Polyembryony: what is it, how does it work, and examples?

Polyembryony is a reproductive curiosity that occurs in some animals, including humans.

All living beings (with the exception of humans) exist and persist on Earth with a single specific objective: 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 in other words, the number of genes that a specimen can pass on during its lifetime to the next generation, either in the form of offspring or Blood relatives.

Many living beings have developed atypical reproduction techniques based on this premise. For example, asexual reproduction partially responds to the dilemma of energy investment: if you reproduce by partitioning, you don't 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 will occur.

The key to reproduction in the world of living beings is to find the most effective middle ground, the balance between leaving lots of offspring and ensuring that these offspring are viable, that is, that they will survive in an environment as demanding as it is dynamic. Today we tell you all about polyembryonya biological phenomenon that will never cease to surprise the human being.

The basis 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 determines the sex, the possible variants being XX (female) and XY (male).

When gametes are formed, the genetic information is "split in half".Otherwise, each generation would have more and more chromosomes than the previous one (2n, 4n, 8n, 16n, etc.). Therefore, the precursor cells of the eggs and sperm must divide by meiosis, in order to remain with only 23 chromosomes. Here, phenomena such as overcrossing or chromosomal permutation occur, which means that each new offspring is 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, which regains diploidy (n+n,2n) and is the product of the paternal and maternal genome, in equal parts. From the zygote derives an embryo, which grows in the maternal placenta, becoming known as a fetus after the twelfth week.

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

What is polyembryony?

Polyembryony is a reproductive mechanism in which two or more embryos develop from a single fertilized gamete.. In other words, one egg and one spermatozoon 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 divides asexually within the maternal environment.

Sounds ideal, doesn't it? A female of a polyembryonic species can have 2, 3 or more offspring in the same reproductive event, and therefore with less energy investment. As positive as it sounds, in nature there is a maxim: if a character has not been fixed among related species, there must be something wrong with it, without exception. If polyembryony were extremely successful, eventually living things with this strategy would spread throughout the world and displace those that are not. As you can see, this has not been the case.

One of the keys to polyembryony is that the offspring are different from the parents. the offspring are different from the parents, but equal to each other.. Since they all come from the same zygote, they have the same genetic information (except for mutations) and the same sex. In this reproductive strategy, quantity is more important than quality, since the fact that all the offspring are the same has a series of repercussions for the species, both good and bad.

Polyembryony is very common in plants, but we find it more interesting to focus on the animal kingdom. For example, all armadillos of the genus Dasypus are polyembryonic. Only one fertilized egg can implant in the maternal environment, but because of this ability to divide, 4 offspring of the same sex and genetically identical result from it. Studies have shown that this does not correlate with increased cooperativity or altruism between siblings, so polyembryony is not explained by kin selection. (or kin selection).

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

Polyembryony in humans

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

The stage at which zygote cleavage occurs is extremely important for the viability of twins.. It is exemplified in the following list:

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

In addition to all this, twins present a growth restriction at birth, 10 to 15% in general.. With all these figures, you can understand why polyembryony is not a viable strategy in mammals, or at least in humans.

Summary

As you may have noticed, polyembryony is a reproductive strategy in the form of a double-edged sword. Having more offspring in a single reproductive event is easier than not doing so, but the offspring are genetically equal to each other and, in species that are not typically polyembryonic, there are also a number of associated complications, ranging from growth retardation to fetal death.

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 the product of different fertilization events. Thus, the genetic variability of the offspring remains intact.