ABSTRACT
Mitochondrial genome organisations were compared to explore supra-phylum affinities of brachiopods, a problem that has been puzzling for more than a century. Published mt-gene maps of 89 metazoans, including two brachiopods, representing nine animal phyla, were examined to identify relationship patterns. Only the arrangements of the 13 protein and the two rRNA genes were considered. For each pair of mt-gene maps, the minimum number of consecutive steps, either of transposition or inversion of a block of genes, required to interconvert the maps, were counted and the gene maps were interconnected to one another assuming minimum evolution. Examined in this way, the echinoid map is connected to the common chordate map with two steps, and the arthropod map is united with the chordate map by three steps. The arthropod map is then connected, by two steps, with the mollusc map, with which the annelid map is linked by a minimum of four steps. The brachiopod map is connected with the mollusc map by a single inversion, which turned out to be the required first step in the minimum routes from the mollusc map to the annelid map. Similarly, the mollusc map lies in one of the shortest routes, which involves three steps, from the arthropod map to the brachiopod map. The ‘tree’, or network, of these mt-gene maps of bilateral animals was rooted by comparison with a diploblastic coral mt-gene map, and the root was inferred to reside between chordates and arthropods. Those observations support the monophyly of lophotrochozoan gene maps, in which the mollusc map is ancestral to the brachiopod and annelid maps, suggesting that brachiopods are more closely related to annelids than to molluscs.
