Insights from stable isotopic tracers on reproductive allocation under stress

Abstract

Fecundity is affected by changes in the nutritional and energetic environment, as a result of changes in acquisition, assimilation, or allocation of macro-nutrients and micro-nutrients. Stable isotopes of carbon and nitrogen offer a window into the processes underlying these changes. In insects that feed on nectar as adults, carbon isotopes can be used to trace allocation of carbon to eggs from larval (capital) and adult (income) sources. If adults are fed sugar-water, there is no source of nitrogen from the adult diet. Thus, nitrogen isotopes in eggs reflect fractionation of larval nitrogen due to protein catabolism and anabolism. We subjected adult females of two butterfly species, Speyeria mormonia and Colias eurytheme, to dietary restriction (DR), larval female S. mormonia to DR, and adult female S. mormonia to extra flight. Females subjected to extra flight were previously found to eat more as adults and to have a higher resting metabolic rate. As predicted, significantly less carbon obtained by feeding as adults was incorporated into eggs in both species under DR when adult. Speyeria mormonia eggs contained significantly more carbon derived from adult feeding under DR as larvae and when subjected to extra flight as adult females. Again as predicted, eggs from females of both species subjected to DR when adults were enriched for (15)N, suggesting that increased protein catabolism or anabolism generated additional carbon compounds. Speyeria mormonia eggs from females subjected to DR when larvae or to additional flight as adults were depleted for (15)N. The result for DR of larvae suggests minimization of protein catabolism when protein reserves are relatively scarce. The results for flight were not as predicted, and deserve further exploration. In most cases, isotopic signature in eggs changed with females' age. Eggs were progressively more enriched for the carbon signature of adults, consistent with a two-compartment mixing model for the carbon sources of larvae and adults. Eggs laid across the life of a female were progressively depleted for (15)N, followed by stabilization. This could be due to high total investment in eggs early in life, as the results are consistent with those for other growing animals. Overall, these results indicate shifts in allocation of incoming and stored (capital) carbon in response to various environmental stresses. The results for nitrogen suggest hypotheses to be tested concerning nitrogen metabolism under environmental stress.