Extended production of the müllerian duct regressor in the American alligator.

Regression of the müllerian ducts in mammals and birds is caused by the testicular hormone, müllerian inhibiting substance (MIS), which is produced for an extended period of time after testicular differentiation. Because it is present long after the ducts regress, it has been proposed to have additional effects other than inducing Mullerian duct regression. A testicular substance also appears to be responsible for müllerian duct regression in the alligator based on studies in which testicular grafts from hatchling males caused regression when implanted into castrated hatchling females. In this study, the approximate ontogeny of the regressor in the alligator was investigated by implanting testicular grafts from both hatchling and yearling males into castrated female hatchlings. Histological quantification of the ducts revealed that regression in both treatment groups was not significantly different indicating that the müllerian duct regressor in the alligator is produced for at least a year after hatching and may have additional nonregressive functions in the alligator as has been proposed for mammals and birds.