Phylogenetic distribution of cysteine proteinases in beetles: evidence foran evolutionary shift to an alkaline digestive strategy in Cerambycidae

We characterized the digestive proteinases of eight species of beetles to i mprove our understanding of the phylogenetic distribution of serine and cys teine proteinases. Serine proteinases function optimally under alkaline pH conditions, whereas cysteine proteinases require acidic pH. The phylogeneti c distribution of cysteine proteinases suggests that they first appeared in an early cucujiform ancestor, however, data for some groups is patchy, and there has been speculation that they have been lost in at least one group, the long-horned beetles (Cerambycidae). The pattern we found supports the hypothesized origin of the proteinases and extends their distribution to an additional superfamily. In addition, we confirmed the presence of cysteine proteinases in some Curculionoidea. Cysteine proteinases were absent, howe ver, from all three species of cerambycids surveyed, supporting the hypothe sis that this group has reverted to the more ancestral serine (alkaline) di gestive strategy. In four species we compared the pH optima for total prote olytic activity to the actual pH of the midgut and found the match between optimal and actual pH to be weaker in the cerambycids. These findings sugge st that either a close correlation between midgut pH and the proteolytic pH optimum is not needed for adequate digestive efficiency, or that midgut pH is a more constrained digestive feature and there has been insufficient ti me for it to shift upwards to maximize serine proteinase activity. (C) 2000 Elsevier Science Inc. All rights reserved.