When a lion and a tiger mate, why are their cubs infertile? Science and Technology improve the future

Two different animal species have a spectrum of reproductive possibilities:

1) Completely incompatible

In many cases, it is logistically impossible for the two species to reproduce; try to imagine a giraffe and a dolphin, for example. Even if one could bring the gametes together (reproductive cells, for example eggs and sperm in humans), fertilization would not occur.

Eggs have barriers around them that contain specific proteins, and in order for a sperm to fertilize the egg, the sperm must have matching enzymes that are able to penetrate this barrier. You could think of it as a “lock and key.” Only sperm of the same species have the correct “key” to allow their DNA to transfer to the egg and allow fertilization to occur.

2) Fertilization occurs but the zygote (fertilized egg) is not viable

This category covers a fairly broad spectrum, but encompasses all the options between fertilization that occurs but subsequent cell division does not occur (that is, remaining as a single-cell zygote), until fertilization occurs and an adult “almost normal ”but sterile, as is the case. case with a liger. The factor that determines where the hybrid will fall on this spectrum is the degree of genetic similarity of parents , particularly with regard to how essential functional genes are assigned to chromosomes.

If two species have similar functions assigned in a similar way, to the same set of chromosomes, it is more likely that you will end up with a viable adult. The greater the difference in the number of chromosomes and the distribution of essential genetic functions on those chromosomes, the less likely the zygote is viable.

3) fertile offspring

If two species can create a hybrid that is fertile, then in reality, by biological definition, are not separate species. [1]

Why is fertility so difficult for hybrids?

Fertility is the part of the process that requires the highest degree of genetic similarity due to the differences between mitosis (asexual cell reproduction) and meiosis (sexual cell reproduction). Mitosis it is the process by which a zygote becomes an adult animal by copying its own cells over and over again and increasing in size; Meiosis it is the process by which that adult creates eggs or sperm to contribute to the next generation.

Chromosomes come in pairs; We have two copies of each chromosome, one from each parent. Humans, for example, have 46 chromosomes, consisting of 23 pairs. Both types of cell division, mitosis and meiosis, require that the chromosomes line up with the analog chromosome of their “pair” before cell division, then the pairs are “divided in half”, to ensure that each new cell ends up with a halves complement and a complete chromosomal complement.

This is a representation of just one pair of chromosomes. The red chromosome has two identical arms, and for the sake of argument it comes from the father. The blue chromosome, from the mother, will have a different code than the red chromosome, but again, the code in the two arms is identical. Of the 23 pairs of chromosomes that humans have, each chromosome has known characteristics. So, the genes for particular functions are found in the same place on the chromosomes for different individuals, even though the code real often differs . For example, in humans, the genes that determine ABO blood type are on chromosome 9. Although there are several variants of the coding that can be found there, you know that if you want to find out what someone’s blood type is (from your DNA – yes, it would be easier to do a blood test!), you could find the DNA sequence at that particular location on chromosome 9.

When considering the chromosomes of two species, rather than two individuals of the same species, there will be some differences in how the functions are distributed on the chromosomes. If the parent animals are quite similar, such as a lion and a tiger, then the general range of functions will be quite similar.

If the chromosome numbers and sizes are “similar enough” to be recognized as pairs by the cellular machinery, then, for mitosis, the pairs can line up along the center of the cell that is about to divide, duplicate, and then separate into two identical groups to form two identical daughter cells.

The crucial distinction that makes fertility extremely difficult for hybrids is that during meiosis, the material is exchanged between the two pairs of chromosomes . (Note that this diagram shows what happens to a only pair of chromosomes during these cell divisions, so it’s actually considerably more complex).


As you can see in the diagram, during meiosis, parts of a pair of chromosomes are exchanged for “pieces” of their opposite number. When your chromosomes and genes match precisely, that’s not a problem; in fact, it is advantageous; This is how genetic diversity is created, but if the lion and the tiger, for example, have a gene for an essential function in a very different place on the chromosome, swapping a bit of one chromosome for a piece of its opposite in the same physical the location on the chromosome almost certainly interferes with the function performed by that gene.

Imagine that the gene that, for example, produces an essential protein has exactly 320,000 bits of data in species A. Species B may have a slightly different protein that performs the same function, but its code is 330,000 bits of data in length. . If you join, by meiosis, the 160,000 that are the first half of the code for species A with the 165,000 bits of code that are the second half of the code for species B, you have a 325,000-long code that does not encode any protein, and it is unlikely to work.

Or it could be that species A and B are very similar, but the genes for this essential function are slightly displaced. If you mix up the code in the gap between the two, you can end up with no copies of that essential gene, in which case the offspring are not viable, or you can end up with two copies, which can be viable but problematic.

As every living organism has thousands of essential functions, the chances of interfering with at least one of them are very high unless the two species are incredibly similar genetically.

In fact, the most common scientific definition of a species takes advantage of this: If a hybrid of two organisms can undergo meiosis and produce viable offspring, then by definition those two organisms are members of the same species. [1]

So to go back to general view for a moment.

  • Lion Dad has meiosis on his testicles. Both copies of its chromosomes are “lion chromosomes,” so meiosis is unproblematic and produces lion sperm that encodes all essential functions.
  • Tiger Mum has meiosis on his ovaries. Both of your chromosomes are tiger chromosomes, so meiosis also produces viable tiger ovules (eggs).
  • Mix Lion Dad’s sperm with Tiger Mum’s eggs, and their properties are “similar enough” for the sperm to fertilize the egg and for the resulting embryo to turn into a liger, which basically “works.” Remember that from fertility to adult only requires mitosis , no meiosis .
  • But when the testicles or ovules of that adult liger try to meiosis so that the liger can reproduce, it is trying to mix chunks of lion DNA with chunks of tiger DNA, and most of the resulting gametes (sperm and eggs) from such mixes will not make it workable

However, there are known cases of ligatures that breed with other big cats, so the hybrids are certainly not totally infertile. [2] What is often understood, in common parlance, when a hybrid such as a liger is labeled “infertile” has less to do with fertility than with highlighting that it is not a new species, that is, hybrids are not capable of Breed freely with other hybrids to produce offspring that have predictable characteristics that are similar to their parents. They generally have substantially limited breeding and longevity challenges.

[1] The American Heritage Science Dictionary, definition of species: “A group of organisms that have many characteristics in common and are classified below a genus. Organisms that reproduce sexually and belong to the same species interbreed and produce fertile offspring ”.

[2] Liger fertility

Mix a zebra and a horse, and you get a zorse. Trade the horse for a donkey, and you have a zonkey. When a pony and a zebra love each other very much, you end up with a zoni.

See also: Getting pregnant naturally while “infertile” – Talz Mag – Medium

It’s fun to think of animal hybrids as nature’s Reeses Peanut Butter Cups. You take a little bit of this and mix it with a little bit of that, and voila, you get a more interesting (and delicious?) End product than the creatures you started with. But hybridization has its costs.

Zebroid is the name of any zebra-based hybrid that is almost always infertile and sometimes suffers from dwarfism. Perhaps this is not so surprising. While horses, zebras, and donkeys look alike and belong to the same genus (Equus), each species has a different number of chromosomes. So just because you can crisscross them doesn’t mean you should.

And that Ian Malcolmian sentiment resonates throughout the hybrid world. Take the garters, the chimera created by a lion and a tiger, popularized at least in part by a cameo in the non-classical-cult-Napoleon Dynamite. While zebroids may be small, ligers suffer from unsustainable growth.

Big Cat Rescue has cared for numerous hybrids over the years, including a liger that has since died. Bass says that any cross-species hybridization of big cats is irresponsible breeding at its worst, a fact done only for greed and without any conservation value.

They are bred to live in a cage. They will never be released in the wild. There is no wild for these cats, she says.

Basically if you put a couple of kittens together long enough something will pop out and people will spend cash to see it. And according to Bass, he likely has birth defects, dies young, or ends up in a rescue organization like Big Cat Rescue.

But it’s safe to say that if you’ve heard of the Camel Reproduction Center in Dubai, it’s probably not for any of the reasons mentioned above. No, you know them for bed. The bed is a hybrid made of a camel and a llama. Because at this point, well, why the hell not?

The beds are like llamas that have nibbled on the Eat Me cake. They are bigger than normal llamas, but not as big as camels. They lack humps, but appear to have thicker bone structure than llamas. Their other characteristics also divide the difference between the parents. Camels have short ears, llamas are long, and beds are medium in size. The camels have round legs, the llamas are split, and the beds are again halfway there. The coat is long-haired and llama while the tail is more camelicious, which is also the name of a popular camel milk brand, FYI.

Lulu Skidmore, CRC scientific director, says her team has managed to raise six beds so far, although all have been infertile. So why bother trying to make the bed happen? If you get the best of both breeds, you’d have an animal that’s bigger than a llama, but more manageable in size than a camel, says Skidmore. It would also have a good quality coat like the llama, and because it is bigger, there would be more.