Mice and Ligers and Genomic Imprinting (Oh My!)


Males and females have been fighting for control since the dawn of time. I’m not talking about Mars vs. Venus stuff. I’m talking about sex.

In many species, males and females do not have the same interests when they reproduce. In polyandrous (females mate with more than one male) species, the offspring in a litter don’t always have the same father. Mothers therefore want multiple equal (smaller) offspring, while fathers want their offspring to be big and outcompete other males’ offspring in the womb.

This is all accomplished by genomic imprinting. In all mammals, some genes are switched off by methylation. Most of the time, all the genes are “re-set” after fertilization, but in about 1% of genes the parental pattern is inherited. This is handy for sexual conflict. Males pass on genes for growth that are turned on, and females pass on ones that are turned off.

This is actually why ligers (yes, the lion-tiger hybrid that Napoleon Dynamite ruined) are so enormous. Lions are polyandrous, so their sperm contains genes imprinted for enormous growth, and their eggs contain genes imprinted to counteract that. Tigers are monogamous, so they don’t have the sex-specific imprinting. So when a male lion fertilizes a female tiger, the paternal genes run wild, causing the offspring to be enormous.

Scientists have been able to create bimaternal (BM) mice by fusing the nuclei of two ova.  This results in baby mice (mouslings?) that have only maternally imprinted genes. As expected, they are much smaller than normal. In addition, according to a new study in Human Reproduction, they live longer. This is interesting, but not  terribly surprising. Small mice usually live longer.

Of course the researchers and science reporters (especially the reporters) overstate their case. They claim that this might shed some light on the difference between male and female lifespans. Bullshit. We know why males don’t live very long, and it’s because of what hangs between our legs. Testosterone wrecks the heart and other organs. In fact, if they’re so inclined, men can remove the offending organs and extend their lifespan a decade or so. Besides, normally-conceived males and females have identical patterns of imprinting, so there’s no imprinting would explain the difference in lifespan.

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