Phylogenetics of the gastropod genus Nucella (Neogastropoda: Muricidae): Species identities, timing of diversification and correlated patterns of life-history evolution

Abstract

Despite the importance of Nucella as a model system in numerous fields of biology, no phylogenetic analysis of the genus, including every widely recognized species, has been conducted. We have analysed about 4,500 bp of DNA from six different genes (three mitochondrial, three nuclear) from each taxon in the genus. Our results showed western Pacific N. heyseana and N. freycinetii as distinct and distantly related, but found no evidence that N. elongata is distinct from N. heyseana. We also resolved N. heyseana as the closest living relative of the North Atlantic N. lapillus and, using the fossil record for calibration, inferred a minimum separation time between Atlantic and Pacific lineages of at least 6.2 Ma, slightly pre-dating the opening of the Bering Strait. Comparative analyses showed egg size to be evolutionarily labile, but also revealed a highly significant negative relationship between egg size and the nurse-egg-to-embryo ratio. The negative correlation indicates that evolutionary changes in egg size among species are balanced by changes in the number of nurse eggs allocated to each offspring, indicating that interspecific variation in the nurse-egg-to-embryo ratio has not been driven by divergent selection on hatching size, but may instead be a response to variation in other factors, such as parent-offspring conflict.