But instead of having the instructions for building a chair or a table, it has the instructions to build living things, like lions and tigers. While sperm and eggs are in the adults, prior to fertilization, their DNA can be modified. These modifications do not change the DNA sequence itself. For example, the instructions might read:.
You can make these modifications without changing any of the words in the manual. The process in which these types of modifications are made to the DNA in eggs or sperm is called imprinting. One of the most well-studied cases of imprinting is in mice. Just like lions, female mice can have litters with pups from multiple fathers. Therefore, male mice want their offspring to be big, so that their pups survive best from the litter. Female mice want all their offspring to be of moderate size.
The version that is inherited from the father is underlined, telling the embryo to grow a lot. The version inherited from the mother is crossed out, telling the embryo not to grow too much. Therefore, only the Igf2 gene inherited from the father is active.
Imprinting does not change the DNA letters themselves. Instead, imprinting causes methylation. Methylation is a chemical change that adds a small molecule on top of the DNA sequence. This chemical change acts like a stop sign, and turns the gene off. Just like lions, a litter of mice can have multiple fathers. Image from Wikimedia.
Because lions and mice have similar mating strategies, they probably have similarly imprinting strategies. Male lions want their offspring to be big, and female lions want their offspring to be medium-sized.
Tigers probably have not evolved similar imprinting strategies. Lions and tigers have different DNA imprinting strategies. Male lions want their cubs to grow as big as possible! But a lioness wants all the cubs to be the same size, so her imprinting counteracts his. This imprinting also explains why a tigon is not nearly as big. They take much longer to grow and are much more difficult to take care of. Currently we do not know for sure which genes are imprinted in lions or tigers, or which of these genes make the biggest differences in terms of growth.
There have been recent studies on big cats to compare genome sequences between tigers, lions and leopards. However, these studies do not give us information about imprinting. The technology exists to find methylation sites in DNA, so maybe in the future we will uncover more about imprinting differences between lions and tigers.
But for now, scientists can infer a lot about these big cats based on evolutionary theories and what we know from mice. 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.
Lions live in prides, while tigers are relatively solitary. Tigers are not subject to this dynamic. A tigress mates with one male, and so he is equally related to all the cubs.
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. Register or Log In. The Magazine Shop. Login Register Stay Curious Subscribe. There is therefore no need for the female to compensate, so the offspring's growth goes uninhibited. When a male lion mates with a tigress, his genes promote large offspring because lions are adapted to a competitive breeding strategy.
The tigress does not inhibit the growth because she is adapted to a non-competitive strategy. Therefore the offspring liger grows larger and stronger than either parent because the effects do not cancel each other out.
Ligers take several years to reach full adult size, but it is a myth that ligers never stop growing. When a male tiger mates with a lioness, his genes are not promoting large growth of the offspring because he is adapted to a non-competitive breeding strategy. However, the lioness is adapted to a competitive strategy and her genes inhibit the growth of the offspring.
This uneven match means that the offspring tigons are often smaller and less robust than either parent. Growth dysplasia has other effects: the size of the placenta may be affected causing miscarriage , the embryo may be aborted at an early stage due to abnormal growth, the cub may be stillborn or may only survive a few days. In some rodents, mating Species A males with Species B females produces offspring half normal size, but mating Species B males with Species A females cause the offspring to be aborted as they try to grow to several times the normal size.
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