Abstract
BackgroundBiased sex ratios are common among dioecious plant species despite the theoretical prediction of selective advantage of even sex ratios. Albeit the high prevalence of deviations from even sex ratios, the genetic causes to sex biases are rarely known outside of a few model species. Here we present a mechanism underlying the female biased sex ratio in the dioecious willow species Salix viminalis.ResultsWe compared the segregation pattern of genome-wide single nucleotide polymorphism markers in two contrasting bi-parental pedigree populations, the S3 with even sex ratio and the S5 with a female biased sex ratio. With the segregation analysis and comparison between the two populations, we were able to demonstrate that sex determination and sex ratio distortion are controlled by different genetic mechanisms. We furthermore located the sex ratio distorter locus to a Z/W-gametologous region on chromosome 15, which was in close linkage with the sex determination locus. Interestingly, all males in the population with biased sex ratio have in this sex ratio distorter locus the same genotype, meaning that males with the Z1/Z3-genotype were missing from the population, thereby creating the 2:1 female biased sex ratio.ConclusionsWe attribute the absence of Z1/Z3 males to an allelic incompatibility between maternally and paternally inherited alleles in this sex ratio distorter locus. Due to the tight linkage with the sex determination locus only male individuals are purged from the population at an early age, presumably before or during seed development. We showed that such allelic incompatibility could be stably maintained over evolutionary times through a system of overdominant or pseudooverdominant alleles. Thus, it is possible that the same mechanism generates the female biased sex ratio in natural willow populations.
Highlights
Biased sex ratios are common among dioecious plant species despite the theoretical prediction of selective advantage of even sex ratios
The S5 population is composed of 182 females and 89 males, and displays an overall 2:1 female biased sex ratio
As we have demonstrated that the biased sex ratio is the result of the absence of a male genotypic class while females are present in two classes and follow Mendelian segregation patterns, we can firmly reject the hypothesis that meiotic drive is causing the sex bias
Summary
Biased sex ratios are common among dioecious plant species despite the theoretical prediction of selective advantage of even sex ratios. Pollen competition (certation) is another phenomenon that can distort progeny sex ratios in plants and has been demonstrated to lead to female biased sex ratios in the male heterogametic species Rumex nivalis [12] as well as in Silene latifolia [15,16,17]. It has been demonstrated that the presence of recessive deleterious alleles, in combination with systematic inbreeding, can cause a greater mortality in the homogametic sex [20]. These results suggest that hemizygous individuals may carry a genetic factor that consistently mask deleterious alleles from expression
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