Genetic diversity and fitness in small populations of partially asexual, self-incompatible plants

Abstract

How self-incompatibility systems are maintained in plant populations is still a debated issue. Theoretical models predict that self-incompatibility systems break down according to the intensity of inbreeding depression and number of S-alleles. Other studies have explored the function of asexual reproduction in the maintenance of self-incompatibility. However, the population genetics of partially asexual, self-incompatible populations are poorly understood and previous studies have failed to consider all possible effects of asexual reproduction or could only speculate on those effects. In this study, we investigated how partial asexuality may affect genetic diversity at the S-locus and fitness in small self-incompatible populations. A genetic model including an S-locus and a viability locus was developed to perform forward simulations of the evolution of populations of various sizes. Drift combined with partial asexuality produced a decrease in the number of alleles at the S-locus. In addition, an excess of heterozygotes was present in the population, causing an increase in mutation load. This heterozygote excess was enhanced by the self-incompatibility system in small populations. In addition, in highly asexual populations, individuals produced asexually had some fitness advantages over individuals produced sexually, because sexual reproduction produces homozygotes of the deleterious allele, contrary to asexual reproduction. Our results suggest that future research on the function of asexuality for the maintenance of self-incompatibility will need to (1) account for whole-genome fitness (mutation load generated by asexuality, self-incompatibility and drift) and (2) acknowledge that the maintenance of self-incompatibility may not be independent of the maintenance of sex itself.

Microsatellite evidence for high clonality and limited genetic diversity in Ziziphus celata (Rhamnaceae), an endangered, self-incompatible shrub endemic to the Lake Wales Ridge, Florida, USA

“…The ability of self-incompatible species to maintain mating-allele diversity is limited in small populations, and genome-wide diversity falls even more rapidly (Byers and Meagher 1992;Navascues et al 2010). Byers and Meager (1992) noted that populations smaller than 50 individuals rapidly lost mating-alleles.…”

Efecto de la reproducción clonal en la estructura genética de Pentaclethra macroloba (Fabaceae: Mimosoideae)

“…Third, there was no significant difference in inbreeding or genetic diversity values between the two habitats or across elevations. Low F IS values, as we observed, are generally expected in a facultatively asexual species with outcrossing due to fixation of heterozygotes (Navascues, Stoeckel, & Mariette, 2010;Stoeckel et al, 2006). If asexual contribution to the adult populations was significant, we would expect to see a reduced amount of variation centered in the swamp, where rare variant alleles would be lost through an overabundance of identical genotypes.…”

Effects of complex life cycles on genetic diversity: cyclical parthenogenesis

“…For example, mean F IS values in moderately clonal populations are nearly the same as those under panmixia (Balloux et al, 2003). Similarly, individual-based simulations to examine multilocus indices of genetic diversity have shown that intermediate levels of clonality have limited effects on r d , a measure of gametic phase disequilibrium (De Meeûs and Balloux, 2004;De Meeûs et al, 2006;Navascués et al, 2010). Furthermore, coalescent models that include partial clonality have demonstrated that genealogies differ from those with full sexuality when the rate of sex is on the order of 1/N, where N is the population size (Bengtsson, 2003;Ceplitis, 2003;Hartfield et al, 2015).…”