La compétition spermatique chez les insectes: les stratégies d'assurance de la paternité et la préséance du sperme

Sperm competition in insects: paternity assurance and sperm precedence. The prediction that insects, as a result of polyandry, extreme sperm longevity within the female and high efficiency of sperm utilisation at fertilisation, are preadapted to sustain a very high level of sperm competition is demonstrated across numerous studies. In many insects, males have evolved strategies to decrease sperm competition risk. Paternity assurance mechanisms such as mating plugs or mate guarding do not necessarily influence the number of eggs laid by the female but are taken by male to reduce the probability of his sperm to be preceded by the sperm of another male. Each of these mechanisms influencing mating has an adaptative significance in promoting male reproductive success. However, female insects are polyandrous and they play an active role in mate choice and in discrimination between the ejaculates of different males. Also, they have co-evolved strategy to increase their own reproductive success and to counteract the costs resulting from paternity assurance mechanisms. They can control paternity before copulation (pre-copulation, pre-insemination), during copulation, and because fertilisation takes place within their bodies after insemination, and after fertilisation through selective abortion. A male's reproductive success can be determined as the product of his mating success (mate per lifetime) and his fertilisation success (average number of progeny sired per mate). Male fertilisation success is generally studied in terms of sperm precedence where the proportion of the female progeny fathered by a given male is examined. Sperm precedence can be studied using different methods, each having advantages and disadvantages. Although female insects behave polyandrously, most sperm competition studies investigate sperm precedence when only two males are mated with a female. To determine if the results obtained in double-mating experiments fit well with reality, it is thus important to examine last-male mating success in experiments where females are mated with more than two males. Moreover, within a species, high fertilisation success variations are observed between males of different populations or even of the same population. These variations result from interaction between factors such as sperm number, sperm length, pre- and/or post-copulatory female choice, paternity assurance mechanism efficiency, female sperm storage organ morphology, etc.