Daniel H Gist

Ph. D.: University of Missouri 1968

Interest in my laboratory is upon the timing of reproductive events in seasonally reproducing organisms. The reproductive events of primary interest to us are the production and maturation of both spermatozoa and eggs as well as their transfer during the mating process; these events must be synchronized for reproductive success.

We use as a model for most of our studies a reptile, the turtle. Despite their evolutionary importance, reptiles remain a poorly studied group, and for many species, little is known about the reproductive events themselves, much less their synchronization. The turtle is a primitive reptile and thus a good model to study the evolution of vertebrate reproductive mechanisms. Additionally, most turtles inhabit the aquatic environment, are abundant, and widespread in their distribution, and as such can serve as indicator species for environmental studies. Our present interest focuses on spermatogenesis, on the subsequent maturation of sperm within the male excurrent canal system (epididymis), and on the storage of sperm within the female reproductive tract. Our studies involve anatomical, physiological, and life history implications of these processes. Our goal is to understand the early stages in the evolution of vertebrate reproductive systems as exemplified by exant turtles. Specific projects addressing that goal include:

Cytology of spermatogenesis in the slider turtle. This study identifies the cell types and their associations in the spermatogenic testis. In addition, the number of sperm produced in a generation and the number of generations per season will be identified.

Annual changes is epididymal histology. The epididymis makes its first appearance in the Reptilia. An androgen-dependent organ in other vertebrates, the turtle epididymis remains devloped and contains sperm for 10 months of the year despite fluctuations in circulating androgens. This study characterizes the anatomical changes in the epididymis throughout the year and correlates those changes to circulating androgens.

Sperm motility and survival. This study examines the ability of spermatozoa from the turtle epididymis to survive under in vitro conditions and the motility of sperm under different physiological conditions. Turtle sperm are unique among the vertebrates in their longevity.

Testicular aromatase activity. Testes of many vertebrates secrete estrogen hormones in addition to androgens. The levels of aromatase, a key enzyme in the synthesis of estrogen hormones, are being estimated in the turtle testis by immunochyochemistry and compared to estrogen presence in blood and seminal fluid.

Steroid receptors. Steroid hormone receptors are nuclear proteins. The presence and quantity of estrogen and androgen receptors in turtle excurrent duct tissues are being examined immunocytochemically and by in situ hybridization. This study will provide information as to the timing and sensitivity of excurrent duct tissues to testicular steroids.

In addition, I am involved in collaborative projects with other faculty and students. One of these involves studies of the reproduction, growth, and development of zebra mussels in the Ohio River. Another collaborative study examines reproductive disorders in fish inhabiting polluted waters.