Increased Posthatching Mortality and Loss of Sexually Dimorphic Gene Expression in Alligators (Alligator mississippiensis) from a Contaminated Environment1

Abstract

A previous study from our laboratory examining development in neonatal alligators from polluted Lake Apopka, Florida, found numerous differences relative to neonates from a reference site, Lake Woodruff National Wildlife Refuge. We postulated that the differences were the result of organizational changes derived from embryonic exposure to environmental contaminants and are related to the poor reproductive success reported in alligators from Lake Apopka. In this study we examine differences in alligators collected as eggs from these two populations and raised under similar conditions for 1 yr. Egg hatch rates did not differ between lake populations; however, posthatching mortality was much higher among Lake Apopka hatchlings. Snout-vent length and body mass were greater in Lake Apopka hatchlings, but no differences were detected between lake populations in thyroid, liver, and spleen mass corrected for body size or in plasma concentrations of testosterone and estradiol. Males from Lake Woodruff exhibited greater relative expression of gonadal mRNA for steroidogenic factor 1 (Nr5a1) and steroidogenic acute regulatory protein (Star) than males from Lake Apopka. Alligators from Lake Woodruff also expressed all genes examined in a sexually dimorphic pattern. In contrast, mRNA expression did not differ between males and females from Lake Apopka for Nr5a1, Star, cytochrome P450 11A1 (Cyp11a1), and hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 1 (Hsd3b1). Our results document persistent differences in development, survivorship, and gene expression in alligators from a contaminated environment. Because these animals were raised under similar laboratory conditions, the differences are most likely of embryonic origin and organizational in nature.

Influence of tissue, age, and environmental quality on DNA methylation in Alligator mississippiensis

“…To further confirm the sex of the juveniles used in this study, gonadal aromatase mRNA expression levels were analyzed by quantitative PCR. Juvenile alligators show clear sexual dimorphic patterns in the expression of aromatase, with females displaying levels at least threefold higher than males , Milnes et al 2008. Both assessments (gross morphology and aromatase expression) classified all juvenile alligators included in this study as female.…”

“…A number of reproductive abnormalities, including altered gonadal development and decreased robustness of sexually dimorphic gene expression, have been reported in alligators originating from Lake Apopka (Milnes et al 2005(Milnes et al , 2008). Yet, understanding of the molecular mechanisms mediating the effects of poor environmental quality on reproductive health remain incomplete.…”

Before senescence: the evolutionary demography of ontogenesis

“…The severity of the environment may influence the magnitude of the losses (e.g. [36,37]), and particular causes of death (e.g. siblicide [38] or juvenile migration [39]), and may drive mortality at one stage of development to exceed that at an earlier stage.…”

Gonadal mRNA Expression Levels of TGFβ Superfamily Signaling Factors Correspond with Post-Hatching Morphological Development in American Alligators

“…RNA isolation and reverse transcription procedures for alligator gonadal tissues have been previously reported from our laboratory [Milnes et al, 2008]. Quantitative real-time PCR (Q-PCR) has been used to measure mRNA expression in the American alligator [Katsu et al, 2004;Gunderson et al, 2006;Kohno et al, 2008].…”