Most often, natural selection will prevent the introgression of foreign alleles into a divergent genomic background. However, in certain circumstances, introgression may be favored if the genomic region confers and adaptive benefit to the species. Although these events may be rare, they offer the genetic variation needed for adaptive evolution that would be unlikely to arise through mutation alone. Understanding the interaction between hybridization and selection is critical for predicting how hybrid populations evolve adaptive traits. Despite a longstanding recognition that hybridization is an important source of novel genetic variation, there is a lack of empirical studies demonstrating how interspecific gene flow influences local adaptation in hybrid populations. In the genomic era, it is becoming increasingly possible to study how divergent genomic background may facilitate local adaptation either in the hybrid population or in one of the two parental species by adaptive introgression.
To identify genomic regions associated with traits under thermal selection (e.g. thermal tolerance) and to track how the allele frequency changes through time in natural hybrid populations.
Differences in thermal tolerance between species are driven by identifiable genomic regions under local adaption. If ancestry at genomic regions associated with thermal tolerance is a function of elevation or water temperature, then we predict a greater proportion of the genome in this region will skew towards the highland parentancestry (i.e. X. malinche) in populations occupying higher elevations/cooler water and towards the lowland parent ancestry (i.e. X. birchmanni) in those occupying warmer water/lower elevation.
To identify the region associated with thermal tolerance in these fish we are conducting a QTL analysis. In January 2020, we conducted the behavioral trials and sequenced 195 intercrossed hybrids. We are currently waiting for the samples to be sequenced. Once they are sequenced, our next steps are to perform the QTL analysis. While we were waiting on the sequence data, we have investigated changes in allele frequency at a priori heat shock protein loci that may be under selection with respect to elevation and water temperature. One of the heat shock protein loci in the highland X. malinche-skewed hybrid population is significantly skewed towards X. birchmanni ancestry.
Previous work conducted in the lab demonstrated physiological adaptation for local thermal conditions in natural populations with X. birchmanni maintaining thermal equilibrium at warmer water temperatures than X. malinche (Culumber et al 2012). It’s unclear exactly which regions of the genome contribute to interspecific differences in thermal tolerance. Therefore, we will perform a QTL analysis for regions associated with thermal tolerance. Recombinant genotypes allow us to use admixture mapping strategies to identify genomic regions under selection and how these genes interact with environmental variation, ultimately shedding light on the genetic basis of local adaptation. In addition, we will leverage genomic time-series datasets from both experimental and natural populations to track changes in allele frequencies over time at putative loci under selection (e.g. heat shock protein loci).