Mark W. J. Ferguson

Temperature of egg incubation determines sex in Alligator mississippiensis.

The factors controlling sexual differentiation in crocodilians are unknown, but heteromorphic sex chromosomes are absent from all species1. Nichols and Chabreck2 speculated that the sex of Alligator mississippiensis was not rigidly determined at the time of hatching but could be influenced by the post-hatching environment. They presented little evidence to support their hypothesis3 (no histological sections of hatchling gonads, no indication of the sex ratio at hatching), and their study failed to take account of habitat preferences of adult male and female alligators4. Here we demonstrate by laboratory and field experiments, that in A. mississippiensis: (1) Sex is fully determined at the time of hatching and naturally irreversible thereafter, and depends on the temperature of egg incubation, temperatures 30 °C producing all females, 34 °C yielding all males. (2) The temperature-sensitive period is between 7 and 21 days of incubation. (3) Natural nests constructed on levees are hotter (34 °C) than those constructed on wet marsh (30 °C), thus the former hatch males and the latter females. (4) The natural sex ratio at hatching is five females to 1 male. (5) Females hatched from eggs incubated at 30 °C weigh significantly more than males hatched from eggs incubated at 34 °C. This weight difference constitutes a possible selective evolutionary advantage of temperature-dependent sex determination (TSD) in alligators in that females become large and sexually mature as early as possible. The occurrence of TSD in alligators has wide-ranging implications for embryological, teratological, molecular, evolutionary, conservation and farming studies as well as for theories relating to the extinction of other Archosaurs.

Egg incubation: Physiological effects of incubation temperature on embryonic development in reptiles and birds

Introduction Generally, the incubation temperature of bird eggs is conservative: within a species there is little variation in incubation temperature at which normal development can proceed. By contrast, the incubation temperature of oviparous reptiles is relatively labile; normal patterns of development in individual embryos can ensue at a wide range of temperatures. In addition, the incubation temperature of avian eggs is usually higher than for reptile eggs which, as a group, have a much wider range of viable incubation temperatures. Average incubation temperatures for birds are tabulated by Rahn (Chapter 21) and comprehensive reviews of the thermal tolerances of avian embryos have been prepared by Drent (1975) and Webb (1987). Comparable data for reptiles are available for turtles (Ewert, 1979, 1985; Miller, 1985 a ), crocodilians (Ferguson, 1985) and squamates (Hubert, 1985). Incubation temperature is very important in determining rates of embryonic growth and development and to a large extent the length of the incubation period. It also has other effects, as yet predominantly observed in reptiles. Incubation temperature determines sex in many species of reptile and also affects the pigmentation pattern of hatchlings, post-hatching growth rates and moulting cycles as well as thermoregulatory and sexual behaviour patterns. These topics are reviewed for reptiles, using primarily the American alligator ( Alligator mississippiensis ) as the example, but the possible effects of temperature on avian development are also examined.

Epithelial-mesenchymal interactions during vertebrate palatogenesis.

Publisher Summary This chapter discusses the epithelial–mesenchymal interactions during vertebrate palatogenesis. During normal development, the palatal epithelium of any vertebrate consists of three distinct regions: nasal, medial, and oral, each with different developmental fates. In most vertebrates, the nasal epithelia differentiate into pseudostratified ciliated columnar cells and the oral epithelia into keratinized stratified squamous cells. However, the fate of the medial edge epithelial cells (MEE) of embryonic palatal shelves varies among different vertebrate species. In mammals, the MEE die; in alligators, they migrate; and in chicks, the MEE keratinize. In each case, the MEE express different and clearly distinguishable phenotypes. It is widely recognized that the epithelia of most tissues and organs are dependent upon tissue interactions with the adjacent mesenchyme for their differentiation and morphogenesis. The differentiation of palatal epithelium could involve epithelial–mesenchymal interactions. However, recent studies have shown that epidermal growth factor influences mouse palatal MEE differentiation via an action on the underlying mesenchyme, so that the role of the mesenchyme in normal MEE differentiation requires reexamination.

Developmental Mechanisms in Normal and Abnormal Palate Formation with Particular Reference to the Aetiology, Pathogenesis and Prevention of Cleft Palate

Palatal development was studied macroscopically, microscopically and ultrastructurally in foetuses of inbred Wistar rats and Alligator mississippiensis. In the rat, elevation of the palatal shelves from a vertical position lateral to the tongue to a horizontal position above the tongue, occurs very rapidly. This reorientation is postulated to be caused by an intrinsic turgor shelf force generated by the hydration of mesenchymal mucopolysaccharides (predominantly hyaluronic acid). Cleft palate was induced in rat foetuses using 5-fluoro-2-desoxyuridine and was associated with greatly decreased mucopolysaccharide synthesis. The alligator is the only animal which develops in an external egg and which possesses a true mammal-like secondary palate: it is therefore a useful animal model system because longitudinal studies and direct surgical and pharmacological manipulations can be performed. The palatal shelves of alligators grow horizontally above the dorsum of the tongue from their first appearance. This de novo horizontal shelf growth is associated with an increased amount of space in the alligator oronasal cavity due to the small, fatty, alligator tongue. It is postulated that the evolution of the large muscular mammalian tongue constrains the palatal shelves to grow vertically until sufficient space can be created to form the common nasal passage simultaneous with shelf elevation.

Extrinsic Microbial Degradation of the Alligator Eggshell

The outer, densely calcified layer of the alligator eggshell shows progressive crystal dissolution, with the production of concentrically stepped erosion craters, as incubation progresses. This dissolution is caused by the acidic metabolic by-products of nest bacteria. Extrinsic degradation serves to gradually increase the porosity and decrease the strength of the eggshell.

Fewer numbers, better science

PIVOT TO SUCCEED Two simple changes could make a big difference. Create a ‘pivot narrative’. Funding applications should give researchers who are in the midst of a shift an opportunity to describe their rationale. The significance and potential of the proposed work should be assessed alongside the researcher’s proven abilities for research in other fields. Alisic, for example, could explain how her work with young people sensitized her to a growing need for evidence-based interventions to treat trauma in children fleeing conflict. A ‘pivot narrative’ would also explain dr y spel ls and the lack of a track record in the proposed area. The simple step of adding a text box to an application form could expand scientists’ willingness to explore, and help assessors to support such exploration. Revise peer review. There is little to no emphasis on peer-review training. Equipping scientists with skills for more nuanced appraisal will help them to consider varied attributes, particularly how to address complex societal challenges and to evaluate broader interdisciplinary questions. This could eventually change institutional cultures. The greatest risk is that innovation will be stifled by failing to invest in the best emerging scientists, who are approaching the peak of their creativity. ■