Alleles at mating genes are diverse and haplotypes are sexually compatible across geographically isolated populations
We explored the boundaries of sexual reproduction with a hypothesis that allopatric and sympatric speciation may erode mate compatibility and conspecific populations are sexually compatible. We examined phylogenetic relationships of two concatenated STE3.2 genes at the pheromone/receptor (PR) locus and HD1 and HD2 genes at the homeodomain locus (HD) (Figs 3A, B). We identified approximately 25–30 alleles at PR and HD loci across the Australian population, which we consider high diversity given that many samples are siblings. Alleles at PR and HD loci were rarely shared among geographically distant populations, and we did not expect to see any geographic pattern beyond this given the high allelic diversity and effects of negative frequency selection, which distributes alleles equally across populations.
Crosses between one haploid culture from the Bunya population were compatible, based on formation of clamp connections, with isolates from the most geographically distant and genetically diverse populations in Tasmania and Victoria. This suggests there are no barriers to reproductive compatibility, even among mating-compatibility loci that have differentiated in populations, and gene flow could occur among geographically limited populations if given the opportunity.
We expected incompatible crosses between haplotypes with identical or near-identical STE3 alleles (e.g., BRIP75264 × BRIP75266, BRIP75402 × BRIP75297) and different alleles at HD loci. A cross of two near identical haplotypes of siblings in the Bunya population (BRIP75388 × POZ13-2) was successful, which is expected in tetrapolar mating systems (25% of siblings are sexually compatible). However, three crosses against BRIP75297 and a cross of BRIP75266 × POZ16-3 were incompatible (based on a lack of clamp connections) despite different alleles at PR and HD loci.