Understanding Sexual Conflict and Environmental Effects on Reproductive Strategies in Mosquitofish (Gambusia holbrooki)

Abstract

Sexual conflict is very common and is considered as a natural phenomenon, which arises when males and females show divergency in their reproductive interests. Environmental factors such as changes in temperature, salinity and water clarity can significantly influence the level of sexual conflict and can also influence the reproductive strategies of each sex. In species, such as mosquitofish (Gambusia holbrooki), where mating is frequently coercive and male harassment is intense, social interactions and environmental conditions both play critical roles in shaping reproductive outcomes. To investigate how sexual conflict and environmental factors (such as salinity, temperature, turbidity) interact to influence reproductive strategies, physiological stress, and life-history traits in males and females, I ran a set of laboratory experiments using mosquitofish. In this thesis, I present four experimental studies examining; the cost related to harassment and multiple mating (Chapter 2); the cost of fighting and sperm production (Chapter 3); the effect of turbidity on female stress level (Chapter 4); and the effect of salinity and temperature on male reproductive strategies (Chapter 5). Chapter 1 introduces the concepts of reproduction, sexual conflict and how different environmental factors can influence reproduction in fish. Chapter 2 differentiates the cost of sexual harassment and multiple mating (polyandry) in female mosquitofish (Gambusia holbrooki). In polyandrous situation both things happen simultaneously, which is why it is difficult to know which one is more expensive. I differentiated the cost by controlling the harassment level and mating rates by using three treatments. I used ablation technique to control the rate of mating. I found that female who faced more harassment, grow more slowly and produce a smaller number of eggs. Chapter 3 differentiates the cost of sperm production from the cost of male-male fighting. As both things usually happen at once, I used the ablation technique (physically cutting the gonopodia) to stop sperm transfer to the female. To find out the most expensive one, I designed three treatments with varying number of rivals and with different sperm ejaculation ability. I found that sperm production is more expensive than fighting and competing with rivals. Chapter 4 investigates the effect of turbid water on the level of male harassment and female stress level and how the level of harassment influences the female stress level. By using four types of treatment (clear water or turbid water; and one male or three males), I found that reduced water turbidity (clear water) elevates female physiological stress (cortisol level), while male harassment levels do not affect female physiological stress. My findings highlight the importance of abiotic conditions in modulating the intensity of sexual conflict. Chapter 5 explores how temperature and salinity influence male life history and reproductive traits, including sperm quantity and velocity. I designed four treatments (two temperatures and two salinity levels) and found that in saline water males produce less sperm than in freshwater and mature earlier at a smaller size. Overall, salinity has more severe effect than temperature, but when salinity and high temperature combine then the effects are more complicated.