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This study examined the effect of malathion-specific resistance, in absence of selective pressure, on the fitness and the reproductive success of Tribolium castaneum, the red flour beetle. Malathion-susceptible and -resistant field strains and isogenic strains, and their hybrids were examined. First, adult reproductive behaviour, male genitalia and sperm precedence were observed. The males initiate more than 85 % of the matings and display mate choice based on female reproductive status. Male genitalia are composed of a retractable brush-like structure that could play a role in sperm competition by stimulating the female during mating. Sperm precedence is affected by inter-mating interval and contact duration between the male and the female. Sperm removal and displacement are not responsible of the last male advantage. The last male to mate possess a reproductive advantage even when more than two males copulate with the same female. The fitness of resistant field strain is higher th an that of susceptibles ones. This reproductive advantage is probably due to the highest fecundity of resistant females. Individuals of the isogenic resistant strain do not possess this reproductive advantage and have a lower fitness than the isogenic susceptible strain. The hybrid strains have the highest fitness due to the heterosis. The highest fecundity of the resistant females from the field populations could be due to gene (s) modifier (s) selected after malathion-specific resistance gene (s). When resistant males (from field or isogenic strain) are in competition with susceptible ones for the insemination of susceptible females, their reproductive success is the highest. The reproductive advantage is thus independent of the presence of gene modifiers. No difference is observed between susceptible and resistant males for success, duration and frequency of mating, genitalia length and sperm precedence. Susceptible males have bigger testis than resistant ones, but it does not give them a reproductive advantage. In average, resistant males from the field strains ejaculate less spermatozoa than susceptible males, but their sperm are longer. When these parameters are observed in the isogenic strains and their hybrids, it appears that malathion-specific resistance genes have a positive effect on sperm number and length. The reproductive advantage observed in resistant males may contribute to the stability and spread of malathion-specific resistance in red flour beetle populations.

Tribolium --- Reproductive disorders --- Fertility --- Malathion --- Resistance to chemicals

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Insecta --- Fertility --- Semen --- Biological competition --- mating systems --- Ancestry

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Tribolium --- reproduction. --- reproduction --- Sexual behaviour