The Evolution of Viviparity in Amphibians: A Study of Homoplasy

胎生——生儿育女的繁殖方式，而不是产卵——在许多动物的谱系中都是独立进化的。因此，这是趋同进化的结果，相同的特征出现在没有共同祖先的分类群中（同质性）。两栖动物的胎生在这些独立进化中有许多不同的表达，包括卵黄被吸收后，在没有母体营养的情况下，输卵管内的受精卵保留在输卵管内，输卵管内同类相食，外部受精卵保留在母体背部的皮肤上等等。本研究计划将通过比较亲代卵巢、输卵管和皮肤的特征与相关的卵层物种的特征来研究胎生产生的机制，以阐明“妊娠”维持的影响；2)不同胎生方式的分类群及其相关产蛋种的胚胎发育模式；3)发育胚胎的保留模式，包括从形态学上测量的激素基质；4)发育中幼崽的营养模式；(5)胎生种幼体与相关卵层的变态特征比较；6)“怀孕”的时间长短以及分娩的环境和生理因素。这项研究将是第一个考虑同类进化机制的研究，而不是简单地记录这种进化模式的实例。

Viviparity-live-bearing modes of reproduction, rather than egg-laying-has evolved independently in lineages of many animals. It therefore is a consequence of convergent evolution, with the same feature occurring in taxa that do not share a common ancestor that bears the trait (homoplasy). Viviparity in amphibians has many different expressions of these independent evolutions including retention of the internally fertilized ova in the oviducts with our without maternal nutrition after yolk is resorbed, intraoviductal cannibalism, retention of externally fertilized ova in the skin of the mother's back, and others. This research program will investigate the mechanisms by which viviparity has arisen by comparing 1) features of parental ovaries, oviducts, and skin with those of related species that are egg-layers to elucidate the effects of 'pregnancy' maintenance; 2) patterns of development of embryos of taxa with different modes of viviparity and with related egg-laying species; 3) modes of retention of the developing embryos, including the hormonal substrate as measured morphologically; 4) modes of nutrition of the developing young; 5) the features of metamorphosis of the young of viviparous species as compared with those of related egg-layers; and 6) length of 'pregnancy' and the environmental and physiological cues for birth. The study will be one of the first to consider mechanisms by which homoplasy has evolved, rather than simply documenting instances of such patterns of evolution.