Wake D B, Hanken J
Museum of Vertebrate Zoology, University of California, Berkeley, USA.
Int J Dev Biol. 1996 Aug;40(4):859-69.
Direct development is a widespread alternate reproductive mode in living amphibians that is characterized by evolutionary loss of the free-living, aquatic larval stage. Courtship, mating, and oviposition occur on land, and the terrestrial egg hatches as a fully formed, miniature adult. While it is the most common reproductive mode in urodeles, development outside the reproductive tract of the female that proceeds directly to a terrestrial hatchling occurs in only a single lineage, the lungless salamanders of the family Plethodontidae. Evolution of direct development in plethodontids has contributed importantly to the extraordinary evolutionary success of this speciose, geographically widespread, and morphologically and ecologically diverse taxon. Developmental consequences and correlates include increased egg size and embryonic development time, loss of larval structures and ontogenetic repatterning, and altered pattern formation in organogenesis. Evolutionary and phylogenetic consequences and correlates include the loss of larval constraints and origin of morphological novelty, and frequent homoplasy. Analysis of direct development in an evolutionary context illustrates the complex interplay between processes of phylogenetic divergence and developmental biology, and substantiates the prominent role of developmental processes in both constraining phenotypic variation and promoting phenotypic diversity. Despite the proven suitability of direct-developing plethodontid salamanders for laboratory and field study, knowledge of basic features of their developmental biology remains far below that available for many other urodeles. Examination of such features of these "non-model" organisms is an appropriate and deserving goal of future research.
直接发育是现存两栖动物中一种广泛存在的替代繁殖模式，其特征是自由生活的水生幼体阶段在进化过程中消失。求偶、交配和产卵都在陆地上进行，陆地卵孵化出的是完全发育成形的微型成体。虽然这是有尾目动物中最常见的繁殖模式，但在雌性生殖道外直接发育成陆生成体的情况仅出现在一个谱系中，即无肺螈科的无肺螈。无肺螈科直接发育的进化对这个种类繁多、分布广泛且形态和生态多样的分类群的非凡进化成功起到了重要作用。发育方面的结果和相关因素包括卵大小增加和胚胎发育时间延长、幼体结构丧失和个体发育模式重排，以及器官发生过程中模式形成的改变。进化和系统发育方面的结果和相关因素包括幼体限制的丧失和形态新奇性的起源，以及频繁的同形现象。在进化背景下对直接发育进行分析，说明了系统发育分歧过程与发育生物学之间复杂的相互作用，并证实了发育过程在限制表型变异和促进表型多样性方面的突出作用。尽管已证明直接发育的无肺螈适合进行实验室和野外研究，但对其发育生物学基本特征的了解仍远低于许多其他有尾目动物。研究这些“非模式”生物的此类特征是未来研究的一个恰当且值得追求的目标。
