Balancing selection across mate compatibility loci in differentiated populations inferred from FST, pi, and Tajima’s D
We used FST, pi, and Tajima’s D as measures of gene flow and selection and tested whether the contigs that contain mating-type loci were differentiated among populations. FST was comparable among all populations (Fig. 4A and 4D), but decreased when the South Australian, Tasmanian, and Victorian population was removed (mean FST of all populations at the HD locus = 0.36; mean FST of all populations sans South Australian, Tasmanian, and Victorian population = 0.26). This difference in FST supports gene flow or shared ancestry among the eastern populations of P. subaeruginosa in Australia, which are less differentiated from each other than from the South Australia, Tasmanian, and Victorian population.
Nucleotide diversity (pi) varied greatly across populations for these two regions (Fig. 4B and 4E) and was generally low (<0.2) across the contigs that contained mating-type genes. Mating-compatibility loci did not have higher or lower nucleotide diversity than other parts of the contig, which may reflect genetic drift since populations became isolated.
Tajima’s D was positive across the contigs that contained mating-type loci (Fig. 4C and 4F), which is expected when multiple alleles are maintained in populations under balancing selection.