Polyploidy Is Genetic Hence May Cause Non‐Adaptive Radiations, Whereas Pseudopolyploidy Is Genomic Hence May Cause Adaptive Non‐Radiations

Abstract

There are two ways eukaryotes double number of chromosomes: (1) whole genome duplication (polyploidy), in which all nuclear DNA is replicated, and (2) karyotypic fission (pseudopolyploidy), in which all chromosomes are physically bifurcated. We contrast polyploidy with pseudopolyploidy, highlighting when it is crucial to look at genetic vs. genomic levels. We review history of pseudopolyploidy, including recent mechanisms by which chromosomal bifurcation may occur and outline methods for detecting such genomic changes. We then delve into the evolutionary implications, with particular focus on adaptive potential, of these two forms of doubling chromosome numbers. We address the common assertion that polyploidy induces adaptive radiations, which contains three fallacies. First, while polyploidy causes quantum speciation, evolutionary theory implies that these radiations should be non-adaptive. Polyploidy causes reproductive isolation, minute effective population sizes, and increased mutation rates, which all imply a diminished role for selection. Second, due to lack of karyotyping in recent decades and lack of distinction between genomic and genetic effects, it is usually impossible to detect pseudopolyploids. Third, pseudopolyploids lack minority cytotype exclusion because they readily backcross with their progenitors, which thereby means no reproductive isolation for newly formed pseudopolyploids. Pseudopolyploidy will thereby not result in radiations until pseudopolyploid descendants undergo subsequent chromosome rearrangements or grow new centromeres. Pseudopolyploids may have a modest selective advantage over their progenitors due to diminished linkage disequilibrium. Thus, pseudopolyploidy may induce adaptive non-radiations. We encourage a renaissance of karyotyping to distinguish between these two mechanisms and a renaissance in genomic perspectives in evolution.

Is post-polyploidization diploidization the key to the evolutionary success of angiosperms?

“…There are also arguments for polyploids not contributing to adaptive radiations per se , but rather polyploids simply arising through their immediate reproductive isolation from parental lineages (purely as a result of differences in chromosome number). Where recurrent polyploidization occurs between the same or different parental species, (non‐adaptive) radiations can result (Gorelick & Olson, ). Even if neopolyploids do have higher extinction rates and make a lower contribution to recent species diversification, as has been argued, all angiosperm species nonetheless have (often multiple rounds of) polyploidy in their ancestry; the ramifications of this are significant and an important focus of research.…”

Karyological patterns in the European endemic genus Soldanella L.: Absolute genome size variation uncorrelated with cytotype chromosome numbers

“…This extraordinary diversity in the DNA amount is not entirely dependent on the number of genes or overall organism complexity (e.g., Gregory, 2001), but can be mostly attributed to differences in repetitive DNA, especially, to transposable elements (Sanmiguel and Bennetzen, 1998; Hawkins et al, 2006). It has been shown that a substantial part of the AGS variability in plants is linked with variation in chromosome numbers, the number of polyploidization events or less dramatic changes such as rearrangements of existing chromosomes (e.g., Leitch and Bennett, 1997, 2004; Otto and Whitton, 2000; Bennetzen et al, 2005; Gorelick and Olson, 2013). Multiple recent studies point out that although AGS initially increases through whole‐genome duplication or accumulation of transposable elements, these processes are usually followed by genome downsizing by means of deletion of transposable elements, illegitimate recombination, or removal of large blocks of DNA (Sanmiguel and Bennetzen, 1998; Hawkins et al, 2006; Vitte and Bennetzen, 2006; Ross‐Ibarra, 2007; Tiley and Burleigh, 2015).…”

Do Micrognathozoa have micro-genomes?

“…Double the nuclear genome size via polyploidy (i.e. whole genome duplication; see Gorelick & Olson, ) and the nucleus will be roughly twice the volume. Polyploid cells are larger than their diploid progenitors (Lomax et al ., ).…”