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Title: The evolution of maturation in Daphnia
Author: Harney, Ewan
ISNI:       0000 0004 2745 8847
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2013
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Maturation is a key life history transition for many organisms, due to the importance of age and size at maturity in determining fitness. Understanding how maturation phenotypes evolve requires an appreciation of the underlying ontogenetic mechanisms, including the maturation threshold, which determines when an individual ‘decides’ to mature. Maturation thresholds are poorly understood, and little is known about how phenotypically plastic or genetically variable they are, or how variable thresholds influence fitness. In this thesis the evolution of maturation thresholds is investigated using the crustaceans Daphnia magna and D. pulex. A comprehensive approach to modelling the maturation process found that the maturation threshold was a developmentally plastic trait in response to variable resource availability, and more closely resembled a process with a rate than a discrete switch. The maturation threshold also differed between genotypes for both species, and these differences were more apparent in D. magna than D. pulex. A second study of maturation in D. magna identified clone-specific parental effects in the threshold. Furthermore, these parental effects influenced growth, and reaction norms for age and size at maturity were a product of interacting effects between both growth and maturation threshold. A microarray study of gene expression changes in D. pulex found that most gene expression changes during maturation were continuous, further supporting the idea that the threshold is better thought of as a rate than a switch. This study also identified increases in vitellogenin transcripts, indicating the allocation of resources towards reproduction, and potential mechanisms for epigenetic inheritance and endocrine control of maturation. Finally the fitness consequences of variation in the maturation threshold were investigated in D. magna. Genotypes with a smaller threshold had a higher intrinsic rate of population increase, but threshold size did not correlate well with competitive success when five clones were directly competed with each other, suggesting that interactions with other factors were influencing fitness. The findings of this thesis suggest that maturation thresholds are not based on a single fixed state, but are responsive to environmental variation. The presence of heritable variation and transgenerational effects in these developmentally plastic traits suggests an important and adaptive role for them in the evolution of age and size at maturity.
Supervisor: Plaistow, Stewart; Paterson, Steve Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available