A gonozooid is any of the reproductive individuals of tunicate, bryozoan, or hydrozoan colonies that produce gametes. Gonozooids may play a role in labour division or in alternation of generations. A gonozooid typically has hardly any other function than reproduction, amounting to little more than a motile gonad.
The production of gonozooids amounts to one aspect of certain classes of alternation of generations. In biological terms the various forms are examples of evolutionary strategies and are largely analogous rather than homologous; the gonozooid phases of tunicates and bryozoans, for instance, did not originate from a common ancestor.
Examples of species with gonozooids
Hydractinia symbiolongicarpus
Polyps from the cnidarian Hydractinia symbiolongicarpus form colonies where labour is divided between reproduction (gonozooids), feeding (gastrozooids) and defense (dactylozooid). Even though they are genetically identical, there is a distinct morphological difference between those polyps. The gonozooids have gametes-producing organs called gonophores on their body column. They lack the mouth and tentacles present on gastrozooids, and the stinging cells present on the dactylozooid. They can only survive as part of the colony.
Doliolum nationalis
The tunicate Doliolum nationalis does not form colonies, but is a free swimming-organism. It exhibits alternation of generation in its life cycle. In this organism, the gonozooids are hermaphrodites. They both produce and fertilize ova, which in turn develop into oozooids, the asexual stage. This oozoid develops further and takes on a quite different form than the gonozooid. Through asexual reproduction called budding, new gonozooids are produced and the life cycle closes.
Celleporella hyalina
Another colonial gonozooid-forming animal is the bryozoan Celleporella hyalina. Male and female gonozooids are budded from a layer of sterile gastrozooids. The gonozooids then produce larva, which will undergo metamorphosis and become the founder of another colony.
Roles of gonozooid individuals in the life cycle
Division of labour
In animals such as Hydractinia symbiolongicarpus, gonozooids are part of a colony. Each individual has a role that benefits the colony as a whole. The gonozooids that produce the gametes can sexually reproduce. As the whole colony is genetically identical, it is not necessary for each polyp to individually reproduce, allowing more energy and resources to be used for other tasks such as feeding and defense.
Sexual/asexual reproduction frequencies
In certain animals, stressful environments have been known to lead to an increase in gonozooid frequencies. Higher gonozooid frequencies lead to more sexual reproduction and thus more offspring being genetically different from their parents. The resulting higher genetic variance increases the chances of beneficial phenotypes appearing in the population. This beneficial phenotype may in turn spread throughout the population and increase its resistance against the stressing factor.
References
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- ^ Hughes, D.J. (1992). "Genotype-environment interactions and relative clonal fitness in a marine bryozoan". Journal of Animal Ecology. 61 (2): 291–306. doi:10.2307/5322. JSTOR 5322.
- ^ Sanders, Steven M.; Shcheglovitova, Mariya; Cartwright, Paulyn (2014-05-28). "Differential gene expression between functionally specialized polyps of the colonial hydrozoan Hydractinia symbiolongicarpus (Phylum Cnidaria)". BMC Genomics. 15 (1): 406. doi:10.1186/1471-2164-15-406. ISSN 1471-2164. PMC 4072882. PMID 24884766.
- ^ Terry, Robert M. (March 1961). "Investigations of Inner Continental Shelf Waters off Lower Chesapeake Bay. Part III. The Phorozooid Stage of the Tunicate Doliolum nationalis". Chesapeake Science. 2 (1/2): 60–64. doi:10.2307/1350721. JSTOR 1350721.
- Andrey N. Ostrovsky (16 December 2013). Evolution of Sexual Reproduction in Marine Invertebrates: Example of gymnolaemate bryozoans. Springer Science & Business Media. pp. 271–. ISBN 978-94-007-7146-8.
- Piraino, S (1991). "The adaptive pattern of growth and reproduction of the colonial hydroid Clavopsella michaeli". Coelenterate Biology: Recent Research on Cnidaria and Ctenophora. pp. 229–234. doi:10.1007/978-94-011-3240-4_32. ISBN 978-94-010-5428-7.
- Crow, James F. (1994). "Advantages of sexual reproduction". Developmental Genetics. 15 (3): 205–213. doi:10.1002/dvg.1020150303. ISSN 1520-6408. PMID 8062455.