Mullerian Inhibiting Substance in the Caudate Amphibian Pleurodeles waltl

Mullerian inhibiting substance (MIS, also known as anti-Mullerian hormone), is a key factor of male sex differentiation in vertebrates. In amniotes, it is responsible for Mullerian duct regression in male embryos. In fish, despite the absence of Mullerian ducts, MIS is produced and controls germ cell proliferation during gonad differentiation. Here we show for the first time the presence of MIS in an amphibian species, Pleurodeles waltl. This is very astonishing because in caudate amphibians, Mullerian ducts do not regress in males. Phylogenetic analysis of MIS P. waltl ortholog revealed that the deduced protein segregates with MIS from other vertebrates and is clearly separated from other TGF-beta family members. In larvae, MIS mRNA was expressed at higher levels in the developing testes than in the ovaries. In the testis, MIS mRNA expression was located within the lobules that contain Sertoli cells. Besides, expression of MIS was modified in the case of sex reversal: it increased after masculinizing heat treatment and decreased after estradiol feminizing exposure. In addition to the data obtained recently in the fish medaka, our results suggest that the role of MIS on Mullerian ducts occurred secondarily during the course of evolution.