Egg size-dependent expression of growth hormone receptor accompanies compensatory growth in fish

Abstract

Large egg size usually boosts offspring survival, but mothers have to trade off egg size against egg number. Therefore, females often produce smaller eggs when environmental conditions for offspring are favourable, which is subsequently compensated for by accelerated juvenile growth. How this rapid growth is modulated on a molecular level is still unclear. As the somatotropic axis is a key regulator of early growth in vertebrates, we investigated the effect of egg size on three key genes belonging to this axis, at different ontogenetic stages in a mouthbrooding cichlid (Simochromis pleurospilus). The expression levels of one of them, the growth hormone receptor (GHR), were significantly higher in large than in small eggs, but remarkably, this pattern was reversed after hatching: young originating from small eggs had significantly higher GHR expression levels as yolk sac larvae and as juveniles. GHR expression in yolk sac larvae was positively correlated with juvenile growth rate and correspondingly fish originating from small eggs grew faster. This enabled them to catch up fully in size within eight weeks with conspecifics from larger eggs. This is the first evidence for a potential link between egg size, an important maternal effect, and offspring gene expression, which mediates an adaptive adjustment in a relevant hormonal axis.

Maternal effects on male weaponry: female dung beetles produce major sons with longer horns when they perceive higher population density

“…Alternatively, egg composition could in theory affect offspring horn growth without a detectable effect on offspring body size. Egg composition is known to affect offspring phenotype in birds [52] and fish [53]. In the seed beetle Stator limbatus, females change the size and probably the composition of their eggs according to the host plant they encounter while maturing eggs, and the survivorship of their offspring is greatly affected by this plasticity in egg production [54].…”

Juvenile exposure to predator cues induces a larger egg size in fish

“…To linearize the growth trajectory, we analysed the natural logarithm of standard length and included the square of the age of the fish as a covariate in the model. We included the mean egg mass of the clutch of origin in the model because previous work on another mouthbrooding cichlid showed that egg size affects juvenile growth [39]. For model selection, we used step-down protocols [40].…”

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

“…Despite the widely accepted view that egg size can affect offspring fitness (Bernardo, ; Mousseau & Fox, ), only one study has reported a link between egg size and offspring gene expression in fishes. Segers et al () found the expression levels of the growth hormone receptor ( GHR ) to be lower in smaller than larger cichlid embryos (originating from small and larger eggs, respectively), a pattern that was reversed after hatching. Such egg size‐dependent gene expression demonstrates the potential of maternal effects to influence developmental plasticity.…”

“…The growth‐related candidate genes were selected based on a literature search, whilst skeletal‐related genes were chosen to expand upon previous findings (Ahi et al, ; see below for further details). We studied patterns of gene expression across four developmental stages to test whether expression of growth‐related genes is higher in larger offspring (Segers et al, ) and whether, in contrast, expression of genes related to skeletogenesis is higher in smaller offspring (Ahi et al, ). These predictions are based on the hypotheses that larger individuals (i.e., originating from larger eggs) invest more energy toward growth, whilst smaller offspring (i.e., originating from smaller eggs) invest more in ossification (Eiríksson et al, ; Segers et al, ).…”