Gene expression in the phenotypically plastic Arctic charr (Salvelinus alpinus): A focus on growth and ossification at early stages of development

Abstract

Gene expression during development shapes the phenotypes of individuals. Although embryonic gene expression can have lasting effects on developmental trajectories, few studies consider the role of maternal effects, such as egg size, on gene expression. Using qPCR, we characterize relative expression of 14 growth and/or skeletal promoting genes across embryonic development in Arctic charr (Salvelinus alpinus). We test to what extent their relative expression is correlated with egg size and size at early life‐stages within the study population. We predict smaller individuals to have higher expression of growth and skeletal promoting genes, due to less maternal resources (i.e., yolk) and prioritization of energy toward ossification. We found expression levels to vary across developmental stages and only three genes (Mmp9, Star, and Sgk1) correlated with individual size at a given developmental stage. Contrary to our hypothesis, expression of Mmp9 and Star showed a non‐linear relationship with size (at post fertilization and hatching, respectively), whilst Sgk1 was higher in larger embryos at hatching. Interestingly, these genes are also associated with craniofacial divergence of Arctic charr morphs. Our results indicate that early life‐stage variation in gene expression, concomitant to maternal effects, can influence developmental plasticity and potentially the evolution of resource polymorphism in fishes.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

“…Such maternally driven differences in developmental trajectories can promote different trophic morphologies in offspring and contribute to the evolution of resource polymorphism. In Arctic charr, there is some evidence for maternally mediated differential expression of genes related to craniofacial development (Ahi et al ., ; Beck et al ., ). In brook charr ( Salvelinus fontinalis ), exposure of larger embryos to stressful environments resulted in greater plasticity and a wider developmental trajectory than in smaller embryos (Penney, Beirão, & Purchase, ).…”

Differences among families in craniofacial shape at early life-stages of Arctic charr (Salvelinus alpinus)

“…4b), two structures which differ between benthic and pelagic morphs (40). This is one of the first studies to document a correlation between gene expression and shape in fishes, and also adds to the mounting body of evidence that Sgk1 may play an important role in intraspecific diversification (24,27).…”

“…Vatnshlíðarvatn is a physically simple, small (70 ha) and shallow (average depth 2-3 m) lake (42). Arctic charr is the only fish species in the lake and has diverged into two weakly divergent morphs (silver and brown; (42,72) The clutches and rearing protocols used in this study are described in (24). In short, mature females (N = 7) and males (N = 5) were caught using gill nets in September 2014.…”

“…The extent of intraspecific divergence can be biased along both genetic and developmental lines of least resistance (18,19), expanding upon existing phenotypic variation, but limited by those available resources (20). Early development provides a more malleable phenotype than later in life (18), as gene expression is more dynamic (21)(22)(23)(24) and can influence developmental trajectories (discussed in: (24)(25)(26)). Studying gene expression combined with morphology at early life-stages can thus provide insight into developmental processes underlying the initiation of phenotypic variation.…”

“…We use eight growthrelated genes chosen from the literature based on their involvement during early development, and six genes related to skeletogenesis based on findings in a previous study (27). Combining this previously collected gene expression data (24) with data on offspring morphology, we test the following predictions: 1) if there are genetic or parental effects in shape at early life-stages, we should see differences among families in craniofacial features; 2) if early life-stage variation is related to genes involved in growth and skeletogenesis of trophic structures, offspring craniofacial shape should covary with the expression of such genes; 3) if maternal investment (i.e. egg size) influences early life-stage phenotypes, female mean egg size or individual offspring size should correlate with offspring morphology; and finally 4) relative expression of the studied growth and skeletal related genes early in life can result in shape variation later in life.…”