Parthenogenesis; non-sexual reproduction that is the development of the ovum into an individual without fertilization by a spermatozoon.
Parthenogenesis is very common in the insect world and in fish, and is routine in animals such as the aphids. Among the reptiles there is strong evidence that parthenogenesis can be a successful strategy for lizards in an environment with low and unpredictable rainfall1. In the Lancet in 1955 it was reported that a woman had a daughter where parthenogenesis could not be disproved. It has been produced in mammals experimentally. There is, however, no certain record of the birth of a parthenogenetic mammal: The most that has been achieved is that parthenogenetic mice and rabbit embryos have developed normally to about halfway through pregnancy but have then died and been aborted.
In humans a recent research study was carried out on ‘The development and systematic study of the parthenogenetic activation and early development of human oocyte’.2 In this study, human oocyte, both freshly retrieved and remaining unfertilized after exposure to spermatozoa, were exposed to alcohol or calcium ionophore and examined for evidence of activation. The outcome of this study was that human oocyte can be activated partheno genetically using calcium ionophore, but at lower rates than seen for mouse oocyte. Human parthenotes can complete division to the 8-cell stage. This data raises the possibility that some early human pregnancy losses may involve oocyte that have been parthenogenetically activated spontaneously.
An incident of partial parthenogenesis in a human was reported in the New Scientist of 7 October, 1995 under the heading, ‘The boy whose blood has no father’.3 In the case of males all cells should have a Y chromosome, but in this particular case study of a three year old boy the white blood cells were found to contain only XX chromosomes. The reporter also mentions that occasionally, chromosomal females carry one X chromosome which includes the maleness gene and that the researchers had at first assumed that their case study was an example of this syndrome. But when they used extremely sensitive DNA technology they were not able to detect any Y chromosome material in the boy’s white blood cells. However, the boy’s skin was discovered to be genetically different from his blood, having both X and Y chromosomes.
A more detailed analysis of the X chromosomes in the boy’s skin and blood revealed that all his X chromosomes were identical and derived entirely from his mother. Similarly, both members of each of the 22 other chromosome pairs in his blood were identical, coming entirely from the mother. The explanation given by the researchers for this phenomenon is that the unfertilised ovum self-activated and began dividing itself into identical cells; one of these cells was then fertilised by a spermatozoon from the father and the resultant mixture of cells began to develop as a normal embryo.
This illustrates that cells created parthenogenetically in mammals are not always disabled. In the case of this boy they were able to create a normal blood system.
Hermaphroditism; a sex anomaly in which gonads for both sexes are present; the external genitalia show traits of both sexes and chromosomes show male female mosaicism (xx/xy).
In a study in the Netherlands in 1990 called ‘Combined Hermaphroditism and Auto-fertilization in a Domestic Rabbit,’ a true hermaphrodite rabbit served several females and sired more than 250 young of both sexes. In the next breeding season, the rabbit which was housed in isolation, became pregnant and delivered seven healthy young of both sexes. It was kept in isolation and when autopsied was again pregnant and demonstrated two functional ovaries and two infertile testes. A chromosome preparation revealed a diploid number of autosomes and two sex chromosomes of uncertain configuration.
A study was carried out on a human hermaphrodite at the Department of Obstetrics and Gynaecology, Chicago, Lying-in Hospital, Illinois.4 The objective of this research was to determine the conceptional events resulting in a 46xx, 46xy true hermaphrodite and to report the first pregnancy in a 46xx, 46xy true hermaphrodite with an ovotestis.
The design of this study involved chromosome studies performed on patient’s lymphocytes and fibroblasts, red cell antigens, human leucocytes antigens and the presence of y-chromosome deoxyribonucleic acid were analysed. Findings were compared with parental and sibling blood group data.
The result of these studies demonstrated that our patient is a chimera; an organism in which there are at least two kinds of tissue differing in their genetic constitution, thus with dual maternal and paternal contributions. In addition, despite the presence of an ovotestis, she conceived and delivered a child.
1 Genetics: 1991 Sept 129(1):211–9
2 Fertility—Sterility—1991 Nov; 56(5):904–12
3 This report concerned the research of David Bonthron et al. and refers to the Oct. 1995 issue of Nature Genetics where their report is to be found.
4 Journal of Fertility and Sterility— JC: evf 57(2): 346–9 1992 Feb.