When does interbreeding fail? Can a human’s sperm enter and fertilize a goat’s egg? Will something start to grow, and die soon after, or will it not even get that far?

  1. Interbreeding fails when organisms are sufficiently different, the more closely related they are, the better chance they have. Typically the organisms have to be within 2 chromosomes of each other in terms of total chromosome count. Humans have 46 chromosomes, goats have 60. Also, if the animals are of different genera it is quite unlikely that they will interbreed successfully. Goats are most closely related to sheep, and sheep only have ~~56~~54 chromosomes, and are a different genus. Hybrids of these are stillborn usually.

    The degree to which a potential offspring can grow in utero is quite different depending on how related the animals are. Some will be stillborn, others won’t be able to conceive at all (such as humans and goats). There is no real way to broadly generalize the level of potential development.

    Edit: lost a word.

  2. It’s actually variable depending on the species involved. There are basically two categories, prezygotic isolation mechanisms and postzygotic isolation mechanisms. Prezygotic means before what you’re talking about, the male and female gametes fusing together. Postzygotic means after that happens.

    During this explanation, I will be defining “species” by the Biological Species Concept, which says that two individuals are of different species if they cannot produce viable offspring. Viable offspring means a healthy offspring that can reproduce itself. For example, according to this concept, domesticated dogs, dingoes and all wolves are currently a subspecies of Canus lupus, and are not separate species.

    Prezygotic Isolation Mechanisms

    • Geographical: This is the mechanism that contributes to allopatric speciation (when speciation occurs after two or more groups of a species have been geographically separated for such a long time that they become separate species according to the Biological Species Concept, which I will define below). It basically means that the organisms live in different places and cannot reproduce. Maybe it’s because those places are too far away, or maybe one lives in antarctica and one lives in the Sahara desert, and either would die in the other’s climate. They will never interact, much less reproduce.

    • Temporal: Species have different mating seasons or times of day (think of plants, or insects) that don’t overlap.

    • Behavior: If your species has evolved a highly specific mating ritual, you will likely not mate with an individual that has a different ritual. This also applies to unattractive physiological qualities in potential mates.

    If our two species are not affected by these mechanisms, for one reason or another, at this point they may attempt mating.

    • Mechanical Isolation: The sex organs are not compatible.

    • Gametic Isolation: The gametes interact (in humans, the sperm gets to the egg), but for some reason are unable to unite. Reasons include: the gametes do not attract one another, the gametes cannot physically fuse, or the male gamete is inviable inside of the female reproductive tract.

    So let’s say that the gametes have fused and we now have a zygote.

    Postzygotic Isolation Mechanisms

    • Hybrid Inviability: The fertilized egg fails to develop past the early stages.

    • Hybrid Sterility: The hybrid cannot reproduce. This is often due to an unusual number of chromosomes that produce non viable gametes. Think of mules, ligers, etc.

    • Hybrid Breakdown: This hybrid is not sterile, but as generations continue to reproduce their offspring are increasingly inviable.

    In your example, humans and goats, there are several mechanisms at play. Behavioral isolation, for example, prevents goats and humans from wanting to mate generally. The stopping point would probably be gametic isolation, for several reasons (I suspect that there would not even be gametic attraction). If not, it would certainly be Hybrid inviability. I’ve done some research based on your question, but fortunately no one is studying when exactly gametes of goats and humans become inviable . . . I don’t think it would even be legal in the US. We know however, that we are different species from goats, therefore one of these mechanisms interrupts along the way.

  3. The average divergence time to complete hybrid inviability between mammal species is 2-4 million years with an upper bound of around 10 million years (Prager and Wilson, 1974; Fitzpatrick 2004). While hybrid sterility, especially hybrid male sterility arises much quicker, OP’s question seems to be concerned with the rates of evolution of inviability instead. Intriguingly, the average divergence time to complete hybrid inviability in non-mammal vertebrates (birds and amphibians specifically, though I would expect similar results in fish and reptiles) is closer to 20 million years with an upper bound of around 50 million years (Prager and Wilson, 1975; Fitzpatrick, 2004). Understanding the causes of these differences is an important question in the field of speciation even today, however we have accumulated some data on the subject.

    First off, many responses so far have discussed the role of karyotypic evolution in the cause of hybrid inviability. In this I think many are mistaken. While it is certainly true that differences in the number of chromosomes can cause problems for hybrids, those problems are almost universally in the sterility category. This is because meiosis requires chromosome pairing and if there are not pairs present, meiosis will fail resulting in sterility. Examples of this include mules (Wodsedalek, 1916), and many others. While meiosis is indeed extremely sensitive to chromosome complement, development does not seem to be adversely affected. This is clear in the heterosis present in mules and others. It also makes biological sense as even though their locations may differ, a full double complement of genes should be present in all F1 hybrids (with the notable exception of sex-linked genes). Furthermore, I know of no study that has found a difference in autosome number to cause hybrid inviability (though of course there are many studies showing that sterility is strongly affected).

    So it remains to be seen why is there such a difference in the rates of hybrid inviability between birds/amphibians and mammals.

    In the mid 1970s A.C. Wilson and colleagues proposed that differences in the rate of the evolution of inviability may be due to a higher rate of regulatory evolution in mammals than other taxa (Wilson et. al. 1974). This is a pretty good hypothesis and seems to be supported by other research, however it really only pushes the question back one step: why is there a higher rate of regulatory evolution in mammals than other vertebrates?

    One likely answer to this question is: mammals have something that other vertebrates lack – a placenta.

    To understand why the presence of a placenta is likely to cause rapid regulatory evolution, we need to talk briefly about mammalian reproduction and how it differs from reproduction in other taxa. Mammal offspring are obligate parasites of their mothers: all of the nutrients they need for survival and development are drawn directly from the mothers tissue. This results in a really strong conflict between the mother and her offspring over nutrient allocation. Mothers of any taxa have an evolutionarily selected level of care (resources) that they prefer to give, while offspring would often prefer more resources (Trivers, 1974). In things that lay eggs, the offspring have no say in the amount of resources they get from their mothers – females allocate resources to the eggs before they are fertilized and then lay the egg shortly after fertilization. In mammals however, the offspring can have a say because of the placenta. In mammals the offspring can send signals to the mother effectively demanding more resources. This conflict is thought to have led to an arms race between mothers and offspring wherein offspring develop strategies to take more resources while mothers have developed counterstrategies to defend their finite resource pool (Zeh and Zeh, 2000; Crespi and Semeniuk 2004). Furthermore, mammalian pregnancies are highly dependent on down-regulating the mother’s immune system (Siiteri, 1982). If proper down-regulation does not occur, the mother will mount an immune response and reject the offspring (similar to rejection of an organ in a failed transplant). Both resource garnering strategies and counterstrategies and immune system deregulation are often in the form of regulatory evolution and likely explain why mammals have a higher rate of regulatory evolution than other taxa.

    Now, parent-offspring conflict played out in the placenta is a solid hypothesis for increased rates of hybrid inviability in mammals, and I have given a number of a priori reasons to expect that it would play a major role, but I have not given any evidence yet. So here we go: Elliot and Crespi (2006) showed that the invasiveness of a placenta is highly correlated within mammals to the rate of hybrid inviability. (Invasiveness is best thought of as one of the offspring’s strategies for taking more resources). This is strong evidence that inviability is very closely tied to placenta function in mammals and easily explains why mammals may evolve complete hybrid inviability faster than other vertebrate taxa.

    edit: spelling and a point of clarification

    Sources (sorry most of these are behind a paywall):

    Crespi, B. J., and C. Semeniuk. 2004. Parent‐offspring conflict in the evolution of vertebrate reproductive mode. Am. Nat. 163:635–653.

    Elliot, M. G., and B. J. Crespi. 2006. Placental invasiveness mediates the evolution of hybrid inviability in mammals. Am. Nat. 168:114–120.

    Fitzpatrick, B. M. 2004. Rates of evolution of hybrid inviability in birds and mammals. Evolution 58:1865.

    Prager, E. M., and A. C. Wilson. 1975. Slow evolutionary loss of the potential for interspecific hybridization in birds: a manifestation of slow regulatory evolution. Proceedings of the National Academy of Sciences 72:200–204.

    Siiteri, P. K., and D. P. Stites. 1982. Immunologic and endocrine interrelationships in pregnancy. Biol. Reprod. 26:1-14.

    Trivers, R. L. 1974. Parent-Offspring Conflict. Integrative and Comparative Biology 14:249–264.

    Wilson, A. C., L. R. Maxson, and V. M. Sarich. 1974. Two types of molecular evolution. Evidence from studies of interspecific hybridization. Proceedings of the National Academy of Sciences 71:2843–2847.

    WODSEDALEK. 16AD. CAUSES OF STERILITY IN THE MULE. Biological Bulletin 30:1–57.

    Zeh, D. W., and J. A. Zeh. 2000. Reproductive mode and speciation: the viviparity-driven conflict hypothesis. Bioessays 22:938–946.

  4. Related question, and somewhat anecdotal.
    In my High School biology class I was taught that in most cases the sperm and ovum of different species will simply not fuse together.
    The species in question had to be very closely related for the surface proteins on the sperms to be able to react with the surface proteins on the ovum. I was given the lock-key analogy for it. So, a pigs sperm cell will never be able to fuse with a human ova on its own, while if the neanderthals existed today, there would be a good chance that their sperm would fuse with a human ova.

    Is what I learned in school, 20 years ago, correct? Or is it a simple generalization, or has been invalidated by our current understanding of reproduction?

  5. Chromosome count, and the egg itself, and the sperm stop this.

    Chromosome counts must match, we have 23 split chromosomes in our sperm, where as apes for example have 24. Fusion may occur, but nothing will come of it.

    Then theres the sperm and the egg. The sperm will give out enzymes to break down the outer coating of the egg, (multiple sperm are needed to reach the concentration to do this). The sperm also have a specific protein on them, which the egg will recognize, and once recognized, will allow the sperm to ‘endocytose’, (be engulfed) by the sperm, and fertilization has occured.

    These variables differ from species to species, allowing only (usually) one species to fertilize its own.

  6. Wow. I just remembered reading that us humans share a lot of similarities with Dolphins.

    And after a quick google search i`ve found that dolphins have 44 chromosomes and humans have 46.

    That being said, would it be more likely to have a successful mating between a human and a dolphin than a human and monkey?

    And if not, why?

  7. I actually did a report on this years ago in eighth grade. In 2003 human cells were fused successfully with a rabbit egg, and after being grown for several days they terminated it to harvest the stem cells.

    In Minnesota the Mayo Clinic managed to replace the blood of a pig with human blood also. I believe he lived out a full life.

    The term that’s most commonly used is “Human Animal Chimeras.” It’s led to a lot of medical discoveries, however it blurs moral and ethical lines. Very similar to the soviet scientist who tried to create an army of ape people for Stalin.

    There’s more information here. http://news.nationalgeographic.com/news/2005/01/0125050125chimeras.html

    Edit: I totally forgot about that picture of the cat-dog that was circulating reddit a while back. It’s the perfect example of interspecies breeding. The hybrid made it to fetal stages, but because the DNA was such a mishmash of cells, it was extremely underdeveloped and was stillborn. It had characteristics of both the cat and the dog. It was really creepy actually. If anyone shows interest I’ll try and find the picture.

  8. Hybridization is not always disadvantageous. In some plants/animals it is selected for.

    However, if hybrids are less fit, this is called a post-mating barrier to hybridization. This can be due to things like genetic disorders (inviability/sterility/ or other non-reproductive disorders) or hybrids not looking sexy to either species.

    If hybrids are unfit, the post-mating barriers can be “reinforced” with pre-mating barriers. This can be anything from changing mating seasons so they don’t overlap, having the wrong shaped genitals, or even preferentially ejecting sperm when it is from the wrong species.

    If species diverge in allopatry, meaning not in contact with each other, then they have no reason to create these reproductive isolating mechanisms. This is how we can get ligers and tigons. However, if the species come back into contact before they are completely divergent, and if the hybrids are less fit, then these reproductive isolating mechanisms are evolved to keep the species apart.