Summary
The short generation time of the zebrafish (Danio rerio) was exploited to investigate the effects of selection for body size at age on early life-history traits and on the transcriptional response to a growth stimulus in skeletal muscle of adult fish. Replicate populations were either unselected (U-lineage) or subjected to four generations of experimental selection for small (S-lineage) or large body size (L-lineage) at 90 days post-fertilization. Body mass was on average 16.3% and 41.0% higher in the L- than the U- and S-lineages respectively. Egg diameter was 6.4% lower with 13% less yolk in the S- compared to the other lineages. Maternal transcripts for igf2r, bmpr1aa, igf1ar, igf2a, igfbp5a, ghra and igfbp3 in 2-4 cell stage embryos were higher in the L- than S-lineage. Larvae from the L-lineage were significantly larger, but survivorship at the end of the first month was similar between lineages. Gene expression was measured in the fast muscle of adult fish fasted for 7d and then re-fed to satiation for 48h. The expression of 11 insulin-like growth factor pathway genes and 12 other nutritionally-responsive genes was similar for the S- and L-lineages as was gut fullness with feeding. Transcript abundance for 4 genes (igf1a, igf2r, igfbp1a and igfbp1b) showed either regulated or constitutive differences between the S- and L-lineages. For example, igf2 receptors transcripts were higher and igbp1a/b transcripts lower in the L- than S-lineages consistent with an effect of selection on insulin-like growth factor signalling.
