Interspecies comparison of a gene pair with partially redundant function: the rst and kirre genes in D. virilis and D. melanogaster.

The D. melanogaster rst and kirre genes encode two highly related immunoglobulin-like cell adhesion molecules that function redundantly during embryonic muscle development. The two genes appear to be derived from a common ancestor by gene duplication. Gene duplications have been proposed to be of major evolutionary significance since duplicated redundant sequences can accumulate mutations without detrimental effects for the organism and leave the duplicated genes free to assume novel functions. To address the issue of conservation of the duplicated sequences and their putative redundancy, as well as to identify putative functional divergence of the paralogs during drosophilid evolution, we performed an interspecies comparison of the rst and kirre genes from D. virilis and D. melanogaster. The D. virilis genome contains orthologues of both rst and kirre and hence the duplication took place before the split of the two lineages and has subsequently been conserved. However, whilst the Rst orthologues show a high degree of sequence similarity, this similarity is lower in Kirre orthologues. Especially the intracellular domains of D. virilis and D. melanogaster Kirre sequences are highly divergent: the D. virilis kirre gene lacks the 3'-most exon present in D. melanogaster, which contains motifs conserved between kirre and rst in D. melanogaster. Hence, while each of the two genes is highly conserved at the level of its exon-intron organization, the selection forces acting on the rst and kirre coding sequences are different. These findings are discussed in the light of general evolutionary mechanisms.