Bottlenecks and inbreeding depression in autotetraploids

Abstract

Inbreeding depression is dependent on the ploidy of populations and can inhibit the evolution of selfing. While polyploids should generally harbor less inbreeding depression than diploids at equilibrium, it has been unclear whether this pattern holds in non-equilibrium conditions following bottlenecks. We use stochastic individual-based simulations to determine the effects of population bottlenecks on inbreeding depression in diploids and autotetraploids, in addition to cases where neo-autotetraploids form from the union of unreduced gametes. With a ploidy-independent dominance function based on enzyme kinetics, inbreeding depression is generally lower in autotetraploids for fully and partially recessive mutations. Due to the sampling of more chromosomes during reproduction, bottlenecks generally reduce inbreeding depression to a lesser extent in autotetraploids. All else being equal, population bottlenecks may have ploidy-dependent effects for another reason-in some cases matings between close relatives temporarily increase inbreeding depression in autotetraploids by increasing the frequency of the heterozygous genotype harboring the most harmful mutations. When neo-autotetraploids are formed by few individuals, inbreeding depression is dramatically reduced, given extensive masking of harmful mutations following whole genome duplication. This effect persists as nascent tetraploids reach mutation-selection-drift balance, providing a transient period of permissive conditions favoring the evolution of selfing.

Selfing ability and drift load evolve with range expansion

“…Moreover, contemporary levels of inbreeding depression may not reflect those immediately following bottlenecks, because such declines in inbreeding depression should be erased relatively quickly after populations rebound in size (Kirkpatrick & Jarne ; Balick et al. ; Layman & Busch ).…”

“…Inbreeding depression, which is mainly caused by partially or fully recessive mutations (Charlesworth & Wills 2009), is expected to decline following a population's recovery from a bottleneck when homozygous mutations are purged by natural selection (Kirkpatrick & Jarne 2000;Glemin 2003;Balick et al 2015). This expectation follows from theoretical work in diploid populations (Glemin 2003;Balick et al 2015;Peischl & Excoffier 2015), yet the purging of recessive mutations is less effective in tetraploids (Ronfort 1999;Layman & Busch 2018). Moreover, contemporary levels of inbreeding depression may not reflect those immediately following bottlenecks, because such declines in inbreeding depression should be erased relatively quickly after populations rebound in size (Kirkpatrick & Jarne 2000;Balick et al 2015;Layman & Busch 2018).…”

“…This expectation follows from theoretical work in diploid populations (Glemin 2003;Balick et al 2015;Peischl & Excoffier 2015), yet the purging of recessive mutations is less effective in tetraploids (Ronfort 1999;Layman & Busch 2018). Moreover, contemporary levels of inbreeding depression may not reflect those immediately following bottlenecks, because such declines in inbreeding depression should be erased relatively quickly after populations rebound in size (Kirkpatrick & Jarne 2000;Balick et al 2015;Layman & Busch 2018).…”

High-Resolution Linkage Map With Allele Dosage Allows the Identification of Regions Governing Complex Traits and Apospory in Guinea Grass (Megathyrsus maximus)

“…For example, an autotetraploid genotype with the "abcd" alleles at a locus that can form six expected combinations of gametes, namely, "ab", "ac", "ad", "bc", "bd", and "cd", but a homozygous gamete can be generated, e.g., "aa", "bb", "cc", or "dd" (Haldane, 1930;Mather, 1935;Haynes and Douches, 1993). How to accommodate DR and its implications in a breeding program as well as the use of these marker loci in linkage mapping has been discussed for a long time (Butruille and Boiteux, 2000;Luo et al, 2000;Xu et al, 2013;Layman and Busch, 2018;Bourke et al, 2019) for some economically important species, such as potato (Bradshaw, 2007;Bourke et al, 2015) and alfalfa (Julier et al, 2003), but has not yet been reported in guinea grass.…”

High-resolution Linkage Map with Allele Dosage Allows the Identification of an Apomixis Region and Complex Traits in Guinea Grass (Megathyrsus maximus)

“…For example, an autotetraploid genotype with the 'abcd' alleles at a locus that can form six expected combinations of gametes, namely, 'ab', 'ac', 'ad', 'bc', 'bd', and 'cd', but a homozygous gamete can be generated, e.g., 'aa', 'bb', 'cc', or 'dd' (Haldane, 1930;Mather, 1935;Haynes and Douches, 1993). How to accommodate DR and its implications in a breeding program as well as the use of these marker loci in linkage mapping has been discussed for a long time (Butruille and Boiteux, 2000;Luo et al, 2000;Xu et al, 2013;Layman and Busch, 2018;Bourke et al, 2019) for some economically important species, such as potato (Bradshaw, 2007;Bourke et al, 2015) and alfalfa (Julier et al, 2003), but has not yet been reported in guinea grass.…”