parthenogens

Parthenogenesis is a form of asexual reproduction in which embryos develop from unfertilized eggs. Parthenogens are organisms or lineages that reproduce in this way, without fertilization by a male gamete. They can occur across a range of taxa and ecological contexts.

Parthenogenesis can be obligate or facultative. In obligate parthenogenesis, females reproduce exclusively by parthenogenesis, with males often absent or nonfunctional. In facultative parthenogenesis, individuals can reproduce either sexually or asexually depending on conditions. Mechanistically, parthenogenesis occurs via apomixis or automixis. Apomixis involves mitotic divisions that produce offspring genetically identical to the mother. Automixis involves meiosis followed by diploid restoration, which can increase homozygosity relative to the mother.

Invertebrates show parthenogenesis widely. Aphids, some crustaceans, nematodes, rotifers, and several insect groups exhibit parthenogenetic modes. Bdelloid rotifers are a notable example of obligate parthenogens. Some species alternate between sexual and parthenogenetic generations, a pattern known as cyclic or facultative parthenogenesis.

Vertebrates reproduce parthenogenetically in rare cases. All-female species of whiptail lizards in the genus Aspidoscelis reproduce exclusively by parthenogenesis. There are also occasional reports of parthenogenetic reproduction in other fishes and reptiles, but many such cases involve related processes such as gynogenesis, where sperm triggers development without contributing genetic material.

Ecological and evolutionary implications include the potential for rapid population growth without mates, balanced against reduced genetic diversity and long-term adaptability. Some parthenogenetic lineages persist for extended periods, sometimes aided by mutations, occasional genetic exchange, or hybridization events.