Molecular Evolution of the Cecropin Multigene Family in Drosophila: Functional Genes vs. Pseudogenes

Approximately 4 kb of the Cecropin cluster region have been sequenced in nine lines of Drosophila melanogaster and one line of the sibling species D. simulans, D. mauritiana, and D. sechellia. This region includes three functional genes (CecA1, CecA2, and CecB), which are involved in the insect immune response, and two pseudogenes (CecΨ1 and CecΨ2). The level of silent polymorphism in the three Cec genes is rather high (0.028), and there is no excess of nonsynonymous polymorphism. There is no evidence of gene conversion in the history of these genes. The interspecific comparison has revealed that in the three species of the simulans cluster the CecA2 gene is partially deleted and has therefore lost its function and become a pseudogene; in each of the species, subsequent deletions have accumulated. Divergence estimates indicate that the CecΨ1 and CecΨ2 pseudogenes are highly diverged, both between themselves and relative to the other three Cec genes. However, both CecΨ1 and CecΨ2 have conserved transcriptional signals and splice sites, and they present an open reading frame; also, correctly spliced transcripts have been detected for both CecΨ1 and CecΨ2. The data support that these genes are either active genes with some null alleles or young pseudogenes.