Parthenogenesis means “virgin birth” or the production of offspring by one parent (a female) without genetic input from a male: the female’s egg is not fertilized by sperm. Thus, the offspring have only their mother’s genes and in most cases will be just like their mother. This asexual mode of reproduction is widespread in plants and animals, although it happens in various ways, and the resulting offspring may differ, depending on the way they were made. For example, in alpine bistort (see my recent essay in this section of the paper), the offspring are tiny plantlets, in honeybees only the males are produced parthenogenetically, and in fishes and salamanders this way of reproduction produces only females.
There are so many different life histories and such a variety of organisms that engage in parthenogenesis that it is hard to know where to start a short essay on the subject. So, of necessity, I will simplify things by, first, dealing with some basic distinctions and, second, by focusing chiefly on vertebrates.
Individuals of some organisms can reproduce both sexually and asexually. For example, a honeybee queen makes her worker broods (all females) from fertilized eggs (with two sets of chromosomes) but toward the end of the season, she produces males from unfertilized eggs (with only one set of chromosomes). Dandelions produce most of their seeds by asexual means but reportedly a small fraction of their seeds can be the result of pollination and fertilization.
Some organisms have complex life cycles in which a sexual generation alternates with an asexual, parthenogenetic generation. This pattern is found among lots of plants, such as mosses and ferns, in which the spores are produced asexually but give rise to individuals that are sexual. The pattern is also found in a variety of invertebrates, such as some crustaceans and aphids. Many aphids, for example, reproduce parthenogenetically (and viviparously) in spring and summer, producing only female offspring. As fall approaches, however, female aphids start to produce some males (with one fewer chromosome than females) also. Males and females mate, females lay overwintering, fertilized eggs, and a new generation of parthenogens emerges the following spring.
Among vertebrates, regular parthenogenesis occurs in certain populations of lizards, salamanders, a frog, a snake, and some small fishes. Most of these populations are reported to be hybrids between two or even three other species, and some are polyploid (having more than the usual two sets of chromosomes). Typically, all the offspring are female. Interestingly, in some of these all-female species, females apparently need to go through the courtship process with a male of a related species and may even need to mate with him, but the male’s genes never contribute to the ensuing offspring. The intricate genetics of how all those females make young with two sets of their own chromosomes are complex and differ among species; I will leave all that aside!
However, for mammals and birds, I have to deal with some genetics. In mammals, each of the normal two sets of chromosomes includes two chromosomes that determine the gender of the offspring. Females have two X chromosomes and males have one X and one Y chromosome (the labels are arbitrary). If a female were to reproduce parthenogenetically, all her offspring would also be female (XX), lacking the necessary Y chromosome to be a male. However, this mode of reproduction is unknown in naturally reproducing mammals, although reportedly it can be induced by experimental tinkering with laboratory mice.
Birds normally reproduce sexually, but here it is the males that have a pair of similar sex chromosomes (called ZZ) and the females that have dissimilar ones (ZW). I know of no reports of parthenogenesis in wild populations of birds, but the females of some varieties of domestic turkeys are known to reproduce parthenogenetically. They produce only male offspring, because, in the process, somehow the W chromosomes get lost.
Why do these organisms reproduce parthenogenetically? They have relinquished the advantages of sexual reproduction, which provide new genetic combinations every generation and hence the ability to adapt to new and changing conditions. Organisms that are sometimes sexual and sometimes parthenogenetic retain this adaptability. In contrast, organisms that are strictly parthenogenetic produce offspring just like themselves (barring mutation), so their offspring typically require exactly the same living conditions as the parents and there is little or no ability to adapt to changing circumstances. Therefore, in general, parthenogenetic reproduction is thought to have a limited evolutionary future, as each lineage meets unsuitable conditions and dies out. So it is not surprising that strict parthenogenesis is not very common in nature.
• Mary F. Willson is a retired professor of ecology.
