Evolutionary and proximate constraints on egg size in butterflies
Arthropod egg and thus progeny size is an evolutionary and ecologically significant trait, showing tremendous variation within and across species. The high variation found is caused by a complex set of interacting proximate and evolutionary factors, but despite increasing effort this network is still only partially resolved. Using butterflies as model organisms, this study focuses on two main factors that are assumed to strongly shape variation in offspring size and number: maternal size and maternal nutrition. Both are assumed to affect reproductive traits and thereby maternal and offspring fitness. Phenotypic correlations between maternal size and egg size within various butterfly species as well as a two-trait selection experiment on simultaneous changes in both traits in the butterfly Bicyclus anynana clearly demonstrated that the importance of maternal size in shaping variation in egg size is limited, both in sense of a morphological as well as an evolutionary constraint. These results strongly contrast to the general assumption of a positive scaling relationship between both traits. This is one of the few studies addressing the issue of evolutionary constraints directly, employing artificial two-trait selection which has been proven to be a powerful tool to unravel genetic variances and covariances that underlie the evolution of traits. While the importance of maternal size in shaping variation in egg size is limited, proximate factors including larval and adult crowding as well as the quantity and quality of available food during the larval and adult stage affect variation in reproductive traits to a high degree. In the butterfly B. anynana, larval and adult densities had surprisingly little effects on female reproduction, whereas dietary limitations yielded strong responses in female reproductive output. Larval food stress reduced fecundity and reproductive investment (mediated through a reduction in body size), but effects on egg size were overall marginal. Additional negative effects of adult food stress on fecundity were largely confined to females being fed as larvae ad libitum, while those being previously starved showed reduced performance regardless of adult income. When abundantly fed during the larval stage, a limitation of adult resources reduced reproductive output, proving the need for adult feeding in B. anynana for egg production. Thus, restricted food access in different developmental stages of B. anynana sets different limits to reproduction, either posed by shortage of larval-derived storage reserves (i.e. nitrogenous compounds) or adult income (i.e. carbohydrates). Consequently, restrictions in both, larval- and adult-derived resources, limit reproduction in B. anynana. Further, this study deals with questions regarding effects of different adult dietary compounds for a fruit-feeding butterfly being novel in its reductionist approach and in the breadth of different nutrient classes considered. This study demonstrates that B. anynana relies to a large extent on adult feeding in order to realise full reproductive output. Female B. anynana require adult-derived carbohydrates for egg production and exhibit a tremendous gain in reproductive output when fed on fruit as compared to sucrose solutions. Contrary to initial expectations, I could not pinpoint a single pivotal substance (in addition to sucrose) that was able to elicit a comparably high reproductive performance as banana, although I tested the micronutrients being most abundantly available in banana (minerals: potassium and magnesium chloride; a mixture of vitamins and a combination of both) and all major substances known to be involved in insect egg production (amino acids, cholesterol, polyunsaturated fatty acids). Further, it is also excluded that the growth of microorganisms and fungi (associated with the production of fermentative products like organic acids and alcohols, thereby providing access to additional resources) explains the found results. In conclusion, reproduction does not only depend on a small number of adult-derived nutrients, but on a larger number having relatively small effects each. Thus, resource congruence (the use of nutrient types in a specified ratio) rather than any specific component may be the key to answer the question.
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