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 s pecies D. simulans, D. mauritiana, and D. sechellia. This region inclu des three functional genes (CecA1, CecA2, and CecB), which are involve d in the insect immune response, and two pseudogenes (Cec Psi 1 and Ce c Psi 2). The level of silent polymorphism in the three Cec genes is r ather high (0.028), and there is no excess of nonsynonymous polymorphi sm. There is no evidence of gene conversion in the history of these ge nes. The interspecific comparison has revealed that in the three speci es of the simulans cluster the CecA2 gene is partially deleted and has therefore lost its function and become a pseudogene; in each of the s pecies, subsequent deletions have accumulated. Divergence estimates in dicate that the Cec Psi 1 and Cec Psi 2 pseudogenes are highly diverge d, both between themselves and relative to the other three Cec genes. However, both Cec Psi 1 and Cec Psi 2 have conserved transcriptional s ignals and splice sites, and they present an open reading frame; also, correctly spliced transcripts have been detected for both Cec Psi 1 a nd Cec Psi 2. The data support that these genes are either active gene s with some null alleles or young pseudogenes.