  Animal mitochondrial genomes usually have two transfer RNAs for Leucine: one,
with anticodon UAG, translates the four-codon family CUN, whilst the other,
with anticodon UAA, translates the two-codon family UUR. These two genes must
differ at the third anticodon position, but in some species the genes differ at
many additional sites, indicating that these genes have been independent for a
long time. Duplication and deletion of genes in mitochondrial genomes occurs
frequently during the evolution of the Metazoa. If a tRNA-Leu gene were
duplicated and a substitution occurred in the anticodon, this would effectively
turn one type of tRNA into the other. The original copy of the second tRNA type
might then be lost by a deletion elsewhere in the genome. There are several
groups of species in which the two tRNA-Leu genes occur next to one another (or
very close) on the genome, which suggests that tandem duplication has occurred.
Here we use RNA-specific phylogenetic methods to determine evolutionary trees
for both genes. We present evidence that the process of duplication, anticodon
mutation and deletion of tRNA-Leu genes has occurred at least five times during
the evolution of the Metazoa - once in the common ancestor of all Protostomes,
once in the common ancestor of Echinoderms and Hemichordates, once in the
hermit crab, and twice independently in Molluscs.
