Meeting Abstract
Researchers have presumed that 2 distinct enzymes, orthologs of mammalian sucrase-isomaltase (SI) and maltase-glucoamylase (MGAM), are responsible for sucrasic and maltasic activities in vertebrates. Using phylogenetic analyses on genomic data and enzymatic assays, we uncovered a single ancestral α-glucosidase (AG) gene (which we here call AAG), homologous to the one primarily annotated as SI in available genomes. AAG appears to be widespread among vertebrates and to have given rise to additional AG genes in mammals and some birds: the enzyme called MGAM is not shared by all vertebrates, but is specific to mammals. The majority of Passeriformes (a group including almost half of all birds) appear to have only AAG and its product has both maltasic and sucrasic activity. The existence of many granivorous songbird species indicates that the presence of two AGs is not a necessary condition for reliance on starchy food. Proteomic and biochemical assays in isolated brush border membrane (bbm) of 3 songbirds and chickens demonstrate that songbirds express a single enzyme but chickens have a duplicated AG present in the bbm, as predicted by genomic data. Data also revealed that birds in a large songbird clade, the starlings and relatives, lack sucrasic activity because their AAG gene has undergone a functional shift, and lost sucrasic but retained maltasic activity. Our findings suggest greater diversity and different evolutionary history of bbm AGs than previously presumed, with widespread implications for our understanding of the digestive physiology of the majority of vertebrates. Supported by NSF IOS-1354893
