No abstract

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.

Summary The Closterium peracerosum–strigosum–littorale complex (Closterium, Zygnematophyceae) has an isogamous mating system. Members of the Zygnematophyceae are the closest relatives to extant land plants and are distantly related to chlorophytic models, for which a genetic basis of mating type (MT) determination has been reported. We thus investigated MT determination in Closterium. We sequenced genomes representing the two MTs, mt+ and mt−, in Closterium and identified CpMinus1, a gene linked to the mt− phenotype. We analyzed its function using reverse genetics methods. CpMinus1 encodes a divergent RWP‐RK domain‐containing‐like transcription factor and is specifically expressed during gamete differentiation. Introduction of CpMinus1 into an mt+ strain was sufficient to convert it to a phenotypically mt− strain, while CpMinus1‐knockout mt− strains were phenotypically mt+. We propose that CpMinus1 is the major MT determinant that acts by evoking the mt− phenotype and suppressing the mt+ phenotype in heterothallic Closterium. CpMinus1 likely evolved independently in the Zygnematophyceae lineage, which lost an egg–sperm anisogamous mating system. mt− specific regions possibly constitute an MT locus flanked by common sequences that undergo some recombination.

Here, we cloned the CpRLK1 gene, which encodes a receptor-like protein kinase expressed during sexual reproduction, from the heterothallic Closterium peracerosum-strigosum-littorale complex, one of the closest unicellular alga to land plants. Mating-type plus (mt(+)) cells with knockdown of CpRLK1 showed reduced competence for sexual reproduction and formed an abnormally enlarged conjugation papilla after pairing with mt(-) cells. The knockdown cells were unable to release a naked gamete, which is indispensable for zygote formation. We suggest that the CpRLK1 protein is an ancient cell wall sensor that now functions to regulate osmotic pressure in the cell to allow proper gamete release.