Volvox is a genus of chlorophytes, a type of green algae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in 1700. Volvox developed its colonial lifestyle 200 million years ago.

Description

Volvox colony: 1) Chlamydomonas-like cell, 2) Daughter colony, 3) Cytoplasmic bridges, 4) Intercellular gel, 5) Reproductive cell, 6) Somatic cell.

Volvox is the most developed in a series of genera that form spherical colonies.^[1] Each mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total,^[2] and embedded in the surface of a hollow sphere or coenobium containing an extracellular matrix^[2] made of a gelatinous glycoprotein.^[3] The cells swim in a coordinated fashion, with distinct anterior and posterior poles. The cells have eyespots, more developed near the anterior, which enable the colony to swim towards light. The individual algae in some species are interconnected by thin strands of cytoplasm, called protoplasmates.^[4] They are known to demonstrate some individuality and working for the good of their colony, acting like one multicellular organism. The flagellates on its outside resemble Euglena.

Reproduction

An asexual colony includes both somatic (vegetative) cells, which do not reproduce, and gonidia near the posterior, which produce new colonies through repeated division. The daughter colonies are initially held within the parent coenobium and have their flagella directed inwards. Later, the parent disintegrates and the daughters invert. In sexual reproduction two types of gametes are produced. Volvox species can be monoecious or dioecious. Male colonies release numerous microgametes, or sperm, while in female colonies single cells enlarge to become oogametes, or eggs.^[1][5]

Habitats

Volvox is a freshwater alga and is found in ponds and ditches, even in shallow puddles.^[5] According to Charles Joseph Chamberlain,^[6]

"The most favorable place to look for it is in the deeper ponds, lagoons, and ditches which receive an abundance of rain water. It has been said that where you find Lemna, you are likely to find Volvox; and it is true that such water is favorable, but the shading is unfavorable. Look where you find Sphagnum, Vaucheria, Alisma, Equisetum fluviatile, Utricularia, Typha, and Chara. Dr. Nieuwland reports that Pandorina, Eudorina and Gonium are commonly found in summer as constituents of the green scum on wallows in fields where pigs are kept. The flagellate, Euglena, is often associated with these forms."

History

Antonie van Leeuwenhoek first reported observations of Volvox in 1700.^[7]

Evolution

Ancestors of Volvox transitioned from single cells to form multicellular colonies at least 200 million years ago, during the Triassic period.^[2][8] An estimate using DNA sequences from about 45 different species of Volvox and related species suggests that the transition from single cells to undifferentiated multicellular colonies took about 35 million years.^[2][8]

References