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.