Parthenogenesis is a natural form of asexual reproduction where an embryo develops from an unfertilized egg. The term originates from Greek, combining “parthenos” (virgin) and “genesis” (creation). This reproductive strategy is observed across various forms of life, from lower plants and invertebrates to some vertebrates.
The Mechanics of Parthenogenesis
The biological processes underlying parthenogenesis vary, but they all bypass the need for sperm to initiate embryonic development. One common mechanism is apomixis, where mature egg cells are produced through mitotic divisions, rather than meiosis. In this process, the offspring are genetically identical clones of the mother, as seen in aphids.
Another mechanism is automixis, which involves meiosis but restores the diploid chromosome number through various means, such as the duplication of chromosomes or the fusion of haploid cells. While meiosis still occurs, leading to some genetic recombination, these offspring are not exact clones but rather “half-clones” of the mother.
Creatures That Reproduce Asexually
Parthenogenesis occurs in a diverse array of animal species, both in the wild and in captivity. Many invertebrates, such as aphids, water fleas (Daphnia), rotifers, and certain bees and wasps, commonly reproduce this way. For instance, some honey bee subspecies, like Apis mellifera capensis, can produce diploid female offspring parthenogenetically.
Among vertebrates, cases of parthenogenesis have been documented in certain fish, amphibians, and birds, but it is more commonly observed in reptiles. Komodo dragons have been known to reproduce parthenogenetically, with instances recorded in zoos where females laid eggs despite being isolated from males for years. The New Mexico whiptail lizard (Aspidoscelis neomexicanus) is an all-female species that reproduces exclusively through parthenogenesis.
Evolutionary Reasons for Parthenogenesis
Parthenogenesis provides several evolutionary advantages in specific environmental or ecological contexts. One significant benefit is reproductive assurance, allowing a species to continue reproducing even when mates are scarce or absent. This is particularly advantageous for solitary species or those with limited mobility, as it eliminates the need to expend energy and time searching for a partner.
Parthenogenesis also enables rapid population growth, as every individual female can produce offspring. This can be a considerable advantage for colonizing new habitats or recovering quickly from population declines. While sexual reproduction offers the benefit of genetic diversity, parthenogenesis can preserve successful gene combinations in stable environments.
Can Humans Reproduce Through Parthenogenesis?
Natural human parthenogenesis, leading to the development of a viable offspring, does not occur. Mammalian development, including humans, requires genetic contributions from both a maternal and paternal genome for proper growth. This requirement is due to a biological phenomenon known as genomic imprinting.
Genomic imprinting involves specific genes being “marked” or epigenetically modified in either the egg or sperm, leading to their expression from only one parental allele. Without both maternal and paternal genetic contributions, a full set of appropriately imprinted genes is not present, which results in early embryonic death in mammals. While unfertilized human egg cells can sometimes begin to develop, they usually form disorganized growths called teratomas rather than a viable embryo.