No tengo tiempo de traducirlo. El asunto tratado es, una vez más, la enorme distancia entre la aparente claridad de los conceptos biológicos, y la enorme diversidad que podemos encontrarnos en la realidad. Si bien gran parte de estas variaciones se deben a que los mecanismos de formación de los individuos no han funcionado como se esperaba.
The concept of chimerism seems so impressive… having cells from two or more different organisms in the same body, what could be stranger? However this concept also includes another condition for animals, if they are born as chimeras, they have to come form an early fusion of two zygotes. It is the tetragametic chimerism. Accordingly, these individuals are, in general, very similar to the rest of their species, they don’t use to call many attention.
There are other possibilities in multigestation pregnancies (and even in embryo development inside an bird’s or insect’s egg) in which the development of one of the siblings goes much further than in chimerism. Luckily parasitary twins and cojoined twins are very rare.
There are a few exceptions to this rule, that is animals that give away their condition of chimeras only because of their appearance.
The most striking are some cases of gynandromorphism in birds and other animals -gynandromoprhs are organism with both male and female characteristics- . Stay with me, the traditional explanation for this phenomenom was mosaicism
, but… in 2010 a study found that three bilateral chickens were genuine male-female chimeras[1].
A very particular way of chimerism. At first, it was supposed that these individuals came from the fusion of two developing embryos, separately fertilised; but now scientists talk about a more complex scenario. When female birds produce an egg cell, this cell is meant to discard half its chromosomes in a bag called the 'polar body'. If the egg keeps this body of DNA however, it can form its own nucleus, and when fertilised (remembering that female birds are heterochromosomic for sex) with two different sperm cells, there could be two different sex embryos that would fuse in one[2]. Rather strange, I know.
The cases in these pics belong to the so called bilateral gynandromorphism
, a particular case in which the animal seems to be divided in two halves by their middle line. A very common question is whether these birds can breed, and since birds only have a functional ovarium, the left one, if the female side of the gynandromorh is on the left, it may be possible.
There are another types of gynandromorphs not so striking, for example some birds may pose male and female characters only in some feathers. There are also bilateral gynandromorphs in insects, aracnids, and some other animals, but not among mammals, because have a complex regulation of sex character that would not allow to have both a female and male side fused together[3].
However, this image of animals so strikingly divided in two by their midline, which are often referred to as “half siders” has lead to a very common mistake. Very often people take mammals with two different colored sides for chimeras. So…
This is the most striking example of what is not a chimera (or might not be):
Well, this beautiful and lovely cat is not chimera. Havig each side of the head with different colors can occur for simple mosaicism, and it is important to remind that she is a regular female. To understand it better, and not being rather exhaustive here, you can watch this video[4], or a2a me (for some weird reason I do like cats genetics).
I would like to add that these animals, halfsiders, are a good reference to study the embrionary development of the very midline[5][6], because there are many birt defects linked to this midline, such as cleft palates, congenital heart diseases.
Turning back to chimeras, there is another condition that can also affect mammals, and of course this include humans, which can be discovered since birth. Chimeric disorders of sexual development, previously known as “true hermaphroditism”[7] in many countries for a long time. Animals and also people born with this condition have gonadal tissue of both types, they can have one ovary and one testicle but it is much more common to have an ovotestis[5].
These people are sometimes fertile (only with one sex). Autofecundation or self fertilization is impossible. Since 2010 there have been at least 11 reported cases of fertile human "true hermaphrodites" in the scientific literature.[8]
However it is far more common that the condition of chimerism is discovered after some strange event in the life of current people and make it to the headlines:
- "Man fathers his dead twin brother's kid": Human Chimera: Paternity Test Reveals Child Fathered By Long Lost 'Vanished Twin' Absorbed In The Womb
- After a paternity test failed, a man was discarded as his IFV baby father, but after subsequent tests it was discovered that this man’s spem DNA didn’t match his own blood DNA. He was diagnosed with chimerism, and in a way his absorbed twin was the genetical father of the baby.
- A very similar case of a woman who was discarded as the genetical mother of her children despite being conceived naturally. In the end, she also is a chimera, and her egg cells DNA do not match her own skin DNA. And once again the absorbed twin is the genetical mother of the children (in a way). [9]
There are other types of chimerism. Microchimerism
occurs is the presence of a small number of cells that originate from another individual and are therefore genetically distinct from the cells of the host individual.
Also, chimerism can be attained in lab. By genetic engineering such is the case of the goats that expresse silk proteine in their milk[10]:
And in some other occassions with more radical and bizarre methods:[11]
And yes, we have reached the end. These are only some intriguing examples of chimerism. But, as I said in the beginning, tetragametic chimerism is not so strinking as long as the fusion occurs very early in the development of the embryos.
Parasitic twin occurs when the absorbed twin has defined some organs and tissues and it is far more… scaring:
Footnotes
[2] Biologists Find Spectacular Bird That's Both Male And Female, Split Down The Middle[5] Embryonic left–right separation mechanism allows confinement of mutation‐induced phenotypes to one lateral body half of bilaterians[6] Familial Recurrence of Midline Birth Defects—A Nationwide Danish Cohort Study[10] Scientists breed goats that produce spider silk [11] Vacanti mouse - Wikipedia
excelente
ResponderEliminarJeje, me voy a hacer una tarjeta Patreon y así en 10 o 120 años me dará para pagar un café :D
EliminarGrcias!
ya te invitaré no a un café, sino a comer cuando estés en Barcelona por dejarme disfrutar tus escritos
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