Maternal effects have the potential to alter early developmental processes of offspring and contribute to adaptive diversification. Egg size is a major contributor to offspring phenotype, which can influence developmental trajectories and potential resource use. However, to what extent intraspecific variation in egg size facilitates evolution of resource polymorphism is poorly understood. We studied multiple resource morphs of Icelandic Arctic charr, ranging from an anadromous morph – with a phenotype similar to the proposed ancestral phenotype – to sympatric morphs that vary in their degree of phenotypic divergence from the ancestral anadromous morph. We characterised variation in egg size and tested whether egg size influenced offspring phenotype at early-life stages (i.e. timing of- and size at- hatching and first feeding [FF]). We predicted that egg size would differ among morphs and be less variable as morphs diverge away from the ancestral anadromous phenotype. We also predicted that egg size would correlate with offspring size and developmental timing. We found morphs had different egg size, developmental timing and size at hatching and FF. Egg size increased as phenotypic proximity to the ancestral anadromous phenotype decreased, with larger eggs generally giving rise to larger offspring, especially at FF, but egg size had no effect on developmental rate. The interaction between egg size and the environment may have a profound impact on offspring fitness, where the resulting differences in early-life history traits may act to initiate and/or maintain resource morphs diversification.  

Female size and egg size data were measured on wild-collected females and embryos reared in temperature-controlled laboratory conditions. To characterise variation in size and developmental times, morphs were sampled at four points during development: 1) post-fertilisation (PF); eye stage (E); hatching (H); and first feeding (FF). Mean egg size per female was estimated by measuring egg diameter of embryos at PF and E stages. Average measurements of offspring size and DD were taken for each female at H and FF. See manuscript for further details on methodology.

All data were analysed using R. The scripts of which can be accessed at https://github.com/SamVBeck/egg_size_Icelandic_Arctic_charr.