In botany, a zoid or zoïd /ˈzoʊ.ɪd/ is a reproductive cell that possesses one or more flagella, and is capable of independent movement.[1] Zoid can refer to either an asexually reproductive spore or a sexually reproductive gamete. In sexually reproductive gametes, zoids can be either male or female depending on the species. For example, some brown alga (Phaeophyceae) reproduce by producing multi-flagellated male and female gametes that recombine to form the diploid sporangia.[2] Zoids are primarily found in some protists, diatoms,[1] green alga, brown alga,[3] non-vascular plants,[4] and a few vascular plants (ferns,[1] cycads,[5] and Ginkgo biloba[6]). The most common classification group that produces zoids is the heterokonts or stramenopiles. These include green alga, brown alga, oomycetes, and some protists.[7] The term is generally not used to describe motile, flagellated sperm found in animals. Zoid is also commonly confused for zooid which is a single organism that is part of a colonial animal.
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,[7] while the Ginkgo biloba produce zoids with tens of thousands of flagella.[6] The position of the flagella and the arrangement of the microtubules varies among species as well. The following sections will briefly outline general characteristics of the zoids found in each subset as well as provide specific examples.
Heterokonts are a diverse group of eukaryotic organisms that include diatoms, green algae, and brown algae. The defining characteristic of this group is their bi-flagellate, motile sperm (zoid).[8] The two flagella are most commonly positioned apically or sub-apically depending on the type of heterokont. One flagella, the tinsel flagella, is generally longer and covered with bristles. The other flagella is typically shorter, potentially even shortened to just a basal body, and is generally smooth and whip-like.[7]
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]
Brown algae (Phaeophyceae) reproduce both sexually and asexually depending on the species. However, all motile reproductive cells in the Class are flagellated and there are no free-living flagellate organisms.[1] The structure of brown algae varies depending on Family and Genus, thus zoids are produced in a variety of ways. Gametes or asexual zoospores can be produced in plurilocular zoidangia in the larger thalli of brown alga.[1] However, in smaller thalli, unilocular zoidangia produce the sexually or asexually reproductive cells.[2] Below is some vocabulary associated with brown algal zoid production:[1]
Brown alga zoids have the same two basic flagella discussed in the heterokont section. However, orientation of the flagella is unique in the Phaeophyceae. In general, the flagella are both inserted laterally.[1]