Ligers and Tigons
A liger is a cross between a male lion and a female tiger,
a tigon is the converse. Either hybrid crossing is very unlikely to occur in the
wild, though the range of the asiatic lion did in the past overlap with that of
the tiger in India. Panthera hybrids by convention are named with the species of
the father first and the mother second (e.g. male tiger
and female jaguar = tiguar ).
The Liger
A liger can live for over twenty years. They look
a bit like a lioness (they tend not to have much mane)
with light coloured stripes. Males are infertile; female hybrids
may be fertile but they do not represent a separate species; this is similar
to the status of a mule or hinny.
The liger normally
grows to a much larger size than either of its parents; adult males can be over
350kg in weight and 3m in length. This is likely due to a genetic phenomenon
called genomic imprinting.
The Tigon
Tigons are smaller and weaker than either of their parents. They used to
be bred fairly frequently in zoos and collections, though they are now more
scarce.
Genomic
Imprinting
All the
mammals have two copies of each of their chromosome (except the X and Y
chromosomes) - one inherited from the mother and one from the father. This means
that each mammal has two copies of every gene. Most of the time both copies are
used to make proteins (this is called expression), but in certain genes only the
copy inherited from one parent is used. This phenomenon is known as genomic
imprinting.
Imprinting is seen in a very small
number of genes - less than 1% of the mammalian genome. It is not entirely clear
why this happens, but it is important: certain diseases are the result of a
breakdown of imprinting. These include the Prader-Willi and Angelman syndromes.
There are two theories for the origin of imprinting, and they are not mutually exclusive. The
first is that it is a defence against foreign pieces of genetic code spliced
into our DNA by viruses. Imprinting can silence this code and prevent viruses
hijacking our cellular machinery. The second theory is that imprinting
represents the struggle between the interests of the father and the mother.
From the father's evolutionary perspective it is better if his offspring
grow quickly, so they become strong and can protect themselves. If they survive
the father's genes will survive too. From the mother's perspective she must also
protect herself. Rapidly growing fetuses and newborns use more of her energetic
resources, and might threaten her survival. She therefore needs to keep this
growth in check so she can live to have more offspring.
This theory is supported by the
observation that imprinted genes that come from the father tend to promote rapid
growth, and that those from the mother tend to inhibit growth.
The exact
basis of the liger's great size is unknown, but imprinted genes involved in
growth and development (for example the insulin-like growth factor (IGF) family)
are probably responsible. At
a simple level the father lion's genes would normally be balanced out by
the mother lion's imprinted genes, but the female tiger does
not exert the same balance.