Mechanism of an Evolutionary Change in Development

大多数无脊椎动物门的物种在其生命周期中都有一个幼虫期。然而，在某些情况下，有改变或消除幼虫期的趋势。在进化过程中，发育中的变化是如何产生的，我们知之甚少。在这一建议中，开发了一个模型系统，并利用它来研究发展过程中进化变化的机制。该模式系统由两个密切相关的海鞘物种组成，它们表现出不同的幼虫形式，并能够进行种间杂交。Molgula oculata（尾尾虫种）发育为常规尾尾虫（或尾尾虫）幼虫，其含有脑色素细胞、脊索和尾肌细胞。相比之下，Molgula occulta（一年生物种）发育为无尾（或一年生）幼虫，缺乏脑色素细胞、脊索和分化的肌肉细胞。由无性系物种的卵子与无性系物种的精子受精产生的杂交种发育出脑色素细胞和脊索细胞，但不形成分化的肌肉细胞，这表明在无性系发育的进化过程中发生了母体和合子的变化。本研究将结合胚胎学和分子学的方法：1)描述从尾海鞘到海鞘幼虫发育转变过程中的母系和合子变化；2)研究导致海鞘缺乏肌肉细胞分化的分子机制；3)确定海鞘和尾海鞘的系统发育关系。研究结果有望对发育过程中进化变化的机制提供更全面的理解。

Most invertebrate phyla contain species which include a larval phase in their life cycle. In certain instances, however, there has been a tendancy to modify or eliminate the larval phase. Little is known about how modifications in development arise during evolutoin. In this proposal, a model system is developed and exploited to study the mechanisms underlying evolutionary changes in development. This model system consists of two closely related species of ascidians that exhibit different larval forms and are capable of interspecific hybridizatrion. Molgula oculata (the urodele species) develops a conventional tailed (or urodele) larva, which contains brain pigment cells, a notochord, and tail muscle cells. In contrast, Molgula occulta (the anural species) develops a tailless (or anural) larva, which lacks brain pigment cells, a notochord, and differentiated muscle cells. Hybrids produced by fertilizing eggs of the anural speies with sperm of the urodele species develop a brain pigment cell and notochrod but do not form differentiated muscle cells, suggesting that both maternal and zygotic changes have occured during the evolution of anural development. This proposal combines embryological and molecular approaches 1) to characterize the maternal and zygotic changes involved in the switch from urodele to anural larval development, 2) to examine the molecular mechanisms responsible for lack of muscle cell differentiation in the anural species and 3) to determine the phylogenetic realtionship of anural and urodele ascidians. The results are expected to provide a more comprehensive understanding of the mechanisms underying evolutionary changes in development.