Chlamydomonas reinhardtii is a unicellular volvocine alga having two mating types: mating type plus (mt+) and mating type minus (mt−), which are controlled by a single, complex mating-type locus. Sexual adhesion between the gametes is mediated by sex-specific agglutinin molecules on their flagellar membranes. Cell fusion is initiated by an adhesive interaction between the mt+ and mt− mating structures, followed by localized membrane fusion. the essential function of the sex-determining genes of the isogamous a unicellular Chlamydomonas reinhardtii is conserved in the multicellular oogamous b Volvox carteri. The sexual reproduction of the unicellular charophycean alga, Closterium peracerosumstrigosum-littorale complex, is also focused on here. The sexual reproductive processes of heterothallic strains are controlled by two multifunctional sex pheromones, PR-IP and PR-IP Inducer, which independently promote multiple steps in conjugation at the appropriate times through different induction mechanisms. The molecules involved in sexual reproduction and sex determination have also been characterized. 1
For successful fertilization, an intimate interaction between eggs and sperm is required. Fertilization has been studied in many organisms, including animals, fungi, and plants. In higher plants, considerable insight into the molecular mechanisms that control interactions between male and female cells has been obtained. In contrast to our increasing understanding of sexual reproduction in higher plants, the evolution of these processes is still unclear. Land plants are thought to have evolved from ancestral green algae. The green algae are divided into two major lineages: the chlorophyte and the charophyte. The chlorophycean algae include the majority of species traditionally called green algae. Among the chlorophycean algae, which are relatively distant from land plant lineages, the sexual reproductive processes and sexualities of freshwater volvocine algal species have been studied in detail.
The charophycean algae form a relevant monophyly with land plants. Their cellular features and metabolism are more similar to those of land plants than the chlorophycean algae. Charophyceans comprise five lineages (orders) of freshwater green algae: Charales, Coleochaetales, Zygnematales, Klebsormidiales, and Chlorokybales. De novo transcriptomic analyses of representative charophyceans have identified putative homologous genes related to the major plant hormones. Among the charophyceans, the genome sequence of Klebsormidium flaccidum is now available . Based on recent phylogenetic analyses, Zygnematales are the closest charophycean algae to land plants.
Sexual reproduction of the isogamous unicellular volvocine alga, Chlamydomonas reinhardtii
Among the volvocine algae, most studies have focused on Chlamydomonas reinhardtii, which is a model unicellular organism in plants. Ch. reinhardtii was first isolated as a green soil alga. It has two mating types: mating type plus (mt+) and mating type minus (mt−), which are controlled by a single complex mating-type locus (MT+ or MT−) (Ferris et al. 2002). These proliferate asexually when there is an adequate source of nitrogen in the environment. When nitrogen levels fall below a certain threshold, respective vegetative cells differentiate into gametes.
The life cycle of Chlamydomonas reinhardtii.
Vegetative cells (V) differentiate into mt+ and mt− gametes (G) during nitrogen starvation (−N). Mating types are restricted by mating-type loci (+ and −). When gametes are mixed, the plus and minus agglutinin molecules on their flagellar surfaces adhere to each other, and this adhesion results in increased intracellular cAMP levels. The signal triggers gamete cell wall release and mating-structure activation. Cells then fuse to form binucleate quadriflagellated cells. Zygotes with thick cell walls germinate in response to light and nitrogen supplementation, and undergo meiosis to release four haploid vegetative cells
Heterozygous mt+/mt− diploids, which are occasionally formed after mating, always mate as mt− gametes, indicating that MT− is dominant over MT+. The core of the two MT loci encompasses 200–300 kb. The MT loci contain highly rearranged DNA sequences, characterized by several large inversions and translocations, which act to suppress recombination. Some genes are specifically linked to one or the other MT locus. The FUS1 gene on the MT+ locus, and the MT locus region d (MTD1) and MINUS DOMINANCE (MID) genes on the MT− locus, have been assigned mating type-specific functions in gametogenesis and mating. MID encodes an RWP-RK family putative transcription factor and is necessary both to activate mt− gene expression and to prevent mt+ gene expression, allowing the conversion of wild-type mt+ gametes to mt− gametes. Mt+ cells transformed with the MID gene differentiate as mt− gametes and a functional mutant in an mt− background differentiates into an mt+ gamete having all of the molecules required of an mt+ gamete, with the exception of the FUS1 protein. Vegetative mt− cells express basal levels of MID. A pulse of upregulated expression(level 1) occurs 30 min after nitrogen removal, followed by a return to the basal level at 1 h. The expression is strongly upregulated (level 2) at 4–6 h together with the acquisition of mating competency. Knockdown of MTD1 in mt− cells results in a failure to differentiate into gametes of either mating type after nitrogen removal. From these results, it was proposed that the first increase in Mid (level 1) is sufficient to activate MTD1 transcription and to repress mt+ gamete-specific genes, and that MTD1 expression in turn allows the second increase (level 2), which is necessary to switch on mt− gamete-specific genes.
a Isogamy is a form of sexual reproduction that involves gametes of similar morphology (similar shape and size)
b Oogamy is the familiar form of sexual reproduction. It is a form of anisogamy (heterogamy) in which the female gamete (e.g. egg cell) is significantly larger than the male gamete and is non-motile.