A Molecular Phylogeny of Gastropods

In an ongoing project, Karen and Dr. Tim Collins have determined nucleotide sequences from small-subunit ribosomal RNA genes (s-rRNA genes) from many species within the Class Gastropoda (snails, slugs). We are using them to reconstruct the history of evolutionary divergence of the snails and also to learn about the rates and patterns of change of s-rRNA genes.

DNA sequences are often useful indicators of relationship among organisms, especially when morphologies have diverged so greatly that it is difficult to find a sufficient number of strictly comparable (homologous) morphological features on which to base an analysis. Knowledge of the pattern of branching of organismal lineages through time, known as a phylogeny, is a prerequisite to answering most evolutionary questions.

Gastropods are diverse in form, number, habitat, and ecology. Their evolutionary history, which spans most of the last 600 million years, is well represented in the fossil record. This outstanding fossil record allows us to compare the history of divergence inferred from the features of living animals with that inferred from the fossil record and to calibrate rates and patterns of evolution.

Although studied for decades, an understanding of gastropod relationships based on a rigorous analysis of specific characters and how those characters are distributed is lacking. It has been suggested that many of the traditionally recognized groups within gastropods represent grades of organization (groups of related and unrelated species that share similar features), rather than clades (a group comprising an ancestor and all of its descendants). Our ultimate goal is to have a phylogeny in which all recognized groups are clades. To do this, we look for the presence of novel (newly derived) characters that are shared because of inheritance from a common ancestor. By discovering hierarchically nested sets of shared-derived characters among organisms, we develop hypotheses about the branching history of the lineages leading to them.

We have obtained complete s-rRNA gene sequences from 24 species, representing most of the major taxonomic groups of gastropods. The genes were amplified in vitro by the polymerase chain reaction (PCR).

We are currently aligning the sequences. This is a critical stage in phylogenetic analysis, as alignment is an inference about which nucleotide positions are homologous among the organisms being compared. Preliminary alignments have been searched for sets of shared-derived sequence characters that diagnose clades. Our results indicate that, indeed, many of the currently recognized groups within gastropods are grades, rather than clades.

Future work will include a refined analysis of relationships after a final alignment is reached. The resulting phylogeny will be the baseline for studies of diversification, extinction, and evolutionary trends within gastropods. In addition, studies of the pattern of nucleotide change of the s-rRNA gene over evolutionary time will be useful for understanding which DNA characters and types of character change are the most useful for identifying clades within gastropods.