The myth says that a long, long time ago, a woman gave birth to a child without having known a man. Today we know that, in the human being, such a thing is not possible. And not only because a woman’s sex chromosomes are XX and therefore, without the intervention of a sperm that contributes a Y chromosome, her offspring could only have X chromosomes. We also know that parthenogenesis is not viable in humans .
Parthenogenesis is the name given to the process by which the female of an animal manages to have offspring without fertilization occurring. Animals generally have diploid cells, that is, with paired chromosomes . It is well known that, normally, gametes – ova and spermatozoa – are produced respectively in the sexual organs of females and males, from cells that we call germinative cells , and through a process called meiosis, which unpairs the pairs of chromosomes of diploid cells.
Unlike mitosis, which forms exact copies of the same cell, meiosis also recombines chromosomes with each other before unpairing them, giving rise to haploid cells (with sets of single chromosomes) with greater genetic variability. When an egg and sperm meet, the chromosomes combine, pairing up again and forming a diploid embryo.
In humans, this is the only way to produce embryos . However, evolution has endowed a significant number of species with this curious ability. In fact, there are various forms of parthenogenesis. Today we present five curious examples of virgin mothers in nature.
Generations and generations of clone aphids
Some species of aphids have a life cycle that spans several consecutive generations from virgin mothers . In aphids, the females have XX chromosomes, but the males have X0, that is, they lack a chromosome of the sexual pair. But since the egg predates the chicken, we’ll start at the beginning.
After the cold winter, in the first weeks of spring, the eggs of the previous year begin to hatch. From these eggs are born wingless females, that is, without wings. The female takes just a week to mature and, when ready, reproduces. But not for eggs. Give birth to live young. Between 50 and 100 pups each female. And all of them female. This type of parthenogenesis is called apomictic , and no meiosis occurs in the process. Germ cells simply reproduce by mitosis , giving rise to embryos that are clones of the mother.
As indicated, a female takes barely a week to mature, so all surviving offspring may have a second parthenogenetic generation, and then a third and a fourth… in some species, up to forty. There are cases in which the young are already pregnant when they are born . Depending on the species, in some generations they may have wings. Either way, the trend continues with consecutive generations of females, until a drastic change in the environment occurs. At that time, the next generation has two peculiarities that differentiate it. On the one hand, they all have wings, and on the other hand, a part of the offspring is born with one less chromosome . Instead of all being born XX, females, now part of the population is X0, male. This is when meiosis takes place, animals produce their gametes, and reproduce sexually to lay eggs, which will wait all winter before hatching again.
Bees, virgin mothers of males
The sexual differentiation of bees is also different from ours. In this case, the key is not in the presence or absence of a chromosome, but in the total number of chromosomes. As stated at the beginning, it is common for all animals to have paired chromosomes, that is, their cells are diploid . In bees we find an exception: males are haploid.
This type of parthenogenesis is called arrhenotoquia , and is characterized by the fact that sexual differentiation occurs depending on whether the egg is fertilized or not. In the female, meiosis occurs normally, forming haploid ovules. These eggs can be fertilized with a sperm to form a diploid embryo, or remain unfertilized. When a queen bee lays a fertilized egg, the animal that is born is diploid, and therefore female . But if she lays an unfertilized egg, the chromosomal load will be haploid, and a male will be born.
There is, therefore, the curiosity that the female can reproduce without the need for a male, but if she does, she will only have males as offspring, and to have more females, she will need a male.
Mealybugs, keeping the surprise of the baby’s gender
Mealybugs are a group of small plant parasitic insects, including those farmed by humans for the extraction of cochineal red food dye. In some of the species there is a different parthenogenesis than those we have seen so far: amphithochy . Females are capable of producing diploid eggs by parthenogenesis, but both males and females can hatch from these eggs.
It is a phenomenon similar to what happens in the last parthenogenetic generation of aphids. In addition, in this case, similarly to what happened in those cases, sexual reproduction also occurs. However, the main difference here is that parthenogenesis does not occur by simple mitosis , as it did with other insects. In this case, meiosis occurs normally, including the crossing over of chromosomes, but before the daughter cells can separate, their nuclei fuse again, forming diploid eggs.
Stick insects, the unnecessary male
Female stick insects also undergo meiosis, but in some species, when the egg is formed, it is fertilized with another egg from the same female . In this form of parthenogenesis, which we call thelytokya , the creature that is born is always a female, for a male to be born, fertilization with sperm must occur.
There are species for which sexual reproduction is occasionally necessary in order to increase the genetic variability of the population, but those that live in particularly stable environments do not. To such an extent that we find species of stick insects in which there are only parthenogenetic females and the male is unknown, and probably non-existent.
whiptail lizard; a hybrid species of females
Among the species of the genus Aspidoscelis there is a high evolutionary proximity. When a female and a male of two different species try to reproduce with each other, something similar happens to what happens between a horse and a donkey; a hybrid is born . But unlike the mule, which is usually sterile, in this case the hybrid female that comes out is not. It is parthenogenetic. Thus starting from a first mother, generation after generation, it can give rise to a whole lineage of parthenogenetic females.
If the need arises, even so, they can choose to reproduce with males. But not hybrid males, but males of other species, which may belong to the parents that gave rise to the hybrid in the first place, or to some other. The resulting mixtures can have very strange chromosome sets, even triploid or tetraploid —with three or four sets of chromosomes instead of two. In fact, the descendants of these new hybridizations can themselves have their new genetic configuration, different from the previous ones, and form a new species by parthenogenesis.
As in the stick insect, the type of parthenogenesis is thelytokya , but with a new peculiarity. Because these females have multiplied chromosomes, during meiosis the chromosomes can recombine not with their homologues, but with their sister chromosomes , providing new genetic diversity to the offspring. Although not as much as would be expected from sexual reproduction, at least the females that are born are not simple clones of their mothers.
This particular case not only breaks down the walls of sexuality understood in a traditional way, showing us authentic virginal mothers of nature, but also shakes the foundations on which the definition of “species” is based.
REFERENCES:
Judson, O. P., & Normark, B. B. 1996. Ancient asexual scandals. Trends in Ecology & Evolution, 11(2), 41-46. DOI: 10.1016/0169-5347(96)81040-8
Manríquez-Morán, NL 2007. Clonal diversity in unisexual lizards of the genus Aspidoscelis. Bulletin of the Mexican Herpetological Society.
Maynard Smith, J. 1978. The evolution of sex. Cambridge University Press. Simon, J.-C., Delmotte, F., et al. 2003. Phylogenetic relationships between parthenogens and their sexual relatives: the possible routes to parthenogenesis in animals: ROUTES TO PARTHENOGENESIS IN ANIMALS. Biological Journal of the Linnean Society, 79(1), 151-163. DOI: 10.1046/j.1095-8312.2003.00175.x
