F ST between haploids and diploids in species with discrete ploidy phases

Many organisms alternate between distinct haploid and diploid phases, which generates population structure according to ploidy level. In this research, we consider a haploid-diploid population using statistical approaches developed for spatially subdivided populations, where haploids represent one "patch" and diploids another "patch." In species with alternating generations, sexual reproduction causes movement from diploids to haploids (by meiosis with recombination) and from haploids to diploids (by syngamy). Thus, an allele in one ploidy phase can be said to "migrate" to the other ploidy phase by sexual reproduction and to "remain" in the same ploidy phase by asexual reproduction. By analyzing a coalescent model of the probability of identity by descent and by state for a haploid-diploid system, we define FST-like measures of differentiation between haploids and diploids and show that these measures can be simplified as a function of the extent of sexuality in each ploidy phase. We conduct simulations with an infinite-alleles model and discuss a method for estimating the degree of effective sexuality from genetic data sets that use the observed FST measures of haploid-diploid species.