Broadly, the goal of the proposed study is to understand the genetic basis of postzygotic reproductive isolation. Two fundamental problems will be addressed by this study: 1) how hybrid incompatibilities arise within genetic and developmental pathways, and 2) how incompatibility alleles become established in natural populations. The first part of the proposed study will use high-resolution genetic mapping to find and characterize genes that cause hybrid male sterility between Drosophila virilis and D. americana, two species in the early stages of divergence. Classic studies have shown that both D. virilis and D. americana exhibit substantial within-species heterogeneity for factors that contribute to hybrid male sterility when the two species are crossed. Therefore, this system provides an opportunity to identify those genes at which hybrid incompatibilities first appear. In addition, understanding the genetic basis of hybrid male sterility may shed light on the genetics of human infertility. The second part of the proposed study will take a comparative mapping approach to examine variation for hybrid male sterility genes within and between natural populations of D. americana. Ultimately, this research aims to take a molecular population genetics approach to investigate the evolutionary dynamics of hybrid incompatibility and species divergence. Some of the hybrid male sterility genes discovered in Drosophila might show functional conservation with human fertility genes. The proposed study is relevant to public health because studies of Drosophila fertility can be applied to understanding the genetics of human fertility.