Evolutionary Biological Studies of Morphological Bipolarization in Conjugating Gametes

真核生物的性别最初是同配生殖的，并且假定异配生殖/卵配生殖在许多谱系中独立地进化，包括动物、陆生植物和卷藻绿色藻类。负责从同花受精进化为卵受精的确切进化机制知之甚少。在本研究中，我们介绍了分子遗传学数据的使用，阐明了在殖民volvocacean Pleodorina starrii（Nozaki et al. 2006，Cuff.生物学）。在近亲莱茵衣藻中，有性生殖是同配子生殖，交配型为正和负。交配型为负代表一种“显性性别”，这是因为C. reinhardtii是使细胞分化为负交配型的同配子所必需的，也是足够的。在我们成功克隆星状藻MID基因之前，一直没有在volvocine绿色藻类中鉴定出性别特异性基因。这种“OTOKOGI”（PlestMID）基因只存在于男性基因组中，并编码一种蛋白质，该蛋白质大量存在于成熟精子的细胞核中。因此，星斑蝶雄性是从其同花受精祖先的优势性别（交配型减）进化而来的。本研究首次发现的雄性特异性基因“OTOKOGI”为克隆雄性交配型位点和确定其基因组成迈出了有力的第一步。比较衣原体和Pleodorina之间的其他交配型连锁基因座可能会揭示哪些其他基因在团藻类雄性进化中起重要作用。“对这些进化基因的研究应该揭示雄性和雌性配子进化的机制，这些配子在大小和运动性上有区别。

Eukaryotic sex was initially isogametic and it is assumed that anisogamy/oogamy evolved independently in many lineages including animals, land plants and volvocine green algae. The exact evolutionary mechanisms that were responsible for the evolution of oogamy from isogamy were poorly understood. In the present study, we introduced the use of molecular-genetic data in elucidating the evolutionary origin of oogamy from isogamy in the colonial volvocacean Pleodorina starrii (Nozaki et al. 2006, Cuff. Biology). In the close relative Chlamydomonas reinhardtii, sexual reproduction is isogametic with mating-types plus and minus. Mating type minus represents a "dominant sex" because the MID ("minus-dominance") gene of C. reinhardtii is both necessary and sufficient to cause the cells to differentiate as isogametes of the minus mating type. No sex-specific genes had been identified in the volvocine green algae until we successfully cloned the MID gene of P. starrii. This "OTOKOGI" (PlestMID) gene is present only in the male genome, and encodes a protein localized abundantly in the nuclei of mature sperm. Thus, P. starrii maleness evolved from the dominant sex (mating type minus) of its isogamous ancestor. The male-specific gene "OTOKOGI", first identified in this study, provides a powerful first step toward cloning the male mating type locus and determining its gene composition. Comparison of other mating type-linked loci between Chlamydomonas and Pleodorina may reveal which other genes have been important in the evolution of volvocacean "maleness." Studies of such evolved genes should shed light on the mechanisms underlying the evolution of male and female gametes that are differentiated in size and motility.