Zygnematophyceae (or Conjugatophyceae) is a class of
green algae in the paraphylum streptophyte algae, also referred to as
Charophyta,[2] consisting of more than 4000 described species.[3] The Zygnematophyceae are the sister clade of the
Embryophyta (land plants).[4][5][6]
Common members of the Zygnematophyceae include the filamentous algae Spirogyra and Mougeotia, as well as
desmids, which are microscopic algae characterized by symmetrical and elaborately ornate cells.[7]
Morphology
The body plan of Zygnematophyceae is simple, and the group appears to have gone through a secondary loss of morphological complexity.[8] The most basal members are unicellular, but filamentous species have evolved at least five times,[9] and a few species form colonies.[7] Each cell contains a single
nucleus.[7]
The
chloroplasts of the Zygnematophyceae are large, typically axile but sometimes parietally located, and contain one or several
pyrenoids. One or several chloroplasts may be present in a single cell. The shape of the chloroplast may be star-shaped (in Zygnema), ribbon-shaped (in Spirogyra), or elaborately lobed and dissected. In some taxa, particularly Mougeotia, the chloroplast is able to move in response to different light conditions.[7]
In the Zygnematophyceae, cell walls are composed of three layers: one outer layer consisting of mucus, a primary wall consisting of
microfibrils, and an innermost layer of cellulosic
microfibrils. Some species shed their primary wall and retain only the innermost layer. The cell wall may be variously decorated with striations, granules, or spines. In the Desmidiales, there are pores in the cell wall which allow the cell to extrude a layer of
mucilage for protection.[7]
Several Zygnematophyceae contain genes involved in protection from desiccation that appear to have been derived by
horizontal gene transfer from bacteria; the genes are found in plants, Zygnematophyceae, and
bacteria, but no other organisms. The genes may have helped to enable plants to make the transition to life on land.[10][11]
A new subclass called Spirogloeophycidae, represented by the species Spirogloea muscicola, was established after the unicellular
subaerial algae, resembling a "gelatinous blob", was rediscovered on a rock close to a river bank near
Cologne in 2006, after first being discovered in France in 1845.[12]
Reproduction
The Zygnematophyceae are able to reproduce both asexually and sexually. Asexual reproduction takes place via fragmentation,
cell division, akinete formation, or parthenospores.[7]
Sexual reproduction in the Zygnematophyceae takes place through a process called conjugation.[13] Unusually, the gametes lack
flagella,[7] and they also lack
centrioles which suggest that flagella were not secondarily lost. This lack of flagella sets the Zygnematophyceae apart from nearly all groups of algae, except for
red algae and
cyanobacteria (also known as blue-green algae, if they are classified as such).[1] The gametes are scarcely differentiated from vegetative cells.[7]
In conjugation, cells or filaments of opposite sex line up, and tubes form between corresponding cells. The male cells then become amoeboid and crawl across the female, or sometimes both cells crawl into the connecting tube. The cells then meet and fuse to form a
zygote, which later undergoes
meiosis to produce new cells or filaments. As in land plants, only the female passes its
chloroplasts on to the offspring.[14]
Taxonomy
The Zygnematophyceae is
monophyletic. Traditionally, it has been known to contain two orders, the
Zygnematales and
Desmidiales.[7] However, the Zygnematales were found to be
paraphyletic with respect to the Desmidiales, and so it was split up. As of 2023[update], five orders and two subclasses are recognized: the Spirogloeales in Spirogloeophycidae, and the Serritaeniales, the
Zygnematales, the Spirogyrales, and the
Desmidiales in Zygnematophycidae.[9]
Members of the Zygnematophyceae are common in nearly all freshwater habitats, particularly filamentous genera such as Spirogyra and Mougeotia. Some Spirogyra species can tolerate disturbed habitats. On the other hand, desmids (the Desmidiales) often prefer bogs, peatlands, and lakes.[17]
Fossil record
Vegetative cells of the Zygnematophyceae are fragile and usually not preserved, but the zygospores are resistant to decay and can become fossilized. The Zygnematophyceae have been recorded from at least the middle
Devonian period, and before the
Carboniferous period, all the major groups had diverged. Fossils of zygospores are indicators of warming spring conditions and shallow, stagnant mesotrophic habitats.[7]
Genomes
The first genomes published for Zygnematophyceae were Mesotaenium endlicherianum and Spirogloea muscicola.[11] Since then, genomes have been published on Penium margaritaceum,[18]Zygnema spp.,[19] and the Closterium peracerosum–strigosum–littorale complex.[20] Furthermore, the gene models of Mesotaenium endlicherianum have been updated.[21]
References
^
abGuiry, Michael D. (15 March 2013). "Taxonomy and nomenclature of the Conjugatophyceae (= Zygnematophyceae)". Algae. 28 (1): 1–29.
doi:
10.4490/algae.2013.28.1.001.
hdl:10379/11755.
^Wickett, Norman J.; Mirarab, Siavash; Nguyen, Nam; Warnow, Tandy; Carpenter, Eric; Matasci, Naim; Ayyampalayam, Saravanaraj; Barker, Michael S.; Burleigh, J. Gordon; Gitzendanner, Matthew A.; Ruhfel, Brad R.; Wafula, Eric; Der, Joshua P.; Graham, Sean W.; Mathews, Sarah; Melkonian, Michael; Soltis, Douglas E.; Soltis, Pamela S.; Miles, Nicholas W.; Rothfels, Carl J.; Pokorny, Lisa; Shaw, A. Jonathan; DeGironimo, Lisa; Stevenson, Dennis W.; Surek, Barbara; Villarreal, Juan Carlos; Roure, Béatrice; Philippe, Hervé; dePamphilis, Claude W.; Chen, Tao;
Deyholos, Michael K.; Baucom, Regina S.; Kutchan, Toni M.; Augustin, Megan M.; Wang, Jun; Zhang, Yong; Tian, Zhijian; Yan, Zhixiang; Wu, Xiaolei; Sun, Xiao; Wong, Gane Ka-Shu; Leebens-Mack, James (11 November 2014).
"Phylotranscriptomic analysis of the origin and early diversification of land plants". Proceedings of the National Academy of Sciences. 111 (45): E4859–E4868.
Bibcode:
2014PNAS..111E4859W.
doi:10.1073/pnas.1323926111.
PMC4234587.
PMID25355905.
^de Vries, Jan; Stanton, Amanda; Archibald, John M.; Gould, Sven B. (June 2016). "Streptophyte Terrestrialization in Light of Plastid Evolution". Trends in Plant Science. 21 (6): 467–476.
doi:
10.1016/j.tplants.2016.01.021.
PMID26895731.
^Hall, John D.; McCourt, Richard M. (2014). "Chapter 9. Conjugating Green Algae Including Desmids". In Wehr, John D.; Sheath, Robert G.; Kociolek, J. Patrick (eds.). Freshwater Algae of North America: Ecology and Classification (2 ed.). Elsevier Inc.
ISBN978-0-12-385876-4.