S Mall Subunit R Dna and the P Latyhelminthes : Signal , Noise , Conflict and Compromise

The strategies of gene sequencing and gene characterisation i frequently determined by a balance between cost and benefit, terms of the amount of phylogenetic signal resolved for a giv taxonomic level. Generally, cost is far easier to predict than b databases is a cost-effective means by which molecular data m addressing systematic problems. As technology advances and affordable and accessible to many researchers, it may be surp gene products remain favoured targets for systematic and phy ribosomal DNA (rDNA), and the various RNA products trans utility in wide ranging groups of organisms. The small (SSU) fragments especially lend themselves to study as they provide sites that enable multiple alignments between homologues, an phylogenetic signal (Hillis and Dixon 1991; Dixon and Hillis (rRNA) is also the commonest nucleic acid in any cell and th sequencing in both eukaryotes and prokaryotes during the ear based molecular systematics (Olsen and Woese 1993). In par and gene product (SSU rRNA) have become such establishe systematic markers among some taxa that databanks dedicate developed and maintained with international and government Database Project II, Maidak et al. 1999; the rRNA WWW Ser One or more species from all metazoan phyla, except the Lor genes characterised in part or fully and if SSU rDNA appears outset of a phylogenetic study, these databases (including EM provide a head start. In addition to raw sequences many of th aligned against models of secondary structure and/or against Addresses of the WWW pages for all these databases are giv development of our knowledge of molecular evolution as it re reconstruction has been influenced greatly by the genes chose played its part such that features of the gene have been charac Abouheif et al. 1998; Zrzavy et al. 1998). The first flatworm species to have had its SSU sequen fragments, was Dugesia tigrina (Field et al. 1988). The first f perhaps not surprisingly considering its medical importance,