Gene Expression

Why ligers are huge

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Believe it or not, tigers are not the largest big cat. Ligers are (you might remember ligers from Napoleon Dynamite). Why? It has to do with the weirdness that occurs when you hybridize across two lineages which have been distinctive for millions of years, but not so long so as not to be able to produce viable offspring (in fact, many ligers are fertile as well). Here’s the explanation:

Imprinted genes are under greater selective pressure than normal genes. This is because only one copy is active at a time. Any variations in that copy will be expressed. There is no “back-up copy” to mask its effects. As a result, imprinted genes evolve more rapidly than other genes. And imprinting patterns — which genes are silenced in the eggs and sperm — also evolve quickly. They can be quite different in closely related species.

Lions and tigers don’t normally meet in nature. But they can get along very well in captivity, where they sometimes produce hybrid offspring. The offspring look different, depending on who the mother is. A male lion and a female tiger produce a liger – the biggest of the big cats. A male tiger and a female lion produce a tigon, a cat that is about the same size as its parents.

The difference in size and appearance between ligers and tigons is due in part to the parents’ differently imprinted genes. Other animals can also hybridize, with similar results. For example, a horse and a donkey can produce a mule or a hinny.


Imprinting generally emerges due to competition between the interests of males and females within a given species because of complex social structures. What’s good for father may not be good for mother. Lions live in prides, while tigers are relatively solitary. Apparently lionesses may mate with multiple males, so any given male has genes which tend to encourage growth in his own cubs as to as maximize his genes’ share of finite resources in a competitive environment. In contrast, the female’s genes tend to fight against this tendency, because she’s equally related to all the potential cubs, and so wants to equally distribute resources as to maximize the number who might survive. Tigers are not subject to this dynamic. A tigress mates with one male, and so he is equally related to all the cubs. His genes would not want to “encourage” growth because there isn’t competition between cubs from the male perspective, they’re all of a piece. So the female does not need to evolve anti-growth imprinting defenses.

You bring a female tiger, who has no defenses against paternally inherited genes which tend to encourage growth, with a male lion who will contribute exactly those genes. And voilà , you get the liger, whose growth is a consequence of this asymmetry at the endpoint of different evolutionary histories.

Related Genetics ∩ Sociology ∩ Evolution = Genomic Imprinting, Kinship theory and genomic imprinting and The Evolutionary Context of Genomic Imprinting.

Comments

  1. #1 Tony P
    September 27, 2009

    Impressive beast though you wouldn’t want one chasing you.

  2. #2 Paul Jones
    September 27, 2009

    Has anyone determined which genes actually govern these effects?

  3. #3 lars
    September 28, 2009

    does this effect occur when different human races produce offspring? i mean is their any diff in offspring between say, nigreian father/chinese mother, and chinese father/nigerian mother?

  4. #4 Tom Bri
    September 28, 2009

    Just a note, tigers and lions don’t share much range now, but up until a few hundred years ago they did. Asiatic lions are nearly extinct, a handful in India, I believe. So the comment above about not normally meeting in nature is only partially true. They probably did meet fairly frequently in the past, but don’t now.

    Makes me wonder if there wasn’t some gene flow that we can’t now observe, because the remnant populations are those that would have experienced it the least. Indian tigers are jungle creatures, while lions savanna.

  5. #5 Hars
    September 28, 2009

    lars

    Biracial children with a white mother and black father have significantly higher IQ’s than the reverse. Perhaps this is a similar dynamic. Like lionesses, black women are statistically much more likely to have multiple fathers of their kids, so the fathers genes would work harder to promote positive traits like IQ, whereas the mother’s would work to “even things out”.

    It is interesting that the reproductive strategies of these two big cats are similar to the indigenous humans that locally habitate with them. The dangerous life of the savannah seems to promote the quantity over quality, and the seasonal temperate and harsh climates promote less offspring with higher quality across species – at least dominant species in the food chain like big cats and humans.

  6. #6 Seth
    September 28, 2009

    You wouldn’t expect variation in human offspring to be the result of differences in imprinting. The sizes of the parents certainly will influence their offspring, but that is not what happens here. Generally speaking, the imprinting status of specific genes is similar across the species. In the case of the liger, you have lions upregulating the paternal allele while silencing the maternal. Since the tigers apparently do not imprint this loci, the maternal allele is expressed, and the offspring are larger.

    A very similar effect has been shown in hybrid mice. Species such as Mus caroli are monandrous while Mus musculus females may mate with multiple males. Crossing a female M musculus and male M caroli you find the reverse of the liger situation. Mus caroli male’s genomes do not show the same upregulation as male Mus musculus. However, the female Mus musculus still imprints those growth related genes. Comparing embryos and placenta of age matched Mus caroli x Mus musculus to normal Mus musculus x Mus musculus you see retarded growth of the embryo, and poor development of the maternal tissues.

  7. #7 ABM
    September 28, 2009

    Biracial children with a white mother and black father have significantly higher IQ’s than the reverse. Perhaps this is a similar dynamic. Like lionesses, black women are statistically much more likely to have multiple fathers of their kids, so the fathers genes would work harder to promote positive traits like IQ, whereas the mother’s would work to “even things out”.

    LOLWUT? Is this a deliberate attempt to be inflammatory, or an exceptionally daft attempt at sociobiology? Even if social conditions in the US are currently such that many black women have children by multiple fathers… man, 400 years is a drop in the evolutionary bucket, even for a young genus like Homo. You also assume that intelligence is so simply inherited that a few imprinted genes would have a big effect, AND that IQ tests measure anything except one’s ability to do well on IQ tests. Both of which range from doubtful to utter nonsense.

  8. #8 Dringle
    September 28, 2009

    “Biracial children with a white mother and black father have significantly higher IQ’s than the reverse. Perhaps this is a similar dynamic. Like lionesses, black women are statistically much more likely to have multiple fathers of their kids, so the fathers genes would work harder to promote positive traits like IQ, whereas the mother’s would work to “even things out”.”

    Last time I checked, human beings were the same species. It’s amazing how pseudo-scientists such as Lars/Hars can sit behind their computers, say untrue, unscientific, poorly thought out mumbo jumbo and call it fact. You should try publishing this somewhiere or just passing it amoungst your homogenous group of weak, unmasculine friends who get their kicks from degrading other races due to deep seeded racism, resentment, jealousy and fear.

  9. #9 DD
    September 28, 2009

    That huge cat looks a bit like a (faded) leopard or jaguar as it does a lion or tiger.

  10. #10 lars
    September 29, 2009

    dringle, i asked a question, i didn’t call anything a fact.
    ‘hars’ supplies no source, so i’m not sure whether to believe him about his assertions about biracial kids. any objective study would probably probably require analyzing stats from adopted biracial kids where records are kept about their father and mother.

  11. #11 ktbug ladydid
    September 29, 2009

    WTF Lars and Hars? Are you really that ridiculously inane to think that?? I’m hoping you were joking, inappropriate as it was.

  12. #12 Douglas Knight
    September 29, 2009

    Does the imprinting information really give us enough information to deduce things about the reproductive habits, or are these just-so stories? A few years ago, Vassar tried to take this size differential and the social structures and conclude the sexes of the parents, but got it backwards. Tigers have more variety of mates; lions may have a few mates in a pride, but they’ll be the same the next year. Maybe competition within a litter trumps competition between litters, but it’s not obvious.

  13. #13 tommy
    September 30, 2009

    Impressive beast though you wouldn’t want one chasing you.

    In spite of their size, ligers are said to be docile compared to their parents.

    Tigons are exceedingly rare. I’ve heard that’s because lionesses usually engage in rather aggressive “foreplay” which tigers find frustrating. Lions must find tigresses an easy score.

  14. #14 Major Cus
    October 1, 2009

    He’s bewdeefull What a magnificiant creature. Where can I get one. How much would a Liger eat ?

  15. #15 rijkswaanvijand
    October 1, 2009

    Multiple mates per lioness?
    According to Packer & Pusey (“Cooperation and competition within coalitions of male lions: kin selection or game theory?” 1982 nature 296, 740-2), male lions tend to respect ownership over, or consortship with oestrus females.
    This suggests multiple mating partners per oestruscycle isn’t common for lionesses..

  16. #16 Squire
    October 1, 2009

    Actually, lions and tigers still share overlapping natural ranges in the Gir Forest of India. There are, however, still no recorded instances of them breeding in the wild, potentially because they occupy distinct ecological niches within this overlapping range.

  17. #17 rijkswaanvijand
    November 3, 2009

    Conflict here might not be in equally dividing resources to cubs.
    There is a major conflict in interest of male and female per se, a male is best of if his cubs grow fast and large, while a female can only spend a certain amount of resources per litter in order to obtain maximal lifetime reproductive succes. So all this additional cub-growth might well impose serious reduction in future reproduction for the female in question, while for the male this future reproduction mostly benefits from the same additional cub-growth.