Abstract
Newly formed polyploids often show extensive meiotic defects, resulting in aneuploid gametes, and thus reduced fertility. However, while many neopolyploids are meiotically unstable, polyploid lineages that survive in nature are generally stable and fertile; thus, those lineages that survive are those that are able to overcome these challenges. Several genes that promote polyploid stabilization are now known in plants, allowing speculation on the evolutionary origin of these meiotic adjustments. Here, I discuss results that show that meiotic stability can be achieved through the differentiation of certain alleles of certain genes between ploidies. These alleles, at least sometimes, seem to arise by novel mutation, while standing variation in either ancestral diploids or related polyploids, from which alleles can introgress, may also contribute. Growing evidence also suggests that the coevolution of multiple interacting genes has contributed to polyploid stabilization, and in allopolyploids, the return of duplicated genes to single copies (genome fractionation) may also play a role in meiotic stabilization. There is also some evidence that epigenetic regulation may be important, which can help explain why some polyploid lineages can partly stabilize quite rapidly.
Highlights
Polyploids, which result from whole genome duplication (WGD) events, are thought to have enhanced adaptability, which might explain why they are so pervasive in nature [1]
It could be that meiosis genes are especially strongly co-evolved, and would show biased fractionation towards one parent, or it could be that the heterozygosity of these genes is inherently bad. Does this fractionation contribute to meiotic stabilization? In keeping with the idea that dosage can be a factor, knocking out one of the two copies carried by B. napus of MSH4, the meiotic gene with the fastest fractionation pattern observed in eukaryotes, prevents illegitimate crossovers between homoeologous chromosomes in allohaploids while levels of legitimate homologous crossovers are maintained in euploids [51]
It was shown that meiotic stability of A. arenosa neo-auto-octoploids generated by whole genome quadruplication of diploids displayed extensive meiotic aberrations, whereas neo-auto-octoploids produced by WGD of an established tetraploid show markedly more regular meiosis [9]
Summary
Polyploids, which result from whole genome duplication (WGD) events, are thought to have enhanced adaptability, which might explain why they are so pervasive in nature [1]. (b–h) Schematic cytological diploidization (with and(shapes without crossovers involving than or two chromosomes, representations of putative genes (rectangles) and factors including circles, squares,more hexagons trianHomologous chromosomes theare same sub-genome are shown in the same color gles) taking differentrespectively). The types of adjustment that occur differ by polyploid type, with autopolyploids (originated from within-species WGD) generally evolving bivalent formation in the absence of partner preferences (Figure 1a) [13], and allopolyploids (in which WGD is preceded or followed by inter-specific hybridization), generally evolving crossover preferences among more similar homologs from the same sub-genome (Figure 1a) [17]. The specific defects of polyploids, and how they are corrected, have been discussed elsewhere [13,17,28,29], here I discuss where these genes come from; in other words, the “genetic route” to stable polyploidy
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