Sex-specific aging in two-spotted spider mite Tetranychus urticae: Effects of diet, social environment and predator-induced stress

Abstract

Aging is associated with progressive decline in physiological performance and increasing vulnerability to disease with age and is almost universal and inevitable for multicellular organisms. Although our knowledge about aging has advanced greatly during the last few decades, our current understanding of how and why we age remains incomplete. My research examines how abiotic and biotic factors including food regime, social environment and predator-induced stress affect the fitness traits (i.e., survival and reproduction) of the organism. Additionally, I attempted to explore how these factors influenced the fitness of both sexes. Meanwhile, I tested two evolutionary hypotheses, antagonistic pleiotropy hypothesis of George Williams and disposable soma theory of Kirkwood and Holliday, which postulated that the survival and reproduction trade off with each other. To address these questions, I employed spider mite Tetranychus urticaeas my model organism, along with its specialist predator Phytoseiulus persimilis to generate predator-induced stress. The life-history traits (e.g. survival rate, adult lifespan and reproductive parameters) were compared between treatments to determine the responses of males and females to various environmental factors. Furthermore, the relationship between longevity and lifetime fecundity were assayed with linear regression and path analysis in testing the two evolutionary theories of aging. To determine the starvation tolerance of spider mites, the lifespan of mites was assayed with mites fed ad libitum as control. It showed the females and males have a median lifespan of 6 and 3 days, respectively (Chapter2). Therefore, in further studies, the spider mites under food regimen were subjected to fasting at most two days. When exposed to four levels of intermittent fasting (IF): 33%, 50%, 67% with control fed ad libitum, I found a sex-specific response to IF in lifespan, whereby females showed a significant increase in lifespan at the intermediate level of IF, while males showed a shorter lifespan when subjected to each level of IF. The females under IF decreased or even ceased reproduction in comparison with ad libitum fed mites during the reproductive stage. The lifetime fecundity of females fed at 50% IF was significantly lower than that of control (Chapter 3). For both sexes isolated individuals outlived their counterparts when paired with conspecifics regardless of sex, indicating that the cost of interaction with the same sex and the opposite sex is significant (Chapter 4). Additionally, the time and frequency of sexual interaction can have significant fitness consequence on spider mites. Females and males showed sex-specific plasticity of longevity (Chapter 5). With a comprehensive investigation on the relationship between survival and reproduction traits through path analysis including traits of longevity, female lifetime reproduction, age at first reproduction, early reproductive efforts and late reproductive efforts, I discovered no evidence for trade-offs between these life-history traits (Chapter 6). I showed that predator-derived odour prolonged the immature duration of both sexes, shortened female adult lifespan (18.8%) but not those of males, and reduced lifetime reproductive outputs of the females (29.8%). Using a full factorial design, I exposed offspring from both risk-experienced mothers and control mothers randomly to leaf discs with predator odour and control. I found that parental effects were significant in the early developing stage, but not in later life stages. There was no significant inter-generational stress influence on adult lifespan and reproduction, and those of the offspring were strongly negatively affected by the stress they directly exposed. Additionally, parental effects in the earlier life stage were sex-specific, with daughters exhibiting delayed hatching when parents were exposed to predation risk, but not sons. These results suggested that predator-induced stress is strongly detrimental and decreases the fitness of prey by accelerating the onset of aging, but the effects are sex-specific. The influence of predator-stress spanned to the next generation and constrained early developmental stages by influencing the hatching age of offspring from risked-experienced parents (Chapter 7). My thesis highlights the pronounced influence of food regimen, social interaction with conspecifics and predator-induced stress on the pace of aging and reproductive output of an organism. It also demonstrates clear sex differences in longevity and responses to environmental variation.