Parthenogenees

Parthenogenesis is a form of reproduction in which offspring develop from an egg that has not been fertilized by a sperm. It occurs across a broad range of organisms, from invertebrates such as rotifers, aphids, and some crustaceans to several vertebrate groups, and in plants as part of apomixis. In many species, parthenogenesis complements or replaces sexual reproduction under certain ecological conditions, such as low mate availability or stable environments.

Two broad routes are recognized. In automixis, meiosis occurs but diploidy is restored by fusion or duplication of meiotic products, yielding offspring with varying degrees of genetic variation. In apomixis-like pathways, the egg develops without meiosis, producing clonal offspring. When parthenogenesis is the only mode of reproduction, the species is obligate; when it occurs alongside sexual reproduction, it is facultative. In vertebrates, mammalian parthenogenesis is rare and, due to genomic imprinting, most embryos do not develop to term; some reptiles and certain sharks have produced viable parthenogenetic offspring.

Notable examples include the New Mexico whiptail lizard, Aspidoscelis neomexicana, an obligate parthenogenetic species. There have been reports of parthenogenetic reproduction in Komodo dragons under captivity. In plants, parthenogenesis is closely related to apomixis, enabling seed formation without fertilization, observed in species such as Taraxacum (dandelions) and various grasses.