In biology, hybrid has two meanings.[1]
The first meaning is the result of interbreeding between two animals or plants of different taxa. Hybrids between different species within the same genus are sometimes known as interspecific hybrids
or crosses. Hybrids between different sub-species within a species are known as intra-specific hybrids. Hybrids between
different genera are sometimes known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known
to occur (such as the guineafowl hybrids).
The second type of "hybrid" are crosses between populations, breeds or cultivars within a single species. This second meaning is often used in plant and animal breeding. In plant and
animal breeding, hybrids are commonly produced and selected because they have desirable characteristics not found or
inconsistently present in the parent individuals or populations. This rearranging of the genetic material between populations or
races is often called hybridization.
Interspecific hybrids
An example of an intraspecific hybrid is a hybrid between a Bengal tiger and an Amur (Siberian) tiger. 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 very often are sterile; thus, hybrid
sterility prevents the movement of genes from one species to the other, keeping both species distinct.[2] 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.[3] Most often
other mechanisms are used by plants and animals to keep gametic isolation and species distinction. Species often have different
mating or courtship patterns or behavours, the breeding seasons maybe 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,
1/2^(2×2)=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.
- Hybrids examples
- 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.
A "Zeedonk", a zebra/donkey hybrid
A "Liger", a Lion/Tiger hybrid
A "Jaglion", a Jaguar/Lion hybrid
-
- 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, Kodiak and polar bears.
- Felid hybrids
- Fertile Canid hybrids occur between coyotes, wolves, dingoes,
jackals and domestic dogs. Dogs and wolves may be considered the
same species, making wolfdogs a non-hybrid.
- 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.
- Wolphin, 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 is genetic integrity. [5]
- Blood parrot cichlid, which is probably created by crossing a Gold
Severum and a Midas Cichlid or Red Devil Cichlid
Some dog hybrids are becoming increasingly popular and are
bred selectively.
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. Animals, being more
mobile, have developed complex mating behaviors that maintain the species boundary and when hybrids do occur, natural selection
tends to weed them out of the population since these hybrids generally can not find mates that will accept them or they are less
adapted and fit for survival in their habitats. 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.[4] 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. Plant hybrids, especially, are
often stronger than either parent variety, a phenomenon which when present is known as hybrid
vigour (heterosis) or heterozygote advantage.[5]
Plant breeders make use of a number of techniques to produce hybrids, including line
breeding and the formation of complex 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.
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 polar bear/grizzly bear hybrids.[6] 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 evolution favors the Sika
Deer genes. There is a similar situation with White-headed Ducks and Ruddy Ducks.
Genetic pollution and Extinction
-
Regionally developed ecotypes can be threatened with extinction when new genes are introduced that alter that ecotype, this is sometimes called Genetic Pollution.[7]
Hybridization and introgression of new genetic material can lead to the replacement of
local genotypes when the hybrids with new genes are more fit and have breeding advantages over the indigenous ecotype or species.[8] Nonnative species can alter native plants and animals by hybridization and
genetic integration. These genetic changes can result from the purposeful or accidental 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, the resulting hybrids might replace the original species genotype, ultimately
effecting the population to such a degree than none of the originally genetically distinct population remains.[9][10].
Effect on biodiversity and food security
-
In agriculture and animal husbandry, the green
revolutions use of conventional hybridization increased yields by creating "high-yielding varieties". Local governments and industry have been pushing hybridization,
effecting the use and genetic distinctiveness of several wild and indigenous breeds that have been selected locally. Often the
indigenous breeds after many years of selection have developed higher resistance to local extremes in climate and immunity to
local diseases etc. The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and
crossbreeding (genetic pollution), has reduced the gene pools of various wild and
indigenous breeds resulting in genetic erosion and genetic pollution and the loss of
genetic diversity. [11]
A genetically modified organism (GMO) is an organism whose genetic material has been altered using the genetic engineering techniques
generally known as recombinant DNA technology. Genetic Engineering today has become
another serious and alarming cause of genetic pollution because artificially created and genetically engineered plants and
animals in laboratories, which could never have evolved in nature even with conventional hybridization, can live and breed on
their own and what is even more alarming interbreed with naturally evolved wild varieties. Genetically Modified (GM) crops today
have become a common source for genetic pollution, not only of wild varieties but also of other domesticated varieties derived
from relatively natural hybridization.[12][13][14][15][16]
It is being said that genetic erosion coupled with genetic pollution is destroying that needed unique genetic base thereby creating an unforeseen hidden crisis which will result in a severe threat to our
food security for the future when diverse genetic material will cease to exist to be able
to further improve or hybridize weakening food crops and livestock against more resistant diseases and climatic changes.[17]
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 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.[18]
Mythical and legendary hybrids
In ancient folktales many fictional hybrids have become part of the mythological narrative. Many mythological creatures are
simple composites of known animals:
Some mythological hybrids were said to be the result of two species mixing.
- Centaur - the offspring of Centaurus
and the mares of Thessaly. Has the body of a
horse with its neck and head replaced by the torso and head of a man.
- Harpy - the torso of a woman with the wings and feet of a bird.
- Hippogriff - the offspring of a griffin and a horse, typically a male griffin and a
mare.
- Minotaur - the offspring of Pasiphaë and a white bull.
Has the body of a man and the head of a bull.
- Nephilim - the offspring of a fallen angel and human
woman.
Etymology
The word has a Latin root: hybrida (or ibrida) which meant "the offspring of a tame sow and wild boar". 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.[19]
See also
References
- ^ Keeton, William T. 1980. Biological science. New York: Norton. ISBN
0-393-95021-2 page A9.
- ^ ISBN 0-393-95021-2 Page 800
- ^ McBeath S, Tan PP, Bai Q, Speed RM. (1988).
- ^ http://www.colorado.edu/MCDB/MCDB2150Fall/notes00/L0033.html
- ^ Evaluating the utility of Arabidopsis thaliana as a model for
understanding heterosis in hybrid crops Journal Euphytica Publisher Springer Netherlands ISSN 0014-2336 (Print) 1573-5060
(Online) Issue Volume 156, Numbers 1-2 / July, 2007 DOI 10.1007/s10681-007-9362-1 Pages 157-171
- ^ "Hybrid bear shot dead in Canada", BBC News, 2006-05-13.
- ^ Hybridization and Introgression; Extinctions; from "The evolutionary impact of invasive
species; by H. A. Mooney and E. E. Cleland" Proc Natl Acad Sci U S A. 2001 May 8; 98(10): 5446–5451. doi: 10.1073/pnas.091093398.
Proc Natl Acad Sci U S A, v.98(10); May 8, 2001, The National Academy of Sciences
- ^ Glossary: definitions from the following publication: Aubry, C., R. Shoal and V. Erickson. 2005.
Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest
Service. 44 pages, plus appendices.; Native Seed Network (NSN), Institute for Applied Ecology, 563 SW Jefferson Ave, Corvallis,
OR 97333, USA
- ^ EXTINCTION BY
HYBRIDIZATION AND INTROGRESSION; by Judith M. Rhymer , Department of Wildlife Ecology, University of Maine, Orono, Maine 04469,
USA; and Daniel Simberloff, Department of Biological Science, Florida State University, Tallahassee, Florida 32306, USA; Annual
Review of Ecology and Systematics, November 1996, Vol. 27, Pages 83-109 (doi: 10.1146/annurev.ecolsys.27.1.83), [1]
- ^ Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the
RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; by Brad M. Potts, Robert C. Barbour, Andrew B. Hingston; September
2001; RIRDC Publication No 01/114; RIRDC Project No CPF - 3A; ISBN 0 642 58336 6; ISSN 1440-6845; Australian Government, Rural
Industrial Research and Development Corporation
- ^ “Genetic Pollution: The Great Genetic Scandal”; Devinder Sharma., [2]
- ^ THE YEAR
IN IDEAS: A TO Z.; Genetic Pollution; By MICHAEL POLLAN, The New York Times, December 9, 2001
- ^ Dangerous Liaisons? When Cultivated Plants Mate with Their Wild Relatives; by Norman C.
Ellstrand; The Johns Hopkins University Press, 2003; 268 pp. hardcover , $ 65; ISBN 0-8018-7405-X. Book Reviewed in: Hybrids
abounding; Nature Biotechnology 22, 29 - 30 (2004) doi:10.1038/nbt0104-29; Reviewed by: Steven H Strauss & Stephen P DiFazio;
1 Steve Strauss is in the Department of Forest Science, Oregon State University, Corvallis, Oregon 97331-5752, USA.
steve.strauss@oregonstate.edu; 2 Steve DiFazio is at Oak Ridge National Laboratory, Bldg. 1059, PO Box 2008, Oak Ridge, Tennessee
37831-6422 USA. difazios@ornl.gov.
- ^ “Genetic pollution: Uncontrolled spread of genetic information (frequently referring to
transgenes) into the genomes of organisms in which such genes are not present in nature.” Zaid, A. et al. 1999. Glossary of
biotechnology and genetic engineering. FAO Research and Technology Paper No. 7. ISBN 92-5-104369-8
- ^ “Genetic pollution: Uncontrolled escape of genetic information (frequently referring to
products of genetic engineering) into the genomes of organisms in the environment where those genes never existed before.”
Searchable Biotechnology Dictionary. University of Minnesota., [3]
- ^ “Genetic pollution:
Living organisms can also be defined as pollutants, when a non-indigenous species (plant or animal) enters a habitat and modifies
the existing equilibrium among the organisms of the affected ecosystem (sea, lake, river). Non-indigenous, including transgenic
species (GMOs), may bring about a particular version of pollution in the vegetable kingdom: so-called genetic pollution. This
term refers to the uncontrolled diffusion of genes (or transgenes) into genomes of plants of the same type or even unrelated
species where such genes are not present in nature. For example, a grass modified to resist herbicides could pollinate
conventional grass many miles away, creating weeds immune to the most widely used weed-killer, with obvious consequences for
crops. Genetic pollution is at the basis of the debate on the use of GMOs in agriculture.” The many facets of pollution; Bologna
University web site for Science Communication. The Webweavers: Last modified Tue, 20 Jul 2005
- ^ “Genetic Pollution: The Great Genetic Scandal”; Devinder Sharma can be contacted at: 7 Triveni
Apartments, A-6 Paschim Vihar, New Delhi-110 063, India. Email: dsharma@ndf.vsnl.net.in. CENTRE FOR ALTERNATIVE AGRICULTURAL
MEDIA (CAAM)., [4]
- ^ Barriers to hybridization of Solanum bulbocastanumDun. and S.
VerrucosumSchlechtd. and structural hybridity in their F1 plants Journal Euphytica Publisher Springer Netherlands ISSN 0014-2336
(Print) 1573-5060 (Online) Issue Volume 25, Number 1 / January, 1976 Category Articles DOI 10.1007/BF00041523 Pages 1-10
- ^ Oxford English Dictionary Online, Oxford University Press 2007.
External links
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