In all organisms, fitness is maximised by trade-offs between life-history traits. It is commonly recognised that inter-specifically, relationships between life-history traits are often negative. Yet, when such relationships are explored intra-specifically, the direction of such relationships cannot be easily predicted. Among the many suits of life-history traits, one of the most commonly studied is that of reproductive trade-offs between offspring size and maternal fecundity. It is recognised that to increase offspring survival, mothers often manipulate allocations between propagule size and number to better suit the environment. However, within a species, such differences between this set of life history traits can be influenced by the maternal environment and condition, which in turn influences offspring phenotypes. This phenomenon is commonly recognised as maternal effects. One such example is maternal sex allocation whereby in sexually reproducing species, limited resources can be further allocated to the reproduction of either sons or daughters. The Trivers–Willard hypothesis makes the following specific predictions about maternal sex allocation that mothers in good conditions will display greater investment in sons than daughters while mothers in poor conditions will instead invest greater towards daughters than sons.
In Chapter 2, I tested the patterns of maternal sex allocation in Gambusia affinis (the western mosquitofish). The patterns I investigated are primary offspring sex ratio and sex-specific offspring investment. To accomplish these aims, I used a sex-specific genetic marker to determine offspring sex. In addition, the associations between maternal size and offspring size, and number in G. affinis were explored as a comparison against previous studies. I identified little evidence to support either patterns of sex allocation in relation to maternal size in G. affinis. It is likely that no sex allocation occur in this species. However, it is also possible that sex allocation was occurring at a level that is too subtle to be detected. Additionally, although, the relationship between maternal size and offspring size in G. affinis is weak and positive, the association between maternal size and offspring number was strong and positive.
Following on in Chapter 3, the two relationships between maternal size and offspring size, and number are further expanded upon. Here, the magnitudes, directionalities and generality of the two intra-specific correlations were meta-analytically quantified across 146 species from 6 phyla. Results revealed strong and positive correlations for both relationships, indicating that intra-specific differences in individual quality are prevalent across species. Moreover, the lack of correlated patterns between closely related species further suggest that such positive associations between competing reproductive traits within a species are also likely to be universal across species.
Using an empirical and a meta-analytic approach, this thesis addresses two aspects of maternal effects. Taken together, the results from this study have provided answers for previous gaps in knowledge. In addition, this study has also opened further avenues of research, which could provide greater understanding on sex allocation theory and patterns of life-history relationships.
