Stewart James R, Mathieson Ashley N, Ecay Tom W, Herbert Jacquie F, Parker Scott L, Thompson Michael B
Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee 37614, USA.
J Morphol. 2010 Nov;271(11):1342-51. doi: 10.1002/jmor.10877.
The eggshell of lizards is a complex structure composed of organic and inorganic molecules secreted by the oviduct, which protects the embryo by providing a barrier to the external environment and also allows the exchange of respiratory gases and water for life support. Calcium deposited on the surface of the eggshell provides an important nutrient source for the embryo. Variation in physical conditions encountered by eggs results in a tradeoff among these functions and influences eggshell structure. Evolution of prolonged uterine egg retention results in a significant change in the incubation environment, notably reduction in efficiency of gas exchange, and selection should favor a concomitant reduction in eggshell thickness. This model is supported by studies that demonstrate an inverse correlation between eggshell thickness and length of uterine egg retention. One mechanism leading to thinning of the eggshell is reduction in size of uterine shell glands. Saiphos equalis is an Australian scincid lizard with an unusual pattern of geographic variation in reproductive mode. All populations retain eggs in the uterus beyond the embryonic stage at oviposition typical for lizards, and some are viviparous. We compared structure and histochemistry of the uterus and eggshell of two populations of S. equalis, prolonged egg retention, and viviparous to test the hypotheses: 1) eggshell thickness is inversely correlated with length of egg retention and 2) eggshell thickness is positively correlated with size of shell glands. We found support for the first hypothesis but also found that eggshells of both populations are surprisingly thick compared with other lizards. Our histochemical data support prior conclusions that uterine shell glands are the source of protein fiber matrix of the eggshell, but we did not find a correlation between size of shell glands and eggshell thickness. Eggshell thickness is likely determined by density of uterine shell glands in this species.
蜥蜴的蛋壳是一种复杂的结构，由输卵管分泌的有机和无机分子组成，它通过对外界环境形成屏障来保护胚胎，同时还允许呼吸气体和水进行交换以维持生命。沉积在蛋壳表面的钙为胚胎提供了重要的营养来源。卵所遭遇的物理条件的变化会导致这些功能之间的权衡，并影响蛋壳结构。延长子宫内卵滞留时间的进化导致孵化环境发生显著变化，尤其是气体交换效率降低，自然选择应有利于蛋壳厚度相应减小。这一模型得到了一些研究的支持，这些研究表明蛋壳厚度与子宫内卵滞留时间呈负相关。导致蛋壳变薄的一种机制是子宫壳腺尺寸减小。平等赛氏蜥是一种澳大利亚石龙子蜥蜴，其繁殖模式具有不寻常的地理变异模式。所有种群在产卵时都会将卵保留在子宫内超过蜥蜴典型的胚胎阶段，有些种群是胎生的。我们比较了平等赛氏蜥两个种群（延长卵滞留时间和胎生）的子宫和蛋壳的结构及组织化学，以检验以下假设：1）蛋壳厚度与卵滞留时间呈负相关；2）蛋壳厚度与壳腺大小呈正相关。我们支持第一个假设，但也发现与其他蜥蜴相比，这两个种群的蛋壳都出奇地厚。我们的组织化学数据支持了先前的结论，即子宫壳腺是蛋壳蛋白质纤维基质的来源，但我们没有发现壳腺大小与蛋壳厚度之间存在相关性。在这个物种中，蛋壳厚度可能由子宫壳腺的密度决定。
