Mitochondrial genomics of platyhelminths

Abstract

Sequences generated from the mitochondrial (mt) genome provide excellent molecular markers for defining population groups, for tracing the genetic history of an individual or a particular group of related individuals, and for constructing deep­ branch taxonomic phylogenies. In addition, metazoan mtDNAs exhibit an abundance of genetic novelties that include modified mt genetic codes; an unequalled variety in the secondary structures of ribosomal RNAs; variable base composition (A+T and G+C content) which for vertebrates mostly differs from invertebrates; the characteristic replication mode of the mtDNA molecule; the codon bias in usage for protein-encoding genes; the variable and modified structural forms of mt transfer RNAs; the presence of unassigned sequence(s) known as non-coding regions that are rich in repeated sequences, and the link of mutations in mtDNA to apoptosis and genetic disorders. Information from mt sequences will be as valuable in studies on genetic variation in parasitic helminths as it has been on vertebrates or insects. Helminth populations are clearly able to respond to selective pressures and therefore, are genetically heterogenous. The use of conventional methods (e.g. isozyme analysis) to obtain direct evidence for variation and to link this variation to evolutionary responses have generally been less successful. Mitochondrial DNA markers offer more hope in this direction than do nuclear sequences. To date, over 130 complete metazoan mt genomes of different phyla have been reported (as of February, 2001), but only limited information on the mt molecules of parasitic flatworms (Phylum Platyhelminthes: Classes Trematoda and Cestoda) has been available. At the commencement of this study, a partial large (16S) subunit mt ribosomal RNA and 3466 bp of mt sequence of Fasciala hepatica, short fragments for the mt cox1 gene for the cestode Taenia saginata and cox1 and nad1 for several Schistosoma spp. were available. Only in 1999, was the first complete mt genome of a flatworm, that of Echinococcus multilocularis, deposited in the GenBank database (see Chapter 1). …………………………….