The Zygnematales ( (''zygós'') and νῆμα (''nḗma'') ( nom.), νήματος (''nḗmatos'') ( gen.)), also called the Conjugatales, are an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

green alga The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( ...

e, comprising several thousand different

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

in two families. The larger family Zygnemataceae, with well-known

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

such as ''Zygnema'' and ''

Spirogyra ''Spirogyra'' (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous charophyte green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is charact ...

'', includes members that grow as unbranched filaments, which grow longer through normal

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

. This group includes the

desmids Desmidiales, commonly called the desmids (''Gr.'' ''desmos'', bond or chain), are an order in the Charophyta, a division of green algae in which the land plants (Embryophyta) emerged. Desmids consist of single-celled (sometimes filamentous or colo ...

. Most members of both families live in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

, and form an important component of the algal scum that grows on or near plants and rocks. Systematically they fall within the division

Charophyta Charophyta () is a paraphyletic group of freshwater green algae, called charophytes (), sometimes treated as a division, yet also as a superdivision. The terrestrial plants, the Embryophyta emerged deep within Charophyta, possibly from terres ...

Streptophyta Streptophyta (), informally the streptophytes (, from the Greek ''strepto'' 'twisted', for the morphology of the sperm of some members), is a clade of plants. The composition of the clade varies considerably between authors, but the definition e ...

, in which the land plants (

Embryophyta The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophy ...

) emerged.

Sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

in Zygnematales takes place through a process called ''

conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form *Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change o ...

''. Here filaments of opposite gender line up, and tubes form between corresponding cells. The male cells then become amoeboid and crawl across, or sometimes both cells crawl into the tube. The cells then meet and fuse to form a

zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...

, which later undergoes

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

to produce new filaments. As in plants, only the female passes

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s on to the offspring. Other conjugating algae are the

Mesotaeniaceae The Mesotaeniaceae are a small family of unicellular green algae known as the "saccoderm desmids". The Mesotaeniaceae appear to be sister or ancestral to the Zygnemataceae. The desmids are a deep branching group of Zygnemataceae. ''Spirotaenia'' ...

, sister of the Zygnematales, and ''

Spirotaenia ''Spirotaenia'' is a genus of basal unicellular green algae that may be sister to the Chlorokybophyceae. It was previously considered to be part of the Zygnemataceae. It is sexually Isogamy#Conjugation, conjugating, a mode of reproduction that ...

'', a basal green algae. Additionally, the

Desmidiales Desmidiales, commonly called the desmids (''Gr.'' ''desmos'', bond or chain), are an order in the Charophyta, a division of green algae in which the land plants ( Embryophyta) emerged. Desmids consist of single-celled (sometimes filamentous or ...

appear to have emerged deep within the Zygnematales, and are also conjugating.

Gallery

File:Netrium oblongum var. cylindricum.jpg, ''

Netrium ''Netrium'' is a genus of algae belonging to the family Mesotaeniaceae The Mesotaeniaceae are a small family of unicellular green algae known as the "saccoderm desmids". The Mesotaeniaceae appear to be sister or ancestral to the Zygnemataceae. ...

oblongum''. A saccoderm "desmid" File:Spirogyra cell.jpg, Single ''

'' cell File:20090328 2222 Spirogyra.jpg, ''

''. Each numbered tick = 122 μM File:20090328 2225 Spirogyra.jpg, ''

''. Each numbered tick = 20 μM File:SpirogyraConjugation100x.jpg, Microscopic view of ''

'' conjugation File:20090906 022641 Algae.jpg, ''

Zygnema ''Zygnema'' is a genus of freshwater filamentous thalloid alga comprising about 100 species. A terrestrial species, ''Z. terrestre'', is known from India. ''Zygnema'' grows as a free-floating mass of filaments, although young plants may be found ...

''. Each numbered tick = 122 μM File:20100417 005205 Zygnema.jpg, ''

''. Each numbered tick = 20 μM

References

{{Authority control Charophyta orders