Abstract

Primary reproductive traits (those traits directly associated with sexual reproduction) have evolved both rapidly and divergently. Despite considerable advances in our understanding of the mechanisms that drive the evolution of these traits, we still have a very limited understanding of how male and female traits interact and how environmental factors influence the expression of these traits. Previous research has shown developmental temperature to affect the ejaculatory characteristics of male Callosobruchus maculatus, which subsequently affected copulatory behaviour and the outcome of sperm competition. However, given there is some evidence that both copulatory behaviour and the outcome of sperm competition are a product of male-female interactions, I here examine the effect of developmental temperature on the expression of female reproductive traits and determine its effects on copulatory behaviour. Populations of C. maculatus were raised at different temperatures (17oC, 27oC and 33oC) in order to investigate the effects of developmental temperature on: 1) female reproductive trait anatomy, 2) copulatory behaviour and 3) the outcome of sperm competition.
Developmental temperature significantly affected female reproductive architecture. The bursa copulatrix of females grown at 17oC tended to be longer and thinner than those grown at 27oC or 33oC. This was evident from linear and shape measurements. The bursa copulatrix is where females receive the spermatophore, therefore variation in the shape of this structure could influence how copulation proceeds in this species. The spermathecal duct length (a key feature of sperm-female coevolution) was shortest in those females grown at 17oC, although not significantly so.
The developmental temperature experienced by both males and females affected the duration of copulation. Females from the 27oC treatment group experienced shorter copulations than those from the 17oC and 33oC temperature groups, whilst males reared at 17oC copulated for longer than males reared at the two warmer temperatures. Females reared at 17oC were very reluctant to mate at all, with only 14% of 111 attempts resulting in copulation. This may be related to the bursa copulatrix of 17oC females being abnormally shaped.
The effect of both male and female developmental temperature on the outcome of sperm competition was estimated using the genetic marker technique (colour polymorphisms). Both female and male developmental temperature had a significant effect on the outcome of sperm competition. P2 was highest in those females reared at 33oC whilst males from the 27oC
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treatment achieved higher P2 values than males reared at either 17oC or 33oC. There was no effect of male nor female development temperature on P1.
In conclusion, this research illustrates how a change in developmental temperature can affect the expression of primary reproductive traits in females and affect copulatory behaviour and the outcome of sperm competition. Given temperature can vary on a micro-climate scale as well as local and global scales, I argue that these results are important for those considering the evolution of primary reproductive traits, as variation in female reproductive architecture is likely to help maintain genetic variation in high-fitness male reproductive traits.