Sexual selection in aphidophagous ladybird beetles: a case study in Menochilus sexmaculatus

Variation in mate quality drives selection in favour of mates that exhibit better quality thus signalling better genes. However, with recent resurgence in the field of sexual selection, pre-copulatory selection no longer assures post-copulatory success, thus, exemplifying the various modes operating at both levels. To test this, studies were carried out to investigate pre- and post-copulatory strategies of sexual selection, if any, in the zig-zag ladybird beetle, Menochilus sexmaculatus (Fabricius). Pre-copulatory study involved mate choice trials where typical and melanic morphs taken from the pure morph lines were placed in male and female mate choice trials at 15, 25 and 35°C. Post-copulatory study involving no choice trials was performed at two levels. In the first study, effect of relatedness on reproductive output was evaluated by disrupting matings at different time intervals from a mating of 2.50 minutes to complete mating (108.00±0.39 minutes). The second level involved no choice double mating experiment with typical and melanic morphs to reveal last male sperm precedence, if present. Results reveal that at pre-copulatory level melanic adults were predominantly selected as mates at 15 and 25°C; however at 35°C this pattern was lost. In the post-copulatory study, females avoided mating with related males by remaining longer in copula with unrelated ones. Also the reproductive output was enhanced when females mated with unrelated males. The study on double mating clearly indicates that second male had higher fertilisation success, which was independent of the morph of second male. Thus, our study concludes that while mate choice study indicates sexual selection at a pre-copulatory level, there are also complex post-copulatory strategies of at play in ladybirds, a fact which has not been much investigated.