Gonadal morphology and difference in reproductive development of two isolated populations of Astyanax rivularis (Teleostei, Characidae).

Individuals of the same species may present different reproductive tactics depending on the environment in which they develop and mature. The present study aimed to define the gonadal development phases of males and females of Astyanax rivularis and to carry out a comparative analysis of the reproductive development of specimens captured in two isolated environments of the São Francisco River basin in Serra da Canastra, Brazil (Point 1: low vegetation and river showing calm and crystalline waters with small well formations; Point 2: current waters, and well-established areas of arboreal vegetation). Thus, the gonads of A. rivularis specimens were collected, fixed and processed with techniques for light microscopy. Five maturation phases of the females' reproductive cycle were established: immature, developing, spawning capable, regressing and regenerating. Three maturation phases of the males' reproductive cycle were observed: spawning capable, regressing, and regenerating. There are differences in the phases of gonadal development of A. rivularis between the two sampling points so that, possibly, animals upstream of the waterfall demonstrate a delay in the reproductive cycle in relation to animals downstream.

The efficiency of spermatogenesis and the support capacity of Sertoli cells in Characiformes.

This stereological analysis of the types of germ cells and the number of Sertoli cells per cyst in Astyanax altiparanae testes during spermatogenesis is the first such report in Characiformes. Testes of 25 male A. altiparanae were examined. Based on the number of spermatogonia B per cyst (469.2 ± 9.92), we estimated that spermatogonia undergo at least nine mitotic divisions before differentiating into primary spermatocytes. There are four spermatogonia types: undifferentiated spermatogonia A*, undifferentiated spermatogonia, differentiated spermatogonia, and type B spermatogonia. The number of Sertoli cells increased gradually from 1.41 ± 0.51 in the single undifferentiated spermatogonium A* to 9.25 ± 0.50 in cysts of spermatocytes in the leptotene/zygotene stage, possibly related to greater complexity of cellular events during the meiotic stage. The number of germ cells rose dramatically from spermatogonia A (1.0 ± 0) to spermatogonia B (469.2 ± 9.92); however, the quantity of spermatocytes inside the cysts in the leptotene/zygotene stage decreased (300.6 ± 6.97) relative to spermatogonia B, representing a loss of approximately 36% of the former number of cells. This was probably the result of apoptosis, which promotes successful development of the remaining cells during sperm production. The support capacity of Sertoli cells increased gradually during spermatogenesis.