Ligers (male lion + female tiger) are bigger than both lions and tigers, tigons (male tiger + female lion) are not. If I had to guess why, I would have said something to do with XY-chromosome sex differentiation. But female ligers, who have a lion X-chromosome and a tiger X-chromosome, are still big. What the hell?
For most chromosomes, a liger gets one from each parent. In highschool we’re taught that whether the mother’s gene or father’s gene gets activated depends on which one is “dominant”. That’s a gloss; genes don’t have dominance: both genes get activated and sometimes the effect of one is so powerful that it doesn’t matter what the other gene does. But if the lion gene says “grow to x” and the tiger gene says “grow to y”, it shouldn’t be the case that the liger grows to larger than both x and y.
I recently found out that the answer is a mechanism called genome imprinting: Chromosomes from each parent have different markers added by chromatin remodeling or methylation (together called epigenetics: I get the impression that which mechanism is unknown in almost all cases). There are genes that act differently or act on other genes differently depending on who they were inherited from.
So either lions have a maternally-imprinted gene and/or tigers have a paternally-imprinted gene that limits growth. And that’s why ligers are bred for their skills in magic.
Tags: genetics
I should ask Crystal, because she was explaining this to me. Lions have sperm/embryo competition, I believe, so male lions developed a gene that made their offspring grow really big so they could outcompete litter-mates from other males. Female lions, getting no benefit from having huge, hard-to-carry offspring that starve their other babies developed a gene that reduces the impact of the male lion’s gene, causing regular-sized cubs.
Tigers do not mate with multiple males, so the males never developed the gene for growth, and the females never developed the gene to counteract it.
So when ligers are created, the lion male’s gene goes unsuppressed, and the liger is huge at the expense of any possible littermates. What I’m a bit fuzzy on: is it competition in the womb or after birth that the gene affects, and does the female lion suppress the expression of the male’s signal, or does she suppress growth (making tigons smaller than either lions or tigers)?
Tigons are the same size, which is how we know it’s genome imprinting rather than competing giantism and dwarfism genes.
Oh, so my last paragraph is backwards.
Ok, after consulting with my colleagues (i.e. Crystal came in to steal printer paper), I realise I had it a bit wrong: it is due to embryo competition, but what it is is that usually, both chromosomes express a “grow” gene (creating insulin-like growth factor II, to be exact). In lions, males developed genes that say “produce lots of growth factor”, so the females started not expressing the gene that produces growth factor at all. In tigers, both sexes contribute growth factor signals, so ligers get the “produce growth factor” from their mom and the “produce shitloads of growth factor” from their dad and grow huge.
Wikipedia thinks there’s growth-inhibator involved somehow, though.
So to put it in genome imprinting terms: lions have a gene that turns off the lion-grow gene if it’s on a maternal-imprinted chromosome. The turn-off gene does nothing when presented with a maternal-imprinted tiger chromosome.
See why people like Creationism? It makes everything so much easier: Ligers are devil-spawn wizard-creatures. End of story, lock and load. Where’s my
letter of marquehunting permit?[Edited to add:] Never mind! I didn’t realize that by “bigger” you meant HUGEr. I, for one, salute our new Liger overlords.
The Wikipedia articles say that ligers can be fertile, and female tigons are fertile. So: Why are there no ligons? I glossed over Haldane’s rule, is that why?
Also: Is Frankenstein too popular? Is Oryx and Crake popular enough?