Molecular adaptation of a leaf-eating bird: stomach lysozyme of the hoatzin

This report describes a lysozyme expressed at high levels in the stomach ofthe hoatzin, the only known foregut-fermenting bird. Evolutionarycomparison places it among the calcium-binding lysozymes rather than amongthe conventional types. Conventional lysozymes were recruited as digestiveenzymes twice in the evolution of mammalian foregut fermenters, and theseindependently recruited lysozymes share convergent structural changesattributed to selective pressures in the stomach. Biochemical convergenceand parallel amino acid replacements are observed in the hoatzin stomachlysozyme even though it has a different genetic origin from the mammalianexamples and has undergone more than 300 million years of independentevolution.