Spea hybridization gene expression study

Interbreeding species can generate hybrids with lower fitness than their parents. Such low hybrid fitness is often caused by genetic incompatibilities between parental genomes. These incompatibilities might be driven either by fixed allelic differences between hybridizing species, or, alternatively, by standing variants that segregate within them. If incompatibilities derive from standing genetic variation, then incompatibilities in hybrids will not only vary across different populations but they can also evolve. Here, we use gene expression as a proxy for possible incompatibilities, and contrast hybrids and pure-species types from populations where hybridization is on-going (sympatry) and populations where hybridization has not occurred (allopatry). We examined gene expression at 10,695 protein-coding genes. We found that hybrids were less likely to differ in expression between allopatry and sympatry when the pure-species types were themselves different in expression. Such a pattern is expected if species differences in expression reflect fixed allelic differences, and such differences contribute to incompatibilities in hybrids. Nevertheless, we find that population had substantial effects on gene expression in hybrids but not the pure-species: 2,837 genes (27% of the transcriptome) differed between the hybrids in sympatry versus allopatry whereas only 311 genes (3% of the transcriptome) differed within each species between sympatry and allopatry. These results suggest that incompatibilities in hybrids derive from segregating variation within the species. Moreover, our results reveal that incompatibilities can evolve. Taken together our results have important implications for the evolutionary maintenance––or breakdown––of reproductive barriers between species.