Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs. In plants, it means reproduction via embryos, such as buds, that develop from the outset without interruption, as opposed to germinating externally from a seed.
|Animal reproduction topics|
- Ovuliparity: fecundation (fertilisation) is external. It occurs in aquatic arthropods such as some Crustacea; also in many fishes and Amphibia
- Oviparity: fecundation is internal, the female lays zygotes as eggs with large vitellus; occurs in all birds, most reptiles, some fishes.
- Ovoviviparity: this is oviparity with retention of zygotes in the female’s body or in the male’s body, but without major trophic interactions between zygote and parents (there may be minor effects, such as maintenance of water and oxygen levels). Anguis fragilis is an example of ovo-viviparity. In seahorses, zygotes are retained in the male’s ventral "marsupium". In the frog Rhinoderma darwinii, the zygotes develop in the vocal sac. In the frog Rheobatrachus, zygotes developed in the stomach.
- Histotrophic viviparity: the zygotes develop in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (intra-uterine cannibalism in some sharks or in the black salamander Salamandra atra).
- Hemotrophic viviparity: nutrients are provided by the female, often through some form of placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills. The skink Pseudemoia entrecasteauxii and most mammals exhibit a hemotrophic viviparity.
The relatively less developed form of animal vivipary, ovoviviparity, occurs in most vipers for instance, and also in most live-bearing bony fishes (Poeciliidae); the more developed form of vivipary is called placental viviparity. Placental mammals are the best example, but other animals have also adapted by incorporating this principle. Examples include some species of scorpions2 and cockroaches,3 certain genera of sharks and snakes, and velvet worms.
Some viviparous species receive nutrients via the placenta (present in therians, some skinks and some fish). In such species the placenta is attached directly to the mother. In at least one species of skink in the large genus Trachylepis this process has gone essentially to completion. In the uterus the eggs are very small, about 1mm in diameter, with very little yolk and very thin shells. The shell membrane is vestigial and transient; its disintegration permits the absorption of nutrients from uterine secretions. The embryo then produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels. As it penetrates the lining, the embryonic tissue grows aggressively till it forms sheets of tissue beneath the uterine epithelium. They eventually strip it away and replace it, making direct contact with maternal capillaries. In several respects the phenomenon is of considerable importance in theoretical zoology. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.4
There is no relationship between sex-determining mechanisms and whether a species bears live young or lays eggs. Temperature-dependent sex determination, which cannot function in an aquatic environment, is seen only in terrestrial viviparous reptiles. Therefore, marine viviparous species, including sea snakes and, it now appears, the mosasaurs, ichthyosaurs, and plesiosaurs of the Cretaceous, use genotypic sex determination (sex chromosomes), much as birds and mammals do.5 Genotypic sex determination is also found in most reptiles including many viviparous ones (such as Pseudemoia entrecasteauxii), whilst temperature dependent sex determination is found in some viviparous species such as the montane water skink (Eulamprus tympanum). 6
|This section requires expansion. (June 2011)|
Viviparous plants produce seeds that germinate before they detach from the parent. In many mangroves, for instance, the seedling germinates and grows under its own energy while still attached to its parent. Some drop into the water and are dispersed by currents, but others develop a heavy, straight taproot that commonly penetrates mud when the seedling drops, thereby effectively planting the seedling. In some trees, like Jackfruit and some citrus, the seeds can be found already germinated while the fruit goes overripe; strictly speaking this condition cannot be described as vivipary, but the moist and humid conditions provided by the fruit mimic a wet soil that encourages germination. However, the seeds can germinate under moist soil too.7
- Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris
- Capinera, John L., Encyclopedia of entomology. Springer Reference, 2008, p. 3311.
- Costa, James T., The Other Insect Societies. Belknap Press, 2006, p. 151.
- Blackburn, D. G. and Flemming, A. F. (2011), Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (scincidae). Journal of Morphology. doi:10.1002/jmor.11011
- Chris L. Organ et al. (2009) "Genotypic sex determination enabled adaptive radiations of extinct marine reptiles", Nature 461, 389-392 (17 September 2009)
- Robert, Kylie A., and Michael B. Thompson. "Sex determination: viviparous lizard selects sex of embryos." Nature 412, no. 6848 (2001): 698-699.
- UCLA: The Mildred E. Mathias Botanical Garden