GENETIC-BASIS OF SPERM AND TESTIS LENGTH DIFFERENCES AND EPISTATIC EFFECT ON HYBRID INVIABILITY AND SPERM MOTILITY BETWEEN DROSOPHILA-SIMULANS AND D-SECHELLIA

Results are reported from a genetic study of hybrid inviability and th ree 'fertilization traits' (sperm motility and length, and testis size ) that affect hybrid sterility between the sibling species Drosophila simulans and D. sechellia. The main findings are as follows. (i) For s perm length there was a dominant effect of the D. simulans genome over that of D. sechellia, and the Y chromosome of D. sechellia in the bac kground of D. simulans reduced the sperm length. (ii) In contrast, tes tis length, in spite of its generally high correlation with sperm leng th, showed an additive effect. (iii) We found a strong asymmetric inco mpatibility between the D. sechellia X in chromosome and D. simulans a utosomes: D. sechellia X chromosome with D. simulans autosomes, but no t the reverse, showed a significant reduction in testis length as well as in hybrid inviability compared to the parental species. (iv) Betwe en the two autosomes, chromosome 3 had a greater effect on these trait s than chromosome 2, and there was additionally an epistatic effect be tween these chromosomes with respect to their parental vs. recombinant status: recombinant chromosomes 2 and 3, together, had lower viabilit y than any other combination. (v) The testis size in the backcross gen eration was greater than the parental species, suggesting that some mo difier genes are being released from their species-specific genetic co ntrol. (vi) The species-specific homogeneity of the genome was importa nt for all three traits--offspring viability, hybrid male fertility an d testis length. These results are discussed with respect to the role of sexual selection and genetic divergence during speciation.