Evolution of Acidic Mammalian Chitinase Gene (CHIA) is Related to Insectivory Feeding in Rodents

Insect exoskeletons rich in chitin form a key dietary component for numerous rodents, yet the molecular evolutionary mechanisms of chitin digestion mediated by the acidic mammalian chitinase gene CHIA remain poorly resolved in this hyper-diverse mammalian order. We systematically analyzed CHIA sequences from 37 rodent species spanning distinct insect/chitin intake levels to link gene evolution with insectivorous adaptation. Obligate herbivorous rodents with negligible dietary chitin frequently exhibited CHIA deletion or pseudogenization; pseudogenes in Microtus ochrogaster underwent significantly relaxed selection, while pseudogenized Rhynchomys soricoides retained stable selective constraint, possibly due to occasional invertebrate feeding and residual physiological functions of CHIA. Genome-wide site tests identified five robust positively selected residues concentrated in the catalytic and disulfide bond domains critical for chitinase activity. The universal one-ratio model revealed pervasive purifying selection across all rodents (ω = 0.258), whereas free-ratio, branch-site and aBSREL models detected lineage-specific episodic positive selection in omnivorous taxa with sustained chitin consumption including Rattus rattus. Multi-ratio branch models further confirmed divergent evolutionary constraints: lineages ingesting chitin bore stronger purifying selection than zero-chitin feeders, with selection intensity correlated to dietary invertebrate proportion. This study provides the first comprehensive rodent-wide evidence that dietary insectivory drives adaptive divergence of CHIA, uncovering distinct molecular degeneration and selection patterns shaped by varying chitin intake and illuminating the genomic basis of mammalian dietary adaptation.