Zygnema C.Agardh, 1817: xxxii, 98

Classification:
Empire Eukaryota
Kingdom Plantae
Phylum Charophyta
Class Conjugatophyceae
Order Zygnematales
Family Zygnemataceae

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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Taxonomic status: currently recognized as a distinct genus.

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.

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.

Common names

(as Zygnema)
Swedish: Tvestjärntrådar (Tolstoy & Österlund 2003).

NCBI Nucleotide Sequences

Numbers of names and species: There are 443 species (and infraspecific) names in the database at present, of which 181 have been flagged as currently accepted taxonomically.

Names: ('C' indicates a name that is currently accepted taxonomically; 'S' a homotypic or heterotypic synonym; 'U' indicates a name of uncertain taxonomic status, but which has been subjected to some verification nomenclaturally; 'P' indicates a preliminary AlgaeBase entry that has not been subjected to any kind of verification. For more information on a species click on it to activate a link to the Species database):

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: http://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. 2013. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org; searched on 19 June 2013.

Algaebase taxon LSID: urn:lsid:algaebase.org:taxname:6833