Evolutionary Biological Studies of Morphological Bipolarization in Conjugating Gametes

真核生物的性别最初是等配子的，据推测，在包括动物、陆地植物和藻绿藻类在内的许多谱系中，异配子/卵配是独立进化的。导致同卵制从同卵制进化到同卵制的确切进化机制尚不清楚。在本研究中，我们介绍了使用分子遗传学数据来阐明种群volvocacean Pleodorina starrii （Nozaki et al. 2006, Cuff）从同婚制到卵制的进化起源。生物学)。在近亲莱茵衣藻（Chlamydomonas reinhardtii）中，有性生殖是等配型的，有正配型和负配型。负交配型代表“优势性”，因为莱茵梭菌的MID（“负优势”）基因是使细胞分化为负交配型的同配子的必要和充分条件。在我们成功克隆出星状藻的MID基因之前，还没有在藻属绿藻中发现性别特异性基因。这种“OTOKOGI”（PlestMID）基因仅存在于男性基因组中，并编码一种在成熟精子细胞核中大量存在的蛋白质。因此，星状假单胞虫的雄性是从其同配型祖先的优势性别（交配型）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提供了强有力的第一步。比较衣藻和多毛藻之间其他交配类型相关的基因位点，可能会揭示哪些其他基因在volvolacean的“雄性”进化中起了重要作用。对这些进化基因的研究将有助于揭示雄性和雌性配子在大小和运动上的分化的进化机制。

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.