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.
Contents |
Five modes of reproduction can be differentiated in animals[1] based on relations between zygote and parents:
A relatively less developed form of animal vivipary is called ovoviviparity, which, for instance, occurs in most vipers and 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 scorpions[2] and cockroaches[3], certain genera of sharks and snakes, and velvet worms.
Certain lizards, in particular some species of skink, also reproduce in this way, referred to as viviparous matrotrophy. Examples include the species Tiliqua rugosa and members of the genus Corucia. In such species the placenta is attached directly to the mother. Viviparous offspring live independently and require an external food supply from birth. This contrasts with some egg laying animals, such as fish and amphibia in which the recently hatched young bear yolk sacs. Precocial birds, such as domestic fowl, typically also bear internal yolk sacs at birth.
Recent investigation has revealed that in at least one species of skink in the large genus Trachylepis this process has gone essentially to completion. In several respects the phenomenon is of considerable importance in theoretical zoology. 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. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.[4]
There is a relationship between sex-determining mechanism 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 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 like birds and mammals.[5]
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 before dropping into the water in order to transport away. In some trees, like Jackfruit, the seeds can be found already germinated while the fruit goes overripe; this condition cannot be said as vivipary since the moist and humid conditions provided by the fruit mimics a wet soil that encourages germination. However, the seeds can germinate under soil too.[6]
Wang Y, Evans SE. 2011. A gravid lizard from the Cretaceous of China and the early history of squamate viviparity. Naturwissenschaften DOI: 10.1007/s00114-011-0820-1