Gene Expression Divergence between Subspecies of the House Mouse and the Contribution to Reproductive Isolation

Under the Biological Species Concept species are groups of interbreeding individuals that are reproductively isolated from other such groups. A common form of isolation in animals is intrinsic postzygotic isolation via hybrid sterility/inviability. There is a strong consensus that hybrid dysfunctions are caused by epistatic interactions between incompatible alleles from different loci (Dobzhansky-Muller incompatibilities). The identification of genes that contribute to reproductive isolation between taxa is critical to the understanding of the process of speciation, but identifying such genes has proven to be difficult. It appears that regulatory evolution might play an important role in postzygotic isolation and the formation of species. The present study employs a whole genome microarray approach to identify genes with regulatory differences between three subspecies of the house mouse, Mus musculus musculus, M. m. domesticus and M. m. castaneus. The within-locus mode of inheritance for gene expression was assessed for three different tissues (brain, liver and testis) by studying the subspecies and their male reciprocal F1 hybrids. The vast majority of transcripts are additively expressed in the hybrids with only few transcripts showing dominance or overdominance in expression except for one direction of one cross, which shows large misexpression in the testis. The reliability of the observed pattern was ensured by three different analysis methods as well as control experiments. The results suggest that additivity is the general mode of inheritance regarding gene expression changes between house mouse subspecies. Differentially expressed transcripts provide promising candidate genes that could be related to reproductive isolation through regulatory incompatibilities. Several transcripts with expression differences between M. m. musculus and M. m. domesticus were selected for further investigation. A validation approach using quantitative Real-Time PCR strongly emphasizes the need for confirmation of microarray candidate genes. The results show that sequence differences even between closely related taxa have the potential to influence expression data from both microarray and follow-up validation approaches. As divergent gene expression evolution between taxa may be entirely neutral, samples from a transect of the natural musculus-domesticus hybrid zone in Bavaria were analyzed in order to assess functional consequences for two candidate genes. Both genes show large expression differences between the subspecies. The analysis revealed that it is unlikely that the two genes contribute to reproductive isolation between the subspecies as no sign of limited introgression is evident. Rather, the hybrid zone approach in combination with population genetic analyses suggests adaptive introgression of those alleles that are associated with high expression levels. In both cases, the high expression phenotype represents the derived state and is associated with reduced levels of nucleotide polymorphism and a negative Tajima's D. For both genes, regulatory and protein-coding evolution is decoupled and the expression difference results from cis- rather than trans-acting changes.