Variation in egg size and offspring phenotype among and within seven Arctic charr morphs

Abstract

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 characterized 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.

Development of resident and migratory three-spined stickleback, Gasterosteus aculeatus

“…The migratory and resident forms are known to vary greatly in morphology and genetics, so a difference in development is not unexpected [ 10 ], and diverged Arctic charr morphs have been found to vary in egg size, development timing and size at hatching [ 15 ]. Migratory stickleback have larger clutches than resident [ 13 ] and there is a trade-off between clutch size and egg size [ 13 , 14 ], suggesting that the eggs of migratory fish are likely smaller than those of residents.…”

“…High levels of reproductive isolation are maintained between the ecotypes, with an estimated hybridisation rate of ~1% [ 10 ]. Migratory and lagoon resident ecotypes vary in both morphology and genetics [ 10 ], so there are likely also differences in egg size and developmental timing, as found between Arctic charr morphs [ 15 ].…”

Patterns, processes and conservation management consequences of intraspecific diversity, illustrated by fishes from recently glaciated lakes

“…Disruptive selection leading to differential fixation of alleles associated with particular phenotypic traits may be well documented (Schluter, 2009), as are transgenerational parental effects, such as gamete provisioning (Räsänen & Kruuk, 2007). Egg size, for example, has been shown to contribute significantly to diversification (Beck et al, 2022; Leblanc et al, 2023), while contrasting omega‐3 fatty acid compositions in walleye ova have been linked to adult phenotypic characteristics in three populations in Canadian lakes (Wiegand et al, 2007). However, a number of promising areas of molecular research have only recently opened up with new approaches and techniques associated with genomics, transcriptomics and epigenetics.…”