MEGHAN A. GUINNEE
The University of Edinburgh and Department of Zoology, Oxford Unversity, Oxford OX1 3PS, UNITED KINGDOM.
Within the same species, different individuals have different life-history characteristics. Much of this is due to genetic differences – but much is due to ‘phenotypic plasticity’, defined by Pigliucci (2001) as “the property of a given genotype to produce different phenotypes in response to distinct environmental conditions.” A ‘maternal effect’ is a specific type of phenotypic plasticity where an individual’s characteristics are influenced by the environment or condition of its mother. Phenotypic plasticity and maternal effects can be adaptive, but need not be. In this thesis, I explore how an individual’s environment, or the environment of its mother, affects its reproductive life-history characteristics (age at maturity, size at maturity, offspring size, offspring number). I attempt to explain observed responses using adaptive reasoning and/or mathematical modelling.
The trade-off between offspring size and number is one of the most studied areas of evolutionary biology, but we still do not have a complete understanding of why offspring (or egg) size variation exists within species. I explored a particular type of small clutch model, the life-history invariants proposed by Charnov and colleagues (Charnov & Downhower 1995; Charnov et al. 1995; Downhower & Charnov 1998), which make quantitative predictions regarding the relationship between clutch size and range in offspring (egg) size. I tested their predictions using the waterflea, Daphnia magna (freshwater crustacean), under closely-controlled, laboratory conditions. I found qualitative support for the model in that the range of mean egg sizes (averaged over clutch) decreased with increasing clutch size. However, this decrease was slower than predicted. I also tested the model using parasitic wasp data previously collected by other researchers. This allowed me to test the generality of the model over multiple species, and to examine whether the relationship holds for sexual species. Again, I found qualitative support for the model in that range in offspring size decreased with increasing clutch size. However, the decrease did not follow the quantitative predictions, and how well the data fit the model varied from species to species. It is likely that genetic differences, along with the fact that the mothers produced both female and male offspring, added variability to the system.
I found that mean egg size decreases with increasing clutch size in Daphnia, and explored possible causes of this using a mathematical model. This pattern could be an adaptive response, if larger offspring have greater fitness advantages in foodlimited environments. Alternatively, such a pattern can result from a minimum viable egg size. I also examined the fitness effects of hatching from a small or large egg in Daphnia. I found that offspring from food-limited mothers are larger, but that they mature later, and produce fewer, smaller offspring. This might be because offspring from food-limited mothers are programmed to be more cautious, investing resources into survival rather than reproduction.
I found that the nematode parasites Strongyloides ratti and Nippostrongylus brasiliensis mature at different rates depending on the efficacy of the host immune response, but that differences are species-dependent. In addition, female N. brasiliensis suffer decreased fecundity at higher densities, but only in hosts with fully-functioning immune systems. This suggests that the density-dependent effects often observed in parasitic nematodes are mediated by the host immune system.
This thesis reminds us that small differences in an individual’s surroundings, or even its mother’s surroundings, can profoundly affect when, how, and how successfully an animal reproduces. Often, these effects can be explained using adaptive reasoning, and/or mathematical modelling. When and how an animal reproduces is certain to have consequences for its fitness, and even the evolution of species. The implications of this, and possible future research directions are discussed.