The effects of food rationing on phenotypic plasticity in reproductive resource allocation in female wild-type zebrafish, Danio rerio

Abstract

Any resources obtained by an organism must be divided between life history components, namely growth, reproduction and survival. Resources are limited, thus creating trade-offs between competing traits. One trade-off is the relationship between the size and the number of offspring (fecundity) produced by a mother. Although adaptive phenotypic plasticity in this trade-off has been demonstrated across the Animal Kingdom, understanding of the mechanisms behind this is lacking. For example, female zebrafish (Danio rerio) subjected to different feeding regimes have shown this phenotypic plasticity, but studies have not determined the mediators of this change. This thesis aimed to examine the phenotypic plasticity in resource allocation to reproduction in wild-type zebrafish and elucidate mechanisms allowing the change. Female zebrafish were fed either 1.5% (Low) or 3% (High) of body weight daily and phenotypic changes in fecundity, offspring size and gene expression of five candidate genes in the ovary were evaluated. Representative ovary samples were also analysed by RNA-Seq. There were trends for increases in lrp8 and esr2a mRNA expression levels in the ovaries of the food-limited females when compared with food-abundant females. This supported previous literature demonstrating increases in vitellogenin in larger eggs. In the food-abundant females, there was an increase in fshr mRNA expression levels. This could be a mechanism to increase fecundity, through increased follicles entering vitellogenesis. A second experiment was performed to assess the sensitivity and timing of the resource allocation decisions. Female zebrafish were exposed to an initial feeding regime for four weeks and then switched to the other feeding regime for another four weeks. These females were analysed for differences in resource allocation and the trade-off between follicle size and reproductive investment was analysed regardless of feeding regime. Across both experiments, differences in phenotype were hard to discern due to the effects of maternal condition. There were marked differences in females within the same tanks, possibly due to dominance effects, and this may have obscured the effects of the individual feeding regimes. Overall, there was no adaptive phenotypic plasticity in offspring size or offspring number evident in the experiments presented. Future gene expression studies on females clearly demonstrating differences in phenotype are necessary.