크레스티드게코 Reptiles usually reproduce sexually, but some can also produce asexually. This is called parthenogenesis, and it has been observed in six families of lizards and snakes.
Reptiles lay their eggs through an opening in their body called the cloaca. This is where waste, sperm and the eggs leave. Some lizards, like the popular blue-tongue skink (Zootoca vivipara), give birth to live young.
Fertilization
Fertilization is the fusion of male and female gametes to create an embryo. This process is essential for all organisms to reproduce. A sperm and egg are specialized sex cells that carry hereditary information. When they fuse together, they form a single cell with double the number of chromosomes of a typical individual of that species.
Most reptiles are oviparous, meaning they lay eggs that develop outside their bodies. However, some lizards and snakes are viviparous, which means they give birth to live young. The ovulation of these animals happens in the external membranes of their skin, and they are usually born covered in a protective fluid called amniotic fluid.
The ovulation of a viviparous reptile can be triggered by hormones, including prostaglandins. Prostaglandins are a type of steroid hormone, and they act on the ovaries in mammals to prepare the uterus for implantation of fertilized eggs. The ovarian cycle of a viviparous reptile is more irregular than that of an oviparous one.
Reptiles have a variety of reproductive strategies that evolved over millions of years 크레스티드게코 to help them survive in their varied environments. They can be oviparous (lizards, geckos, tortoises and snakes), viviparous (skinks) or ovoviviparous (some chameleons and boas). Some lizards can even reproduce asexually. However, this type of reproduction is extremely rare. It is also called parthenogenesis, and it occurs in some lizards, fish and insects.
Parthenogenesis
Parthenogenesis is a form of asexual reproduction in which a female reptile produces fertile eggs without the assistance of a male. It occurs in scaled reptiles, such as the lizard Heteronotia binoei. However, parthenogenic lizards have lower fecundity than their sexual progenitors.
Scientists have found that a number of environmental factors can trigger parthenogenesis in reptile eggs. These include changes in ion concentration and abiotic stress. However, the underlying molecular mechanisms are not yet known.
Asexual reproduction can be advantageous in the short term, allowing an animal to reproduce without the need for mating. It also allows a species to colonize new environments. However, high extinction rates and high rates of reversal in parthenogenetic species suggest that this reproductive strategy is self-destructive in the long term.
There are two forms of parthenogenesis in reptiles, automixis and apomixis. In automixis, the egg cell fuses with a polar body, which shifts the mother’s genes. As a result, the offspring are similar to the mother but not a full clone. In apomixis, the sperm cell does not fuse with the egg cells, so the offspring have a single set of chromosomes.
Scientists have recently discovered that a solitary Asian water dragon female was capable of producing fertile offspring without the need for a male. This was the first time that a viable parthenogenic reptile had been recorded. This reproductive process could help to save endangered species by enabling them to produce in poor habitats or where there is insufficient male population.
Oviparous
Among the animals that follow internal sexual reproduction, those that lay eggs are called oviparous. The eggs of these animals are fertilized with sperms within the mother’s body. This method of fertilization is different from the viviparous internal fertilization mode, wherein an embryo develops inside the mother’s body and gets nourished through the yolk sac. It is also referred to as ovoviviparity.
In oviparous reptiles, sperm from the male penetrates the ova of the female and fertilizes them in her cloaca. The resulting fertilized egg is then laid by the female and sealed in a hard shell. This is the reproductive strategy followed by most fish, amphibians, and reptiles. It is also the most common method of reproduction in birds, all monotremes, and dinosaurs.
In oviparous reptiles, the eggs are very fragile and are often hidden from predators. The mother usually lays the eggs in a safe place, like a nest, where they are protected from heat and other environmental conditions. Some oviparous animals, such as hens, sit on their eggs to keep them warm and others bury them in the ground. Some snake species, such as king cobras and mambas, are ovoviviparous and give birth to live young ones rather than laying eggs. These young ones are also known as fetuses. Their young are not able to hide and protect themselves from predators, so they rely on the mother to take care of them.
Vivoviviparous
Viviparous reptiles are animals that give birth to live young rather than eggs. Their offspring are connected to the mother through a special supply line called the placenta, which is similar to the umbilical cord in mammals. Viviparous offspring develop in the mother until they are ready to leave her body and survive on their own. During this time, viviparous offspring have to be fed by the mother’s blood. This process is referred to as intrauterine growth or nidotrophic nutrition.
Despite their different reproductive modes, both oviparous and viviparous reptiles have certain similarities. These include internal fertilization, paired maternal oviducts that house the eggs between fertilization and oviposition, a thin eggshell that resists dehydration, a highly vascularized chorioallantois that lines the eggshell, and a yolk sac that surrounds a relatively large yolk (see figure).
In addition, the ovarian structure is not a rigid tube, but rather a flattened cleft in the uterine wall. This allows for a close fit between the oviduct and the fetal membranes, and it facilitates gas exchange between maternal and fetal tissues. This same feature also accounts for the absence of an eggshell in viviparous reptiles.
Viviparity first emerged in reptiles during the Pleistocene period, and it has since been confirmed in several lizard species, including Lacerta vivipara and Sceloporus aeneus. However, there is no proof that viviparity was the dominant reproductive mode in the Mesozoic or in mosasauroids, lepidosauriformes, sauropterygids, or choristoderans.