EVOLUTION OF THE DEC-1 EGGSHELL LOCUS IN DROSOPHILA .3. SEQUENCE COMPARISONS OF THE SIMULANS COMPLEX REPEATED DOMAIN REVEAL NONCONCERTED EVOLUTION

The X-linked Female sterile locus dec-1 (defective chorion-1) was exam ined in the closely related species D. simulans, D. mauritiana, and D. sechellia (the simulans complex). This locus encodes important eggshe ll proteins produced in the follicle cells during stages 9 and 12 of o ogenesis. In D. melanogaster four variant protein forms have been foun d, differing in 2-3 kDa each. The variation is due to deletions of 1, 2, or 3 units of a 5-times repeated sequence (78 bp long) of the centr al coding region. The same type of deletions were found in two variant s of D. simulans; in this species, however, the maximum number of repe ats observed so far is four. The island species D. mauritiana and D. s echellia both have the repeat sequence repeated three times. Sequence comparisons revealed that the repeats in the simulans complex have bee n less homogenised by the forces of concerted evolution than the repea ts in D. melanogaster. Two domains of the repetitive region that evolv e at different rates and are subject to different mechanisms of DNA tu rnover were also defined.