Zygnema C.Agardh, 1817, nom. et typ. cons.

Holotype species: Zygnema cruciatum (Vaucher) C.Agardh

Original publication and holotype designation: Agardh, C.A. (1817). Synopsis algarum Scandinaviae, adjecta dispositione universali algarum. pp. [i]-xl, [1]-135. Lundae [Lund]: Ex officina Berlingiana.

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Description: Thalli comprised of unbranched uniseriate filaments intertwined to form skeins. Cells cylindrical, 10 to 50 µm in diameter, most 20-30 µm; length equal to or several times width; cell wall two-layered with inner cellulose, outer mucilage layer; external mucilage sheath ranges from very thin to wider than cell, but generally thinner than that of related Spirogyra; end walls plane; no flagellated stages. Cells uninucleate; chloroplasts usually two (rarely four) per cell, each plastid with large pyrenoid at center of radiating short branches in stellate pattern. Nucleus in cytoplamic bridge between chloroplasts. Chromosomes (n=14 to 82) typically small, dotlike (<1.5 µm). Haploid chromosome number 14 to 82. Asexual reproduction by mitosis and fragmentation. Akinetes very common as overwintering form; aplanospores and fragments may also persist through winter. Life cycle haplobiontic, meiosis zygotic. Sexual reproduction by conjugation, most often scalariform; lateral conjugation also observed (see definitions under Spirogyra). Majority of species anisogamous ; one gamete amoeboid, the other stationary; some species isogamous (both gametes motile); zygospores forming inside one cell wall of conjugating pair (anisogamous); but many species isogamous with zygospores forming in conjugation tube between cells; in some of these zygospores are cut off from gametangia by distinct cross wall. Most species homothallic; one heterothallic species known. Chloroplast inherited from receptive (female) cell in anisogamous in few species studied. Zygospore median wall colored brown or blue; ornamention crucial in species identification. Zygospores compressed-globose to ovoid, some with keels or ridges.

Information contributed by: R.M. McCourt and R.W. Hoshaw. The most recent alteration to this page was made on 2022-09-26 by M.D. Guiry.

Taxonomic status: This name is of an entity that is currently accepted taxonomically.

Gender: This genus name is currently treated as neuter.

Most recent taxonomic treatment adopted: Hess, S., Williams, S.K., Busch, A., Irisarri, I., Delwiche, C.F., de Vries, S., Darienko, T., Roger, A.J., Archibald, J.M., Buschmann, H., von Schwartzenberg, K. & de Vries, J. (2022). A phylogenomically informed five-order system for the closest relatives of land plants. Current Biology 32: 1-10.

Comments: One of three commonly encountered genera in Zygnemataceae, including equally common Mougeotia and far more widespread and abundant Spirogyra. Collected from all continents, from sea-level to mountain habitats, tropical to arctic climates. Habitats range widely from quiet to running water; fresh to brackish. Filaments usually found as free-floating masses. No known economic use or nuisance problems. Fossil zygospores known from Holocene, but may have diverged from other zygnematacean genera as early as Carboniferous. Experimental uses: pyrenoid isolation; zygospores used in paleoecological studies as markers for clean, oxygen-rich, shallow stagnant, mesotrophic water in habitats subject to seasonal warming. Phylogenetic relationships to other Zygnematales unclear; conflicting phylogenetic hypotheses have postulated relation to other taxa with stellate chloroplasts, either unicells (Cylindrocystis) or filaments. Along with other Zygnematales, Zygnema member of Charophyceae and thus related to lower land plants.

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Contributors
Some of the descriptions included in AlgaeBase were originally from the unpublished Encyclopedia of Algal Genera, organised in the 1990s by Dr Bruce Parker on behalf of the Phycological Society of America (PSA) and intended to be published in CD format. These AlgaeBase descriptions are now being continually updated, and each current contributor is identified above. The PSA and AlgaeBase warmly acknowledge the generosity of all past and present contributors and particularly the work of Dr Parker.

Descriptions of chrysophyte genera were subsequently published in J. Kristiansen & H.R. Preisig (eds.). 2001. Encyclopedia of Chrysophyte Genera. Bibliotheca Phycologica 110: 1-260.

Linking to this page: https://www.algaebase.org/search/genus/detail/?genus_id=43566

Citing AlgaeBase
Cite this record as:
M.D. Guiry in Guiry, M.D. & Guiry, G.M. 26 September 2022. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 22 November 2024