Hybrid (biology)
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, subspecies, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents such as in blending inheritance, but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.
Species are reproductively isolated by strong barriers to hybridization, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.
A form of often intentional human-mediated hybridization is the crossing of wild and domesticated species. This is common in both traditional horticulture and modern agriculture; many commercially useful fruits, flowers, garden herbs, and trees have been produced by hybridization. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy. It is also more occasionally done in the livestock and pet trades; some well-known wild × domestic hybrids are beefalo and wolfdogs. Human selective breeding of domesticated animals and plants has also resulted in the development of distinct breeds ; crossbreeds between them are sometimes also imprecisely referred to as "hybrids".
Hybrid humans existed in prehistory. For example, Neanderthals and anatomically modern humans are thought to have interbred as recently as 40,000 years ago.
Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women.
Significance
In evolution
Hybridization between species plays an important role in evolution, though there is much debate about its significance. Roughly 25% of plants and 10% of animals are known to form hybrids with at least one other species. One example of an adaptive benefit to hybridization is that hybrid individuals can form a "bridge" transmitting potentially helpful genes from one species to another when the hybrid backcrosses with one of its parent species, a process called introgression. Hybrids can also cause speciation, either because the hybrids are genetically incompatible with their parents and not each other, or because the hybrids occupy a different niche than either parent.Hybridization is a particularly common mechanism for speciation in plants, and is now known to be fundamental to the evolutionary history of plants. Plants frequently form polyploids, individuals with more than two copies of each chromosome. Whole genome doubling has occurred repeatedly in plant evolution. When two plant species hybridize, the hybrid may double its chromosome count by incorporating the entire nuclear genome of both parents, resulting in offspring that are reproductively incompatible with either parent because of different chromosome counts.
In conservation
has resulted in an increase in the interbreeding between regional species, and the proliferation of introduced species worldwide has also resulted in an increase in hybridization. This has been referred to as genetic pollution out of concern that it may threaten many species with extinction. Similarly, genetic erosion from monoculture in crop plants may be damaging the gene pools of many species for future breeding.The conservation impacts of hybridization between species are highly debated. While hybridization could potentially threaten rare species or lineages by "swamping" the genetically "pure" individuals with hybrids, hybridization could also save a rare lineage from extinction by introducing genetic diversity. It has been proposed that hybridization could be a useful tool to conserve biodiversity by allowing organisms to adapt, and that efforts to preserve the separateness of a "pure" lineage could harm conservation by lowering the organisms' genetic diversity and adaptive potential, particularly in species with low populations. While endangered species are often protected by law, hybrids are often excluded from protection, resulting in challenges to conservation.
Etymology
The term hybrid is derived from Latin hybrida, used for crosses such as of a tame sow and a wild boar. The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids. Examples of this include the cama, pumapard, sturddlefish, and wholphin.
As seen by different disciplines
Animal and plant breeding
From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different breeds. Single cross hybrids result from the cross between two true-breeding organisms which produces an F1 hybrid. 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. Typically, 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 an F1 hybrid and an inbred line. Triple cross hybrids result from the crossing of two different three-way cross hybrids. Top cross hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average.
Population hybrids result from the crossing of plants or animals in one population with those of another population. These include interspecific hybrids or crosses between different breeds. In biology, the result of crossing of two populations is called a synthetic population.
In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen produces offspring that are "true to type" with respect to phenotype; i.e., a true-breeding organism.
Biogeography
Hybridization can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis and L. a. astyanax. The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridization occurs between a narrow area across New England, southern Ontario, and the Great Lakes, the "suture region". It is at these regions that the subspecies were formed. Other hybrid zones have formed between described species of plants and animals.Genetics
From the point of view of genetics, several different kinds of hybrid can be distinguished.A genetic hybrid carries two different alleles of the same gene, where for instance one allele may code for a lighter coat colour than the other. 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 results when only the heterozygous genotype occurs, as in Oenothera lamarckiana, because all homozygous combinations are lethal. In the early history of genetics, Hugo de Vries supposed these were caused by mutation.
Genetic complementation
Genetic complementation is a hybridization test widely used in genetics to determine whether two separately isolated mutants that have the same phenotype are defective in the same gene or in different genes. If a hybrid organism containing the genomes of two different mutant parental organisms displays a wild type phenotype, it is ordinarily considered that the two parental mutant organisms are defective in different genes. If the hybrid organism displays a distinctly mutant phenotype, the two mutant parental organisms are considered to be defective in the same gene. However, in some cases the hybrid organism may display a phenotype that is only weakly wild-type, and this may reflect intragenic complementation.Taxonomy
From the point of view of taxonomy, hybrids differ according to their parentage.Hybrids between different subspecies are called intra-specific hybrids. Interspecific hybrids are the offspring from interspecies mating; these sometimes result in hybrid speciation. Intergeneric hybrids result from matings between different genera, such as between sheep and goats. Interfamilial hybrids, such as between chickens and guineafowl or pheasants, are reliably described but extremely rare. Interordinal hybrids are few, but have been engineered between the sea urchin Strongylocentrotus purpuratus and the sand dollar Dendraster excentricus.