Evolution of the gene of the lectin, Phalera flavescens agglutinin (PFA)

The larva of the Japanese buff-tip moth (Phalera flavescens) has a novel N-acetyllactosamine-binding lectin, termed Phalera flavescens agglutinin (PFA). It has been suggested that the PFA gene evolved from a lysozyme gene through the loss of lysozyme activity sites. In this study, we investigated whether the PFA gene is evolving through positive selection or by relaxation of functional constraint. For this purpose, we sequenced several PFA-orthologous genes from P. assimilis, P. takasagoensis, and P. bucephala, and compared synonymous and non-synonymous changes to examine the state of natural selection. Although a number of non-synonymous changes were observed on several branches of the gene tree, these effects may have been caused by relaxation of a functional constraint rather than a mechanism of positive selection. Therefore, it is reasonable to conclude that a lysozyme gene in the common ancestor of the Phalera (genus) species lost its lysozyme activity and accumulated mutations during the evolution of the species. Although the actual function of PFA in the moth species is as yet unknown, the gene probably obtained the N-acetyllactosamine-specific lectin activity, collaterally.