Concepts of Hybrid Vigor and Inbreeding Depression

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  Concepts of hybrid vigor and inbreeding depression Heterosis may be defined as the increase in size, vigour, fertility, and overall productivity of a hybrid plant, over the midparent value (average performance of the two parents). Heterosis ={[F 1   − (P 1  +P 2 )/2] / [(P 1  +P 2 )/2]} or F1-MP/MPx100 Term, heterosis, was coined by G. H. Shull The estimate is usually calculated as a percentage (i.e., ×100). The synonymous term is hybrid vigour. It should be pointed out immediately that, as it stands, heterosis is of no value to the breeder (and hence farmer) if a hybrid will only exceed the midparent in performance. The practical definition of heterosis is hybrid vigor that greatly exceeds the better or higher parent in a cross. For commercial purposes, heterosis is calculated as superiority of F 1  over the best commercial variety of the concerned crop (Economic or useful heterosis). Heterosis occurs when two inbred lines of outbred species are crossed, as much as when crosses are made between pure lines of inbreeders. Heterosis can be manifested in several ways: ã   Increased size and productivity (cotton hybrids need wider spacing) ã   Increased growth rate not accompanied by large size at maturity ã   Increase in number of pods, leaves, nodes as compared to parents but overall size may not increase ã   Earlier in maturity ã   Greater resistance to biotic/abiotic stresses, may have better fitness Heterosis, though widespread in the plant kingdom, is not uniformly manifested in all species and for all traits. It is manifested at a higher in tensity in traits that have fitness value, and also more frequently among cross-pollinated species than self-pollinated species. All breeding methods that are preceded by crossing make use of heterosis to some extent. However, it is only in hybrid cultivar breeding and the breeding of clones that the breeder has the opportunity to exploit the phenomenon to full advantage. Hybrids dramatically increase yields of non-hybrid cultivars. By the early 1930s (before extensive use of hybrids), maize yield in the USA averaged 1,250 kg/ha. By the early 1970s (following the adoption of hybrids), maize yields quadrupled to 4,850 kg/ha. The contribution of hybrids (genotype) to this increase was estimated at about 60% (the remainder being attributed to pro- duction practices). Inbreeding depression   Heterosis is opposite and complementary to inbreeding depression (reduction in fi tness as a direct result of inbreeding). In theory, the heterosis observed on crossing is expected to be equal to the depression upon inbreeding, considering a large number of crosses between lines derived from a single base population. Reduction in fitness  is usually manifested as a reduction in vigor, fertility, and productivity. The effect of inbreeding is more severe in the early generations (generations 5  – 8).  Degree of inbreeding depression: ã   High inbreeding depression: A large proportion of plants produced by selfing show lethal characteristics and do not survive. Loss in vigour and fertility is so great that few lines can be maintained after 3-4 generations of selfing. eg. alfalfa and carrot ã   Moderate inbreeding depression: many lethal and sublethal types appear in the selfed progeny but a substantial proportion of the progeny can be maintained under self pollination eg maize, sorghum, bajra ã   Low inbreeding depression: sunflower, cucurbits, and rye are more tolerant of inbreeding with minimal consequences of inbreeding depression. ã   No inbreeding depression: Most of Self pollinated, some cross pollinated crops like cucurbits. Effects of inbreeding: ã   A number of lethal and sublethal appear in the early generations of selfing. Maximum decrease in first generation in maize ã   The material rapidly separates into distinct lines which become increasingly uniform for differences in morphological and functional traits ã   Most of the lines decrease in vigour and fecundity till they cannot be maintained even under most favourable cultural conditions ã   The lines that survive show a general decline in size and vigour  Purpose of inbreeding ã   Production of inbred lines for use in hybrid seed production. Inbreds can be maintained through self pollination without change in genetic constitution ã   To reduce frequency of deleterious recessive genes in genotypes that serve as parents of synthetics or a vegetatively propagated cultivar ã   Inbreeding increases genetic variability among individuals in a population. Increased variability among inbred progeny can increase the effectiveness of selection Selfing and BC (inbred recurrent parent) lead to most intense form of inbreeding followed by Full sib mating; BC( non inbred recurrent parent) and the least effective being Half Sib mating Genetic basis of heterosis Two schools of thought have been advanced to explain the genetic basis for why fitness lost on inbreeding tends to be restored upon crossing. The two most commonly known are the dominance theory and the overdominance theory. A third theory, the mechanism of epistasis (non-allelic gene  interactions) has also been proposed. A viable theory should account for both inbreeding depression in cross- pollinated species upon selfing, and increased vigor in F1 organisms upon hybridization.   Dominance theory : Proposed by C. G. Davenport in 1908; Bruce (1910); Keeble and Pellew (1910). The dominance theory assumes that vigor in plants is conditioned by dominant alleles, recessive alleles being deleterious or neutral in effect. It follows then that a genotype with more dominant alleles will be more vigorous than one with few dominant alleles. Consequently, crossing two parents with complementary dominant alleles will concentrate more favorable alleles in the hybrid than either parent. Inbreeding depression occurs upon selfing because the deleterious recessive alleles that are protected in the heterozygous condition (heterozygous advantage), become homozygous and are expressed leading to loss of vigour and fitness. The recessives are also expressed in open pollinated populations but not at frequencies which reduce productivity severely. The increase in their frequency upon inbreeding appears to provide an explaination for part of the injurious effects due to inbreeding. Unfavourable genes segregate on inbreeding and upon fixation brought about by homozygosity, produce lines which possess different genes or gene complexes. Some lines receive more of the favourable genes than others accounting for diffrences observed in the degree of inbreeding depression in different lines. Inbreeding depression is thus not a process of degeneration but a consequence of Mendelian segregation. Under this hypothesis, when two inbreds are crossed leading to formation of hybrids, the deleterious recessive genes contributed by one parent are again hidden by dominant alleles contributed by the other inbred line. This, of course, will depend on the genotype of the two inbreds. Some genotypes should complement eaach other nicely to produce hybrids better than the average of the srcinal open polinated variety, whereas some will not 'nick' well by virtue of the particular combination of dominant and recessive genes they happened to receive during segregation. This hypothesis seemed to be consonant with the observed facts and appeared to explain the effets of inbreeding and outcrossing in their relation to one another. Inbred A Inbred B AAbbccDDEE x aaBBccddee F 1 AaBbccDdEe It has more dom alleles than either of the parent, and thus more vigourous Objections to dominance hypothesis 1) If this hypothesis is correct it should be possible to obtain individuals homozygous for all the dominant factors. Such lines will be like F1 in vigour but will be true breeding. Such lines have not been found. Jones(1918) explained that chances of getting such true breeding lines are remote. He pointed out that many genes probably affect growth and that each chromosome would be expected to include some favourable dominants and some unfavourable recessives. A series of precisely placed crossovers will be  required to obtain all the dominant genes in one gamete. However, much progress has been made in obtaining more favourable combinations of favourable dominant genes since this theory was proposed. Especially in corn, the inbred lines used now a days in hybrid seed production are greatly improved in vigour and productivity over those used earlier. Now single cross hybrid seed production is not a problem and single cross hybrids are dominating maize cultivation now. 2. The other objection was directed at the symmetrical distributions that were observed for heterotic characters in F 2 . If heterosis is solely due to dominance of independent factors, the F 2  distribution curve should be skewed, rather than symmetrical, since the dominant and recessive phenotypes would be distributed according to the expansion of the binomial (3/4+1/4) n  . Jones was able to reconcile the symmetrical distribution actually observed with the hypothesis on the basis that linkage between groups of favourable and unfavourable genes would lead to theoretically symmetrical distribution. Collins(1921) pointed out that even in the absence of linkage, skewness will be difficult to detect, and moreover, chances of recovering a completely homozygous type would be remote so long as the number of genes involved were at all large. If the two parents differ by n genes, probability of recovering a F2 plant homozygous for all dominants is (¼) n If n=20, this probability is one in 4 20  It is further reduced if linkage between desurable and undesirable genes Another objection against dominance hypothesis is that although inbred lines have been improved over the decades, the magnitude of heterosis has not decreased but increased slightly. If heterosis was caused by complementation of deleterious recessive alleles and inbreds have been purged of such alleles, absolute amount of heterosis is expected to decrease somewhat. This slight increase in hybrid vigour over the years may be due to selection of alleles at loci that produce best combination of hybrids Another objection to dominance hypothesis is progressive heterosis in tetraploids (Bingham et al 1994). Diploids can have two different alleles at a locus; tetraploids can have four In autotetraploids amount of heterosis increases with increase in number of different alleles at a locus ( AABB or ABCD) For simple complementation to explain heterosis, each new stepwise combination of genomes would need to supply increasingly superior alleles to complement the pre existing rate limiting alleles without introducing deleterious alleles at other loci. The probability that this situation would occur is quite low. Moreover, inbreeding depression in tetraploids in many species is faster than expected based on increase in homozygosity. This suggests that allelic dosage plays a important role in tetraploids for generating inbreeding depression than does complete homozygosis itself. The increasing number of identical alleles appears to have negative dosage effect on vigour These several ideas came to be called as the dominance or the dominance of linked genes hypothesis. Overdominance hypothesis: Proposed independently by Shull and East (1908). It aassumes that there is a physiological stimulus to development that increases with the diversity of uniting gametes. In Mendelian terms, it means that there are loci at which the heterozygote is superior to either homozygote and that vigour increases in proportion to the amount of hetrozygosis. This idea has been
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