Complete sequence of the mitochondrial DNA of the primitive opisthobranch gastropod Pupa strigosa: Systematic implication of the genome organization

The complete sequence (14,189 bp) of the mitochondrial DNA of the opisthobr anch gastropod Pupa strigosa was determined. The genome contains 13 protein , 2 rRNA, and 22 tRNA genes typical of metazoan mtDNA. The Pupa mitochondri al genome is highly compact and shows the following unusual features, like pulmonate land snails: (1) extremely small genome size, (2) absence of leng thy noncoding regions (with the largest intergenic spacer being only 46 nt) , (3) size reduction of encoded genes, and (4) many overlapping genes. Seve ral tRNA genes exhibit bizarre secondary structures with reduced T or D ste ms, and many tRNA genes have unstable acceptor stems that might be correcte d by posttranscriptional RNA editing. The Pupa mitochondrial gene arrangeme nt is almost identical to those of pulmonate land snails but is radically d ivergent from those of the prosobranch gastropod Littorina saxatilis and ot her molluscs. Our finding that the unique gene arrangement and highly compa ct genome organization are shared between opisthobranch and pulmonate gastr opods strongly suggests their close phylogenetic affinity.