A laboratory investigation into features of morphology and physiology for their potential to predict reproductive success in male frogs

Abstract

Amphibian populations are declining globally, however, the contribution of reduced reproduction to declines is unknown. We investigated associations between morphological (weight/snout-vent length, nuptial pad colour/size, forelimb width/size) and physiological (nuptial pad/testis histomorphology, plasma hormones, gene expression) features with reproductive success in males as measured by amplexus success and fertility rate (% eggs fertilised) in laboratory maintained Silurana/Xenopus tropicalis. We explored the robustness of these features to predict amplexus success/fertility rate by investigating these associations within a sub-set of frogs exposed to anti-androgens (flutamide (50 μg/L)/linuron (9 or 45 μg/L)). In unexposed males, nuptial pad features (size/colour/number of hooks/androgen receptor mRNA) were positively associated with amplexus success, but not with fertility rate. In exposed males, many of the associations with amplexus success differed from untreated animals (they were either reversed or absent). In the exposed males forelimb width/nuptial pad morphology were also associated with fertility rate. However, a more darkly coloured nuptial pad was positively associated with amplexus success across all groups and was indicative of androgen status. Our findings demonstrate the central role for nuptial pad morphology in reproductive success in S. tropicalis, however, the lack of concordance between unexposed/exposed frogs complicates understanding of the utility of features of nuptial pad morphology as biomarkers in wild populations. In conclusion, our work has indicated that nuptial pad and forelimb morphology have potential for development as biomarkers of reproductive health in wild anurans, however, further research is needed to establish this.

DNA damage and repair in differentiation of stem cells and cells of connective cell lineages: A trigger or a complication?

“…Furthermore, Orton et al found nuptial pad features (size/color/number of hooks/androgen receptor mRNA) are positively associated with amplexus success, and demonstrated the central role for nuptial pad morphology in reproductive success in Xenopus tropicalis. 28 Our Western blot results show that the protein levels of AR, ERα, ERβ and aromatase are higher during pre-hibernation than those during the breeding period, and the protein levels of AR, ERα and ERβ in two periods show significant difference. Fasano et al .…”

Immunoreactivities of AR, ERα, ERβ and aromatase in the nuptial pad of Chinese brown frog (<em>Rana dybowskii</em>) during pre-hibernation and the breeding period

“…Furthermore, Orton et al found nuptial pad features (size/color/number of hooks/androgen receptor mRNA) are positively associated with amplexus success, and demonstrated the central role for nuptial pad morphology in reproductive success in Xenopus tropicalis. 28 Our Western blot results show that the protein levels of AR, ERα, ERβ and aromatase are higher during pre-hibernation than those during the breeding period, and the protein levels of AR, ERα and ERβ in two periods show significant difference. Fasano et al studied the intratesticular feedback mechanisms in the regulation of steroid profiles in the R. esculenta, and concluded that testosterone shows high values during early spring (between February and March) and autumn (between October and November), with a rapid decrease thereafter.…”

Nuptial pad (“breeding gland”) morphology is related to non‐random mating in wild male common frogs (Rana temporaria)

“…However, although males of wild Columbia frogs found in amplexus possessed a larger nuptial pad than lone males (Greene & Funk, 2009), the importance of this feature for breeding success has not previously been experimentally demonstrated in wild anurans. In laboratory reared Silurana tropicalis , nuptial pad morphology was shown to be central to breeding success (Orton et al, 2020). Thus, the importance of this feature to breeding success has now been demonstrated in three anuran species, which employ a range of breeding modes (i.e.…”

“…It is not surprising, therefore, that here no correlation between body mass and nuptial pad length/colour was observed (see Figures S2 and S3). Similarly, male size was unimportant in achieving success for amplexus in the prolonged breeding tropical clawed frog Silurana tropicalis (Orton et al, 2020). This is in contrast to many Bufonid (toad) species, where a larger male body size has been reported as an advantage to better enable males to dislodge competitors or to maintain their grip in amplexus ( Bufo bufo : Bókony et al, 2018; Bufo bufo : Davies & Halliday, 1977; Bufo bufo : Davies & Halliday, 1979; Bufo (rhinella) marinus : Lee & Price, 2001; Bufo bufo : Orton et al, 2014).…”

“…As androgens have also been shown to control the development and maintenance of nuptial pads, or “breeding glands” (Brizzi et al, 2003; Epstein & Blackburn, 1997), these are a likely contributor to non‐random mating in anurans. Supporting this, amplecting wild male Columbia spotted frogs ( Rana luteiventris ) have been reported to possess larger nuptial pads compared to lone males (Greene & Funk, 2009) and in a laboratory study male, Silurana tropicalis that were successful in amplexing females had a darker and longer nuptial pad compared to those that did not achieve amplexus (Orton et al, 2020). In the latter study, nuptial pad morphology was also related with androgen status, evidenced by greater numbers of spermatogonia in the testis, and higher plasma androgen levels.…”