THE PHYLOGENETIC POSITION OF THE BRACHIOPODA: NEW EVIDENCE FROM NUCLEAR HOUSEKEEPING GENES AND MICRORNAS

Resolving the correct phylogenetic relationships of extant clades allows for a determination of the morphological characters present in the last common ancestor at a specific node. This information is essential to the placement of fossil taxa in their correct position in various stem-lineages. It also reveals the sequence of character acquisition during the early history of the clade and leads to hypotheses regarding evolutionary process. Determining the phylogenetic relationships of the Spiralia or Lophotrochozoa (annelids, brachiopods, molluscs, flatworms and their allies) has been notoriously difficult, both in morphological and molecular phylogenetic studies. Here, we demonstrate using northern analysis that brachiopods and phoronids share specific microRNAs (mir-750 and mir-745) which are found in annelids, molluscs and flatworms but not in deuterostomes and ecdysozoans. Since microRNAs appear to be almost never secondarily lost once they are added to the genome, this result confirms the inference originally based on 18S ribosomal DNA that brachiopods sit within the Spiralia rather than Deuterostomia. Within the Spiralia, the three-taxon phylogenetic problem of brachiopods, annelids and molluscs remains unresolved. We have sequenced and analyzed a new dataset consisting of seven nuclear housekeeping genes from brachiopods and phoronids that helps place constraints on the position of these taxa within the Spiralia. These results provide a phylogenetic framework for the study of biomineralization in relation to the Cambrian radiation.