes thus obtained and the tendency of the gene-loss later WGD was investigated. Interestingly, it was institute that not simply the developmental genes but also prison cell cycle regulators are tend to exist conserved as paralogs afterward WGD.

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Large-Scale Gene and Aboriginal Genome Duplications

YVES VAN DE PEER , AXEL MEYER , in The Development of the Genome, 2005

MECHANISMS OF LARGE-Scale DUPLICATION

In some groups, polyploids appear to class frequently and repeatedly, with a large percentage of species showing signs of recent polyploidization (see Chapters vii and viii). At that place is besides increasing evidence that many organisms are "paleopolyploids"—that is, ancient polyploids whose genome duplications have been masked past subsequent molecular and chromosomal evolution (see later department). The mechanisms involved in the initial duplications are thought to be the same every bit those occurring in more than contempo polyploidization events. Other mechanisms of big-scale duplications, above the level of individual genes but less than entire genomes, are also recognized. The most important of these are described briefly in the following sections. Additional details regarding mechanisms of polyploidization tin exist institute in Chapters vii and 8, whereas rediploidization, the evolutionary process in which a tetraploid species "decays" to become a diploid, is discussed in more detail past Wolfe (2001).

AUTOPOLYPLOIDY

Polyploidy tin occur when an fault during meiosis leads to the production of unreduced (i.e., diploid) gametes rather than haploid ones, as shown in Figure half-dozen.1. If 2 diploid gametes fuse, an autotetraploid volition be created whose nucleus contains 4 copies of each chromosome. Autopolyploids are frequently viable because each chromosome all the same has a homologous partner and can therefore class a bivalent during meiosis. This mechanism allows an autopolyploid to reproduce successfully, but prevents interbreeding with the original organism from which information technology was derived because a cross betwixt a tetraploid and a diploid would give triploid offspring. Different tetraploids, triploids are very oftentimes sterile because one total prepare of its chromosomes lacks homologous partners to form the bivalents necessary for segregation (come across Chapter viii for more on the meiotic consequences of polyploidy).

Figure 6.1. The basis of autopolyploidization. Autopolyploidization can occur when the pairs of homologous chromosomes accept not separated into different nuclei during meiosis. The resulting gametes volition be diploid rather than haploid.

 Based on Brown (1999), reproduced by permission (© BIOS Scientific Publishers). Copyright © 1999

ALLOPOLYPLOIDY

Polyploidy can also result from hybridization of two closely related species, leading to feasible hybrids when the genomes are very similar, or to sterile hybrids when the chromosomes are insufficiently similar to synapse (pair) during meiosis. Still, if the new combined genome undergoes a chromosomal doubling, 2 identical sets of chromosomes are bachelor to pair during meiosis. As a result, a fertile tetraploid is produced (Fig. 6.2 ). For the most function, ancient polyploids are assumed to have been formed through allopolyploidy rather than autopolyploidy (e.g., Spring, 2003), although some semiancient polyploids such every bit the salmonid fishes are believed to exist autopolyploid (see Chapter eight).

Figure half dozen.2. The basis of allopolyploidization. Allopolyploidy can consequence from hybridization of two closely related species, perhaps leading to sterile hybrids, because chromosomes tin non synapse (pair) during meiosis because they are not similar plenty. Yet, when the newly combined genome undergoes a chromosomal doubling, two identical sets of chromosomes are available to pair during meiosis, and a fertile tetraploid is produced.

ANEUPLOIDY

Aneuploids have a chromosome number that differs from an exact multiple of the haploid chromosome set. In such cases, a single chromosome is either lost or added from a normal diploid set of chromosomes. Duplication of individual chromosomes, described extensively for humans (and Drosophila), is either lethal or results in serious genetic diseases such as Downwardly syndrome, which is caused by the possession of three copies (trisomy) of Chromosome 21.

Cake DUPLICATIONS

Comparative studies take suggested that "cake" (or "segmental") duplications—the duplication of large Dna segments—take been a continuing process during evolution. Block duplications are those in which many genes and their upstream regions are duplicated in a single event. Yet, for a long time it was unclear how such duplications could be generated, whether near of them occur intra- or interchromosomally, whether these tend to be found in directly or inverted orientation, and what sort of sequences are involved at the junctions. Recently, Koszul et al. (2004) accept used a cistron dosage assay for growth recovery in Saccharomyces cerevisiae to accost these questions. They demonstrated that a majority of the revertant strains resulted from the spontaneous duplication of large Dna segments, both intra- and interchromosomal, ranging from 41 to 655 kilobases (kb) in size. In fact, in many cases dozens of genes were duplicated in a unmarried effect. The types of sequences at the breakpoints too every bit their superposition with the replication map suggest that spontaneous large segmental duplications mainly issue from replication accidents (Koszul et al., 2004).

TANDEM DUPLICATIONS

Tandem duplications are duplications where the two copies of the duplicated region are located immediately adjacent to one another. The process of unequal recombination (or crossing-over) is widely viewed as responsible for the creation of tandem duplications. Well-known examples of cistron complexes created by tandem duplications are the Hox gene clusters (discussed elsewhere in this affiliate) and ribosomal RNA genes (run across also Chapter 5).

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Found Reproductive Biological science

Michael One thousand. Simpson , in Plant Systematics (Second Edition), 2010

Hybridization, Introgression, and Polyploidy

Hybridization is usually defined as sexual reproduction between dissimilar species, specifically termed interspecific hybridization (although the term tin be used for sexual reproduction betwixt different populations or infraspecific taxa within a species). Hybridization is thought to be relatively common in plants, more so than in nigh groups of animals.

2 different species of plants will not interbreed if they are geographically isolated or if they exhibit one or more than of a variety of genetically determined traits that prevent or inhibit gene exchange. These genetic, reproductive isolating features include differences in habitat; differences in timing of reproduction; differences in floral morphology; or differences in pollinators. (For example, one species that is adapted to a wet environment may non be capable of interbreeding with i that is adapted to a dry surroundings, but because the two species are rarely in shut enough proximity to permit pollination.)

Hybridization between two plant taxa may only occur if these taxa are genetically similar enough. Whatever hybrid progeny that are produced may be fertile (capable of sexual reproduction) or sterile, the latter oftentimes the result of irregularities in meiosis, resulting in sterile or noncompatible gametes.

Introgression is hybridization between two species followed by backcrossing to i or both parents. The importance of introgression is that information technology can be a mechanism of promoting some gene period between ii different species, ultimately increasing the genetic variability or fettle of i or the other species.

Polyploidy is a mutation in which offspring have a multiple of some bequeathed set of chromosomes. Polyploidy tin occur either within a species ( autopolyploidy ) or betwixt different species (allopolyploidy).

Polyploidy can occur in two general means. One way that polyploidy can occur is past the production of gametes that have more than one set of chromosomes (Effigy 13.7A,B). Diploid gametes tin consequence from an irregularity during meiosis termed nondisjunction, in which homologous chromosomes do not segregate; if this occurs with all homologous chromosome pairs, and so the daughter cells may exist unreduced (i.e., diploid, non haploid). [An unreduced (diploid) pollen grain can sometimes be detected microscopically, whereby only ii, larger microspores (run into Chapter 12) are detected in the tetrad phase (Effigy 13.7D).] If both parents (either of the aforementioned or different species) produce diploid gametes, and then the offspring possesses iv sets of chromosomes, which is a tetraploid (Figure xiii.7A). Tetraploids are normally fertile, as they can produce viable, diploid gametes. If, withal, i parent contributes a haploid gamete and the other a diploid gamete, the offspring will be triploid (Effigy 13.7B). Triploids are generally sterile, as any gametes they produce volition by and large lack a full complement of chromosomes considering of meiotic irregularities, forming groupings of three homologs (trivalents; e.g., Figure thirteen.7E), instead of the normal ii (bivalents). Triploids might persist as a population, however, if they tin can keep to reproduce asexually.

Figure 13.7. Polyploidy. A,B. Mechanisms past which tetraploidy and triploidy can ascend in nature by meiotic nondisjunction, resulting in diploid gametes. C. Mechanism by which a tetraploid individual can arise by somatic chromosome doubling in a sterile hybrid. D. Pollen development in Cylindropuntia sp., showing normal tetrad of haploid microspores (left) and aberrant dyad of diploid microspores (right), the latter precursors to diploid pollen grains. E. Chromosome squash (Cylindropuntia prolifera, a sterile triploid), showing groupings of three homologous chromosomes (trivalents) during meiosis, indicative of triploidy. F. Evolution of wheat, Triticum spp., via polyploid events.

 Photos at D and E courtesy of Jon Rebman.

A 2nd way that polyploidy can occur is by the spontaneous doubling of chromosome number in an individual establish after normal sexual reproduction. For example, hybridization between ii unlike species might produce living diploid offspring, but the offspring often cannot produce feasible gametes because of the genetic dissimilarities betwixt the two parents (Figure 13.7C). However, if this sterile offspring tin can persist, e.g., if information technology can also reproduce vegetatively, it might (rarely) undergo a rare somatic (i.e., in a nonreproductive cell) chromosome doubling during mitosis in a critical region, east.k., the apical meristem of a shoot, such that this entire shoot becomes tetraploid. The tetraploid is at present potentially capable of producing viable, diploid gametes and, therefore, fertile offspring (Figure 13.7C). This blazon of polyploid consequence may be rare, but has been documented in species of Spartina (cordgrass), in which a new tetraploid species evolved from two, separate diploid parents.

Polyploidy is thought to exist a major mechanism in plant evolution, as chromosome studies take demonstrated that nearly plants have undergone a polyploid event during some fourth dimension of their history. The evolution of both emmer wheat and bread wheat occurred via ancestral, allopolyploidy events, resulting in tetraploid (4n) and hexaploid (6n) individuals (Figure xiii.7F).

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FUNGI | Classification of the Peronosporomycetes

M.W. Dick , in Encyclopedia of Food Microbiology, 1999

Sexual Reproduction

Gametangia may be developed terminally, subterminally, in an intercalary position on main or branch hyphae, as final or lateral appendages to a not-mycelial thallus, or from the unabridged thallus. Gametangia are coenocytic meiogametangia, in which synchronous meioses occur. The numbers of nuclei inbound the oogonial initials are greater than the numbers of mature uninucleate haploid female gametes, which in turn are more numerous than the numbers of zygotes in the oogonium. A comparable reduction in the number of nuclei occurs in the antheridium. In paired gametangia the meioses are too either simultaneous, or nearly so, between the 2 protoplasts. Afterward meiosis the contents of the receptor gametangium (oogonium) become separated every bit one or several uninucleate and initially unwalled gametes, but there is no differentiation of the male gametangial contents.

The evolution of the Peronosporomycetes is based on vegetative diploidy, and thus differs from that of other fungi. The occurrence of multiple synchronous meioses in a gametangium makes it possible for karyogamy to take place between ii haploid nuclei from adjacent meioses in the aforementioned gametangium (automictic sexual reproduction). Sexual reproduction can thus occur without a separate male gametangium or antheridium. In the Myzocytiopsidales and a few Pythiales the thallus becomes septate and adjacent segments assume the function of gametangia, whether automictic, or by homothallic pairing of equal-sized segments. In the Olpidiopsidales a face-to-face only independent, smaller thallus (<ten% of the receptor gametangium volume), the companion cell, functions as the heterothallic donor.

The majority of species of near of the genera are homothallic; heterothallism may be secondarily derived. The classic studies of heterothallism in Achlya, Phytophthora and Pythium accept led to some understanding of the mating systems, morphogenesis of directional growth and penetration, and the identification of a C29 steroid sexual practice hormone, antheridiol. Relative sexuality in the Peronosporales is idea to be a result of lethals. Estimates of chromosome numbers may be difficult to establish because of the possibilities of autopolyploidy, polysomy and chromosome polymorphy.

Karyogamy is presumed to occur in the oosphere later on fertilization, or perhaps in the haploid coenocyte of the oogonium (see Fig. 1). The cytoplasmic reorganization of the oospore is indicative that functional or not-functional meioses precede oospore formation.

Many species have oogonia with a smoothen, more than or less spherical outline (Fig. 6), but many others are ornamented. Oogonial form depends on three criteria: initial expansion, secondary primordial initiation and wall deposition. The sequential or simultaneous expression of these criteria results in unlike kinds of oogonial ornament. No single group within the Peronosporomycetes displays all the morphological diverseness, morphogenetic patterns and wall layering that can be establish in the grade (Fig. 7).

Figure half dozen. A typical oogonium and oospore.

Figure 7. The biodiversity of oogonial and oosporic wall structure and oosporic cytoplasmic organization. The diagram is arranged as three partially exploded wheels of sectors: the outermost sector provides examples of oogonial wall structure; the heart sector displays the complex layers of the oospore wall (the usually thick endospore wall is resorbed on germination; the epispore wall is usually thin, but may become convoluted; the exospore wall (of periplasmic derivation) is blocked in); the innermost sector with the dotted line indicating the plasma membrane, indicates the cytoplasmic reorganization in the oospore (zygote). The central ooplast is diagrammed to indicate the stage reversal between a solid ooplast (hatched) with translucent zones and a fluid ooplast with granules in Brownian motility; the outer zone displays the differential coalescence of lipids. *indicates a fluid layer.

The morphology and morphogenesis of the antheridium are simpler than those of the oogonium. When the antheridium is of regular occurrence, the style of application of the antheridium to the oogonium can exist diagnostic. Antheridial origin tin either be feature for a species, or highly variable within a species. Amphigynous antheridial development, in which the oogonial initial grows through the preformed antheridial initial, is feature of some species of Phytophthora. When an antheridium is nowadays, and fertilization occurs, there is the injection of a minor part of the antheridial protoplasm into the oogonium through a fertilization tube. Gametangial copulation (Myzocytiopsidales) occurs when the two gametic protoplasts condense to a mutual pore in the contiguous walls of the gametangia.

The morphogenesis of the oosphere is of major phylogenetic significance. The starting time distinction is between centripetal and centrifugal oosporogenesis; second is the extent of exclusion of part of the oogonioplasm for the oosphere(s) as periplasm. In the Peronosporales and Rhipidiales this periplasm may be substantial, persistent and sometimes initially nucleate. When the entire oogonial cavity is occupied by the oospore, the oospore is plerotic: very few taxa are truly plerotic. In Pythium the concept of an aplerotic index has been proposed to assistance taxonomic assessment of species differences.

Within the oospore, there are two complementary processes of vesicle coalescence which proceed simultaneously. The DBVs gradually coalesce to form a large, single membrane-spring structure, the ooplast. At the same fourth dimension in that location may be a variable degree of coalescence of lipid globules. The mature oospore of some species contains a single ooplast and a unmarried lipid globule. Mean oospore size is very variable (Table iii).

Tabular array 3. Oospore dimensions for a miscellaneous range of 20 taxa to give an indication of intergeneric variation. Annotation that there is approximately a hundredfold departure in oospore volume from the top to the bottom of this list, with a wide spread for all orders and some genera. Overall diameters are used considering nearly of the oospore wall thickness is due to endospore material which is resorbed on germination. Differences presumably reflect inoculum potential in relation to life-history strategies

Species Oospore diameter (μm) Oospore volume (μm)
Calyptralegnia achlyoides 51 (lxxx) 69 456
Achlya megasperma 48 57 905
Aqualinderella fermentans 46 50 965
Pythium polymastum 44 44 602
Basidiophora entospora 42 38 792
Phytophthora megasperma 41 36 086
Verrucalvus flavofaciens 41 36 086
Peronospora media 38 28 730
Pachymetra chaunorhiza 34 xx 579
Phytophthora infestans thirty xiv 137
Phytophthora citricola 22 5 575
Aphanomyces euteiches 22 5 575
Pythium acanthicum 21 4 849
Pythium ultimum var. ultimum 18 3 053
Myzocytiopsis lenticularis 18 3 053
Geolegnia inflata 14 one 436
Pythium parvum 13 1 150

The ooplast has different characteristics within the class. In most of the Saprolegniales the ooplast is fluid, with Brownian movement of granules, but in the Pythiales and Leptomitales the ooplast appears to exist homogeneous.

The evolutionary strategies of straminipilous fungi and mycote fungi are similar in that they depend on large populations with generation times that are ephemeral in the context of the generation times of the macrobiota of the ecosystem (Fig. 8). Diploidy has resulted in the oospore population operation in ecological or population genetics in a way analogous to that of heterokaryotic anamorph spore populations in other fungi; the oospore is not e'er a survival spore.

Figure 8. Congenitally foreshortened life-histories within the genus Pythium.

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Speciation, Process of

Guy L. Bush , in Encyclopedia of Biodiversity, 2001

III.C. Other Modes of Nonallopatric Speciation

III.C.1. Spontaneous Thelytokous Speciation

This fashion of sympatric speciation occurs when a unisexually reproducing taxa arises spontaneously from an unfertilized egg of a diploid bisexual species. Subsequent reproduction in taxa originating in this manner produces just females from unfertilized eggs. Several good examples are provided by White (1978).

3.C.2. Autopolyploid Speciation

Occasionally, polyploidization of a diploid species may occur spontaneously in one or more individuals. Because autopolyploid individuals have three or more than chromosome sets, each chromosome has more than than one homologous pairing partner. During meiosis, multivalents are produced leading to unbalanced gametes and zygotes, sterility, and other problems. Only rarely does autopolyploidy result in the origin of new species, such as in the common tater ( Solanum tuberosum) and its relatives (Grant, 1981). These unremarkably originate from crosses betwixt races whose chromosomes differ only slightly.

3.C.3. Speciation by Interspecific Hybridization

A new species tin ascend two ways by interspecific hybridization (Fig. 1f). Homoploid hybrid speciation results in a diploid-derived species, whereas polyploid hybrid speciation produces a species that combines a complete set of chromosomes from each hybridizing parental species. Hybrid species occupy habitats unlike from those of the parental species, thus reducing competition and the level of gene menstruation between them. Hybrid speciation, which is far more than common in plants than in animals, can occur in at least iv ways.

Three.C.3.a. Introgressive Hybrid Speciation

Individual factor exchange among closely related species provides recombinant offspring that shift to and exploit a new habitat not utilized by either parental species. The hybrid species may be interfertile with one or both parental species, but it is reproductively isolated from them by premating barriers to cistron flow. This style of speciation has been reported in several plant species (Grant, 1981), but confirmation of this mode of speciation remains controversial and requires definitive experimental and analytical studies.

III.C.3.b. Recombinational Hybrid Speciation

A far more common mode of hybrid speciation involves the formation and institution in the progeny of a chromosomally sterile or semisterile species hybrid of a new, structurally homozygous recombination blazon. Individuals are fertile within the line but isolated from other lines and from the parental species by a chromosomal sterility bulwark. It is most likely to occur when the hybrid interface is long and the organisms involved are predominantly selfing, relatively fertile, and possess few structural chromosome differences betwixt the parental species.

III.C.iii.b.i. Hybrid Speciation in Wild Sunflowers

A molecular study of hybrid speciation in the wild sunflowers Helianthus past Rieseberg et al. (1995) revealed that F1 hybrids of H. annus and H. petiolaris are semisterile with pollen viabilities less than 10% and seed fix less than 1%. Fii pollen viability is highly variable, ranging from xiii to 97%. The two species are distinguished by several morphological and chromosomal features, and based on chloroplast Dna and nuclear ribosomal Deoxyribonucleic acid variation they occur in divergent clades. Although the species are sympatric throughout much of the western United states, they have different ecological requirements. Helianthus annus is restricted to heavy, clay soils, whereas H. petiolaris predominantly inhabits dry, sandy soils.

Helianthus anomalus is a rare owned to xeric habitats in northern Arizona and southern Utah. It is well within range of parental species and is a recombinational hybrid resulting from a cross between H. annus and H. petiolaris. The Fane hybrids with parental species are partially sterile because chromosomal structural differences enhance reproductive isolation. A preliminary survey of 126 loci in natural populations of the parental species indicated that H. anomalus has loci derived from both H. annus and H. petiolaris. Some blocks of markers, possibly protected from recombination, are transmitted intact.

Helianthus anomalus combines rDNA repeat units and allozymes of H. annus and H. petiolaris equally predicted for diploid hybrid species, although individuals possess chlDNA haplotypes of H. annus and H. petiolaris rather than a unique haplotype. Genetic linkage maps generated for all three species using random amplified polymorphic Deoxyribonucleic acid markers reveal loci distributed onto 17 linkage groups corresponding to the haploid chromosome number of the three species. Although levels of polymorphisms vary from 212 in H. annus to 400 in H. petiolaris, map density is like among species. Past comparing genomic location and linear order of homologous markers, chromosomal structural relationships were inferred amongst the iii species.

Fifty-fifty though six linkage groups showed no changes in all three species, the remaining xi linkages were not conserved in gene order. The parental species differ from H. anomalus past at least 10 carve up structural rearrangements, iii inversions and a minimum of 7 inter-chromosomal translocations. The genome of H. anomalus is thus extensively rearranged relative to its parents. All 7 novel rearrangements in H. anomalus involve linkage groups that are structurally divergent in parental species, suggesting that structural differences may induce additional chromosomal rearrangements upon recombination.

III.C.3.c. Allopolyploid (Amphiploid) Speciation

Interspecific hybridization tin as well result in combining two or more than consummate chromosome sets. F1 hybrids produced between two established related species are ofttimes sterile because chromosomes lack sufficient homology to pair well at meiosis. Fertility is restored if hybrids persist long enough by asexual reproduction until somatic doubling of the chromosomes can occur in a bloom, or until in that location is a rare union between two unreduced gametes. A new sexually reproducing species is then established that is "instantaneously" isolated from both parental species (Grant, 1981).

Three.C.three.c.i. Allopolyploid Speciation in Spartina anglica

The recent natural rapid evolution of the amphiploid perennial salt marsh grass, Spartina anglica, provides an example of allopolyploid speciation (Raybould et al., 1991). This species originated on the south coast of England at the end of the nineteenth century. It arose equally a issue of chromosome doubling in S. x townsendii, a hybrid between the native British S. maritima and the N American S. alterniflora, introduced past shipping (Fig. 3). Spartina anglica is now widespread along the English coast and is highly successful.

Figure 3. The origin of the allopolyploid marsh grass species Spartina anglica (AABB) that resulted from hybridization between Due south. maritima (AA) and Southward. alternifolia (BB). Each letter in parentheses represents a haploid genome. The sterile hybrid, S. ten townsendii (AB), reproduced vegetatively until the genomes of individual plants doubled by autopolyploidy. This results in the institution of a new fertile sexually reproducing tetraploid species, S. anglica, with a full complement of chromosomes from Due south. maritima and Southward. alternifolia (AABB) that can produce normal gametes (ab).

Although more than half of all plant species are directly or indirectly the by-products of allopolyploid speciation, allopolyploid speciation is relatively rare in animals (White, 1978).

III.C.3.d. Direct and Reticulate Allogenous Speciation

At that place are two modes of allogenous speciation (i.e., combining the genomes of ii singled-out species). In the example of direct allogenous speciation, hybridization between 2 bisexual, closely related species combines the genomes of two distinct parental species giving rise to a new, unisexual species (Bullini, 1994). The hybridization issue produces either an allodiploid or an allopolyploid unisexual species that acquires clonal (parthenogenesis) or hemiclonal (hybridogenesis; i.due east., it must mate with males of a bisexual parental species) modes of reproduction. In most such clonal and hemiclonal organisms, the heterozygous genetic construction of the parental species is retained.

In the example of reticulate allogenous speciation, individuals of unisexual hybrid taxon hybridize with a bisexual relative giving rise to new, unisexual species, frequently with a higher ploidy level than that of the parental species (Bullini, 1994). Most take highly heterozygous genomes that brandish heterosis. Clones produced past direct and indirect allogenous speciation exhibit heterosis and demographic reward over both parental species and individuals.

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