Identification of several members of Hox and ParaHox genes in heterodont bivalves

Homeobox genes encode transcription factors characterized by the homeodomain, a 60-amino acid DNA binding motif. Hox and ParaHox genes are particular subgroups of homeobox genes, which are found in special gene clusters and function in the patterning of body axis. Up to know Hox and ParaHox genes have been studied in several molluscs but the number of the genes identified in the clusters ranges severely in different classes. We carried out a polymerase chain reaction survey using degenerate primers to amplify Hox and paraHox genes in the heteroconch bivalves Abra nitida, Thyasira sp., Hiatella sp., and Astarte elliptica. Our results show that at least one anterior, five central, and one posterior Hox genes exist in these species. Two to four antennapedia orthologs are present in each species. Gene duplication of labial, deformed, Lox2 or Lox4 Hox cognates is apparent in some species. These results suggest that some of the anterior or posterior orthologs may have been lost in some species, while the medial cognates are either lost and/or duplicated. Mollusc-specific signature peptides were recognized in the central part of the homeodomain (20-46 amino acid position) of labial, deformed, Lox4, and posterior genes. We were not able to amplify paralog 3 group (zerknult) gene in any of the species. The entire paraHox cluster genes -including Cdx, Xlox, and Gbx- occur in all species studied here. At least one member of the cluster undergoes duplication in each species. Hiatella sp., e.g., has a duplicated paraHox cluster. These results suggest that the number of genes in both Hox and paraHox clusters differs from species to species in the Heteroconchia, and that loss/duplication events probably occurred independently in different clades.