SEXUAL PROCESSES AND PHYLOGENETIC RELATIONSHIPS OF A HOMOTHALLIC STRAIN IN THE CLOSTERIUM PERACEROSUM–STRIGOSUM–LITTORALE COMPLEX (ZYGNEMATALES, CHAROPHYCEAE)1

Authors

  • Yuki Tsuchikane,

    1. Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112 8681, Japan
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  • Machiko Sato,

    1. Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112 8681, Japan
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  • Tomoko Ootaki,

    1. Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112 8681, Japan
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  • Yume Kokubun,

    1. Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112 8681, Japan
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  • Hisayoshi Nozaki,

    1. Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7–3–1, Hongo, Bunkyo-ku, Tokyo 113 0033, Japan
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  • Motomi Ito,

    1. Department of General Systems Studies, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153 8902, Japan
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  • Hiroyuki Sekimoto

    1. Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112 8681, Japan
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  • 1

    Received 2 January 2009. Accepted 12 October 2009.

  • 3

    Author for correspondence: e-mail sekimoto@fc.jwu.ac.jp.

Abstract

Members of the Closterium peracerosum–strigosum–littorale (C. psl.) complex are unicellular charophycean algae in which there are two modes of zygospore formation, heterothallic and homothallic. A homothallic strain of Closterium (designation, kodama20) was isolated from a Japanese rice paddy field. Based on alignment of the 1506 group-I introns, which interrupt nuclear SSU rDNAs, homothallic kodama20 is most closely related to the heterothallic mating group II-B, which is partially sexually isolated from group II-A. Time-lapse photography of the conjugation process in kodama20 revealed that most of the observed zygospores originated from one vegetative cell. The sexual conjugation process consisted of five stages: (1) cell division resulting in the formation of two sister gametangial cells from one vegetative cell, (2) formation of a sexual pair between the two sister gametangial cells (or between gametangial cells of another adjoined individual), (3) formation of conjugation papillae, (4) release of gametic protoplasts from both members of a pair, and (5) formation of the zygospore by protoplast fusion. For conjugation to progress, the cell density and light condition in the culture was critical. We suggested the presence of a conjugation promotion factor.

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