Specific mitochondrial ss-tRNAs in phylum Chaetognatha

Chaetognaths are marine invertebrate organisms that constitute a small phylum of very abundant animals. Chaetognaths exhibit several morphological, physiological, anatomical and molecular peculiarities. In all known chaetognath mitochondrial (mt) genomes, the number of genes encoding tRNAs is always small. In addition, the primary sequences and secondary structures of chaetognath tRNAs are not conventional. Several metazoan mt-trn genes (encoding tRNAs) exhibit nucleotide triplets corresponding to stop codons (TAG/TAA) and/or start codons (ATG/ATA) at specific conserved positions. The products of genes that bear one or both types of these codons are known as ss-tRNAs (ss for stop/start). Indirect analyses strongly suggest that in chaetognath mt genomes, some of these start and stop codons could be functional. Moreover, taking these codons into account in the algorithms predicting tRNAs makes it possible to identify and correct several potential annotation errors. Furthermore, a type of ss-trn gene appears to have emerged by duplication in order Aphragmophora.