Abstract
AbstractAnt-eating mammals represent a textbook example of convergent evolution. Among them, anteaters and pangolins exhibit the most extreme convergent phenotypes with complete tooth loss, elongated skulls, protruding tongues, hypertrophied salivary glands producing large amounts of saliva, and powerful claws for ripping open ant and termite nests. However, 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 functionalCHIAparalogs (CHIA1-4), Asian pangolins (Manisspp.) have only one functional paralog (CHIA5). Here, we performed a comparative transcriptomic analysis of salivary glands in 33 placental species, including 16 novel transcriptomes from ant-eating species and close relatives. Our results suggest that salivary glands play an important role in adaptation to an insect-based diet, as expression of differentCHIAparalogs is observed in insectivorous species. Furthermore, convergently-evolved pangolins and anteaters express different chitinases in their digestive tracts. In the Malayan pangolin,CHIA5is overexpressed in all major digestive organs, whereas in the southern tamandua, all four functional paralogs are expressed, at very high levels forCHIA1andCHIA2in the pancreas, and forCHIA3andCHIA4in the salivary glands, stomach, liver, and pancreas. Overall, our results demonstrate that divergent molecular mechanisms underlie convergent adaptation to the ant-eating diet in pangolins and anteaters. This study highlights the role of historical contingency and molecular tinkering of the chitin-digestive enzyme toolkit in this classic example of convergent evolution.
Publisher
Cold Spring Harbor Laboratory