Background
Although mitochondrial DNA (mtDNA) of many animals tends to mutate at higher rates than nuclear DNA (nuDNA), a recent survey of mutation rates of various animal groups found that the gastropod family Bradybaenidae (suborder Helicina) shows a nearly 40-fold difference in mutation rates of mtDNA (\(\mu\)m) and nuDNA (\(\mu\)n), while other gastropod taxa exhibit only two to five-fold differences. To determine if Bradybaenidae represents an outlier within Gastropoda, I compared estimated values of \(\mu\)m/\(\mu\)n of additional gastropod groups. In particular, I reconstructed mtDNA and nuDNA gene trees of 121 datasets that include members of various clades contained within the gastropod subclasses Caenogastropoda, Heterobranchia, Patellogastropoda, and Vetigastropoda and then used total branch length estimates of these gene trees to infer \(\mu\)m/\(\mu\)n.
Results
Estimated values of \(\mu\)m/\(\mu\)n range from 1.4 to 91.9. Datasets that exhibit relatively large values of \(\mu\)m/\(\mu\)n (i.e., > 20), however, show relatively lower estimates of \(\mu\)n (and not elevated \(\mu\)m) in comparison to groups with lower values. These datasets also tend to contain sequences of recently diverged species. In addition, datasets with low levels of phylogenetic breadth (i.e., contain members of single genera or families) exhibit higher values of \(\mu\)m/\(\mu\)n than those with high levels (i.e., those that contain representatives of single superfamilies or higher taxonomic ranks).
Conclusions
Gastropods exhibit considerable variation in estimates of \(\mu\)m/\(\mu\)n. Large values of \(\mu\)m/\(\mu\)n that have been calculated for Bradybaenidae and other gastropod taxa may be overestimated due to possible sampling artifacts or processes that depress estimates of total molecular divergence of nuDNA in groups that recently diversified.
