Male genital lobe morphology affects the chance to copulate in Drosophila pachea

Abstract Introduction Male genitalia are thought to ensure transfer of
sperm through direct physical contact with female during copulation.
However, little attention has been given to their pre-copulatory role with
respect to sexual selection and sexual conflict. Males of the fruitfly
Drosophila pachea have a pair of asymmetric external genital lobes, which
are primary sexual structures and stabilize the copulatory complex of
female and male genitalia. We wondered if genital lobes in D. pachea may
have a role before or at the onset of copulation, before genitalia
contacts are made. Results We tested this hypothesis with a D. pachea
stock where males have variable lobe lengths. In 92 mate competition
trials with a single female and two males, females preferentially engaged
into a first copulation with males that had a longer left lobe and that
displayed increased courtship vigor. In 53 additional trials with both
males having partially amputated left lobes of different lengths, we
observed a weaker and non-significant effect of left lobe length on
copulation success. Courtship durations significantly increased with
female age and when two males courted the female simultaneously, compared
to trials with only one courting male. In addition, lobe length did not
affect sperm transfer once copulation was established. Conclusion Left
lobe length affects the chance of a male to engage into copulation. The
morphology of this primary sexual trait may affect reproductive success by
mediating courtship signals or by facilitating the establishment of
genital contacts at the onset of copulation.