SEX AND SPECIATION: GENETIC ARCHITECTURE AND EVOLUTIONARY POTENTIAL OF SEXUAL VERSUS NONSEXUAL TRAITS IN THE SIBLING SPECIES OF THE DROSOPHILA MELANOGASTER COMPLEX.

Phenotypic divergence in the male reproductive system (genitalia and gonads) between species of the Drosophila melanogaster complex and their hybrids was quantified to decipher the role of these traits in species differentiation and speciation. Internal as well as external, sexual and nonsexual traits were analyzed with respect to genetic variation and trait asymmetry between strains within species, genetic divergence between species, and dominance and asymmetry in species and hybrids. The variation between strains within species was significant among sexual traits, and only external traits were less asymmetric than internal ones, which suggests that sexual traits are not strongly constrained within species. Three main findings show that sexual traits are most divergent between species: (1) testis length and area, and the area of the posterior lobe of the genital arch (sexual traits) showed the highest proportion of variation between species; (2) linear discriminant functions with the highest components associated to sexual traits were better predictors of species membership; and (3) testis length and area revealed a departure from a linear relationship between members of the species group. Examination of interspecific hybrids showed that sexual traits had higher asymmetry in species hybrids than in the parental species and that sexual traits showed additivity or dominance whereas nonsexual traits showed overdominance (with the exception of malpighian tubules length). These results suggest that sexual traits have undergone more genetic changes and, as a result, tend to show higher divergence and stronger hybrid breakdown between species than nonsexual traits. We propose that sexual selection in the broad sense, affecting all aspects of sexuality, may be responsible for the diversified appearance of sexual traits among closely related species and that the genetic architecture underlying sexual traits may be more prone to disruption during the early stages of speciation.