Germ cell development in teleost gonads

Among vertebrate species, fishes are considered to be the most developmentally diverse taxa exhibiting remarkable gonadal plasticity with capacity of sex reversal. Sex determination in teleost fishes is governed via a complex regulatory network involving multiple genetic, environmental (temperature, population density, pH), social interaction/stress factors. Piscine germ-line originates as Primordial germ cells (PGC) from cleavage blastomeres/endoderm and gets specified by maternal inheritance of Vasa, Nanos, Tdrds, Dnd, Dazl etc and subsequently migrates to reach the embryonic gonadal primordium. Post-hatching, mitotically active germ cells (Gc) continue to proliferate via two distinct pathways-firstly, stem cell-like mode followed by meiosis-committed cystic division. In Japanese rice fish medaka, Oryzias latipes sex gets determined on the chromosomal basis, where XY becomes a male because of testicular Dmy transcription and XX develops as female owing to Gc restricted Foxl3 expression. Sex specific germ-line stem cell systems also have been found in adult O. latipes. Intriguingly in teleost fishes, the developmental fate of transplanted Gc do not depend on the sex of the donor rather solely relies upon the sex of the recipient. Furthermore, sexually mature piscine gonads remain developmentally plastic showing extensive responsiveness towards sex steroids. The critical regulation of gonadal differentiation is directed by female specific 17β-estradiol (E2) and bio-active male androgen 11-keto-testosretone (11-kT). Various transcription factors (TFs) e.g.- Dmrt1, DAX1, Ad4BP/SF-1, Foxl2 regulate the bio-conversion rate of testosterone (T) to either E2 or 11-kT. Similarly, multiple endocrine factors (EFs) e.g.- LH, FSH, E2, 11-kT and growth factors (GFs) e.g- AMH, GSDF, IGF-1/3, GDF-9/BMP-15 critically induce Gc differentiation to regulate the gametogenic output. In this article we have precisely discussed the endocrine regulation of Gc development during gonadal maturation of teleosts.