Reproductive Endocrinology of Fishes
 

B.S., Richmond College

M.S., New York University

Ph.D., New York University
 
 

Research Interests

I am most interested in the investigation of the neuroendocrine control of reproduction in teleost fishes with a particular emphasis
upon the hypothalamo-pituitary-gonad axis. My earlier work focused on the genetic control of size polymorphism and time of
sexual maturation in the pygmy swordtail, Xiphophorus pygmaeus. I was able to show that a sex linked gene exerted a
pleiotropic effect, i.e., it controlled the differentiation of both somatrotrops and gonadotrops in the pituitary gland which accounted
for differences in growth rate and age of maturity among the different morphs of Xiphophorus.

My recent work has focused on the neuroendocrine control of female to male sex reversal in the protogynous marine fish,
Thalassoma bifasciatum.. In this species, sex change is initiated by social cues, that is, a change in the primary/terminal phase
individuals in a population. I have shown that the social stimulus triggers an internal signal that is responsible for the
transformation; the signal involves changes in the hypothalamo-hypophysial-gonad axis. I have traced the signal to the level of the
hypothalamus. However, undoubtedly the initial site of action probably lies at a level above the hypothalamus.

At present, I am utilizing immunocytochemistry to investigate changes in the number and/or activity of the gonadotrops in the
pituitary gland of this fish as it undergoes gender change, since I have shown that at least one gonadotropin (LH) induces
precocious reversal in this species.

Questions arise concerning the activity of the gonadotrops and serve as the basis for future research. As in higher vertebrates,
these cells are under hypothalamic control via GnRH. In addition, I have found that, as in some other teleost species, the
gonadotrops of T. bifasciatum are subjected to inhibitory influence via dopaminergic innervation. Perhaps other chemical
messengers influence the activity of the gonadotrops in T. bifasciatum and thus affect the timing of sex reversal. Such factors
could include NPY and endothelin, both of which have been shown to influence the gonadotrops in teleosts. Also, although the
initial signal for reversal is a social change, by what pathway(s) is this external stimulus transduced? For example, are the visual
and/or olfactory senses employed?

Publications

C.R. Kramer, M.T. Caddell and L. Bubenheimer-Livolsi.  (1993).   sGNRH-A ((D-Arg6, Pro9, Net-)LHRH) in combination with
domperidone induces gonad reversal in protogynous fish, the bluehead wrasse, Thalassoma bifasciatum.  J. Fish Biology 42:
185-195.

S. Koulish, and Kramer, C.R.  (1989).   Human chorionic gonadotropin (hCG) induces gonad reversal in protogynous fish, the bluehead wrasse, Thalassoma bifasciatum (Teleostei, Labridae). J. Expt'l. Zool. 242: 156-168.

S. Koulish, and Kramer, C.R.  (1988).   hCG induces sex reversal in Thalassoma bifasciatum, a protogynous fish., J. Cell Biol. 107:483a.

C.R. Kramer, S. Koulish and P.L. Bertacchi.  (1988).  The effects of testosterone implants on ovarian morphology in the bluehead wrasse, Thalassoma bifasciatum (Block) (Teleostei: Labridae). J. Fish Biology 32: 397-407.

F.A. Kincl, C.R. Kramer and S. Koulish.  (1987).  Sex reversal in wrasses. I Uptake of testosterone by the gonads and the central nervous system and its aromatization in the CNS of Thalassoma duperrey (Teleostei: Labridae). Endocrinol. Experimentalis 21, 115-123.

C.R. Kramer and K.D. Kallman.  (1985).  Sex differentiation of somatic tissue in the unsexualised gonad primordia of the embryos of three species of poeciliid fish. J. Anatomy 140: 269-277.