Zygnematales
Encyclopedia
The Zygnematales also called the Conjugales, are an order
of green algae, comprising several thousand different species
in genera
such as the well-known Zygnema and Spirogyra
. All the members of this group develop into unbranched filaments, one cell thick, which grow longer through normal cell division
. Most live in freshwater
, and form an important component of the algal scum that grows on or near plants, rocks, and various debris.
Systematically they fall within the Charophyta
(Streptophyta) Division, which includes algae that are closer related to the higher plants than they are to most of the other algae (and including land plants themselves in Streptophyta classification).
Sexual reproduction
in Zygnematales takes place through a process called conjugation
. 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 spore, which later undergoes meiosis
to produce new filaments. Only the female passes chloroplast
s on to the offspring, as in higher plants.
The only other group of conjugating algae are the desmid
s, which live as individual cells often striking for their symmetrical appearance. The two orders are definitely close relatives, and sometimes the desmids are included among the Zygnematales, or the two are grouped in their own Class
(Zygnematophyceae
).
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of green algae, comprising several thousand different species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
in genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as the well-known Zygnema and Spirogyra
Spirogyra
Spirogyra is a genus of filamentous green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is diagnostic of the genus. It is commonly found in freshwater areas, and there are more than 400 species of Spirogyra in the world. Spirogyra measures...
. All the members of this group develop into unbranched filaments, one cell thick, which grow longer through normal cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
. Most live in freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
, and form an important component of the algal scum that grows on or near plants, rocks, and various debris.
Systematically they fall within the Charophyta
Charophyta
The Charophyta are a division of green algae, including the closest relatives of the embryophyte plants. In some groups, such as conjugating green algae, flagellate cells do not occur. The latter group does engage in sexual reproduction, and motility does not involve flagella, since they are...
(Streptophyta) Division, which includes algae that are closer related to the higher plants than they are to most of the other algae (and including land plants themselves in Streptophyta classification).
Sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
in Zygnematales takes place through a process called conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...
. 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 spore, which later undergoes meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
to produce new filaments. Only the female passes chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s on to the offspring, as in higher plants.
The only other group of conjugating algae are the desmid
Desmid
Desmids are an order of green algae, comprising around 40 genera and 5,000 to 6,000 species, found mostly but not exclusively in fresh water. Most are unicellular, and are divided into two compartments separated by a narrow bridge or isthmus...
s, which live as individual cells often striking for their symmetrical appearance. The two orders are definitely close relatives, and sometimes the desmids are included among the Zygnematales, or the two are grouped in their own Class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
(Zygnematophyceae
Zygnematophyceae
Zygnematophyceae is an class of green algae. It contains two orders: Zygnematales and Desmidiales....
).