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KMID : 0613820170270010100
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Journal of Life Science
2017 Volume.27 No. 1 p.100 ~ p.121
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Sexual Reproduction in Unicellular Green Alga Chlamydomonas
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Lee Kyu-Bae
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Abstract
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The sexual reproduction of the unicellular green alga Chlamydomonas is reviewed for a comprehensive understanding of the complex processes. The sexual life cycle of C. reinhardtii is distinguished into five main stages: gametogenesis, gamete activation, cell fusion, zygote maturation, and meiosis and germination. Gametogenesis is induced by nitrogen starvation in the environment. C. reinhardtii has two mating types: mating type plus (mt＋) and mating type minus (mt－), controlled by a single complex mating type locus (MT+ or MT－) on linkage group VI. In the early gametogenesis agglutinins are synthesized. The mt＋ and mt－ agglutinins are encoded by the autosomal genes SAG1 (Sexual AGglutination1) and SAD1 (Sexual ADhesion1), respectively. The agglutinins are responsible for the flagellar adhesion of the two mating type of gametes. The flagellar adhesion initiates a cAMP mediated signal transduction pathways and activates the flagellar tips. In response to the cAMP signal, mating structures between two flagella are activated. The mt＋ and mt－ gamete-specific fusion proteins, Fus1 and Hap2/Gcs1, are present on the plasma membrane of the two mating structures. Contact of the two mating structures leads to develop a fertilization tubule forming a cytoplasmic bridge between the two gametes. Upon fusion of nuclei and chloroplasts of mt＋ and mt－ cells, the zygotes become zygospores. It is notable that the young zygote shows uniparental inheritance of chloroplast DNA from the mt+ parent and mitochondrial DNA from the mt－ parent. Under the favorable conditions, the zygospores divide meiotically and germinate and then new haploid progenies, vegetative cells, are released.
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KEYWORD
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Chlamydomonas, fertilization tubule, mating structure, sexual reproduction, uniparental inheritance
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