Gene expression of secretory proteins in the nuptial pads of three Lithobates species (Anura: Ranidae)

Abstract

Many amphibian species possess male-specific secretory breeding glands. In anurans, such specialized glands are often present as nuptial pads at the first digit of the hand, which are pressed against the female during amplexus. However, the role of nuptial pad secretions remains largely unknown. Here, we investigate the inner morphology as well as the RNA expression patterns of the nuptial pads of several Central American Lithobates species (Ranidae). As shown for the breeding glands of other amphibian species, the Lithobates nuptial pads are composed of large specialized mucus glands, excreting proteinaceous content to the surface. Whole-transcriptome sequencing revealed that for one of the species the most highly expressed transcripts encoding secretory proteins in the nuptial pads are sodefrin precursor-like factor (SPF) proteins, known to have a pheromone function in multiple amphibian species. The other two species, however, lack high expression of SPF transcripts but express other secretory proteins, whose roles in amphibian breeding glands remain unknown. Several of these proteins are highly expressed in all samples, suggesting a direct role during courtship or for the maintenance/development of the glands.

Sexy fingers: Pheromones in the glands of male dendrobatid frogs

“…It is also highly expressed in the male breeding glands of frog species belonging to families other than Dendrobatidae (Table 1). In addition to the high expression of SPFs in the swollen fingers, we found several differentially expressed genes known to be highly expressed in other male frog macroglands (Bossuyt et al, 2019;Schulte et al, 2021Schulte et al, , 2022.…”

“…This expression is both, strongly differential (compared to control tissues) and involves not one but many SPF transcripts. In other amphibians, SPFs are highly expressed in male salamander and frog macroglands and are most likely produced and stored in SMGs (Bossuyt et al, 2019;Janssenswillen, Willaert, et al, 2015b;Schulte et al, 2021Schulte et al, , 2022. In dendrobatid frogs, SMGs are responsible for the male swelling (macroglands) in the fingers and wrist (Cavalcanti et al, 2022).…”

“…Molecular data also support the reproductive role of these sexually dimorphic glands. Some specialised glands in frogs have been found to secrete volatiles or proteins resembling known pheromones (Bossuyt et al, 2019;Schulte et al, 2021Schulte et al, , 2022Schulz et al, 2021).…”

“…Although high expression of SPFs has been found in male macroglands of some species of frogs (Bossuyt et al, 2019;Schulte et al, 2021Schulte et al, , 2022, their role in mating, association and evolution with courtship behaviours, and general reproductive function in frogs remain poorly understood.…”

Pheromones for amphibian conservation – considering new approaches in conservation breeding programmes

“…breeding glands; Houck & Sever, 1994;Brizzi, Delfino, & Jantra, 2003). Such breeding glands can be found in countless amphibian species, including many frogs which produce proteins closely related to salamander courtship pheromones (Bossuyt et al, 2019b;Schulte et al, 2021Schulte et al, , 2022. The ubiquitous nature of these glands in amphibians suggests a significant role of the herein produced proteins for mating success, and calls for a collective effort to try to incorporate such potential courtship pheromones into conservation breeding programmes.…”