Comparative transcriptomics reveals divergent paths of chitinase evolution underlying dietary convergence in ant-eating mammals

Ant-eating mammals represent a textbook example of convergent morphological evolution. Among them, anteaters and pangolins exhibit the most extreme convergent phenotypes with complete tooth loss, elongated skulls, protrusive tongues, and powerful claws to rip open ant and termite nests. Despite this remarkable convergence, comparative genomic analyses have shown that anteaters and pangolins differ in their chitinase gene (CHIA) repertoires, which potentially degrade the chitinous exoskeletons of ingested ants and termites. While the southern tamandua (Tamandua tetradactyla) harbors four functional CHIA paralogs (CHIA1, CHIA2, CHIA3, and CHIA4), Asian pangolins (Manis spp.) have only one functional paralog (CHIA5). These two placental mammal lineages also possess hypertrophied salivary glands producing large quantities of saliva to capture and potentially digest their social insect prey. We performed a comparative transcriptomic analysis of salivary glands in 23 representative species of placental mammals, including new ant-eating species and close relatives. Our results on chitinase gene expression suggest that salivary glands play a major role in adapting to an insect-based diet with myrmecophagous and insectivorous species highly expressing CHIA paralogs. Moreover, convergently-evolved pangolins and anteaters express different chitinases in their hypertrophied salivary glands and other additional digestive organs. CHIA5 is overexpressed in Malayan pangolin, whereas the southern tamandua exhibits high levels of CHIA3 and CHIA4 expression. Overall, our results demonstrate that divergent molecular mechanisms underlie convergent adaptation to the ant-eating diet in pangolins and anteaters. This work highlights the role of historical contingency and molecular tinkering of the chitin-digestive enzyme toolkit in this classical example of convergent evolution. ### Competing Interest Statement The authors have declared no competing interest.