Phylogenetic analysis of molluscan mitochondrial LSU rDNA sequences and secondary structures

Mollusks are an extraordinarily diverse group of animals with an estimated 200,000 species, second only to the phylum Arthropoda. We conducted a compa rative analysis of complete mitochondrial ribosomal large subunit sequences (LSU) of a chiton, two bivalves, six gastropods, and a cephalopod. In addi tion, we determined secondary structure models for each of them. Comparativ e analyses of nucleotide variation revealed substantial length variation am ong the taxa, with stylommatophoran gastropods possessing the shortest leng ths. Phylogenetic analyses of the nucleotide sequence data supported the mo nophyly of Albinaria, Euhadra herklotsi + Cepaea nemoralis, Stylommatophora , Cerithioidea, and when only transversions are included, the Bivalvia. The phylogenetic limits of the mitochondrial LSU rRNA gene within mollusks app ear to be up to 400 million years, although this estimate will have to be t ested further with additional taxa. Our most novel finding was the discover y of phylogenetic signal in the secondary structure of rRNA of mollusks. Th e absence of entire stem/loop structures in Domains II, III, and V can be v iewed as three shared derived characters uniting the stylommatophoran gastr opods. The absence of the aforementioned stem/loop structure explains much of the observed length variation of the mitochondrial LSU rRNA found within mollusks. The distribution of these unique secondary structure characters within mollusks should be examined. (C) 2000 Academic Press.