Molecular evolution of the small subunit ribosomal DNA in woodlice (Crustacea, Isopoda, Oniscidea) and implications for Oniscidean phylogeny

The small subunit ribosomal DNA (ssu rDNA) of 13 isopods was sequenced. The entire length of the ribosomal gene is unusually long, resulting from the presence of five expansion elements accounting for more than 40% of the gen e. We found that in terrestrial isopods the length of the ssu rDNA ranges f rom 2414 bp (Ligidium hypnorum) to 3537 bp (Cubaris murina). This is the lo ngest metazoan ssu rDNA reported to date. The conserved regions are highly informative far analysis of the early nodes of the tree, whereas the variab le expansion elements are better suited to reconstruction of the branching pattern between closely related taxa. The suggested relationship among Syno chaeta, Crinochaeta, and Diplochaeta based on the conserved regions confirm s that based on previous morphological analyses. In contrast, the phylogeny within the Crinochaeta based on the entire ssu rDNA including the variable domains is in conflict with that based on most of the morphological analys es. The phylogenetic analyses of the ssu rDNA support a repeated independen t evolution of the three different types of pleopodal lungs in the Crinocha eta. (C) 2001 Academic Press.