Adaptive phenotypic change over the lifetime of an invader - Transcriptomics of a Cottus lineage of hybrid origin
Central idea about hybrid speciation is that recombination of parental traits allows hybrids to colonize new ecological niches, however, the connections between ecological context and the genetic traits underlying adaptive evolutionary changes remain elusive in most systems. I study a young hybrid lineage of Cottus fish that has invaded an environment that is significantly warmer in the summer and where its parent species do not occur. This suggests that hybrids have adapted to new ecological niche, coarsely related to water temperature. I tested temperature preference of the three Cottus lineages in the laboratory conditions and found that hybrids, despite living in warmer waters, prefer same water temperature as their parental species. Then I used RNAseq to compare, in an unprecedented detail, transcriptomes of wild populations over the course of a complete year in order to shed light on the nature of local adaptation. I found genes that respond to changes in temperature were enriched for metabolism-related GO terms and suggest thermal adaptation in the hybrid lineage. The analysis was complemented by sampling of transcriptomes from fish raised in controlled laboratory conditions, which revealed heritable components of divergence. Some patterns distinguish hybrid Cottus from both parents and are likely to be involved in evolutionary novelty after hybridization. Transgressive segregation is thought to be main source of potential to expand to new ecological niches. I found ecologically relevant transgressive traits in hybrids and those were mainly connected to metabolic functions suggesting that hybrids have altered the metabolism relative to the parental species. Another interesting candidate gene shows transgressive expression pattern likely due to an increase in copy number and may be involved in the extended red light vision. This possibly constitutes an adaptation to life in the murky waters of downstream habitats of the invasive Cottus.