Gene Expression Profiling Using cDNA Microarray Analysis of the Sexual Reproduction Stage of the Unicellular Charophycean Alga Closterium peracerosum-strigosum-littorale Complex

The desmid Closterium peracerosum-strigosum-littorale complex, which is the closest unicellular sister to land plants, is the best characterized of the charophycean green algae with respect to the process of sexual reproduction. To elucidate the molecular mechanism of intercellular communication during sexual reproduction, we created a normalized cDNA library from mixed cells of the sexual and the vegetative phases and generated a cDNA microarray. In total, 3,236 expressed sequence tags, which were classified into 1,615 nonredundant groups, were generated for cDNA microarray construction. Candidate genes for key factors involved in fertilization, such as those that encode putative receptor-like protein kinase, leucine-rich-repeat receptor-like protein, and sex pheromone homologs, were up-regulated during sexual reproduction and/or by the addition of the purified sex pheromones, and the expression patterns of these genes were confirmed by quantitative real-time polymerase chain reaction analysis. This first transcriptome profile of Closterium will provide critical clues as to the mechanism and evolution of intercellular communication between the egg and sperm cells of land plants.