Department of Chemistry and Center for Life Science and Technology, Keio University, Hiyoshi, Yokohama 223-8522, Japan.
Integr Comp Biol. 2003 Apr;43(2):242-6. doi: 10.1093/icb/43.2.242.
Many metazoans convert the reproductive modes presumably depending upon the environmental conditions and/or the phase of life cycle, but the mechanisms underlying the switching from asexual to sexual reproduction, and vice versa, remain unknown. We established an experimental system, using an integrative biology approach, to analyze the mechanism in the planarian, Dugesia ryukyuensis (Kobayashi et al., 1999). Worms of exclusively asexual clone (OH strain) of the species gradually develop ovaries, testes and other sexual organs, then copulate and eventually lay cocoons filled with fertilized eggs, if they are fed with sexually mature worms of Bdellocephala brunnea (an exclusively oviparous species). This suggests the existence of a sexualizing substance(s) in sexually mature worms. Random inbreeding of experimentally sexualized worms (acquired sexuals) produces an F1 population of spontaneous sexuals (innate sexuals) and asexuals in a ratio of approximately 2:1. All regenerants from various portions of innate sexuals become sexuals. In the case of acquired sexuals, head fragments without sexual organs regenerated into asexuals though regenerants from other portions became sexuals. Thus, we conclude that neoblasts, the totipotent stem cells in the planarians, of acquired sexuals remain "asexual" and the worms require external supply of a sexualizing substance for the differentiation of sexual organs and gametes. On the other hand, some, if not all, neoblasts in innate sexuals are somehow "sexual" and do not require external supply of a sexualizing substance for the eventual differentiation of themselves and/or other neoblasts into sexual organs and gametes. It is also shown that sexuality in acquired sexuals is maintained by the putative sexualizing substance(s) of their own. The sexualization is closely coupled with cessation of fission, and the worms seem to have an unknown way of controlling the karyotype. Our integrative approach integrates multiple fields of study, including classic breeding, regeneration, and genetics experiments, as well as karyotyping, and biochemical and molecular biological analyses; none of which would have revealed much about the intricate mechanisms that regulate sex and fission in these animals.
许多后生动物根据环境条件和/或生命周期阶段转换生殖模式，但从无性繁殖到有性繁殖的转变机制，以及反之亦然的机制仍然未知。我们建立了一个实验系统，使用整合生物学方法来分析扁形动物（Dugesia ryukyuensis）（Kobayashi 等人，1999 年）的机制。该物种的无性克隆（OH 株）的蠕虫逐渐发育出卵巢、睾丸和其他性器官，然后交配并最终产下充满受精卵的茧，如果它们喂食性成熟的 Bdellocephala brunnea（一种专性卵生种）的蠕虫。这表明性成熟蠕虫中存在一种或多种性化物质。实验性性化蠕虫（获得性）的随机近亲繁殖产生自发有性（先天有性）和无性的 F1 群体，比例约为 2:1。所有先天有性的各个部分的再生体都成为有性体。在获得性的情况下，没有性器官的头部碎片再生为无性体，尽管来自其他部分的再生体成为有性体。因此，我们得出结论，获得性的多能干细胞（neoblasts）仍然是“无性”的，蠕虫需要外部供应性化物质来分化性器官和配子。另一方面，先天有性的一些（如果不是全部）neoblasts 在某种程度上是“有性”的，并且不需要外部供应性化物质来最终分化成性器官和配子。还表明，获得性的有性是由其自身的假定性化物质维持的。性化与分裂的停止密切相关，蠕虫似乎有一种未知的方式来控制核型。我们的综合方法整合了多个研究领域，包括经典的繁殖、再生和遗传学实验，以及核型分析以及生化和分子生物学分析；这些方法都没有揭示出多少关于调节这些动物的性和分裂的复杂机制。
