Genetics of hybrid male sterility among strains and species in the Drosophila pseudoobscura species group

Abstract

Taxa in the early stages of speciation may bear intraspecific allelic variation at loci conferring barrier traits in hybrids such as hybrid sterility. Additionally, hybridization may spread alleles that confer barrier traits to other taxa. Historically, few studies examine within- and between-species variation at loci conferring reproductive isolation. Here, we test for allelic variation within Drosophila persimilis and within the Bogota subspecies of D. pseudoobscura at regions previously shown to contribute to hybrid male sterility. We also test whether D. persimilis and the USA subspecies of D. pseudoobscura share an allele conferring hybrid sterility in a D. pseudoobscura bogotana genetic background. All loci conferred similar hybrid sterility effects across all strains studied, although we detected some statistically significant quantitative effect variation among D. persimilis alleles of some hybrid incompatibility QTLs. We also detected allelism between D. persimilis and D. pseudoobscura USA at a second chromosome hybrid sterility QTL. We hypothesize that either the QTL is ancestral in D. persimilis and D. pseudoobscura USA and lost in D. pseudoobscura bogotana, or gene flow transferred the QTL from D. persimilis to D. pseudoobscura USA. We discuss our findings in the context of population features that may contribute to variation in hybrid incompatibilities.

Figure 1. Backcross Design

Males from each line of D. persimilis were crossed to white-eyed D. pseudoobscura bogotana (bog) females. The fertile F1 hybrid females were backcrossed to white-eyed D. pseudoobscura bogotana males. The white-eyed backcross male progeny (BCbogw) were then dissected and genotyped.

Figure 2. Percent fertility of individuals with a D. pseudoobscura USA copy of QTL 2

Genotypes for each QTL (Q2, Q3 and Q4) are listed vertically. Sample sizes for each genotype combination are listed below the genotypes. "bog": D. pseudoobscura bogotana allele, "per/bog": heterozygous for D. persimilis and D. pseudoobscura bogotana alleles, "ps/bog": heterozygous for D. pseudoobscura USA and D. pseudoobscura bogotana alleles.