The Evolution of Uterine Function

How did the amniote egg evolve? Are all amniote eggs made equal? How do you make a dinosaur egg? What are the morphological and physiological adaptations required for the evolution of amniote viviparity? These are some of the question that I have tried to address in collaboration with a number of undergraduate and graduate students. To begin to answer these questions, we work with reptiles exhibiting various reproductive modes and producing eggs with differing characteristics. What makes a reptile egg unique compared to that of amphibians or birds?

Nigel Noriega, working with his graduate student mentor Drew Crain, has used electrophoretic and amino acid sequencing techniques to show that eggs from various crocodilian species have some egg white (albumen) and yolk proteins similar to those of birds. However, he found that they also have proteins that are dramatically different in amino acid sequence and molecular weight. One protein, the major protein of the albumen by weight, is unlike any published in the literature. Nigel's observations suggest that all eggs are not made equal. Could this major protein have been found in its archosaurian relatives, the dinosaurs?

Where do the egg white proteins come from? What controls their synthesis? Jocelyn Rathbun and Michael Gilmore, working with graduate student mentors Cathy Cox and Dan Pickford, have demonstrated that estrogens are important in the growth of the glands that secrete albumen proteins. Importantly, they have also demonstrated that estrogen-induced uterine growth is partially indirect as estrogens stimulate the synthesis of growth factors such as epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I). These students have shown that the production of these growth factors induced uterine gland differentiation and growth in alligators and lizards.

Do these observations shed any light on the evolution of the placenta and viviparity in amniotes? Recent evidence from our laboratory indicates that a major characteristic associated with the evolution of viviparity is a reduction in the number of egg shell-secreting glands. The observations made by our research team suggest that growth factors (IGF-I, EGF) may control the recruitment of uterine glands and thus, the number of shell glands present in the uterus.

Finally, what other factors control uterine function? Recent evidence suggests that many environmental contaminants act as hormones or antihormones, such as estrogens (DDT) and antiestrogens (Dioxin), and may disrupt the development and functioning of the vertebrate reproductive system. Kevin Carter and graduate student mentor Andy Rooney are beginning to test this hypothesis. Early data suggest that alligators living in contaminated lakes make eggs that have differing compositions of proteins in the egg white and have modified thyroid and gonadal functioning. In the near future, these questions will be examined in my laboratory by new undergraduate students and their graduate student mentors.