Changes in mitochondrial genetic codes as phylogenetic characters: Two examples from the flatworms

Abstract

Shared molecular genetic characteristics other than DNA and protein sequences can provide excellent sources of phylogenetic information, particularly if they are complex and rare and are consequently unlikely to have arisen by chance convergence. We have used two such characters, arising from changes in mitochondrial genetic code, to define a clade within the Platyhelminthes (flatworms), the Rhabditophora. We have sampled 10 distinct classes within the Rhabditophora and find that all have the codon AAA coding for the amino acid Asn rather than the usual Lys and AUA for Ile rather than the usual Met. We find no evidence to support claims that the codon UAA codes for Tyr in the Platyhelminthes rather than the standard stop codon. The Rhabditophora are a very diverse group comprising the majority of the free-living turbellarian taxa and the parasitic Neodermata. In contrast, three other classes of turbellarian flatworm, the Acoela, Nemertodermatida, and Catenulida, have the standard invertebrate assignments for these codons and so are convincingly excluded from the rhabditophoran clade. We have developed a rapid computerized method for analyzing genetic codes and demonstrate the wide phylogenetic distribution of the standard invertebrate code as well as confirming already known metazoan deviations from it (ascidian, vertebrate, echinoderm͞hemichordate).

The Expression of Tubulin and Tektin Genes in Dicyemid Mesozoans (Phylum: Dicyemida)

“…Several studies have argued that the simple body plan of dicyemids is likely secondarily derived from higher lophotrochozoan animals as a result of their endoparasitic, or endosymbiotic, lifestyle in the cephalopod kidney (Katayama et al, 1995;Pawlowski et al, 1996;Kobayashi et al, 1999;Telford et al, 2000). In the present study, alignment of innexin amino acid sequences, as well as the phylogenetic trees based on the innexin amino acid sequences, showed that dicyemids are lophotrochozoans.…”

“…Katayama et al (1995) suggested platyhelminth affinities, as did Stunkard (1954), Nouvel (1947), and Ginetsinskaya (1988), who proposed the same relationship based on the morphology and life cycles. Telford et al (2000) analyzed the mitochondrial genetic code of concatenated Cox I, II, and III sequences from the dicyemids and showed that the dicyemids have the canonical invertebrate mitochondrial genetic code, suggesting that the dicyemids are at least triploblastic. Zrzavý (2001) reviewed the interrelationships of metazoan parasites, based on recent morphological and molecular phylogenetic analyses, and suggested that the Mesozoa are closely related to the Acoelomorpha.…”

Platyhelminth systematics and the emergence of new characters

“…To date, over 20 mt genomes have been fully characterized for a diversity of parasitic flatworms (Table II). Rhabditophoran flatworms exhibit two changes to the invertebrate mitochondrial genetic code (Telford et al, 2000), yielding mt genome level apomorphies for the group. Unique gene orders amongst species of Schistosoma (Digenea: Schistosomatidae) have provided evidence of shared ancestry and have allowed many new additional mt genes and mtDNA fragments to be amplified for other parasitic, usually closely-related flatworms, albeit not without technical problems (Littlewood et al, unpublished).…”

Stenostomum Leucops Dugès, 1828 (Platyhelminthes, Catenulida): A Putative Species Complex With Phenotypic Plasticity

“…Thus, as only asexual reproduction has been observed, the worms in each culture are clones, therefore, homogeneous mitochondrial sequences are expected. The primers and PCR conditions used to amplify Cytochrome C Oxidase I (COI) were described by Telford et al (2000). A fragment of 497 bp was sequenced using a MegaBace 500 automatic sequencer.…”