In
biology and specifically
genetics,
hybrid has several
meanings, all referring to the offspring of
sexual reproduction.
- In general usage, hybrid is synonymous with
heterozygous: any offspring
resulting from the mating of two distinctly homozygous individuals
- a genetic hybrid carries two different
alleles of the same gene
- a structural hybrid results from the fusion of
gametes that have differing structure in at
least one chromosome, as a result of
structural
abnormalities
- a numerical hybrid results from the fusion of
gametes having different haploid numbers of
chromosomes
- a permanent hybrid is a situation where only
the heterozygous genotype occurs, because
all homozygous combinations are lethal.
From a
taxonomic perspective, hybrid refers
to offspring resulting from the interbreeding between two animals
or plants of different
taxa.
- Hybrids between different subspecies
within a species (such as between the Bengal tiger and Siberian tiger) are known as
intra-specific hybrids. Hybrids between different
species within the same genus (such as between lions and tigers) are sometimes
known as interspecific hybrids or crosses. Hybrids
between different genera (such as between sheep and goats) are
known as intergeneric hybrids. Extremely rare
interfamilial hybrids have been known to occur
(such as the guineafowl hybrids)..
No interordinal (between different orders) animal
hybrids are known.
- The second type of hybrid consists of crosses between populations, breeds or
cultivars within a single species. This
meaning is often used in plant and animal breeding, where hybrids
are commonly produced and selected because they have desirable
characteristics not found or inconsistently present in the parent
individuals or populations. This flow of genetic material between
populations or races is often called hybridization.
Etymology
According to the
Oxford
English Dictionary, the word is derived from Latin
hybrida (or
(h)ibrida), meaning the "offspring of
a tame sow and a wild boar", "child of a freeman and slave", etc.
The term entered into popular use in English in the 19th century,
though examples of its use have been found from the early 17th
century.
Types of hybrids
Depending on the parents, there are a number of different types of
hybrids;
Single cross hybrids - result from the cross
between two
true breeding
organisms and produces an F1 generation called an
F1 hybrid (F1 is short for Filial 1, meaning
"first offspring). The cross between two different
homozygous lines produces an F1 hybrid that is
heterozygous; having two alleles, one
contributed by each parent and typically one is
dominant and the
other
recessive. The F1
generation is also
phenotypically
homogeneous, producing offspring that are all similar to each
other.
Double cross hybrids - result from the cross
between two different F1 hybrids.
Three-way cross hybrids - result from the cross
between one parent that is an F1 hybrid and theother is from an
inbred line.
Triple cross hybrids - result from the crossing of
two different three-way cross hybrids.
Population hybrids - result from the crossing of
plants or animals in a
population with
another population. These include crosses between organisms such as
interspecific hybrids or crosses between different races.
Interspecific hybrids
Interspecific hybrids are bred by mating two species, normally from
within the same
genus. The offspring display
traits and characteristics of both parents. The offspring of an
interspecific cross are very often sterile; thus, hybrid sterility
prevents the movement of genes from one species to the other,
keeping both species distinct. Sterility is often attributed to the
different number of chromosomes the two species have, for example
donkeys have 62
chromosomes, while
horses
have 64 chromosomes, and
mules and
hinnies have 63 chromosomes. Mules, hinnies, and other
normally sterile interspecific hybrids cannot produce viable
gametes because the extra chromosome cannot
make a homologous pair at
meiosis, meiosis
is disrupted, and viable sperm and eggs are not formed. However,
fertility in female mules has been reported with a donkey as the
father.
Most often other processes occurring in plants and animals keep
gametic isolation and species distinction. Species often have
different mating or courtship patterns or behaviors, the breeding
seasons may be distinct and even if mating does occur antigenic
reactions to the sperm of other species prevent fertilization or
embryo development.The
Lonicera fly is
the first known
animal species that resulted from natural hybridization.
Until the discovery of the Lonicera fly, this process was known to
occur in nature only among
plants.
While it is possible to predict the genetic composition of a
backcross
on average, it is not possible to accurately
predict the composition of a particular backcrossed individual, due
to random segregation of chromosomes. In a species with two pairs
of chromosomes, a twice backcrossed individual would be predicted
to contain 12.5% of one species' genome (say, species A). However,
it may, in fact, still be a 50% hybrid if the chromosomes from
species A were lucky in two successive segregations, and meiotic
crossovers happened near the
telomeres. The
chance of this is fairly high: \frac{1}{2}^{(2 \times 2)} =
\frac{1}{16} (where the "two times two" comes about from two rounds
of meiosis with two chromosomes); however, this probability
declines markedly with chromosome number and so the actual
composition of a hybrid will be increasingly closer to the
predicted composition.
Hybrids are often named by the
portmanteau method, combining the names of the
two parent species. For example, a
zeedonk
is a cross between a
zebra and a
donkey. Since the traits of hybrid offspring often
vary depending on which species was mother and which was father, it
is traditional to use the father's species as the first half of the
portmanteau. For example, a
liger is a cross
between a male
lion and a female
tiger, while a
tigon is a cross
between a male tiger and a female lion.
Examples of hybrid animals
- Hybrid Iguana is single cross
hybrid, result of natural inbreeding from
male marine iguana
and female land Iguana since late 2000s.
- Equid hybrids
- Mule, a cross of female horse and a male donkey.
- Hinny, a cross between a female donkey and
a male horse. Mule and Hinny are examples of reciprocal hybrids.
- Zebroids
- Zeedonk or Zonkey, a zebra/donkey cross.
- Zorse, a zebra/horse cross
- Zony or Zetland, a zebra/pony cross ("zony" is a generic term; "zetland" is
specifically a hybrid of the Shetland pony breed with a zebra)
- Bovid hybrids
- Sheep-goat hybrids, such as
the The Toast of
Botswana.
- Ursid hybrids, such as the Grizzly-polar bear hybrid, occur
between black bears, brown bears, and polar bears.
- Felid hybrids
- Fertile Canid hybrids occur between
coyotes, wolves, dingoes, jackals and domestic dogs.
- Hybrids between Black Rhinos &
White Rhinos have been recognized.
- Hybrids between spotted owls and
barred owls
- Cama, a cross between a Camel and a Llama, also an
intergeneric hybrid.
- Wholphin, a fertile but very rare cross
between a False Killer Whale and
a Bottlenose Dolphin.
- A fertile cross between an albino
King Snake and an albino Corn Snake.
- At
Chester
Zoo
in the United Kingdom, a cross between African elephant (male) and Asian elephant (female). The male calf
was named Motty. It died of gut infection
after twelve days.
- Cagebird breeders sometimes breed hybrids between species of
finch, such as Goldfinch x Canary. These
birds are known as Mules.
- Gamebird hybrids, hybrids
between gamebirds and domestic fowl, including Chickens, Guineafowl and
Peafowl, interfamilial hybrids.
- Numerous Macaw hybrids are also
known.
- Red Kite x Black
Kite: 5 bred unintentionally at a falconry center in England. (It is reported that
the black kite (the male) refused female black kites but mated with
two female red kites.)
- Hybridization between the endemic Cuban Crocodile (Crocodilus
rhombifer) and the widely distributed American Crocodile (Crocodilus
acutus) is causing conservation problems for the former
species as a threat to its genetic integrity. [6764]
- Blood parrot cichlid, which
is probably created by crossing a Red Head Cihclid and a Midas
Cichlid or Red Devil Cichlid
- The Mulard duck, hybrid of the domestic
Pekin duck and domesticated Muscovy ducks.
Hybrids should not be confused with
chimaeras such as the chimera between
sheep and
goat known as
the
geep.Wider interspecific hybrids can be
made via
in vitro
fertilization or somatic hybridization, however the resulting
cells are not able to develop into a full organism. An example of
interspecific hybrid cell lines is the
humster (hamster x human) cells.
Hybrid plants
Plant species hybridize more readily than animal species, and the
resulting hybrids are more often fertile hybrids and may reproduce,
though there still exist sterile hybrids and selective hybrid
elimination where the offspring are less able to survive and are
thus eliminated before they can reproduce. A number of plant
species are the result of hybridization and
polyploidy with many plant species easily cross
pollinating and producing viable seeds, the distinction between
each species is often maintained by geographical isolation or
differences in the flowering period. Since plants hybridize
frequently without much work, they are often created by humans in
order to produce improved plants. These improvements can include
the production of more or improved; seeds, fruits or other plant
parts for consumption, or to make a plant more winter or heat hardy
or improve its growth and/or appearance for use in horticulture.
Much work is now being done with hybrids to produce more disease
resistant plants for both agricultural and horticultural crops. In
many groups of plants hybridization has been used to produce larger
and more showy flowers and new flower colors.
Many plant genera and species have their origins in polyploidy.
Autopolyploidy resulting from the sudden multiplication in the
number of chromosomes in typical normal populations caused by
unsuccessful separation of the chromosomes during meiosis.
Tetraploids or plants with four sets of chromosomes are common in a
number of different groups of plants and over time these plants can
differentiate into distinct species from the normal diploid line.
In
Oenothera lamarchiana the diploid species has 14
chromosomes, this species has spontaneously given rise to plants
with 28 chromosomes that have been given the name
Oenthera
gigas. Tetraploids can develop into a breeding population
within the diploid population and when hybrids are formed with the
diploid population the resulting offspring tend to be sterile
triploids, thus effectively stopping the intermixing of genes
between the two groups of plants (unless the diploids, in rare
cases, produce unreduced gametes).
Another form of polyploidy called allopolyploidy occurs when two
different species mate and produce hybrids. Usually the typical
chromosome number is doubled in successful allopolyploid species,
with four sets of chromosomes the genotypes can sort out to form a
complete diploid set from the parent species, thus they can produce
fertile offspring that can mate and reproduce with each other but
can not back-cross with the parent species. Allopolyploidy in
plants often gives them a condition called hybrid vigour, which
results in plants that are larger and stronger growing than either
of the two parent species. Allopolyploids are often more aggressive
growing and can be invaders of new habitats.
Sterility in a hybrid is often a result of
chromosome number; if parents are of differing
chromosome pair number, the offspring will have an odd number of
chromosomes, leaving them unable to
produce chromosomally balanced
gametes.
While this is a negative in a crop such as
wheat, when growing a crop which produces no seeds
would be pointless, it is an attractive attribute in some fruits.
Bananas and seedless
watermelon, for instance, are intentionally bred
to be
triploid, so that they will produce
no seeds. Many hybrids are created by humans, but natural hybrids
occur as well.
Heterosis
Plant hybrids, especially, are often stronger than either parent
variety, a phenomenon which when present is known as
hybrid vigor (heterosis) or heterozygote
advantage.
Plant breeders make use of
a number of techniques to produce hybrids, including line breeding
and the formation of complex hybrids. An economically important
example is hybrid
maize (corn), which provides
a considerable seed yield advantage over open pollinated varieties.
Hybrid
seed dominates the commercial maize seed market in the United States
, Canada
and many
other major maize producing countries.
Examples of Species Hybrids
Some plant hybrids include:
Some natural hybrids are:
Some horticultural hybrids:
- Dianthus
×allwoodii, is a hybrid between Dianthus
caryophyllus × Dianthus plumarius. This is an
"interspecific hybrid" or hybrid between two species in the same
genus.
- ×Heucherella
tiarelloides, or Heuchera sanguinea ×
Tiarella cordifolia is an "intergeneric hybrid" a hybrid
between two different genera.
- Quercus x warei [Quercus robur x Quercus
bicolor] Kindred Spirit
Hybrid Oak
Hybrids in nature
Hybridisation between two closely related species is actually a
common occurrence in nature. Many
hybrid
zones are known where the ranges of two species meet, and
hybrids are continually produced in great numbers. These hybrid
zones are useful as biological model systems for studying the
mechanisms of
speciation (
Hybrid speciation).
Recently DNA analysis
of a bear shot by a hunter in the North West Territories
confirmed the existence of naturally-occurring and
fertile grizzly–polar
bear hybrids. There have been reports of similar
supposed hybrids, but this is the first to be confirmed by DNA
analysis. In 1943, Clara Helgason described a male bear shot by
hunters during her childhood. It was large and off-white with hair
all over its paws. The presence of hair on the bottom of the feet
suggests it was a natural hybrid of Kodiak and Polar bear.
In some species, hybridisation plays an important role in
evolutionary biology. While most hybrids are
disadvantaged as a result of genetic
incompatibility, the fittest
survive,
regardless of species boundaries. They may have a beneficial
combination of traits allowing them to exploit new habitats or to
succeed in a marginal habitat where the two parent species are
disadvantaged. This has been seen in experiments on sunflower
species. Unlike mutation, which affects only one gene,
hybridisation creates multiple variations across genes or gene
combinations simultaneously. Successful hybrids could evolve into
new species within 50-60 generations. This leads some scientists to
speculate that life is a genetic continuum rather than a series of
self-contained species.
Where there are two closely related species living in the same
area, less than 1 in 1000 individuals are likely to be hybrids
because animals rarely choose a mate from a different species
(otherwise species boundaries would completely break down). In some
closely related species there are recognized "hybrid zones".
Some species of Heliconius butterflies exhibit dramatic
geographical polymorphism of their wing patterns, which act as
aposematic signals advertising their unpalatability to potential
predators. Where different-looking geographical races abut,
inter-racial hybrids are common, healthy and fertile. Heliconius
hybrids can breed with other hybrid individuals and with
individuals of either parental race. These hybrid backcrosses are
disadvantaged by natural selection because they lack the parental
form's warning coloration, and are therefore not avoided by
predators.
A similar case in mammals is hybrid White-Tail/Mule Deer. The
hybrids don't inherit either parent's escape strategy. White-tail
Deer dash while Mule Deer bound. The hybrids are easier prey than
the parent species.
In birds, healthy Galapagos Finch hybrids are relatively common,
but their beaks are
intermediate in
shape and less efficient feeding tools than the specialised beaks
of the parental species so they lose out in the competition for
food. Following a major storm in 1983, the local habitat changed so
that new types of plants began to flourish, and in this changed
habitat, the hybrids had an advantage over the birds with
specialised beaks - demonstrating the role of hybridization in
exploiting new ecological niches. If the change in environmental
conditions is permanent or is radical enough that the parental
species cannot survive, the hybrids become the dominant form.
Otherwise, the parental species will re-establish themselves when
the environmental change is reversed, and hybrids will remain in
the minority.
Natural hybrids may occur when a species is introduced into a new
habitat. In Britain, there is hybridisation of native European Red
Deer and introduced Chinese Sika Deer. Conservationists want to
protect the Red Deer, but the environment favors the Sika Deer
genes. There is a similar situation with White-headed Ducks and
Ruddy Ducks.
Expression of parental traits in hybrids
When two distinct types of organisms breed with each other, the
resulting hybrids typically have intermediate traits (e.g., one
parent has red flowers, the other has white, and the hybrid, pink
flowers). Commonly, hybrids also combine traits seen only
separately in one parent or the other (e.g., a
bird hybrid might combine the yellow head of one
parent with the orange belly of the other). Most characteristics of
the typical hybrid are of one of these two types, and so, in a
strict sense, are not really new. However, an intermediate trait
does differ from those seen in the parents (e.g., the pink flowers
of the intermediate hybrid just mentioned are not seen in either of
its parents). Likewise, combined traits are new when viewed as a
combination.
In a hybrid, any trait that falls outside the range of parental
variation is termed
heterotic. Heterotic
hybrids do have new traits, that is, they are not intermediate.
Positive heterosis produces more robust hybrids, they
might be stronger or bigger; while the term
negative
heterosis refers to weaker or smaller hybrids. Heterosis is
common in both animal and plant hybrids. For example, hybrids
between a lion and a tigress ("ligers") are much larger than either
of the two progenitors, while a tigon (lioness × tiger) is smaller.
Also the hybrids between the Common Pheasant (
Phasianus colchicus) and domestic
fowl (
Gallus gallus) are
larger than either of their parents, as are those produced between
the Common Pheasant and hen Golden Pheasant (
Chrysolophus pictus). Spurs are
absent in hybrids of the former type, although present in both
parents.
When populations hybridize, often the first generation
(F
1) hybrids are very uniform. Typically, however, the
individual members of subsequent hybrid generations are quite
variable. High levels of variability in a natural population, then,
are indicative of hybridity. Researchers use this fact to ascertain
whether a population is of hybrid origin. Since such variability
generally occurs only in later hybrid generations, the existence of
variable hybrids is also an indication that the hybrids in question
are fertile.
Genetic mixing and extinction
Regionally developed
ecotypes can be
threatened with
extinction when new
alleles or genes are introduced that alter
that ecotype. This is sometimes called
genetic mixing. Hybridization and
introgression of new genetic material can lead
to the replacement of local
genotypes if
the hybrids are more
fit and have
breeding advantages over the indigenous ecotype or species. These
hybridization events can result from the introduction of non native
genotypes by humans or through habitat modification, bringing
previously isolated species into contact. Genetic mixing can be
especially detrimental for rare species in isolated habitats,
ultimately effecting the population to such a degree that none of
the originally genetically distinct population remains..
Effect on biodiversity and food security
In agriculture and
animal
husbandry, the
green
revolution's use of conventional hybridization increased yields
by breeding "
high-yielding
varieties". The replacement of locally indigenous breeds,
compounded with unintentional cross-pollination and crossbreeding
(
genetic mixing), has reduced the
gene pools of various wild and indigenous breeds resulting in the
loss of
genetic diversity. Since
the indigenous breeds are often well-adapted to local extremes in
climate and have immunity to local pathogens this can be a
significant
genetic erosion of the
gene pool for future breeding. Therefore, commercial plant
geneticists strive to breed "widely adapted" cultivars to
counteract this tendency.
Limiting factors
A number of conditions exist that limit the success of
hybridization, the most obvious is great genetic diversity between
most species. But in animals and plants that are more closely
related hybridization barriers can include morphological
differences, differing times of fertility, mating behaviors and
cues, physiological rejection of sperm cells or the developing
embryo.
In plants, barriers to hybridization include blooming period
differences, different pollinator vectors, inhibition of pollen
tube growth, somatoplastic sterility, cytoplasmic-genic male
sterility and structural differences of the chromosomes.
Mythical and legendary hybrids
Ancient folktales often contain mythological creatures, sometimes
these are described as hybrids (e.g.
Hippogriff as the offspring of a
griffin and a
horse, and the
Minotaur which is the offspring of
Pasiphaë and a white bull. More often they are
kind of
chimera, i.e. a
composite of the physical attributes of two or more kinds of
animals, mythical beasts, and often humans, with no suggestion that
they are the result of interbreeding, e.g.
Harpies ,
mermaids and
centaurs.
See also
References
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