Exactly what causes inter-species breeding to not work?

[wasteofo2]

I know sometimes animals in the same genus can breed, and often, if not always, the offspring is infertile. But what is it exactly that draws the line between which species can and can't breed together?

[Another God]

I'm no professional on this topic, but as an outright educated guess, it has an obvious restriction from the number and structure of the chromosomes: Upon cell division the chromosomes need to pair up. If the chromosomes are different (no pairing), then cell division won't occur. And chromosomes are nearly always different. I think chimpanzees have an extra pair of chromosomes over humans for instance??? And we even share that 98% or whatever of our DNA... It seems that chromosome shuffling is quite common in the evolutionary history.

[wasteofo2]

That makes sense, but there are millions of animals in existance, as far as I know, amounts of chromosomes only range up to a few hundred. There are undoubtedly animals that have nothing in common, perhaps a coyote and a tucan, which will just happen to have the same amounts of chromosomes.

[rockind78]

My answer to this would be that every organism I can think of relies on a protein-protein, and protein-DNA interactions as a fundamental basis for the sustainance (sp??) of life. Assuming for a moment that you could get a viable fetus (I have never had developmental or embryology mind you), it would seem to me that you would have all sorts of incompatibilities regarding the aforementioned interactions. Although bye and large, genes and proteins have large areas of homology from organism to organism (we just proved this with furin in my lab as a project, for example), they are nonetheless, different. Mind you these interactions are extremely specific to that organism. I think tinkering with this would obviously be disasterous.

Sorry I don't have anything more concrete for you. [b(]

[Another God]

Its not just the number of chromosomes, its their pairing up on the mitotic plate.... Just before cell division pairs align so that they can segregate evenly: One of each pair goes into each cell. If the chromosomes aren't aligned, then the division won't occur. So if you get one half of your chromosomes from one species and one half from another, then their are no pairs at all: Even if they have the same number of chromosomes....

Assuming same number of chromosomes, it is also unlikely that the chromosomes will be of the same size. If they are of the same size, then the homology between the pairs need to be close enough so that they 'recognise each other'...they recognise each other by several mechanisms, one being based entirely on a protein holding them together at the centromere, but another mechanism (and the one more relevent here) is that points along the chromosome which are identical can cross over...they...well, let me find a picture of this online. That will be the easiest way to describe it... ok Here is crossover explanation with diagram (http://www.web-books.com/MoBio/Free/Ch8D1.htm). Look at that and that is what I am talking about: For crossover to occur, there needs to be homology between the pairs, without that homology, it won't happen. I am not certain though, how important this is for aligning pairs during meiosis.

I'll tell you what, I'll look into the mechanisms of how the cells identify pairs, and that should explain a lot.

[Another God]

Originally posted by rockind78
My answer to this would be that every organism I can think of relies on a protein-protein, and protein-DNA interactions as a fundamental basis for the sustenance of life.

I'm not certain I know what you mean, but if you are talking about there being certain promotors required to express certain DNA segments, and so forth in a complex web of cause and effect, then I am not sure whether that will be a problem or not. Eukaryotes tend to at least follow similar rules for a start, and then organisms even slightly related will almost certainly use the same gene-protein interaction systems. Since these systems are the basis of life, it is almost impossible for evolution to really change anything. If an elementary system gets changed, than everything that relies on that system won't work: => Death of organism.

So in general, these molecular bases for organisms will stay the same between species.

Assuming for a moment that you could get a viable fetus (I have never had developmental or embryology mind you), it would seem to me that you would have all sorts of incompatibilities regarding the aforementioned interactions. Although bye and large, genes and proteins have large areas of homology from organism to organism (we just proved this with furin for example), they are nonetheless, different. Mind you these interactions are extremely specific to that organism. I think tinkering with this would obviously be disasterous.
[b(] But yes, as you say there are still differences... OK. I think I understand your point now, and it is something I alluded to in my first post: There are complicated inter-relations between, for instance, surface receptors and its effect on genes. If one species expresses a particular gene in presence of ethanol, it doesn't mean another species wants the same response....so there will be intracellular confusion, leading almost certainly to death.

[Another God]

This site (http://www4.ncsu.edu/unity/users/b/bnchorle/www/) has a pretty neat explanation of Meiosis, but it doesn't explain how the homologous chromosomes recognize each other.

still looking....

Well, Alberts et al (Molecular Cell Biology text book) says The set of chromosomes of a typical sexually-reproducing organism consists of austosomes, which are common to all members of the species, and the sex chromosomes, which are differently allocated according to the sex of the individual. A diploid nucleus contains two closely similar versions of each chromosome. For each of the autosomal chromosome pairs, one member was initially inherited from the male parent and the other was initially inherited from the female parent. The two versions, which are very similar but not identical in DNA sequence, are called homologs, and in most cells they maintain a completely seperate existence as independent chromosomes.
....


Nope, sorry, can't find anything yet. I actually have a sneaking suspicion that no one really knows how cells match up the homologous chromosomes, all we know is that they do. But I am still willing to bet that however it is done, that it the primary cause of why different species can't interbredd.

[nautica]

In the plant kingdom, a process called polyploidism often takes place, this is an entire duplication of the chromosomes making the zygote a 2n rather than a 1n. This spp can only pair up with another 2n making the offspring a 4n. This 4n is a new spp which can not breed with the parents. So, in this case for certain it is the pairing of chromosomes.

Nautica

[russ_watters]

Even if you do get a zygote to form, there are a host of biological differences that can cause problems: blood type/ph, proteins/enzymes, nutritional requirements, gestation period/requirements.

Just imagine what would happen if you tried to grow a human embyro in a chicken egg.

[nautica]

Yes, I can.

[Nereid]

Originally posted by nautica
In the plant kingdom, a process called polyploidism often takes place, this is an entire duplication of the chromosomes making the zygote a 2n rather than a 1n. This spp can only pair up with another 2n making the offspring a 4n. This 4n is a new spp which can not breed with the parents. So, in this case for certain it is the pairing of chromosomes. I've been told that cross-species hybridization is much more common in plants than in animals (partly why 'species' isn't so easy to define for plants?); is this one of the reasons why?

[Another God]

Originally posted by russ_watters
Even if you do get a zygote to form, there are a host of biological differences that can cause problems: blood type/ph, proteins/enzymes, nutritional requirements, gestation period/requirements. I was thinking about this before and couldn't make up my mind. Are you sure that these problems would arise, because what struck me was that blood type, nutritional requirements etc are all encoded in the DNA: So assuming the chromosomes could somehow pair up consistently, then DNA combination would express everything according to the two scripts it has. This in itself would probably cause problems, but the least of its issues would be its blood type, or how it gets nutrition etc.

But the womb of the mother is most certainly also a consideration. Actually...I wonder if they have tried playing with that aspect much in developmental biology. Everytime they try to clone an endangered animal, they clone it into the womb of a closely related species so that its developmental environment is similar enough. What does happen if you put...a chicken foetus in a dog????

[Moonbear]

The first barrier would occur long before chromosomes were trying to pair up. The female immune system would attack the sperm of another species before it got very far. If sperm got to the egg, then having the wrong proteins would likely prevent the sperm from recognizing and penetrating the egg. Even sperm from the same species can have trouble at this step. You can get weird occurrences of an ovum starting to divide and implant in the absence of fertilization or in the case of polyploidy, but it never develops into a viable fetus (and in the former case, can lead to tumor formation), so whatever signal terminates the pregancy in that case is likely similar to what would happen if a ovum fertilized by another species got to that stage at all. Since pregnancy is maintained by signals from the fetus, if those signals were screwed up by mixed up genetics, then the mother's body wouldn't be able to "read" them to sustain the pregnancy.