The molecular mechanisms underlying local adaptation to high elevation habitats in the honey bee Apis mellifera
Sensory and Behavioural Biology
Final Report Abstract
In our project, we challenged the hypothesis that chromosomal inversions can be linked to phenotypic adaptation to elevation. We compared honey bees from high and low elevations on different African mountains in Uganda, Tanzania and Kenya to unravel chromosomal rearrangements and single nucleotide polymorphisms for population and subspecies characterization. Whole genome sequences provided insights into subpopulation structure, levels of differentiation, signatures of selection and the distribution of the chromosomal inversions across different elevations and geographic regions. Gene expression (qRT-PCR; RNAseq) deciphered molecular mechanisms underlying adaptation to elevation. We compared the cuticular hydrocarbon (CHC) profiles of bees from different elevations and colonies translocated to the opposite elevation to search for physiological responses to elevation via RNAseq and CHC profiling to test for phenotypic plasticity versus local adaptation. The octopamine receptor gene AmOARβ2 and pigmentation synthesis-related genes ebony and tan were knocked out with CRISPR/Cas9 for functional characterization and their relevance to elevation adaptation. Whole genome data confirmed the presence of previously described chromosomal inversions (r7 and r9), while we identified remarkable differentiation of r7/r9 haplotypes among the different mountain regions. Initial transcriptome analyses using RNAseq and gene expression comparisons using qRT-PCR revealed several candidate genes that differed significantly between bees from different altitudes. Remarkably, chromosomal inversion appears to affect gene expression. Octopamine β receptor genes differed systematically in mRNA expression between highland and lowland honey bees. Cuticular hydrocarbon profiles similarly differed between highland and lowland honey bees. For the first time, we knocked out the two putative genes involved in pigment synthesis in the honey bee, ebony and tan. While ebony knockout mutants were black, tan mutants displayed a yellow pigmentation, demonstrating a clear role for these genes in honey bee pigmentation. AmOARβ2 knockouts suggest an important role for this receptor in thermoregulatory responses. Our project reveals important mechanisms of adaptation to elevation in honey bees and paves the way for unravelling further candidate genes, pathways and networks involved in this form of phenotypic plasticity.
