''Polytomella'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

green algae 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 ...

in the family

Dunaliellaceae Dunaliellaceae is a family of algae in the order Chlamydomonadales. It is widespread in freshwater and saline environments worldwide, less so in marine waters. Members of the family Dunaliellaceae are single-celled, flagellate algae. They are of ...

. It is widespread in freshwater habitats worldwide. ''Polytomella'' consists of single, cells that lack cell walls. Cells are ellipsoid, pyriform, or globose, and slightly metabolic (flexible). Cells have four anterior

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

arising from a central papilla. The posterior of the cell is usually rounded, rarely pointed or with spine-like extensions. Although classified as an alga,

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 and

pyrenoid Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,Giordano, M., Beardall, J., & Raven, J. A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''An ...

s are absent and cells instead have vestigial leucoplasts. The cell accumulates starch in its

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

. A single stigma and two to four apical

contractile vacuole A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists, including unicellular algae. It was previously known as pulsatile or pulsating vacuole. Overview The contrac ...

s are typically present. The single nucleus is located in the central or anterior half of the cell. Asexual reproduction occurs by longitudinal, binary division; sexual reproduction is

isogamous Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), and is found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as ma ...

with the zygotes giving rise to four motile cells. ''Polytomella'' is somewhat similar to '' Tetrablepharis'', another colorless alga with four flagella, but ''Tetrablepharis'' has cell walls. Species are distinguished based on morphological characters such as the presence or absence of a papilla, shape of the cells, and position of intracellular features like starch granules.

Evolution

''Polytomella'' is a free-living, flagellated, nonphotosynthetic green alga with a highly reduced, linear fragmented mitochondrial genome. ''Polytomella'', as it exists today, bears evidence of once having a functional photosynthetic plastid which has over evolutionary time changed such that it would appear now to have no genome or gene expressing mechanisms remaining to it. Having transitioned completely to heterotrophy, ''Polytomella'' uses organic acids, alcohols and monosaccharides as its carbon source. Despite being an evolutionary descendant of the green algae, ''Polytomella'' is a colourless organism because it has lost its photosynthetic ability.

References

Chlamydomonadales Chlamydomonadales genera Freshwater algae {{Chlorophyceae-stub