부산대학교 도서관

Conditions experienced during early development play a key role in determining an organism's adult phenotype. As a result of early environmental effects, an organism may be able to anticipate specific conditions during adulthood, which may be adaptive if actual conditions match those for which it was primed. Rapid changes in environmental factors may, however, cause an organism to experience a mismatch between its early and adult conditions. While phenotypic plasticity (adaptive or not) in response to changing conditions has been demonstrated in a wide range of organisms, the underlying mechanisms of the responses to either matching or mismatching conditions remain largely unclear. Insects (for example, pea aphid) provide a unique system for studying maternal effects, phenotypic plasticity, and stress response, making them an ideal organism for examining environmental match-mismatch scenarios and possible pathways associated with environmental change response. Recent research suggests that altered gene functionality (possibly induced by epigenetic modifications, such as DNA methylation) may play an important role in this process in which differential gene regulation enables an organism to respond to changes in its environment without modifying the organism genetically. In this thesis, focusing on a temperature and host plant quality gradient reflecting good and poor developmental conditions, I have tested key predictions of how matching or mismatching in thermal and nutritional conditions between early and adult environments affect fitness components. A fully factorial experiment was conducted with clonally reproducing pea aphids with different life histories to determine the effects of matching or mismatching developmental conditions on aphid survival and population size. In two separate experiments (manipulating temperature in one and host plant quality in another), pea aphids encountered similar or different conditions [either good or poor] between two different developmental times; during early development (before birth) and during adult development (after birth). As a result of this combination, four environmental scenarios were created: early good - adult good, early good - adult poor, early poor - adult good, and early poor - adult poor. Seven first-instar aphids were used in each of these scenarios at the beginning of the experiment. After 14 days, the number of aphids that survived these environments was counted and their total population size (reflecting the number of surviving aphids and the number of offspring produced by them) was measured. Using the surviving aphids, I further investigated the effects of environmental match/mismatch on transcriptome profiles, gene expression, and endosymbiont density, predicting that variation in gene expression patterns and endosymbiont number contribute to differences in aphid responses. The phenotypic results showed that better early conditions provided a fitness advantage regardless of whether or not these conditions match the adult conditions whereas fitness always improved with better adult conditions - reflecting a silver spoon effect. It was found, however, that the magnitude of the response varied depending on both environmental factors (temperature and plant quality) and aphid genotype. As an example, the difference in temperature during early development had more adverse effects on aphids than the difference in temperature during adult development. In contrast, differences in plant quality during adult development resulted in a profound detrimental effect on aphids. The transcriptome profile of aphids that developed in identical or changing environments revealed a genotype-specific response to environmental conditions as well as distinct environmental effects independent of genotype, which depended on the environmental factor in question. A number of genes were differentially expressed between aphids from good and poor environments, with significant enrichments in growth, metabolism, and stress responses. Additionally, I found associated context-dependent changes in the obligate endosymbiont density of aphids, indicating that environmental impacts on endosymbiont communities directly or indirectly influence the response of aphids. While a poor quality host plant resulted in an increase in Buchnera numbers in aphids, a poor thermal condition resulted in a decrease in Buchnera numbers. In summary, this thesis established how changes in developing environments result in altered gene expression patterns and altered endosymbiont densities that in turn affect aphid life-history traits. In addition to serving as an excellent model for studying early developmental programming, phenotypic plasticity, epigenetic mechanisms, and host-symbiont interactions, the pea aphid can also be used to determine how ecosystem dynamics are influenced by changing environmental conditions.