Parallel evolution of gene expression between trophic specialists despite divergent genotypes and morphologies

Parallel evolution of gene expression commonly underlies convergent niche specialization, but parallel changes in expression could also underlie divergent specialization. We investigated divergence in gene expression and whole-genome genetic variation across three sympatric Cyprinodon pupfishes endemic to San Salvador Island, Bahamas. This recent radiation consists of a generalist and two derived specialists adapted to novel niches: a scale-eating and a snail-eating pupfish. We sampled total mRNA from all three species at two early developmental stages and compared gene expression with whole-genome genetic differentiation among all three species in 42 resequenced genomes. Eighty percent of genes that were differentially expressed between snail-eaters and generalists were up or down regulated in the same direction between scale-eaters and generalists; however, there were no fixed variants shared between species underlying these parallel changes in expression. Genes showing parallel evolution of expression were enriched for effects on metabolic processes, whereas genes showing divergent expression were enriched for effects on cranial skeleton development and pigment biosynthesis, reflecting the most divergent phenotypes observed between specialist species. Our findings reveal that even divergent niche specialists may exhibit convergent adaptation to higher trophic levels through shared genetic pathways. This counterintuitive result suggests that parallel evolution in gene expression can accompany divergent ecological speciation during adaptive radiation.