Gonadal Differentiation and Development in the Snouted Treefrog, Scinax fuscovarius (Amphibia, Anura, Hylidae)
Accurate descriptions of gonadogenesis are necessary to have a baseline to detect abnormalities during development and to assess the time a species may be more vulnerable to the action of pesticides, endocrine disruptors, and pharmaceuticals to which abnormal conditions have been attributed. Here, I describe the histomorphological changes during gonadal differentiation in the Snouted Treefrog Scinax fuscovarius. I sexed and measured a total of 302 tadpoles between Gosner stages 26–46 and 21 juvenile and adults. The results show that 1) the species possesses the undifferentiated type of gonadal differentiation—ovaries differentiate much earlier (at stage 26) than testes (stage 36); 2) oogenesis begins almost simultaneously with ovarian differentiation; and 3) ovaries and testes exhibit different rates of gonadal differentiation with respect to somatic development—the ovaries have an accelerated rate (the ovarian cavity differentiates early during larval development and previtellogenic oocytes develop at early premetamorphic stages), whereas the testes have a decelerated rate (seminiferous tubules differentiate at the juvenile stage). The comparison of these characteristics with those of other co-occurring anuran species reveals important interspecific variation in the relative timing of gonadogenesis. Finally, and as a byproduct of the extensive sampling, I describe several cases of gonadal malformations.Abstract
Superposition of the geographic range of Scinax fuscovarius (modified from BerkeleyMapper) depicted with a white dashed line, with the Gran Chaco extension (continuous white line) on the map of South America forest change in the last 12 years. Red areas show high rates of forest loss attributable to deforestation; green indicates forest extent; and purple indicates forest loss and gain (source: Hansen/UMD/Google/USGS/NASA). The right picture depicts a detail of Salta Province (turquoise line), with yellow stars indicating the main locations were tadpoles were collected.
Sequence of events during ovarian differentiation in Scinax fuscovarius. (A–B) Stage 26: (A) external morphology; (B) differentiated ovary with a distinct central lumen (asterisk), and primordial germ cells in mitotic division placed in the cortex. (C–D) Stage 31: (C) external lobulation becomes evident; (D) the ovarian cavity enlarges; the cortex is formed by several oogonia and few oocytes (prediplotenic oocytes sensu Falconi et al., 2001). (E–F) Stage 33: (E) lobulation of the ovary is evident and the first signs of fat body differentiation occur; (F) the ovary contains numerous diplotene oocytes surrounded by follicular cells (arrowhead), and the location of primary and secondary oogonia is reduced to the outer part of the cortex. Nucleoli in oocytes locate at the periphery of the nuclei. The cortex appeared lined by a clearly discernible basal lamina. (G–H) Stage 36: (G) ovarian size and lobulation increases. Fat bodies grow; (H) diplotene oocytes are larger. Arrowhead indicates follicular cells. (I–J) Stage 38: ovarian size increases attributable to an increase in the number and size of diplotene oocytes. (K–L) Stages 42–43: (K) numerous oocytes can be identified in the ovary; (L) the ovarian sacs have numerous diplotene oocytes reducing the lumen. do = diplotene oocytes, fb = fat bodies, k = kidneys, og = oogonia, ov = ovary. Scale bars = 0.5 mm in A, C, E, G, I, K; 50 μm in B, D, F, H, J; and 100 μm in L.
Juvenile ovary of Scinax fuscovarius. (A) External morphology. (B) Histological section of the ovary showing a similar structure and size to that of metamorphosing individuals. The ovary contained numerous diplotene oocytes. Scale bar = 0.5 mm in A, and 100 μm in B.
Sequence of events during testes differentiation in Scinax fuscovarius. (A–C) Stage 37: (A) externally, testes show no sign of differentiation (fat bodies are well-developed); (B) few primary spermatogonia are distributed throughout the testis; (C) scheme of testis shape with a proximal and a distal part (divided by the dashed line). (D–F) Stage 41: (D) testes are shorter than in previous stages and the proximal and distal parts become recognizable; (E) secondary spermatogonia (arrowhead) appear; (F) the constriction between the proximal and the distal part becomes conspicuous. (G–I) Stage 43: (G, I) testes get thicker, and the distal part becomes acuminate; (H) primary and secondary spermatogonia increase in number; (J–L) juvenile testes: (J, L) the distal part degenerates. (K) Formation of seminiferous tubules begins. For recognition, one seminiferous tubule is outlined with a dashed line. (M–O) Adult testes: (M) adult testes have grown much more than in the previous stages and the numerous seminiferous tubules can be identified as small, rounded structures distributed all along the testes; (N) seminiferous tubules (outlined with a dashed line) with spermatogonia, spermatocytes, spermatids, and spermatozoids; (O) detail of fully developed tubules. d = distal part of testis, fb = fat bodies, k = kidney, p = proximal part of testis, t = testis, spc = spermatocytes, spg = spermatogonia, spt = spermatids, spz = spermatozoids. Scale bar = 1 mm in A, C, D, F, G, I, J, L; 50μm in B, E, H, K; 360 μm in N; and 200 μm in O.
Phalangeal cross sections in Scinax fuscovarius. A) A female juvenile with no LAGs, 23.76 mm SVL. B) A 7-yr old male, 47.60 mm SVL. Scale bar = 50 μm.
Gonadal abnormalities in Scinax fuscovarius. (A) Unpaired ovary in a tadpole at Gosner stage 41. Arrowheads show encysted metacercariae. (B) Unpaired testis in a tadpole at Gosner stage 42. (C–D) Unpaired and segmented gonad in a tadpole at Gosner stage 40. (C) In gross morphology, sex differentiation is not evident and the single gonad appeared with an odd morphology. (D) In transverse sections, the ovarian cavity (asterisk) was detected, and evidenced of a poorly developed ovary. (E–F) Asymmetric ovaries in a tadpole at Gosner stage 42. The right ovary is underdeveloped but has numerous oocytes in diplotene, as does the left one. (G–J) Posterior fusion of both ovaries in a tadpole at stage 42. Anteriorly, both ovaries are separated but become fused in a posterior direction. Dashed lines in G indicate the location of serial cross-sections in H, I, and J. Scale bar = 1 mm in A, B, C, E, G; and 200 μm in D, F, H–J.
Comparison of relative timing of gonadal differentiation in anuran species from semiarid environments of Argentina. Gonadal differentiation and development progress in a general pattern but with heterochronic changes that characterize each species. Dark and light bars indicate ovarian and testicular differentiation, respectively. Black circles indicate differentiation of diplotene oocytes. Black squares indicate differentiation of seminiferous tubules. Differentiation rates are indicated, next to species names, for each gonad. Data for Pseudis platensis, Phyllomedusa azurea, and Phyllomedusa sauvagii was taken from Fabrezi et al., (2010), and data for Dermatonotus muelleri was taken from Fabrezi et al. (2012).
