Insulin-like signaling (IIS) responses to temperature, genetic background, and growth variation in garter snakes with divergent life histories

Abstract

The insulin/insulin-like signaling pathway (IIS) has been shown to mediate life history trade-offs in mammalian model organisms, but the function of this pathway in wild and non-mammalian organisms is understudied. Populations of western terrestrial garter snakes (Thamnophis elegans) around Eagle Lake, California, have evolved variation in growth and maturation rates, mortality senescence rates, and annual reproductive output that partition into two ecotypes: "fast-living" and "slow-living". Thus, genes associated with the IIS network are good candidates for investigating the mechanisms underlying ecological divergence in this system. We reared neonates from each ecotype for 1.5years under two thermal treatments. We then used qPCR to compare mRNA expression levels in three tissue types (brain, liver, skeletal muscle) for four genes (igf1, igf2, igf1r, igf2r), and we used radioimmunoassay to measure plasma IGF-1 and IGF-2 protein levels. Our results show that, in contrast to most mammalian model systems, igf2 mRNA and protein levels exceed those of igf1 and suggest an important role for igf2 in postnatal growth in reptiles. Thermal rearing treatment and recent growth had greater impacts on IGF levels than genetic background (i.e., ecotype), and the two ecotypes responded similarly. This suggests that observed ecotypic differences in field measures of IGFs may more strongly reflect plastic responses in different environments than evolutionary divergence. Future analyses of additional components of the IIS pathway and sequence divergence between the ecotypes will further illuminate how environmental and genetic factors influence the endocrine system and its role in mediating life history trade-offs.

Hormonal and metabolic responses to upper temperature extremes in divergent life-history ecotypes of a garter snake

“…This distinction -whether hormonal profiles change with age or whether certain profiles permit greater growth -has important implications for understanding the role of hormonal regulation of life-history strategies (e.g. Reding et al, 2016).…”

Exploring the mechanistic link between corticosterone and insulin-like growth factor-1 in a wild passerine bird

“…While interest in the ecological and evolutionary relevance of IGF-1 has recently increased (Sparkman, Vleck & Bronikowski, 2009; Sparkman et al, 2010; Palacios, Sparkman & Bronikowski, 2012; Lodjak, Mägi & Tilgar, 2014; Reding et al, 2016; Lodjak, Tilgar & Mägi, 2016; Lodjak et al, 2017), to the best of our knowledge no study has directly tested the effects of stressors on the activity of the HPS axis or investigated the mechanistic link between the HPA and HPS axes in any free-living organisms. Therefore, we aimed at answering whether: (1) acute stress affects plasma levels of IGF-1 and whether (2) glucocorticoids directly affect circulating IGF-1 levels.…”

Among‐individual heterogeneity in maternal behaviour and physiology affects reproductive allocation and offspring life‐history traits in the garter snake Thamnophis elegans

“…Numerous studies have demonstrated that both behavioural and physiological traits in garter snakes are both repeatable and heritable (Arnold and Bennett 1984, Garland 1988, Brodie 1993, King et al 2004, though generally physiological markers seem to be less consistent within individuals Garland 2012, Sparkman et al 2014, this study). Furthermore, common garden experiments reveal that growth, metabolic rates and endocrine function are partially determined by genetic background (Bronikowski 2000, Gangloff et al 2015, Reding et al 2016, Addis et al 2017). More precise estimates of heritability will be needed to assess the potential for correlated selection to shape integrated phenotypes.…”