Editing Green algae (section)

==Reproduction==
[[File:Algaeconjugate.jpg|thumb|right|Green algae conjugating]]
Green algae are a group of photosynthetic, eukaryotic organisms that include species with haplobiontic and diplobiontic life cycles. The diplobiontic species, such as ''[[Sea lettuce|Ulva]]'', follow a reproductive cycle called [[alternation of generations]] in which two multicellular forms, haploid and diploid, alternate, and these may or may not be isomorphic (having the same morphology). In haplobiontic species only the haploid generation, the [[gametophyte]] is multicellular. The fertilized egg cell, the diploid [[zygote]], undergoes [[meiosis]], giving rise to haploid cells which will become new gametophytes. The diplobiontic forms, which evolved from haplobiontic ancestors, have both a multicellular haploid generation and a multicellular diploid generation. Here the zygote divides repeatedly by [[mitosis]] and grows into a multicellular diploid [[sporophyte]]. The sporophyte produces haploid spores by meiosis that germinate to produce a multicellular gametophyte. All [[Embryophyte|land plants]] have a diplobiontic common ancestor, and diplobiontic forms have also evolved independently within [[Ulvophyceae]] more than once (as has also occurred in the red and brown algae).<ref name="Trillo-2015">{{cite book |last1=Trillo |first1=Inaki Ruiz |last2=Nedelcu |first2=Aurora M |date=2015 |title=Evolutionary transitions to multicellular life: Principles and mechanisms |publisher=Springer |isbn=978-94-017-9642-2 }}</ref>

Diplobiontic green algae include isomorphic and heteromorphic forms. In isomorphic algae, the morphology is identical in the haploid and diploid generations. In heteromorphic algae, the morphology and size are different in the gametophyte and sporophyte.<ref name="Bessho-2009">{{cite journal |first1=Kazuhiro |last1=Bessho |first2=Yoh |last2=Iwasa |s2cid=46519857 |title=Heteromorphic and isomorphic alternations of generations in macroalgae as adaptations to a seasonal environment |journal=Evolutionary Ecology Research |date=2009 |volume=11 |pages=691–711 }}</ref>

Reproduction varies from fusion of identical cells ([[isogamy]]) to [[fertilisation|fertilization]] of a large non-motile cell by a smaller motile one ([[oogamy]]). However, these traits show some variation, most notably among the basal green algae called [[prasinophyte]]s.{{cn|date=April 2025}}

Haploid algal cells (containing only one copy of their DNA) can fuse with other haploid cells to form diploid zygotes. When filamentous algae do this, they form bridges between cells, and leave empty cell walls behind that can be easily distinguished under the light microscope. This process is called ''conjugation'' and occurs for example in ''[[Spirogyra]]''.{{cn|date=April 2025}}

===Sex pheromone===
[[File:Freshly harvested algae 01.jpg|thumb|Freshly harvested algae]]
[[Sex pheromone]] production is likely a common feature of green algae, although only studied in detail in a few model organisms. ''[[Volvox]]'' is a genus of [[Chlorophyta|chlorophytes]]. Different species form spherical colonies of up to 50,000 cells. One well-studied species, ''[[Volvox carteri]]'' (2,000 – 6,000 cells) occupies temporary pools of water that tend to dry out in the heat of late summer. As their environment dries out, asexual ''V. carteri'' quickly die. However, they are able to escape death by switching, shortly before drying is complete, to the sexual phase of their life cycle that leads to production of dormant desiccation-resistant [[zygote]]s. Sexual development is initiated by a [[glycoprotein]] pheromone (Hallmann et al., 1998). This pheromone is one of the most potent known biological effector molecules. It can trigger sexual development at concentrations as low as 10<sup>−16</sup>M.<ref name="Hallmann-1998">{{cite journal | vauthors = Hallmann A, Godl K, Wenzl S, Sumper M | title = The highly efficient sex-inducing pheromone system of Volvox | journal = Trends in Microbiology | volume = 6 | issue = 5 | pages = 185–9 | date = May 1998 | pmid = 9614342 | doi = 10.1016/s0966-842x(98)01234-7 }}</ref> Kirk and Kirk<ref name="Kirk-1986">{{cite journal | vauthors = Kirk DL, Kirk MM | title = Heat shock elicits production of sexual inducer in Volvox | journal = Science | volume = 231 | issue = 4733 | pages = 51–4 | date = January 1986 | pmid = 3941891 | doi = 10.1126/science.3941891 | bibcode = 1986Sci...231...51K }}</ref> showed that sex-inducing pheromone production can be triggered experimentally in [[Somatic (biology)|somatic]] cells by [[heat shock]]. Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.<ref name="Hallmann-1998" />

The ''Closterium peracerosum-strigosum-littorale (C. psl)'' complex is a unicellular, isogamous [[Charophyceae|charophycean]] alga group that is the closest unicellular relative to land plants. [[Heterothallic]] strains of different [[mating type]] can conjugate to form [[zygospore]]s. Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.<ref name="Sekimoto-1990">{{cite journal | vauthors = Sekimoto H, Satoh S, Fujii T | title = Biochemical and physiological properties of a protein inducing protoplast release during conjugation in theClosterium peracerosum-strigosum-littorale complex | journal = Planta | volume = 182 | issue = 3 | pages = 348–54 | date = October 1990 | pmid = 24197184 | doi = 10.1007/BF02411384 | s2cid = 1999634 }}</ref>