Development of male-larger sexual size dimorphism in a lizard: IGF1 peak long after sexual maturity overlaps with pronounced growth in males

Abstract

Squamate reptiles have been considered to be indeterminate growers for a long time. However, recent studies demonstrate that bone prolongation is stopped in many lizards by the closure of bone growth plates. This shift in the paradigm of lizard growth has important consequences for questions concerning the proximate causes of sexual size dimorphism. The traditional model of highly plastic and indeterminate growth would correspond more to a long-term action of a sex-specific growth regulator. On the other hand, determinate growth would be more consistent with a regulator acting in a sex-specific manner on the activity of bone growth plates operating during the phase when a dimorphism in size develops. We followed the growth of males and females of the male-larger Madagascar ground gecko (Paroedura picta) and monitored the activity of bone growth plates, gonad size, levels of steroids, expression of their receptors (AR, ESR1), and expression of genes from the insulin-like growth factor network (IGF1, IGF2, IGF1R, and IGF2R) in livers. Specifically, we measured gene expression before the onset of dimorphic growth, at the time when males have more active bone growth plates and sexual size dimorphism was clearly visible, and after a period of pronounced growth in both sexes. We found a significant spike in the expression of IGF1 in males around the time when dimorphism develops. This overexpression in males comes long after an increase in circulating testosterone levels and sexual maturation in males, and it might be suppressed by ovarian hormones in females. The results suggest that sexual size dimorphism in male-larger lizards can be caused by a positive effect of high levels of IGF1 on bone growth. The peak in IGF1 resembles the situation during the pubertal growth spurt in humans, but in lizards, it seems to be sex-specific and disconnected from sexual maturation.

Sex‐specific effects of dietary restriction on physiological variables in Japanese quails

“…Despite the weak response to DR, a distinct difference between females and males was observed: females exhibited 64.7% higher IGF-1 levels than males (Figure 2). Our findings are consistent with previous reports across different species, suggesting that IGF-1 exhibits sex-specific expression (Ashpole et al, 2017;Bacon et al, 1993;Baéza et al, 2001;Johnson et al, 1990;Meter et al, 2022;Tóth et al, 2022;Yuan et al, 2009). Notably, the larger sex exhibits higher IGF-1 levels, particularly during growth and maturation.…”

“…Contrast, in species where males are larger, such as brown anole lizards ( Anolis sagrei ) (Cox et al., 2017), yellow catfish ( Pelteobagrus fulvidraco ) (Ma et al., 2015) and chicken (Yadav et al., 2023), testosterone induced the expression of IGF1 gene, which also showed a male‐biased pattern. In Madagascar ground geckos ( Paroedura picta ), ovarian hormones suppress IGF1 expression in females, resulting in smaller females (Meter et al., 2022). In the latter species, the overexpression of IGF1 in males occurs well after the rise in circulating testosterone levels during sexual maturation.…”

Dietary restriction reveals sex-specific expression of the mTOR pathway genes in Japanese quails

“…It exerts its effects by binding to its specific receptor, that triggers a cascade of signalling, mainly mTOR pathway, a pathway responsible for cellular proliferation 62 , 63 . The IGF-1/IGF1R axis also plays a pivotal role in sexual dimorphism, influencing the distinct physiological and morphological differences between males and females upon interaction with sex steroids 64 – 66 . In both sexes, expression of IGF1 and IGF1R genes was reduced in all restriction levels compared to the ADL group (Fig.…”

Dietary restriction reveals sex-specific expression of the mTOR pathway genes in Japanese quails

“…It exerts its effects by binding to its specific receptor, that triggers a cascade of signalling, mainly mTOR pathway, a pathway responsible for cellular proliferation 62,63 . The IGF-1/IGF1R axis also plays a pivotal role in sexual dimorphism, influencing the distinct physiological and morphological differences between males and females upon interaction with sex steroids [64][65][66] . In both sexes, expression of IGF1 and IGF1R genes was reduced in all restriction levels compared to the ADL group (Fig.…”