Hybridization


In biology, hybrid has two meanings. 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 consists of 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.




Types of Hybrids

Depending on the parents, there are a number of different types of hybrids;




Interspecific Hybrids

An example of an interspecific hybrid is a hybrid between a Bengal tiger and a 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.

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 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 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, (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.




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 Pasiphae 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.

Examples of Hybrid Animals

Hybrid Biology Wikipedia




In the News ...


UK's first hybrid embryos created BBC - April 1, 2008
Scientists at Newcastle University have created part-human, the BBC can reveal. The embryos survived for up to three days and are part of medical research into a range of illnesses. It comes a month before MPs are to debate the future of such research.

The Catholic Church describes it as "monstrous". But medical bodies and patient groups say such research is vital for our understanding of disease. They argue that the work could pave the way for new treatments for conditions such as Parkinson's and Alzheimer's.

Under the microscope the round bundles of cells look like any other three-day-old embryos. In fact they are hybrids - part-human, part-animal.

They were created by injecting DNA derived from human skin cells into eggs taken from cows ovaries which have had virtually all their genetic material removed. So what possible justification can scientists offer for doing what the Catholic Church has branded "experiments of Frankenstein proportion"?The Newcastle team say they are using cow ovaries because human eggs from donors are a precious resource and in short supply.

The hybrid embryos are purely for research and would never be allowed to develop beyond 14 days when they are still smaller than a pinhead. Scientists want to extract stem cells, the body's master cells, from the embryos, in order to increase understanding of a whole range of diseases from diabetes to stroke and ultimately to produce treatments.

Professor John Burn from Newcastle University says the research is entirely ethical. "This is licensed work which has been carefully evaluated. This is a process in a dish, and we are dealing with a clump of cells which would never go on to develop. It's a laboratory process and these embryos would never be implanted into anyone. "We now have preliminary data which looks promising but this is very much work in progress and the next step is to get the embryos to survive to around six days when we can hopefully derive stem cells from them."

The research in Newcastle was approved by the UK's fertility regulator, the Human Fertilisation and Embryology Authority. It acted ahead of the passing of new legislation which will specifically allow the creation of hybrid embryos so as not to hold back research. The bill setting out the new legislation is not due to be debated in the House of Commons until next month. It is highly controversial and last week Prime Minister Gordon Brown gave in to demands for a free vote on the issue.

Critics from the Roman Catholic Church say the creation of hybrids is immoral. "It is difficult to imagine a single piece of legislation which more comprehensively attacks the sanctity and dignity of human life than this particular bill," Cardinal Keith O'Brien, archbishop of St Andrews and Edinburgh declared last week. Dr David King, of Human Genetics Alert, said: "For anyone who understands basic biology, it is no surprise that these embryos died at such an early stage. "Cloning is inefficient precisely because it is so unnatural, and by mixing species it becomes even more unnatural and unlikely to succeed. "The public has been grossly misled by the hype that this is vital medical research. "Even if stem cells were ever to be produced, like cloned animals, they would have so many errors of their metabolism that they would produce completely misleading data." Not for the first time developments in science have outpaced the debate from legislators. For supporters of embryo research the creation of hybrid embryos is a small but significant move forward. For opponents it is a step too far.




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