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

摘要

How self-incompatibility systems are maintained in plant populations is stilla debated issue. Theoretical models predict that self-incompatibility systemsbreak down according to the intensity of inbreeding depression and number ofS-alleles. Other studies have explored the function of asexual reproduction inthe maintenance of self-incompatibility. However, the population genetics ofpartially asexual, self-incompatible populations are poorly understood andprevious studies have failed to consider all possible effects of asexualreproduction or could only speculate on those effects. In this study, weinvestigated how partial asexuality may affect genetic diversity at the S-locusand fitness in small self-incompatible populations. A genetic model includingan S-locus and a viability locus was developed to perform forward simulationsof the evolution of populations of various sizes. Drift combined with partialasexuality produced a decrease in the number of alleles at the S-locus. Inaddition, an excess of heterozygotes was present in the population, causing anincrease in mutation load. This heterozygote excess was enhanced by theself-incompatibility system in small populations. In addition, in highlyasexual populations, individuals produced asexually had some fitness advantagesover individuals produced sexually, because sexual reproduction produceshomozygotes of the deleterious allele, contrary to asexual reproduction. Ourresults suggest that future research on the function of asexuality for themaintenance of self-incompatibility will need to (1) account for whole-genomefitness (mutation load generated by asexuality, self-incompatibility and drift)and (2) acknowledge that the maintenance of self-incompatibility may not beindependent of the maintenance of sex itself.