If this qualifies as a thread hijack, then I apologise, but I think it's time somebody explained a few things. This post isn't exactly G-rated by the way, so... yeah.
Firstly, this thread is getting BLATANTLY unscientific. If the aim of this is to formulate a decent experiment to attempt to hybridise Poeciliidae of different genii (an intergeneric hybrid, eg Poecilia/Xiphophorus, which now seems to be the main topic of conversation) then there is no place for flaming, suppressing of information, statements without proof, or rudely demanding proof. If this was in the scientific section, where it probably belongs since an actual experiment is proposed and not just a talk fest, many of the posts on this thread would have been removed for being personal attacks, unsubstatiated or otherwise unscientific. I just wanted to get that out of the way.
Secondly, on chromosome count - Chromosome count is not all important. Animals can and do live with the 'wrong' number of chromosomes. That is what causes, for instance, Down syndrome - people with Down sydrome have an extra chromosome 21. It's the same with Turner sydrome, children with Turner's are born with an extra sex chromosome giving them XXY (XX is female, XY is male, so they are somewhat ambiguous.)
Horses and donkeys have a different number of chromosomes, but they are able to hybridise, the result being the mule. This is very common - it is done often, intentionally, because mules are useful, combining a donkey's sense with a horse's strength. Mules are almost always sterile although there are some accounts of them raising foals to maturity. (I don't believe there are any reports of fertility in the male, but at least 60 reported for the female. Mule females - horse mare x jack donkey - that bear foals with jack donkeys have foals with 100% donkey genes - they pass on only paternal genes. Interestingly, there was one report of a female hinny - stallion x jenny donkey - that had a foal with a jack donkey. The foal was 1/4 horse in genetic makeup - the hinny mare passed both maternal and paternal genetics to her foal.)
Horse and zebra also have a different number of chromosomes, but there is at least one confirmed account of hybridisation, and I have photographs. As far as I know, the hybrid is still too young to determine whether it is fertile. It is female and resulted from the unintentional mating of a female zebra with a stallion at a zoo. It has patches of white and patches of zebra markings, and is between horse and zebra in physical type.
The Beefalo is a result of a cross between Bos taurus (domestic cattle) and Bison bison (the American bison). These hybrids are both successful and fertile, to the point where genes from domestic cattle have contaminated wild bison populations. There is in fact only one wild herd of american bison that is free from both brucellosis (pathogen that causes abortion at several months gestation and may make humans ill) and cow genes. These species are in different genii, but the same family - so the same level of TAXONOMIC relation as Poecilia/Xiphophorus - but this does NOT mean the chances are equal, because this is just how humans have classified them and may not reflect exactly how closely related they are. A good example of what I mean here is demonstrated by the sheep/goat hybrid. Sheep (Ovis spp.) and goats (Capra spp.) have EXACTLY THE SAME level of TAXONOMIC relation as do bison and cattle, which as I just stated are capable of forming fertile hybrid populations. But the result of a natural mating between a sheep and goat (AS OPPOSED to a chimaera, which is a laboratory-created genetic splice) is virtually always miscarried or stillborn. There is only one confirmed live birth of such an animal that I can find, the 'Toast of Botswana' which was a cross between a male sheep and female goat by natural mating. It was male, and sterile.
Birds (the previous cited example for intergeneric or even interfamilial hybridisation) mate by means of the 'cloacal kiss'. Basically birds have a single genital opening, which is virtually identical in males and females, out of which their combined urine/faeces pass. The male transfers his sperm to the opening of the female, and the sperm find their way inside. This means of insemination is obviously far more open to hybridisation than is that of the livebearer.
You should have a look at some photos of the livebearer's gonopodium that have been taken under a microscope. It is an incredibly detailed organ. It is only ever described as a simple tube when we are explaining, to a relative beginner, how to sex livebearers. It has a complex array of hooks at the end to enable the male to hold onto the female for long enough to ensure his sperm packets are delivered. In fact, in the swordtail these hooks often cause the female physical injury when the male withdraws - it is believed they are this large and sharp to a) prevent a less than willing mate shaking him off and b) discourage her from mating within the next few days because she is sore.
If you take a look at some microscope photos of the gonopodium from several different species, you will notice that they are all different. You might not immediately realise how significant these differences are - but when you think about the fish's size scale, the differences become huge. I don't want to be crude OR racist, but think about how difficult it would be for humans from different races to breed together if there was that sort of variation with the human male's *apparatus*. It would have to be achieved by artificial insemination.
Also, while there ARE cases - many of them - of male livebearers getting it on with the wrong species, they are not common and certainly not reliable. In my opinion, it is utterly futile to put a female Xiphophorus in a tank with a male Poecilia and wait for them to hybridise. You'll be waiting an awfully long time. Gonopodium mutation has already been addressed - yes, it is possible that the male might have a mutant gonopodium and manage to mate with a female from a different genus. But this is an even bigger WILD long shot than a normal male and normal female from different genii managing to produce offspring by natural mating (as you keep maintaining, with no evidence, that they will do.) You also have to take into account that such a male would not be an ideal genetic representative of his species because of the mutation, so your findings could easily be called into question that way.
So if you want to experiment properly with this, rather than just throwing fish around and declaring that whatever gets born is a hybrid (which will earn you ridicule) you might want to attempt to force fertilisation by means of artificial insemination. It has been done with livebearers before. Due to the deformation of the gonopodium in the lyretail swordtail and platy (but interestingly not the molly - in the lyre molly the gonopodium is normal) homozygous lyretails can only be reliably created by AI. Basically what you do is prepare a tiny drop of sterile saline or Water For Injection. Then you knock out the male with clove oil and manually stimulate the gonopodium until milt is released. This is more difficult than it sounds because it is so tiny and hard to do it properly. You suck the milt up with a needle and then get a tiny amount of saline/WFI into the needle to 'loosen' the sperm a bit, this increases the chance that it will take. Then you allow the male to recover. Knock out the female, insert the needle VERY CAREFULLY into her genital opening and inject the contents of the syringe. This takes practise. Sometimes one or both of the fish don't wake up and it's quite common that the sperm does not take, ESPECIALLY when you are mucking around with the sperm of a different genus that probably won't take anyway. I suggest you become familiar with the process using fish of the same species so that you can gauge its success by the percentage of females becoming pregnant when compatible sperm is used. You'll also have to ensure, beyond doubt, that the females are virgin when bred. You'll need a big supply of virgin females, in other words. Unless you do this properly, with a pretty big setup and a LOT of notes, whatever you find out is going to be ignored as 'unconfirmable'.
Adrian_HD (Swamp River Aquatics) had a VERY odd livebearer a few years ago. I believe it was a hybrid, but I forget exactly what it was. It looked like a female guppy in shape and it was the size of a female - I believe he was working with endlers where the males are considerably smaller than the females. It had a gonopodium and male markings but was infertile. I forget whether it showed sexual behavior. I don't know what it was, I'll try to find it again.
Hope this helps.
Laura
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, I'm speechless, literally. I mean providing so much proof that this could not happen. giving info that if I googled it i wouldn't get any results. Finally, answers and errors in my experiment. But what I am really trying to say is that there is no way it is 100% impossible. Too many variables. I mean it might be 99.999999% possible. but its not 100%. NO way. 
' and if you want to be taken seriously then you must be serious.
