Parthenogenesis is a form of reproduction where eggs develop without fertilization, resulting in unisexual species. This phenomenon is closely related with reproductive modes such as hybridogenesis, where fertilization occurs, but the paternal DNA is not passed on. Among amphibians, it is seen in numerous frog and salamander species, but has not been recorded in caecilians. Artificial parthenogenesis Haploid parthenogenesis has been achieved experimentally in amphibians, through approaches that could be considered artificial gynogenesis. Oscar Hertwig first achieved artificial parthenogenesis in frogs in 1911, using eggs fertilized by irradiated sperm. The radiation destroyed the DNA within the sperm, but nearly normal embryos were still produced. Gunther Hertwig repeated this experiment in 1924, using crosses between different frogs. A cross between a toad, Amietophrynus regularis, and a frog, Rana fusca, would not produce a viable embryo, but fertilization of a toad egg by an irradiated frog sperm would produce a haploid larva. Parthenogenesis has also been induced in Pelophylax nigromaculatus by pricking an egg with a needle. This method produced tadpoles which metamorphosed into adult frogs, some of which were albino. Rana japonica, Pelophylax nigromaculatus and Lithobates pipiens have all produced viable haploid adults as a result of artificial parthenogenesis. Parthenogenesis in nature Origins Salamanders are the oldest known parthenogenic vertebrates. Molecular methods date the origins of unisexual salamanders to the Pliocene, from between 3.9 million to approximately 5 million years ago. All known parthenogenic amphibians have been the result of hybridization events between closely related species. Pelophylax esculentus, the edible frog, is the product of crosses between Pelophylax lessonae and Pelophylax ridibundus. Similarly, Ambystoma laterale, Ambystoma jeffersonianum, Ambystoma texanum and Ambystoma tigrinum have been identified as extant parent species to unisexual salamanders within the same genus. However, mitochondrial evidence suggests that the origins of hybrid Ambystoma, on the maternal line, lie in a relative of Ambystoma barbouri. In spite of this, all extant unisexual species of Ambystoma share no nuclear DNA with Ambystoma barbouri. Polyploidy in unisexual amphibians Polyploidy, a numerical change in the number of chromosomes, is common in parthenogenic amphibians. Triploidy (having three sets of chromosomes), tetraploidy (four sets of chromosomes) and pentaploidy (five sets of chromosomes) are common in salamanders. In unisexual salamanders these different levels of polyploidy are a result of multiple hybridization events, involving two to four species. Ambystoma nothagenes is a unisexual, triploid hybrid of Ambystoma laterale, Ambystoma texanum and Ambystoma tigrinum, while hybrids of