Parthenogenesis

Parthenogenesis, a form of asexual reproduction. It involves formation of the new organism from an unfertilized egg and so represents a modification of the normal sexual process. It may be distinguished from other forms of asexual reproduction, such as fission or budding, where the new organism is formed from a multicellular mass that usually includes several tissue types. Parthenogenesis occurs in both animals and plants, although the term is normally only applied to animals, with alternative terms such as parthenocarpy (the production of fruit without fertilization or seeds) used for plants.

Parthenogenesis is found in many animal groups, including insects, crustacea, rotifers, nematodes (see Roundworm), molluscs, and non-mammalian vertebrates. Some groups include examples of both parthenogenesis and other types of asexual reproduction. For example, the sipunculans (see Peanut Worm) normally reproduce sexually, but there are a few species that reproduce by fission and at least one that reproduces by parthenogenesis. But the insects and the vertebrates, in which parthenogenesis has been most studied, have no other forms of asexual reproduction. Some of these, such as the fruit fly Drosophila mangabeirai, or the Surinam cockroach Pycnoscelus surinamensis, or the lizard Cnemidophorus uniparens are exclusively parthenogenetic. In other cases parthenogenesis is incorporated into a more complex life cycle. Honey bees produce female offspring by normal fertilization, and male offspring (drones) by parthenogenesis. The males are therefore haploid (that is, they have just one set of unpaired chromosomes), and so produce haploid sperm with no chromosome reduction step. Many aphids normally reproduce by parthenogenesis during the summer. In the autumn, special females arise called 'stem-mothers' that produce female offspring capable, in their turn, of producing either male or female progeny. These males and females mate to produce winter-resistant eggs that hatch in the spring to produce another generation of parthenogenetic females.

Parthenogenesis involves formation of a diploid organism—one with two, paired sets of chromosomes—from an egg, which would normally be haploid. The doubling of the chromosome number can be achieved in various ways. In some cases there is no meiosis and a diploid egg, with a single polar body, is formed by mitotic division. In other cases the haploid egg is formed by a normal meiosis, and the diploid state is re-established by fusion of the second polar body with the egg pronucleus. In still others, the first two nuclei of the embryo fuse to give a single diploid nucleus. It is often stated that parthenogenetic offspring are clones of the parent, but this is only strictly true where there is no meiotic crossing-over (interchange of genetic material between homologous chromosomes by breaking and rejoining of segments), since crossing-over can lead to loss of alleles into the polar bodies, thereby creating a genetic difference between parent and offspring. It is also sometimes stated that parthenogenetic offspring are all female. This is true so long as the female and not the male is the homogametic sex (that is, possesses two copies of the sex chromosome, as in the female XX, male XY system). So, for example, parthenogenetic lizards are indeed all female. But in birds the male is the homogametic sex, and so the occasional parthenogenetic embryos produced by turkeys are all male. Parthenogenetic males also arise through chromosome loss in the life cycles of aphids and honey bees mentioned above.

Animals that do not normally show parthenogenetic reproduction may often be provoked to do so artificially. For example, eggs of sea urchins or frogs may be activated by a variety of stimuli such as treatment with chemicals or pricking with a fine needle. Particularly if the second polar body is incorporated to give a diploid chromosome number, these can occasionally develop to adulthood.

Unusually, the mammals show neither natural nor artificial parthenogenesis. The reason is that in mammals, but not in most other animals, some genes are only expressed from either the paternal or the maternal chromosomes (a situation known as genetic imprinting). A parthenogenetic egg has only maternal chromosomes, and so will lack altogether the products of genes normally expressed from paternal chromosomes, and have a double dose of those normally expressed from the maternal chromosomes. This situation is confirmed by nuclear transplantation experiments on mice, which show that eggs can develop to term if provided with one paternal and one maternal pronucleus, but they die in mid-gestation if both pronuclei are paternal or both maternal.

Two artificial forms of modern reproductive technology that have been extensively used for experimentation on mice are related to parthenogenesis but respect the same limits in terms of chromosomal origins. One is the cloning of animals by transplantation of a nucleus into the fertilized egg. In this case the normal imprinting is retained because the donor nucleus contains the normal complement of paternally derived and maternally derived chromosomes. Another is the formation of an embryo from embryonic stem cells, reimplanted into a normal embryo, which is the basis for the technology of targeted mutagenesis (the process of mutation) in the mouse. Here, the embryonic stem cells themselves are derived ultimately from a fertilized egg and so again possess the normal complement of maternally derived and paternally derived chromosomes.