Louis J. Guillette Jr.-Professor of Zoology

Research Interests

The mechanisms by which environmental factors influence the evolution of reproduction in vertebrates has been, and continues to be, the focus of my research. Specifically, I have examined the physiological and anatomical modifications required for the evolution of viviparity in amniote vertebrates. This has included studying the endocrine basis of gestation maintenance and the anatomical modifications of the oviduct required for successful placentation. I also have active research programs examining (1) the influence of contaminants on the developmental and reproductive biology of wildlife and (2) endangered/threatened species reproduction. Below I briefly describe these study areas.

Gestation Maintenance

The physiological basis for the retention of developing young in the uterus has been an important evolutionary, agricultural and biomedical question for over a hundred years. In eutherian mammals, the maintenance of pregnancy is induced by a chemical signal from the embryo. This phenomenon of pregnancy maintenance by an embryonic signal is termed "maternal recognition of pregnancy" (MRP). An interesting observation associated with MRP is that no one chemical signal works in the majority of species. For example, differing signals exist in primates, rodents, and sheep. MRP has been considered unique to mammals until work in France and in my laboratory indicated that lizards display MRP, with luteal rescue in response to a embryonic signal that is, at this time, chemically uncharacterized. I proposed a model in 1987 to explain the observed variation in embryonic signals suggesting that evolutionarily embryos made products that served antibacterial, antiviral, antifungal and antipredatory actions that were later adapted for MRP. This hypothesis is now an accepted model being tested.

 

 

Evolution of the Placenta:

The chorioallantoic placenta is a characteristic of amniote viviparity. The presence of a yolk sac placenta is also essential but poorly studied. The structure and function of these placentae have been of interest since the late 1800's but the evolutionary mechanisms leading to placentation in amniotes were poorly defined until recently. Part of the problem was obtaining appropriate model species - that is, closely related species that exhibit various "intermediary" stages of placentation. Lizards are excellent models for this research. My work on closely related lizards with varying reproductive strategies has helped defined the required initial steps in this process - from genetic to morphological levels. Prior to the work done in my laboratory, viviparity was hypothesized to be a two step process, but we have shown that is not the case. A reduction in the thickness of the egg shell is the essential step. Moreover, our work has shown for the first time that the evolution of viviparity is a response to evolutionary forces acting throughout the reproductive cycle not just on the pregnant female. This work continues to provide important insight into the evolution of viviparity in early mammals.

 

 

Reproductive Cycles, Anatomy and Physiology of Reproduction in Various Reptiles:

A central aspect of any reproductive study is understanding the reproductive cycle of the species under study. In collaboration with numerous colleagues from various countries, my students and I has described the reproductive cycle and biology of a number of Mexican high elevation lizards and snakes, the tuatara and several geckos of New Zealand, and the American alligator. These studies have help demonstrate the great diversity that exists in reproductive cyclicity and have also provide important clues to the evolution of the reproductive system in amniotes. For example, work done in collaboration with my former student Dr. Brent Palmer (University of Kentucky) showed that the alligator reproductive tract had a functional anatomy and physiology almost identical to birds. As crocodilians are the earliest extant branch of the Archosaurs, our observations on oviduct function and anatomy suggest that crocodiles, dinosaurs and birds all form(ed) eggs in a similar fashion - a form of archosaurian oviparity. Work in collaboration with Dr. Alison Cree (University of Otago, New Zealand) described the reproductive cycle of the tuatara and reported the longest reproductive cycle currently known in a reptiles - almost 5 years! In addition to the work on reptiles, I also have an interest in reproduction in various other vertebrates and my graduate students have worked on fish, amphibians, birds and mammals.

 

 

Environmental Endocrine Disrupting Contaminants

The effects of contaminants on wildlife have been studied for more than 30 years, especially since the publication of Rachel Carson's Silent Spring. During the last seven years, new data has reminded us of Carson's original focus, the effects of environmental contamination on the development of various organ systems, especially the reproductive system. For the last seven years, I have examined the developmental biology of the reproductive system in control and contaminated populations of fish and alligators. Our data indicate that environmental contaminants are endocrine disruptors capable of causing a wide spectrum of effects, from embryonic death to subtle, but detrimental changes in reproductive anatomy and function. I currently collaborate with the USFWS, CDC and EPA on various studies of wildlife and human populations. This work has received extensive national and international attention, because the study animal is the alligator but also for its interdisciplinary approach - studies from gene to ecosystem. This approach to science has defined my work for the last 18 years and will continue to do so in the future.