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Genome sizes are known to vary within and among closely related species, but the knowledge about genomic factors contributing to the variation and their impacts on gene functions is limited to only a small number of species. This study identified a more than 2-fold heritable genome size variation among the unicellular Zygnematophycean alga, Closterium peracerosum–strigosum–littorale (C. psl.) complex, based on short-read sequencing analysis of 22 natural strains and F1 segregation analysis. Six de novo assembled genomes revealed that genome size variation is largely attributable to genome-wide copy number variation (CNV) among strains rather than mating type-linked genomic regions or specific repeat sequences such as rDNA. Notably, about 30% of genes showed CNV even between strains that can mate with each other. Transcriptome and gene ontology analysis demonstrated that CNV is distributed nonrandomly in terms of gene functions, such that CNV was more often observed in the gene set with stage-specific expression. Furthermore, in about 30% of these genes with CNV, the expression level does not increase proportionally with the gene copy number, suggesting presence of dosage compensation, which was overrepresented in genes involved in basic biological functions, such as translation. Nonrandom patterns in gene duplications and corresponding expression changes in terms of gene functions may contribute to maintaining the high level of CNV associated with extensive genome size variation in the C. psl. complex, despite its possible detrimental effects. [ABSTRACT FROM AUTHOR]

ContentsSummary99I.Introduction99II.Life cycle of Closterium100III.Sexual reproductive processes in the heterothallic Closterium peracerosum‐strigosum‐littorale complex101IV.Homothallism in the C. psl. complex102V.Sexual reproduction and inheritance of mating types in Closterium ehrenbergii102VI.mt‐determining gene of the C. psl. complex103VII.Future perspectives103Acknowledgements103References103 Summary: Closterium occupies a key phylogenetic position as an ancestor of land plants and is the best‐characterized Charophycean alga in terms of the process of sexual reproduction. Zygospores form as a result of sexual reproduction between genetically determined mating type plus (mt+) and mating type minus (mt−) cells in heterothallic strains, or between clonal cells in homothallic strains. Here we review knowledge on the intercellular communication and mating type determination for successful sexual reproduction in Closterium. Using genomic information and transgenic techniques, the genus could be a model organism to study the mechanisms and evolution of sexual reproduction in streptophytes. [ABSTRACT FROM AUTHOR]

Detailed conjugation processes in Penium, a unicellular conjugating green alga, are described for the first time. A homothallic strain of Penium margaritaceum (Ehrenb.) Bréb. (Designation, izu84-10) was isolated from a rice paddy field in Japan. The species was identified based on its morphology, and a molecular phylogeny confirmed that izu84-10 was closely related to another identified strain of this species. Using time-lapse photography, the conjugation processes in P. margaritaceum were observed and then categorized into the following six 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 nearby individual); (3) formation of conjugation papillae by elongation of the cell wall; (4) release of a gamete from one of the pair members; (5) release of a gamete from the other pair member; and (6) formation of the zygospore by gamete fusion. By alcian blue staining, possible involvement of mucilage to facilitate this cell adhesion and cell-cell communication was suggested. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Closterium peracerosum–strigosum–littorale ( C. psl.) complex consists of unicellular algae and is known to be composed of several reproductively isolated mating groups of heterothallic strains. Group I-E is completely isolated from mating groups II-A and II-B, groups II-A and II-B are partially isolated from each other, and only mating-type plus (mt+) cells of group II-A and mating-type minus (mt−) cells of group II-B form zygotes. Based on the alignment of 1506 group I introns, significant phylogenetic relationships were observed among mating groups II-A and II-B, while mating group I-E was distant from groups II-A and II-B. Sexual cell division in both mating-type cells of group II-A was stimulated in conditioned media in which cells of group II-B had been cultured. When mt− cells of group II-B were stimulated in conditioned medium derived from group II-A, mt+ cells of group II-B did not respond to the conditioned medium. Conditioned media derived from group I-E did not exhibit sexual cell division (SCD)–inducing activity against any strain except those within its own group. From the alignment of deduced amino acid sequences from orthologous protoplast-release-inducing protein (PR-IP) Inducer genes, we detected a significant similarity among groups II-A and II-B, and mating group I-E had low similarity to other mating groups. The existing degree of reproductive isolation can be partially explained by differences in molecular structures and physiological activities of sex pheromones of these heterothallic mating groups. [ABSTRACT FROM AUTHOR]

Sexual cell division (SCD) that produces two gametangial cells from one vegetative mother cell is the first step observed morphologically in the sexual reproduction in the Closterium peracerosum–strigosum– littorale complex. SCD-inducing activities specific for each mating-type cells were detected in the medium in which both mating type cells has been cocultured. Mating-type minus (mt- ) cells released SCD-inducing substance specific for mating-type plus (mt+ ) cells and were designated as SCD- inducing pheromone (IP)-minus, whereas mt- specific substances released from mt+ cells were designated as SCD-IP-plus. Culture medium was subjected to gel filtration, and then SCD-IP-plus and SCD-IP-minus chemical were found to have the molecular masses of 90–100 kDa and 10–20 kDa, respectively. It was evident that light was imperative for this type of signaling. Gametangial cells of both mating types were obtained from vegetative cells by treatment with SCD-IPs. Gametangial mt+ cells showed high competency for conjugation with vegetative mt- cells, whereas gametangial mt- cells showed low competency for conjugation with vegetative mt+ cells. These results indicate that SCD in both mating type cells is induced by high molecular weight sex pheromones and that the roles of gametangial cells in the process of conjugation differ by sex. [ABSTRACT FROM AUTHOR]

Sexual cell division (SCD) that produces two gametangial cells from one vegetative mother cell is the first step observed morphologically in the sexual reproduction in the Closterium peracerosum–strigosum– littorale complex. SCD-inducing activities specific for each mating-type cells were detected in the medium in which both mating type cells has been cocultured. Mating-type minus (mt- ) cells released SCD-inducing substance specific for mating-type plus (mt+ ) cells and were designated as SCD- inducing pheromone (IP)-minus, whereas mt- specific substances released from mt+ cells were designated as SCD-IP-plus. Culture medium was subjected to gel filtration, and then SCD-IP-plus and SCD-IP-minus chemical were found to have the molecular masses of 90–100 kDa and 10–20 kDa, respectively. It was evident that light was imperative for this type of signaling. Gametangial cells of both mating types were obtained from vegetative cells by treatment with SCD-IPs. Gametangial mt+ cells showed high competency for conjugation with vegetative mt- cells, whereas gametangial mt- cells showed low competency for conjugation with vegetative mt+ cells. These results indicate that SCD in both mating type cells is induced by high molecular weight sex pheromones and that the roles of gametangial cells in the process of conjugation differ by sex. [ABSTRACT FROM AUTHOR]