From missing genotypes to negative epistasis. Russ Corbett-Detig, Jun Zhou, Daniel Hartl, Julien Ayroles. OEB, Harvard University, Cambridge, MA.

   Negative epistasis, or genetic incompatibilities resulting from interactions between loci, is believed to be an important force in speciation. To date the majority of speciation genes that have been characterized affect hybrids of ancient speciation events. In most cases it is unclear if these loci could have contributed to the initial genetic isolation of taxa. Here we present an alternative approach that aims to identify incompatibiliies segregating within a species. Specifically, we scanned panels of Drosophila melanogaster recombinant inbred lines for inter-chromosomal linkage disequilibria. In total, we identified eightteen pairs of incompatible haplotypes, and conservatively estimate that any two haploid genomes have one in three chance of harboring a pair of incompatible alleles. Building on the genome scan, we showed that one pair of incompatible alleles causes almost complete male sterility. Our results indicate that natural populations are segregating many cryptic incompatible alleles. This suggests that postzygotic isolating barriers exist prior to divergence between populations and that speciation may be a highly polygenic phenomenon.