“Virgin Birth,” meaning giving birth to offspring without mating with a male, has been a mythical story and historical account associated with various ancient deities like Mars from Roman mythology, Horus from ancient Egyptian mythology, Kuanyin from ancient Chinese mythology, and Jesus in some Abrahamic religion. All of them were believed to have been born from virgin mothers. While mythological or religious narratives aside, the concept of giving birth as a virgin mother has its roots in the natural world. In scientific terms, it is called “parthenogenesis” or “অপুংজনি” in Bengali.

Recently, human interest in parthenogenesis has increased. This is due to a published research paper by a group of American scientists [2]. In this article, let’s dive into the scientific evidence of parthenogenesis in nature, as well as compare its pros and cons.

Figure: Recently, a crocodile gave birth without the help of a male partner—image Source BBC.

The case of virgin birth in crocodile

The first evidence of parthenogenesis in crocodiles was discovered in an 18-year-old female crocodile. It was a Crocodylus acutus, that had been kept without any male presence for 15 years at a Costa Rican sanctuary. In 2018, the female crocodiles laid a total of 14 eggs, but none of them hatched naturally as expected. Researchers then observed that six eggs seemed in good condition, with only one showing signs of being a nonviable female embryo. This was confirmed through genomic sequencing, which showed a 99.9% genetic match between the embryo and its mother, further supporting the lack of a paternal contribution and the occurrence of asexual reproduction.

However, it’s important to note that the baby crocodile was not a clone. Like other documented instances of parthenogenesis in reptiles, it was a unique event, making it a fascinating occurrence in the world of parthenogenesis. Essentially, the embryo was created from two copies of the mother’s DNA, similar to regular sexual reproduction. There was no genetic contribution from any male as external temperatures entirely determine the female crocodiles’ gender, and the eggs were incubated at 29.5°C, the correct temperature for forming female embryos. It cannot be called a clone or a daughter with maternal genes; it is the female offspring without any male contribution.

Figure: Parthenogenesis is not very uncommon in living-being.

Parthenogenesis in the animal kingdom

In the animal kingdom, virgin birth or offspring born without mating with a male is not new. It has been documented in various species like the California condor [3], stick insects [4], snakes [5], sharks, and different bird species.

In the living world, all species primarily reproduce in two ways. They either reproduce sexually, where genetic material from both parents combines to form the offspring’s body, or asexually, where only the mother’s genetic material contributes to the offspring’s body. Our single-celled ancient ancestors also reproduced asexually without needing to create clones of themselves through technical means before the existence of sexual reproduction. Many animals and plants [6] still reproduce asexually today. It’s still the primary form of reproduction for single-celled organisms such as archaea and bacteria.

However, a drawback of asexual reproduction is that it creates genetically identical individuals, leading to a lack of genetic diversity among members of the same species. This means that if one member of that species cannot adapt to a changing environment, it can harm all members, potentially leading them to extinction.

In species like humans, sexual reproduction requires the fusion of sperm from the father and egg from the mother. In other words, they each contribute 50% of the genetic material, making it a 50-50 combination. In the context of evolution, sexually reproducing species are considered “advanced” because their descendants have a diverse genetic makeup, where the genetic material from both parents creates a unique blend within their bodies.

The ability to adapt to changing environments plays a crucial role in maintaining genetic diversity [7]. On the other hand, genetic mutations [8] contribute to variations often seen in asexual reproduction.

Figure: Difference between sexual vs. asexual reproduction.

Why parthenogenesis is so rare?

The virgin birth is a form of asexual reproduction, as it does not require genetic information from sperm. However, unlike other forms of asexual reproduction, it requires an unfertilized egg. You can see unfertilized eggs in eggs you eat, for example, unfertilized chicken eggs. If unfertilized eggs are not eaten, they eventually undergo natural degradation and decomposition. However, in the case of virgin birth, the unfertilized egg develops into a complete embryo.

In a fertilized embryo, it must be genetically distinct from the mother. It depends on how the egg develops primarily. A parthenogenetic embryo can also be a complete or half-clone of the mother. Half-clones occur when the cells of the embryo divide before increasing the number. Complete clones occur when all the embryo cells divide before increasing the number.

In comparison to complete clones, half-clones have less genetic diversity. Because they only get half of their mother’s genetic diversity. That is, they lack the genetic diversity of sexual reproduction.

Some species can also perform both sexual and asexual reproduction. They are called facultative parthenogenesis. They primarily depend on sexual reproduction but can use asexual reproduction when needed.

Virgin birth can occur in various situations and for various reasons. For example, if there is no male around, such as in some populations of bonnethead sharks [7]. However, even when enough males are present, parthenogenesis has been observed in reproduction. For instance, a female zebra shark in Chicago’s aquarium [8] had offspring without any contribution from a male’s genetic material. This discovery surprised researchers as there were no apparent reasons why the female zebra shark would not prefer the males around her.

Figure: In reptile, gender is often determined by environmental temperature. Source: Developmental Biology.

In environments where conditions are unfavorable, asexual reproduction through parthenogenesis is more convenient than sexual reproduction, as it saves time and energy for females in search of male mates. For example, parthenogenesis has been discovered in various gecko, snake, and stick insect species [9] that inhabit dry and harsh climatic regions.

The advantage of adapting to changing environments is that it allows female animals to engage in asexual reproduction as well. For instance, marbled crayfish, originally native to the United States, has successfully established itself in many European waterways through parthenogenesis [10]. This is attributed to the role of parthenogenesis in their success.

Parthenogenesis was once considered rare, but scientists have gradually realized it is prevalent among reptiles. Researchers have observed successful cases of parthenogenesis in various snake, lizard, and turtle species [12].

So far, more than 80 species of vertebrates, including stick insects, snakes, sharks, and rays [12], have been reported to undergo virgin birth, although most of these instances occur in captivity.

Since virgin birth has been observed in both reptiles and birds, one might wonder if ancient ancestors, the dinosaurs, also possessed this asexual reproductive capability. Crocodiles and birds belong to a group called archosaurs [11]. Looking back along this branch, we find that dinosaurs and flying archosaurs were also part of it.

So, do males become unnecessary? – Not exactly. The biggest drawback of virgin birth is that it reduces the opportunity for genetic diversity to occur, as I mentioned before. In virgin birth, the chance of genetic diversity through mutation decreases. A mutation is significant for the existence of ticks. Besides, virgin birth is quite rare and happens only under adverse conditions. Among species (in a minimal number), which produce offspring from parthenogenetic eggs, the rate is less than 3% [13].

Figure: It’s possible to change mammals to give virgin birth genetically. Source

Is parthenogenesis possible in mammals?

Is it possible among humans? – No. However, I mentioned earlier that the possibility of events like those in crocodiles exists. So, for a complete embryo formation without sperm in humans, the chromosome in the egg cell has to replicate so that by the end, a ‘normal’ number of chromosomes is formed, similar to those in sexual reproduction. Then, a process of activation or initiation of the egg cell, usually performed by the sperm, has to be triggered. Only then can full-fledged embryo development occur. But the chances of this happening are so minuscule that it is practically impossible [14].

However, virgin birth is impossible in humans or other mammalian species. In tests with rodents and other mammals, it has been observed that for any development to occur, an egg cell must be fertilized with sperm. Simply using chemical or electrical signals does not trigger the egg cell in mammals to undergo natural division. It has been found that a specific type of protein derived from sperm is required in egg cells to set the wave of calcium ions in motion in the dino’s eggs [15]. This leads to various changes required for DNA and chromosome replication and division in the two cells.

But it’s not enough to supply the protein only through sperm. Each chromosome of the egg must have a copy of both sets. Typically, one set is provided by the mother (in the egg’s nucleus), and the other set is provided by the father (in the sperm’s nucleus). Although there has been discussion about the loss of the Y-chromosome earlier, you can refer to my previous writing on that topic on bigganblog.org in Bangla (Link: Degradation of Y-Chromosome).

So, why is virgin birth not possible in mammals, at least not in nature? It has been observed that more than 30 genes are only active when they receive the signal from the father’s sperm. Additionally, more than 30 genes are active only when they come from the mother. Hence, genomic imprinting restricts virgin birth in mammals, including humans [16][17].

However, it is possible to genetically edit mice so that they can give birth without male or sperm, but that’s a topic for other time.

Sources:

Virgin Birth in the Animal Kingdom: A Comparative Exploration of Asexual Reproduction

The case of virgin birth in crocodile

Parthenogenesis in the animal kingdom

Why parthenogenesis is so rare?

Is parthenogenesis possible in mammals?