Heterosis
Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions of its parents. The heterotic offspring often has traits that are more than the simple addition of the parents' traits, and can be explained by Mendelian or non-Mendelian inheritance. Typical heterotic/hybrid traits of interest in agriculture are higher yield, quicker maturity, stability, drought tolerance etc.
Definitions
In proposing the term heterosis to replace the older term heterozygosis, G.H. Shull aimed to avoid limiting the term to the effects that can be explained by heterozygosity in Mendelian inheritance.Heterosis is often discussed as the opposite of inbreeding depression, although differences in these two concepts can be seen in evolutionary considerations such as the role of genetic variation or the effects of genetic drift in small populations on these concepts. Inbreeding depression occurs when related parents have children with traits that negatively influence their fitness largely due to homozygosity. In such instances, outcrossing should result in heterosis.
Not all outcrosses result in heterosis. For example, when a hybrid inherits traits from its parents that are not fully compatible, fitness can be reduced. This is a form of outbreeding depression, the effects of which are similar to inbreeding depression.
Genetic and epigenetic bases
Since the early 1900s, two competing genetic hypotheses, not necessarily mutually exclusive, have been developed to explain hybrid vigor. More recently, an epigenetic component of hybrid vigor has also been established.Dominance and overdominance
When a population is small or inbred, it tends to lose genetic diversity. Inbreeding depression is the loss of fitness due to loss of genetic diversity. Inbred strains tend to be homozygous for recessive alleles that are mildly harmful. Heterosis or hybrid vigor, on the other hand, is the tendency of outbred strains to exceed both inbred parents in fitness.Selective breeding of plants and animals, including hybridization, began long before there was an understanding of underlying scientific principles. In the early 20th century, after Mendel's laws came to be understood and accepted, geneticists undertook to explain the superior vigor of many plant hybrids. Two competing hypotheses, which are not mutually exclusive, were developed:
Image:Heterosis.svg|thumb|300px|right| Genetic basis of heterosis.
Dominance hypothesis. Scenario A.
Fewer genes are under-expressed in the homozygous individual. Gene expression in the offspring is equal to the expression of the fittest parent.
Overdominance hypothesis. Scenario B.
Over-expression of certain genes in the heterozygous offspring.
- Dominance hypothesis. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable recessive alleles from one parent by dominant alleles from the other. It attributes the poor performance of inbred strains to loss of genetic diversity, with the strains becoming purely homozygous at many loci. The dominance hypothesis was first expressed in 1908 by the geneticist Charles Davenport. Under the dominance hypothesis, deleterious alleles are expected to be maintained in a random-mating population at a selection–mutation balance that would depend on the rate of mutation, the effect of the alleles and the degree to which alleles are expressed in heterozygotes.
- Overdominance hypothesis. Certain combinations of alleles that can be obtained by crossing two inbred strains are advantageous in the heterozygote. The overdominance hypothesis attributes the heterozygote advantage to the survival of many alleles that are recessive and harmful in homozygotes. It attributes the poor performance of inbred strains to a high percentage of these harmful recessives. The overdominance hypothesis was developed independently by Edward M. East and George Shull. Genetic variation at an overdominant locus is expected to be maintained by balancing selection. The high fitness of heterozygous genotypes favours the persistence of an allelic polymorphism in the population. This hypothesis is commonly invoked to explain the persistence of some alleles that are harmful in homozygotes. In normal circumstances, such harmful alleles would be removed from a population through the process of natural selection. Like the dominance hypothesis, it attributes the poor performance of inbred strains to expression of such harmful recessive alleles.
Historical retrospective
Which of the two mechanisms are the "main" reason for heterosis has been a scientific controversy in the field of genetics. Population geneticist James Crow believed, in his younger days, that overdominance was a major contributor to hybrid vigor. In 1998 he published a retrospective review of the developing science. According to Crow, the demonstration of several cases of heterozygote advantage in Drosophila and other organisms first caused great enthusiasm for the overdominance theory among scientists studying plant hybridization. But overdominance implies that yields on an inbred strain should decrease as inbred strains are selected for the performance of their hybrid crosses, as the proportion of harmful recessives in the inbred population rises. Over the years, experimentation in plant genetics has proven that the reverse occurs, that yields increase in both the inbred strains and the hybrids, suggesting that dominance alone may be adequate to explain the superior yield of hybrids. Only a few conclusive cases of overdominance have been reported in all of genetics. Since the 1980s, as experimental evidence has mounted, the dominance theory has made a comeback.Crow wrote:
The current view... is that the dominance hypothesis is the major explanation of inbreeding decline and the high yield of hybrids. There is little statistical evidence for contributions from overdominance and epistasis. But whether the best hybrids are getting an extra boost from overdominance or favorable epistatic contributions remains an open question.
Epigenetics
An epigenetic contribution to heterosis has been established in plants, and it has also been reported in animals. MicroRNAs, discovered in 1993, are a class of non-coding small RNAs which repress the translation of messenger RNAs or cause degradation of mRNAs. In hybrid plants, most miRNAs have non-additive expression. This suggests that the small RNAs are involved in the growth, vigor and adaptation of hybrids.'Heterosis without hybridity' effects on plant size have been demonstrated in genetically isogenic F1 triploid plants, where paternal genome excess F1 triploids display positive heterosis, whereas maternal genome excess F1s display negative heterosis effects. Such findings demonstrate that heterosis effects, with a genome dosage-dependent epigenetic basis, can be generated in F1 offspring that are genetically isogenic. It has been shown that hybrid vigor in an allopolyploid hybrid of two Arabidopsis species was due to epigenetic control in the upstream regions of two genes, which caused major downstream alteration in chlorophyll and starch accumulation. The mechanism involves acetylation or methylation of specific amino acids in histone H3, a protein closely associated with DNA, which can either activate or repress associated genes.
Specific mechanisms
Major histocompatibility complex in animals
One example of where particular genes may be important in vertebrate animals for heterosis is the major histocompatibility complex. Vertebrates inherit several copies of both MHC class I and MHC class II from each parent, which are used in antigen presentation as part of the adaptive immune system. Each different copy of the genes is able to bind and present a different set of potential peptides to T-lymphocytes. These genes are highly polymorphic throughout populations, but are more similar in smaller, more closely related populations. Breeding between more genetically distant individuals decreases the chance of inheriting two alleles that are the same or similar, allowing a more diverse range of peptides to be presented. This, therefore, increases the chance that any particular pathogen will be recognised, and means that more antigenic proteins on any pathogen are likely to be recognised, giving a greater range of T-cell activation, so a greater response. This also means that the immunity acquired to the pathogen is against a greater range of antigens, meaning that the pathogen must mutate more before immunity is lost. Thus, hybrids are less likely to succumb to pathogenic disease and are more capable of fighting off infection. This may be the cause, though, of autoimmune diseases.Plants
Crosses between inbreds from different heterotic groups result in vigorous F1 hybrids with significantly more heterosis than F1 hybrids from inbreds within the same heterotic group or pattern. Heterotic groups are created by plant breeders to classify inbred lines, and can be progressively improved by reciprocal recurrent selection.Heterosis is used to increase yields, uniformity, and vigor. Hybrid breeding methods are used in maize, sorghum, rice, sugar beet, onion, spinach, sunflowers, broccoli and to create a more psychoactive cannabis.