WHY BREED HYBRIDS?

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WHY BREED HYBRIDS?

HOW NEW SPECIES ARE CREATED

COMPLEX HYBRIDS

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Hybrid big cats are artificial creations. They are unlikely to occur in the

wild except in unnatural situations e.g. in very isolated populations where

there is no mate of the appropriate species available. Because of the

fertility issues, valuable genes may be lost by breeding dissimilar species

together. Most conservationists condemn deliberate hybridization as wasteful

in terms of genes and in terms of money. So why are they bred?

 

Many are bred out of curiosity. Exotic animals, especially ligers (the

largest big cats on the planet), are great crowd-pullers. Pony-sized striped

big cats and leopard-patterned lions are undeniably magnificent creatures.

Others occur by accident where two animals are housed together from an early

age in the belief that they won't mate with each other. The mating instinct

is strong enough that a puma allowed herself to be mated by an ocelot one

third of her size! This occurs where there is limited accommodation e.g.

private collections, travelling circuses etc. Even experienced zoos have

accidentally bred hybrids this way e.g. the servical. Believing that hybrids

are always sterile, some keepers have housed a hybrid big cat with pure-bred

big cats only to discover that hybrid females are fertile.

 

Private menageries also breed hybrids, sometimes as exotic pets. Some are

bred to bypass restrictions on ownership of purebred big cats. Loopholes in

some legal systems means that hybrids are not subject to the same legal

restrictions on ownership or transportation as pure-bred tigers or lions!

Many privately-owned curiosities end up at rescue centres when they grow too

large, become to expensive to keep or prove to be temperamental. There is

also an element of salacious - as well as genuine scientific - interest in

the act of inter-species copulation.

 

There is a limited amount of hybridization for scientific reasons. This may

be for research into how physical or behavioural traits are inherited or to

discover how closely two species are related. The ability of pumas to

produce offspring with ocelots (South American cat) and also with leopards

(African cat) helps scientists to work out the taxonomy of pumas i.e. how

closely they are related to other cat species. More worryingly, big cats

have been hybridized in an attempt to create domestic big cats.

 

Hybrids do not generally give rise to new species. Because hybrid males are

mostly infertile, female hybrids are mated back to pure-bred animals. In

only a few generations, the " alien genes " are absorbed into the gene pool of

the species she is bred back to. Theoretically, a new sub-species may arise

if the population is isolated, but they will only have subtle differences

such as lions retaining spots into adulthood as a result of a few lurking

leopard genes in the gene pool from a leopon many generations back.

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HOW SPECIES ARE CREATED

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Speciation (one species evolving into two) is usually an excruciatingly slow

process. Different species usually cannot mate and reproduce (reproductive

isolation). If the species are closely related, such as certain cat species,

they can produce hybrids, but those hybrids have reduced fertility. The more

easily two species form hybrids, the more closely they are related in

evolutionary terms. One way reproductive isolation occurs is genetic

mutation. One group of animals might be geographically isolated from others

of the same species. Each group accumulates slightly different mutations

over many generations - some genes affect appearance, others affect

behaviour. Many generations later, the two groups have diverged and are

different enough that even if they can mate, they can't produce fully

fertile offspring.

 

Sometimes, one species splits into two through behavioural isolation. Some

individuals develop behaviour patterns which limit their choice of mates e.g.

they might be attracted to certain colours or might be active at different

times of day. Though they are fully capable of interbreeding with the other

group, their different behaviours keep them apart. If their habitat changed,

behavioural barriers might break down and allow interbreeding; the hybrids

might become new species.

 

Another way reproductive isolation occurs is when fragments of DNA

accidentally jump from one chromosome to another in an individual

(chromosomal translocation) The mutant individuals can only reproduce with

other mutant individuals - not much good unless the individual has mutant

siblings to mate with! There are also " master genes " which govern general

body plan (Hox genes) and those which switch other genes on and off. A small

mutation to a master gene can mean a sudden big change to the individuals

that inherit that mutation. Sometimes, those radical mutations can " undo "

generations of divergent evolution so that two unrelated species can mate

with each other and produce fertile young (so far, this has only been seen

in micro-organisms).

 

Hybridisation is frequently a dead end because the hybrids are not fully

fertile. If the hybrids are fertile, they are usually absorbed back into the

population of one or other parent species and most of the alien genes are

bred out. More rarely, hybrids can become new species or new sub-species. In

the hands of breeders, some domestic/wildcat hybrids can become breeds;

these are not new species because the wildcat genes are largely bred out by

crossing with domestic cats, until only the wildcat pattern remains.

 

Although big cat species rarely, if ever, form hybrids under natural

conditions, in other species, hybridisation might possibly play a larger

role in evolutionary biology than previously believed. Most hybrids face

handicaps as a result of genetic incompatibility, but the fittest survive,

regardless of species boundaries. Life may be a genetic continuum rather

than a series of self-contained species.

 

In wild sunflowers, hybridisation causes an explosion of genetic variation;

some hybrids become new species capable of exploiting new ecological niches.

In this case, hybridisation may be more important than genetic mutations in

causing rapid, widespread evolutionary transitions because hybridisation

creates variations in many genes or gene combinations simultaneously.

Laboratory hybrids of annual sunflowers were back-crossed over one or more

generations to one of the parent species. Enough " alien " genes were retained

in later generations to allow them to thrive in conditions where neither

parent species could live. Computer simulations suggest that the successful

hybrids could evolve into new species within 50 to 60 generations.

Similarly, genes from GM crops will inevitably leak into the wild gene pool.

 

In Heliconius butterflies genes have leaked from one species into another

through hybridisation. Heliconius hybrids are relatively common and are a

long way from the biology textbook stereotype of a sterile and deformed

hybrid. These hybrids can successfully breed with either parental species or

with other hybrids. However, there is natural selection against hybrids.

Pure-bred Heliconius butterflies have warning colouration recognised by

predators. The hybrids, equally unpalatable, have an intermediate pattern

which is not recognised - the predators have not yet adapted and so the

hybrids are disadvantaged.

 

Natural hybrids are found among butterflies, birds and fish. Blue whales

will hybridise with fin whales. Interspecies matings have been witnessed in

dolphins. Wolves, coyotes and dogs all produce fertile hybrids, so much so

that some wild canids are becoming increasingly mongrelised. Usually, where

there are two closely related species living in the same area, less than 1

in 1000 individuals will be hybrids because animals rarely choose a mate

from a different species (unless mates from their own species are in short

supply, the reason some endangered species are further threatened by

hybridisation).

 

So why don't genetic leaks cause species boundaries to break down

altogether? One, seen in butterflies, is because predators may not recognise

the hybrids as inedible. Another is because hybrids cannot compete against

the parent species for resources. Healthy hybrids between Darwin's Galapagos

finches are relatively common, but their beaks are intermediate in shape and

less efficient feeding tools than the beaks of the parental species so they

lose out in the competition for food. In a 1983 storm, changes to the local

habitat meant new types of plant began to flourish and some hybrids had an

advantage over the birds with specialised beaks.

 

The hybridisation of the native European white-headed duck and the

introduced American ruddy duck means that pure white-headed ducks are being

hybridised into extinction. While humans want to protect the white-headed

duck; evolution wants to utilise the ruddy duck genes. Once a species is

introduced into a new habitat and the process starts, any Endangered Species

legislation is trying to work against the inexorable and far more ancient

forces of nature.

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BACKCROSSING

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If a fertile female tigon or liger offspring were mated back to the lion,

the percentage of lions genes in the offspring increases and the percentage

of tiger genes decreases. Assuming that the offspring at each generation

were fertile females, the tiger genes would eventually be swamped by

continually back-crossing to a lion. The end result (after several

generations) would show no visible signs of tiger ancestry. The same would

happen if the tigon or liger was backcrossed to a tiger and the fertile

female offspring of each generation were backcrossed to a tiger etc. After

several generations of backcrossing to tigers, the percentage of lion in the

offspring would decrease to such a point that the offspring would appear to

be wholly tiger. After several generations of backcrossing to one parental

species, the offspring become indistinguishable from that species as the

effect of the alien genes is swamped out. This applies to the nuclear DNA -

the DNA which transmits physical and physiological traits.

 

There is a second type of DNA which is found only in the *mitochondria* and

which is inherited only from the mother. In simple terms, mitochondria are

the energy factories inside cells. They are present in the egg and sperm

cells, but when fertilization occurs the sperm mitochondria are left outside

the fertilized egg. If a female liger and her female offspring were

repeatedly backcrossed to a lion (as detailed above), the nuclear DNA

becomes almost all of lion origin. However, the mitochondrial DNA come from

the original tiger mother. In later generations, the male hybrids become

genetically close enough to being all lion to be fertile and can breed with

lionesses. Only at that point is the original tiger mitochondrial DNA lost.

The same happens when tigons are backcrossed to tigers over many

generations. The mitochondrial DNA is lion DNA until such time as fertile

males are produced and breed with tigresses who pass on tiger mitochondrial

DNA .

 

Where there is suspicion of hybridisation in the past, it is possible to

test the mitochondrial DNA. Unless something prevented all of the later

generation female hybrids breeding (so that they couldn't pass on their

mitochondrial DNA), there will be traces of the other species mitochondria

DNA in what appear to be pure-bred lions or pure-bred tigers.

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EXAMPLE: BACKCROSSING LION TO TIGER

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A female liger is 50% lion and 50% tiger. This is backcrossed to a purebred

male tiger. At each generation, the female offspring is backcrossed to a

purebred male tiger. The percentage of tiger genes goes up in each

generation until they reach 99% at which point it could be considered

purebred. It will never quite reach a round total of 100%.

 

The lion and tiger genes won't be inherited in neat 50/50 splits, so

breeders talk of " pure-bloodedness " instead. How close is each generation to

being a pureblooded tiger? The arithmetic here is rounded up to one or two

decimal places.

 

F1 cross: 50%

F2 backcross: 75%

F3 backcross: 87.5%

F4 backcross 93.75%.

 

Once the hybrid is 90% one species or the other, the male hybrids are likely

to be fertile (based on information from Bengal and Savannah cat breeders).

 

Each successive backcross after that gives:

 

96.9%

98.5%,

99.25%

99.6%

99.8%

99.9%

 

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VERY COMPLEX HYBRIDS

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Question " Can you get really complex hybrids? I mean lion x tiger x leopard

x panther? What would it look like? "

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Because the female hybrids are often fertile, it is theoretically possible

to create a very complex hybrid. It would depend on the females of each

generation being fertile - conceivably, there could be a point where there

are so many different or incompatible genes in the mix that the offspring

are no longer fertile.

 

The most complex hybrid so far was a lijagulep (li-jagleop). First a jaguar

and a leopard were crossed. The female offspring, a jagulep (jagleop) was

crossed to a lion to produce the lijagulep. This was 50% lion and (roughly

speaking because the genes might have been passed on unevenly) 25% leopard +

25% jaguar. It therefore looked more like a lion than like a leopard or a

jaguar.

 

If the lijagulep had been female it might have been fertile. If so, it be

crossed to a tiger. Lion x tiger matings produce offspring, but tiger x

leopard matings have been unsuccessful in captivity. If a female lijagulep

was fertile and produced offspring when mated to a tiger, it would result in

a ti-lijagulep. A ti-lijagulep would be 50% tiger, 25% lion, 12.5% leopard

and 12.5% jaguar. No-one has ever bred one, but based on the percentages of

genes it would probably look like a ti-tigon since it contains more tiger

and lion genes than leopard or jaguar genes and it has twice as many tiger

genes as lion genes.

 

You could cross the ti-lijagulep back to a leopard to get 56.25% leopard,

25% tiger, 12.5% lion, 6.25% jaguar (56.25% = 50% leopard + 6.25% leopard

from the earlier mating). In fact, as long as the female offspring are

fertile, you could go on like this forever. There are dozens of

permutations, but as a rule as the percentage of genes from one species

decreases, the offspring looks less and less like that species and will most

closely resemble the parent whose genes make up the greatest percentage.

Other genes will be more and more dilute - too dilute to show up visually.

 

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