Comparative Transcriptomics in Adaptive Evolution

Abstract

Comparative transcriptomics -- the systematic comparison of gene expression profiles across species, populations, or experimental conditions -- has become a powerful tool for identifying the molecular basis of adaptive phenotypic divergence, revealing how gene regulatory changes rather than amino acid substitutions often underlie morphological and physiological adaptation. This study conducted a multi-species comparative RNA-sequencing analysis across 84 tissue-condition combinations in 18 vertebrate species, specifically targeting three classes of adaptive divergence: thermal adaptation (six fish species pairs spanning cold-warm ecotypic divergence), hypoxia tolerance (six high-altitude versus lowland vertebrate pairs), and dietary specialisation (six carnivore-herbivore divergence pairs). A total of 8,847 individuals contributed tissue samples processed through a standardised RNA extraction, library preparation, and bioinformatic pipeline. Convergent differential gene expression -- genes showing the same direction of expression change across phylogenetically independent species pairs adapting to the same environmental challenge -- was detected significantly more often than expected by chance for all three adaptation classes (thermal: 847 convergent DEGs; hypoxia: 612 convergent DEGs; dietary: 424 convergent DEGs; all permutation p < 0.001). Gene ontology enrichment of convergent DEGs identified heat shock protein networks, membrane lipid desaturases, and ion channel regulators as the primary convergent modules in thermal adaptation; HIF-1 pathway, haemoglobin subunit expression, and mitochondrial electron transport chain genes in hypoxia adaptation; and lipase/protease up-regulation with cellulose-processing enzyme down-regulation in carnivory versus herbivory transitions. cis-regulatory divergence -- as opposed to coding sequence change -- explained 74.8% of convergent expression differences, confirming the primacy of regulatory evolution in rapid adaptive divergence.