Parthenogenesis

SIGNIFICANCE: Parthenogenesis is unisexual reproduction through the development of unfertilized eggs, which produces offspring that are genetically alike. This clonal reproduction strategy is used by a number of species, especially for rapid reproduction under favorable conditions, and it appears to offer a selective advantage to creatures living in disturbed habitats.

The Nature of Parthenogenesis

The term parthenogenesis is derived from two Greek words that mean “virgin” (parthenos) and “origin” (genesis). It describes a form of reproduction in which females lay diploid eggs (containing two sets of chromosomes) that develop into offspring without fertilization—there is no fusion of a sperm nucleus with the ovum nucleus to produce the new diploid individual. This is a form of clonal reproduction because all of the offspring are genetically identical to the mother and to each other.

The mechanisms of parthenogenesis do not show any single pattern and have evolved independently in different groups of organisms. In some organisms, parthenogenesis alternates with normal sexual reproduction. For example, aphids reproduce by parthenogenesis when there is a rich food source, such as new rose bushes emerging in the early spring; however, as the food source decreases by late in the summer, sexually reproducing females appear. The same pattern has been observed in rotifers, in which a decrease in the quality of the food supply leads to the appearance of females that produce haploid eggs by normal meiosis that require fertilization for development. This switching strategy appears to favor the clonal production of large numbers of genetically identical individuals that are well suited to the environment when environmental conditions are favorable, and the production of a variety of different types by the recombination that occurs during normal meiosis and the mixing of alleles from two individuals in sexual reproduction when environmental conditions are less favorable.

In many social insects, such as bees, wasps, and ants, parthenogenesis is a major factor in sex determination, although it may not be the only factor. In these insects, eggs that develop by parthenogenesis remain haploid and develop into males, while fertilized eggs develop into diploid, sexually reproducing females. Researchers have also identified a high incidence of parthenogenesis in insect species that are considered agricultural pests. One hypothesis holds that this is due to the abundance of food resources in agricultural settings, which makes it easier for a single genotype to flourish.

In algae and some forms of plants, parthenogenesis also allows rapid reproduction when conditions are favorable. In citrus plants seed development by parthenogenesis maintains the favorable characteristics of each plant. For this reason, most commercial citrus plants are propagated by asexual means, such as grafting.

Parthenogenesis has also been induced in organisms that do not show the process in natural populations. In sea urchins, for example, development can be induced by mechanical stimulation of the egg or by changes in the chemistry of the medium. Even some vertebrate eggs have shown signs of early development when artificially stimulated, but haploid vertebrate cells lack all of the information required for normal development, so such “zygotes” cease development very early. Genetic engineering has allowed further experimentation with induced parthenogenesis in vertebrates, including mammals such as mice and rabbits, as well as invertebrates. In 2023 a team of researchers from the University of Cambridge notably reported using gene editing methods to induce parthenogenesis in multiple generations of fruit flies, a species not known for natural parthenogenesis. This was hailed as a breakthrough in identifying the exact genes involved in the heredity of parthenogenesis.

Parthenogenesis in Vertebrates

Parthenogenesis has been observed in certain populations of vertebrates such as fish, frogs, and lizards. In many parthenogenetic populations, all organisms are female, so parthenogenesis provides the only means of reproduction. However, parthenogenetic fish often occur in populations along with sexually reproducing individuals. The parthenogenetic forms produce diploid eggs that develop without fertilization; in rare cases, however, fertilization of a parthenogenetic egg gives rise to a triploid individual that has three sets of chromosomes rather than the normal two sets (two from the diploid egg and one from the sperm). In some groups, penetration of a sperm is necessary to activate development of the zygote, but the sperm nucleus is not incorporated into the zygote.

Evidence indicates that in each of these vertebrate situations, the parthenogenetic populations have resulted from a hybridization between two different species. The parthenogenetic forms always occur in regions where the two parental species overlap in their distribution, often an area that is not the most favorable habitat for either species. The hybrid origin has been confirmed by the demonstration that the animals have two different forms of an enzyme that have been derived from the two different species in the region. Genetic identity has also been confirmed using skin graft studies. In unrelated organisms, skin grafts are quickly rejected because of genetic incompatibilities; clonal animals, on the other hand, readily accept grafts from related donors. Parthenogenetic fish from the same clone accept grafts that confirm their genetic identity, but rejection of grafts by other parthenogenetic forms from different populations shows that they are different clones and must have a different origin. This makes it possible to better understand the structure of the populations and helps in the study of the origins of parthenogenesis within those populations. Comparisons using nuclear and mitochondrial DNA also allow the determination of species origin and the maternal species of the parthenogenetic form since the mitochondria are almost exclusively transmitted through the vertebrate egg. Within the hybrid, a mechanism has originated that allows the egg to develop without fertilization, although, as already noted, penetration by a sperm may be required to activate development in some of the species.

The advantage of parthenogenesis appears to be the production of individuals that are genetically identical. Since the parthenogenetic form may, at least in vertebrates, be a hybrid, it is heterozygous at most of its genetic loci. This provides greater variation that may provide the animal with a greater range of responses to the environment. Maintaining this heterozygous genotype may give the animals an advantage in environments where the parental species are not able to reproduce successfully and may be a major reason for the persistence of this form of reproduction. Many vertebrate parthenogenetic populations are found in disturbed habitats, so their unique genetic composition may allow for adaptation to these unusual conditions.

Mechanisms of Development

The mechanisms of diploid egg development are as diverse as the organisms in which this form of reproduction is found. In normal meiosis, the like chromosomes of each pair separate at the first division and the copies of each chromosome separate at the second division (producing four haploid cells). During the meiotic process in the egg, three small cells (the polar bodies), each with one set of chromosomes, are produced, and one set of chromosomes remains as the egg nucleus. In parthenogenetic organisms, some modification of this process occurs that results in an egg nucleus with two sets of chromosomes—the diploid state. In some forms, the first meiotic division does not occur, so two chromosome sets remain in the egg following the second division. In other forms, one of the polar bodies fuses back into the cell so that there are two sets of chromosomes in the final egg. In another variation, there is a replication of chromosomes after the first division, but no second division takes place in the egg, so the chromosome number is again diploid. In all of these mechanisms, the genetic content of the egg is derived from the mother’s genetic content, and there is no contribution to the genetic content from male material.

The situation may be even more complex, however, because some hybrid individuals may retain the chromosomal identity of one species by a selective loss of the chromosomes of the other species during meiosis. The eggs may carry the chromosomes of one species but the mitochondria of the other species. The haploid eggs must be fertilized, so these individuals are not parthenogenetic, but their presence in the population shows how complex reproductive strategies can be and how important it is to study the entire population in order to understand its dynamics fully: A single population may contain individuals of the two sexual species, true parthenogenetic individuals, and triploid individuals resulting from fertilization of a diploid egg.

Key Terms

• adaptive advantageincreased fertility in offspring as a result of passing on favorable genetic information
• diploidhaving two sets of homologous chromosomes
• fertilizationthe fusion of two cells (egg and sperm) in sexual reproduction
• haploidhaving one set of chromosomes
• meiosisnuclear division that reduces the chromosome number from diploid to haploid in the production of the sperm and the egg
• zygotethe product of fertilization in sexually reproducing organisms

Bibliography

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