Abstract

This thesis examines the effects of the thermal rearing environment on larval development and its influence on the expression of primary reproductive traits in Callosobruchus maculatus.
Chapter 1 provided basic outline to the thesis, starting with a general introduction to sexual selection and specifically its role in the evolution of genitalia and primary reproductive traits. These traits appear to exhibit a level of phenotypic plasticity and thus the opening chapter introduces this concept and its role in evolution and diversification. Phenotypic plasticity in the primary reproductive traits of insects and the evolutionary consequences of this plasticity are discussed.
Chapter 2 examined the effect of larval rearing temperature on larval development and offspring size. Larvae go through 4 larval instars prior to pupation. Unsurprisingly larvae reared at 17°C took approximately four times longer to develop than those larvae reared at 33°C but those beetles that completed their development at the coolest temperature were the largest, in agreement with Bergmann’s rule. This chapter paved way for the experiments carried out in subsequent chapters.
Chapter 3 investigated the effect of developmental temperature on the expression of primary reproductive traits (e.g. sperm length and egg size) and life-history traits (e.g. adult longevity and fecundity). Sperm length exhibited phenotypic plasticity, being shortest when larvae developed at the temperature extremes (17°C & 33°C). Beetles raised at 17°C inseminated the fewest sperm despite the fact that these males had the largest absolute testes size. Developmental temperature affected female fecundity; females that underwent larval development at 17°C laid the fewest eggs. These females along with those reared at 33°C also laid the largest eggs. In general egg size tended to increase with decreasing developmental temperature (in agreement with Bergmann’s rule) although sperm size gives a more mixed response being largest at an intermediate temperature. This is in agreement with the effect of developmental temperature on sperm size in dung flies, but in guppies and a species of snail sperm size increased with decreasing temperature.
Chapter 4 used thermal switch experiments to identify when, during larval development, the expression of sperm length was sensitive to temperature. Thermal switching involves an abrupt shift in the thermal environment experienced by the developing larvae, such that development starts at one environment and is completed at either a higher or lower thermal rearing environment. There appeared to be two thermal sensitive periods (TSP) for the expression of sperm length: one during the very stages of ontogeny and one around the time of instar III to instar IV. This is the first demonstration of a TSP in relation to the phenotypic expression of the sperm. The findings of this chapter are discussed in relation to previous shift-once or shift-twice experiments on reptilian embryos and a handful of insects.
Chapter 5 studied the consequences of thermal environment during larval development and as adults on copula duration. Both larval rearing temperature and post-eclosion temperature affected copula duration; males reared at the lowest temperature taking longer to complete copulation. This is the first demonstration that larval rearing temperature affects copulatory behaviour. It is argued that because thermal heterogeneity is likely to be common in nature, this under-examined component of male behaviour could account for much variation in copulatory behaviour in nature.
Chapter 6 examined the effect of larval rearing temperature on the outcome of sperm competition. Males reared at 17°C were less successful at sperm defence and sperm offence, whilst males reared at 33°C were less successful at sperm offence in comparison to males reared at 27°C. This is the first study to report an effect of developmental temperature on the outcome of postcopulatory sexual selection. The results are discussed in light of the physiological consequences of developmental temperature on male ejaculatory characteristics.
Chapter 7 discusses the possible mechanisms and consequences of phenotypic plasticity in reproductive traits as a result of experiencing heterogeneous environments during development, and how these processes might interact with postcopulatory sexual selection. Further work on understanding the role of phenotypic plasticity in the evolution of primary reproductive traits is required.