Zoid
In
Diversity of zoids
A zoid contains one or more flagella for motility. In the various species that produce zoids, there is a high level of diversity in the number of flagella produced. The heterokonts generally produce zoids with 2 flagella,
Zoids in heterokonts
- In Green Algae
Green algae have a life cycle that includes an alternation of generations.[9] Zoids can be found in both the haploid and the diploid phases of this life cycle in certain green alga. Number of flagella is one characteristic that aids in the classification of different types of green alga.[9] Zoids are either released through pores or by lysing of the zoid-producing cells in either the gametangium or the sporangium.[10] A majority of the zoids produced within this group are either bi-flagellate or quadri-flagellate. To represent the diversity of zoids found in green alga, below is a list of genera from the family Monostromataceae which is part of the phylum Chlorophyta.[11]
- Genus Monostroma – produces bi-flagellate gametes and quadri-flagellate zoospores
- Genus Gayralia – produces bi-flagellate zoids in the monomorphic asexual form
- Genus Protomonostroma – produces quadri-flagellate zoids
- Genus Ulvopsis – produces bi-flagellate gametes, bi-flagellate asexual zoids, and quadri-flagellate zoospores
- Genus Ulvaria – produces biflagellate gametes and quadri-flagellate zoospores
- In Brown Algae
Brown algae (
- Plurilocular = many chambered, each chamber produces one zoid
- Unilocular = one chamber, can produce multiple gametes in one chamber
- Plurilocular gametangia = structure that has many chambers that produce haploid gametes
- Plurilocular sporangia = structure that has many chambers that produce diploid spores
- Plurilocular zoidangia = collective term for plurilocular gametangia and plurlocular sporangia
- Plurizoids = zoids produced in a plurlocular zoidangia
- Unilocular sporangia = can produce meiospores or asexual spores
- Unilocular zoidangia = synonym for unilocular sporangia
- Unizoids = zoids produced in a unilocular zoidangia
Brown alga zoids have the same two basic flagella discussed in the
- In Diatoms
Zoids are not as common in the diatoms as in the algal families.
Zoids in non-vascular plants and fungi
Among the non-vascular plants, specifically the
Fungi are a very diverse group of organisms with very diverse life cycles. Most reproduce using spores and many do not utilize zoids for their reproduction. However, one particular class of organisms that is very closely related to fungi use a similar zoid to the
Zoids in vascular plants
Zoids are found in three types of
Evolution
In plants, the zoid, or swimming sperm, is considered to be a trait of the "lower" land plants. In aqueous environments, the necessity for motile reproduction is obvious, but on land this adaptation loses its relevance. The zoid is most common among the non-vascular plants and the "lower" vascular plants. It is hypothesized that as the land plants evolved enclosed ovules, the necessity for a film of water and therefore motile sperm became unnecessary.[4] Motile, flagellated sperm or zoids is rare in angiosperms.
Along the same lines, the Ginkgo is a species that has no close living relative. It is believed to be most closely related to the giant seed ferns which date back to the Jurassic period.[6] This represents what would also be considered a "lower" land plant. Ginkgo were originally classified in the Taxaceae, or yew, family. When it was discovered that ginkgo had motile sperm, they were moved to their own family, Ginkgoaceae.[6]
References
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