Polyembryony: what it is, how it works, and examples

All living beings (with the exception of humans) exist and persist on Earth with a single specific goal: to leave as many offspring as possible.

The conception of the individual in nature does not matter, since what is relevant is biological fitness, or what is the same, the number of genes that a specimen can transmit throughout its life to the next generation, either in the form of offspring or blood relatives.

Many living things have developed atypical reproduction techniques based on this premise. For example, asexual reproduction partially responds to the energy investment dilemma: if you reproduce by partition, you do not spend resources on finding a mate. This mechanism might seem perfect, but the reality is that sexuality is the key to evolution: if all specimens are the same as their parents, no adaptations occur.

The key to reproduction in the world of living things is to find the most effective middle ground, the balance between leave a lot of offspring and that this is viable, that is, that it will survive in an environment as demanding as dynamic. Today we tell you everything about

polyembryony, a biological phenomenon that will never cease to amaze humans.

• Related article: "The 8 phases of meiosis and how the process develops"

The bases of reproduction in the animal kingdom

Reproduction in humans (and in most vertebrates) is fairly straightforward. Our species is diploid (2n), which means that we have two copies of each chromosome in each of our body cells, one inherited from the mother and one from the father. The karyotype, therefore, is as follows: 23 parental chromosomes + 23 maternal chromosomes, 46 total. The last pair of chromosomes is the one that determines sex, the possible variants being XX (female) and XY (male).

When gamete formation occurs, genetic information is "cut in half"Otherwise, each generation would have more and more chromosomes than the previous one (2n, 4n, 8n, 16n, etc.). For this reason, the precursor cells of the ovules and sperm must divide by meiosis, in order to remain with only 23 chromosomes. Here phenomena such as crossover or chromosomal permutation occur, which makes each new offspring not just the sum of its parts.

Once the gametes have formed and both individuals of the opposite sex have reproduced, fertilization occurs. In this event, a zygote is formed that recovers diploidy (n + n, 2n) and is the product of the paternal and maternal genome, in equal parts. An embryo derives from the zygote, which grows in the maternal placenta, and is called a fetus from the twelfth week.

We have described the general reproductive mechanism in mammals, but there are clear exceptions to this rule. Some living beings (such as certain starfish) create copies of themselves by breaking a part of their body (autotomy), while there are living beings that are directly haploid. Without going any further, the males of the ant colonies have half the genetic information that the ant colonies do. queens and workers, since they are the product of a cell that has not been fertilized, or what is the same, they are haploid.

• You may be interested in: "The 3 phases of intrauterine or prenatal development: from zygote to fetus"

What is polyembryony?

Polyembryony is a reproductive mechanism in which two or more embryos develop from a single fertilized gamete. In other words, an egg and a sperm give rise to more than one offspring, unlike what would be expected in the reproductive model mentioned above. The zygote is produced by sexual reproduction, but then it divides asexually within the maternal environment.

Sounds ideal, right? A female of a polyembryonic species can have 2.3 or more children in the same reproductive event, and therefore, with a lower energy investment. As positive as it sounds, in nature there is a maxim: if a character has not been fixed between related species, something bad must have, without exception. If polyembryony were extremely successful, in the end living beings with this strategy would spread throughout the world and displace those who are not. As you can see, this has not been the case.

One of the keys to polyembryony is that children are different from parents, but equal to each other. Since they all come from the same zygote, they present the same genetic information (saving mutations) and the same sex. In this reproductive strategy, quantity prevails over quality, since all the descendants are equal has a series of repercussions for the species, both good and bad.

Polyembryony is very common in plants, but we see more interest to focus on the animal kingdom. For example, all armadillos of the genus Dasypus they are polyembryonic. Only a fertilized ovum can be implanted in the maternal environment, but due to this capacity for division, it produces 4 offspring of the same sex and genetically identical. Studies have shown that this does not correlate with greater cooperativity or altruism among siblings, so polyembryony is not explained by kinship selection (or kin selection).

The only possible explanation for this phenomenon in this species is morphological constrictions. Polyembryonic species are stipulated only out of necessity, not because it is a more viable strategy. A bitch can have a litter of 5 different puppies in a single birth, but the armadillo's uterine implantation site is too small to accommodate 4 zygotes from different fertilizations. Thus, once implanted, only one can divide asexually and give rise to multiple offspring. It is not the ideal scenario, but as they say in animal anatomy, "nature does what it can with what it has."

• You may be interested in: "Chromosomes: what are they, characteristics and how they work"

Polyembryony in humans

We cannot end this space without mentioning that polyembryony exists in humans. The twins are the proof of this, since they both come from the same fertilization event and are genetically identical, again, saving spontaneous mutations that can occur during division or development. It is important not to confuse this biological event with twins, since they are genetically different. Twins arise when two zygotes (products of different fertilizations) are implanted at the same time, so they are not the same.

The phase in which the zygote cleavage occurs is extremely important for the viability of the twins.. We exemplify it in the following list:

• The division occurs before day 5: both twins will have their own pouch (chorion) and placenta. It is the case of ⅓ of the twins and the most ideal scenario. The perinatal abortion and death rate is 2%.
• The division occurs between days 4 and 8: the twins share a placenta, but have two separate chorions. It corresponds to 68% of twin pregnancies.
• The division occurs after day 10: the twins share a bag and placenta. This is the case in 4% of twins, and the survival of both may be compromised. The abortion rate increases up to 10%, in addition to the risk of physiological abnormalities.
• The division occurs after day 13: the babies are Siamese. It is the worst possible scenario, since the survival rate is 5 to 25%.

Besides all this, twins are growth restricted at birth, generally 10-15%. With all these figures, you can understand why polyembryony is not a viable strategy in mammals or, at least, in humans.

Resume

As you may have seen, polyembryony is a reproductive strategy in the form of a double-edged sword. Having more children in a single reproductive event is easier than not, but the offspring are genetically the same among themselves and, in species that are not typically polyembryonic, a series of associated complications also appear, ranging from growth retardation to death of the fetuses.

For all these reasons, polyembryony is a strategy that is very limited in the animal kingdom. Whenever possible, animals resort to having multiple litters, but as a result of different fertilization events. Thus, the genetic variability of the offspring remains intact.