Heterosis refers to the phenomenon that the first generation of hybrids are superior to their parents in body shape, growth rate, fecundity and behavioral characteristics.Animal breeders often cross two different pure line parents with some desired traits, and the hybrid seed generation often shows better traits than their parents.However, if the first generation of hybrid crosses with each other again, this advantage will be weakened.
Heterosis isHeterozygoteIn one or morecharacterSuperior to twoparentPhenomenon.For example, differentstrainThe hybrid generation obtained by hybridization between different varieties or even between different species and genera often shows stronger growth rate and metabolic function than its parents, leading to developed organs, increased body size, increased yield, or disease resistance, insect resistanceResistance to adversityForceBecome energetic, fertility, viability, etc.This is a common phenomenon in the biological world.[1]
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One is often found in someDistant hybridizationThe heterosis in the progeny only shows the enlargement of individuals or some organs, just like the exuberant growth of plants in excellent environments, but their survival and reproduction ability has not improved.This kind of heterosis is calledHybrid vigor。Hybrid vigor may be beneficial to production, but it may not have adaptive significance in evolution, so some scholars studying biological evolution call itPseudoheterosis。
The other type of heterosis is manifested in the improvement of hybrid survival and reproductive capacity, but it does not necessarily exceed its parents in individual growth. This type of heterosis has evolutionary adaptation significance, so it is regarded as a real heterosis.
In very few organisms, the survival ability of hybrids can also be reduced compared with that of their parents. This phenomenon is calledHybrid disadvantage。
Hybrids with heterosis generally showSelfingDecline phenomenon.For example, the plant height of maize shows heterosis, and self pollination decline can also be seen clearly.The mortality rate of newborns and infants who are closely married is higher than that of ordinary marriage,Congenital diseaseAnd the birth rate of mentally retarded babies is also high, which are all humanInbreeding declinePhenomenon.[1]
A brief history of research
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China
As early as 2000 years ago, Chinese people used mares and donkeys to mate with each other to obtain a powerful hybrid service mule, setting a precedent for observing and using heterosis in human history.
foreign country
In plants, German scholar JG. In 1776, Kerroyt first described the heterosis among different species of tobacco, Dianthus, Mirabilis jalapa, Datura, etc.1849 CF. Geithner also found heterosis in 700 species of 80 genera he studied.G. J. Mendel's paper "Plant Hybridization Experiment" published in 1866 mentioned that the progeny plants obtained by crossing two kinds of peas, one foot high and six foot high, without exception, all reached the height of 6-7.5 feet.1876 CR. In his book The Effects of Cross pollination and Self pollination in the Plant World, Darwin summarized the observation results of hybridization and self pollination experiments among 30 families, 52 genera, 57 species and many varieties and lines, and concluded that hybridization is beneficial to plants, while self pollination is harmful to plants.Inspired by Darwin's conclusion, many scholars have carried out extensive research on the theory and application of heterosis in the following 20-30 years.1900Mendel's lawAfter rediscovery, the research and application of heterosis have been further developed.[1]
In the early stage, A5. Bruce and DF. Jones et al.
They think the majorityDominant geneIt is conducive to individual growth and development, relativelyRecessive geneIt is not conducive to growth and development.Self or inbreeding of hybrid individuals will increase offspringHomozygoteThe chance to expose the harmful traits represented by recessive genesInbreeding decline。If we choose these different inbred offspringPure line（inbred line）, then some recessive genes brought into the offspring hybrid by one parent will be transferred by another parentDominant alleleSo as to enhance the growth potential of heterozygous offspring.Because heterosis involves many genes, and harmful recessive genes and favorable dominant genes are inevitably linked, the probability of concentrating a large number of favorable genes in a homozygous state into an individual of inbred offspring is very small, so it is impossible to obtain an inbred line with the same growth potential as the hybrid.In hybridizationOffspring generationHowever, almost all harmful genes are covered by their favorable genes, so heterosis appears.Dominant hypothesisThe explained heterosis of this harmful recessive mutation gene covered by the dominant gene is calledMutagenicityHeterosis, which is a protective measure for offsetting the pressure of harmful mutation, is the basis of the allogamous population.
Wild type genes are generally dominant, dominant genes mostly encode proteins with biological activity, mutant genes are generally recessive, and recessive genes mostly encode proteins that lose or reduce activity.Therefore, the viability of hybrid AaBb is higher than that of pure AAbb or aaBB.This is the biochemical basis of heterosis in the dominance hypothesis.
Hyperdominant hypothesis
First by GH. In 1911, Sar put forward that heterosis was an effect of stimulating development after the combination of gametes with different genotypes.Later EM. In 1918, Easter believed that theAlleleThe interaction (such as A1 and A2) in the heterozygote (A1A2) has the function of stimulating growth, so the heterozygote shows greater growth advantage than the two parental homozygotes (A1A1 and A2A2), and the degree of advantage growth is closely related to the degree of heterozygosity between alleles.according toHyperdominant hypothesisThe heterozygotes A1A2, A1A3, A2A3, etc. always have high adaptability, so A1, A2, A3,... and other genes can be stored in this population with a certain frequency, becoming a balanced polymorphism and making the population have the maximum adaptability.Such heterosis can be calledBalanced heterosis。
Hyperdominant hypothesisThe biochemical basis of the said heterosis has at least two possible situations:
① The two alleles each encode a protein. The interaction of these two proteins is more conducive to the survival of individuals than their independent existence.For example, the sickle shape of humanhemoglobinThere are two kinds of hemoglobin in red blood cells of heterozygote (HbA/HbS): adult hemoglobin (HbA) and sickle cell hemoglobin (HbS).Heterozygotes are neither anaemic nor easy to beplasmodiumInfection, which is more conducive to survival in malaria endemic areas.
② The polypeptide encoded by two heterozygous alleles combines to form a protein with higher activity than the same subunit.This form of allelic interaction was at least onceGlutamate dehydrogenaseGene.[1]
application
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Breeding application
According to the principle of heterosisbreedingThe improvement and innovation of means can make agricultural (livestock) products achieve significant growth.In this regard, hybrid maize was the first to be used and achieved the most remarkable results, generally increasing the yield by more than 20%.Then onsilkworm, poultry, pigscattle、beet、forage grass、sorghum、onion、eggplant, tomato, green peppercotton, sunflower, rape, flowers and treesHybrid generationProduction and utilization.The methods of obtaining heterosis vary according to the reproductive characteristics and available genetic characteristics of different species.Due to the discovery and utilization of male sterility and its restoration genes, many of the above crops, vegetables and wheat can be mass produced and planted with hybrid seeds.Since the mid-1970s, Chinese breeding workers have pioneered the popularization and utilization of hybrid rice in production in a large area, which has received great yield gains and opened up a new way for the application of heterosis.
Asexual reproduction
Heterosis declined greatly from the second generation of hybrids.So it was envisaged to useApomixisThe principle ofHybrid generationFixed dominance can save the trouble of preparing hybrid seeds every year, but so far it has not been successful.Now there are only sweet potatoes and potatoesAsexual reproductionThe seed plants ofSexual hybridization, and then rely on asexual propagules to maintain a certain degree of heterosis.along withPlant tissue culture technologyIt is expected to make a breakthrough in simplifying hybrid seed production procedures by using somatic cell asexual proliferation and differentiation into seedlings.[1]
