Fixation of heterosis

Hybrid vigour has been exploited in both self as well as cross pollinated species. The expression of heterosis is restricted to the hybrid individual (F1 generation) and it dissipates in the advanced generations or in the progenies derived from it due to segregation and recombination which disturb the original heterotic combination of genes. This makes it imperative that hybrid seeds be produced every time since the hybrid vigour breaks down in succeeding generations. A preservation of heterotic expression is, however, possible if fixation of the hybridity i.e. hybrid combination giving heterosis is maintained from one generation to another. There are four principle ways of fixation of heterosis in crop plants.

1. Asexual Reproduction Species which possess the capability of asexual reproduction through vegetative parts offer one of the easiest ways of preserving heterosis expression. Plant breeders have often taken advantage of this situation by faithfully perpetuating heterozygous genotype which remains unaltered since sexual apparatus remains unaffected. For example, Coastal Bermuda grass, which is an important forage crop, is a hybrid between two strains of Bermuda from South Africa. The superior quality of the original hybrid is maintained by propagating it through stolons or rhizomes. Another successful case of heterosis maintenance has been obtained in triploid hybrids of banana namely, Gros, Micheal and Cavendish.

2. Apomixis Apomixis is also a form of asexual reproduction. This offers a mechanism to propagate genotypes asexually through development of seeds in the absence of fertilization. Fixation of heterosis by apomixis is common among citrus fruits, black berries, roses, blue grasses and several other ornamental plants. If F 1 and its progeny produce only apomictic seeds having 2n embryos, the F 2 and the subsequent generations will be identical in genotype to the F 1. Therefore, apomixis will lead to the fixation of heterosis. In order to produce the hybrid seed, one parental inbred should reproduce sexually (female) while the other parent is preferably obligate apomicts (male). Apomixis should be dominant to sexuality so that the F 1 and the subsequent generations produce only apomictic seeds

Obligate apomixis results in complete fixation of heterosis. This has been achieved in Bahia grass (P. notatum) and in buffel grass (P.ciliare ) which are obligate apomicts. If apomixis is facultative, heterosis can be fixed partially. In such cases, at least some of the hybrids may be highly apomictic and used for fixation of heterosis. Such hybrids have been produced in Kentucky blue grass and Guinea grass.

Sexually producing line X Apomictic line ( FEMALE PARENT ) (MALE PARENT) APOMICTIC HYBRID MAINTAINED AND MULTIPLIED THROUGH SEED Fixation of heterosis by obligate apomixis governed by dominant genes..

Ideal Apomictic System All the progeny of plants should be apomictic so that progeny have the same genotype as the maternal parent ie 2n+0. Obligate adventitious embryony, somatic embryony, somatic apospory and obligate diplospory have this feature. The apomictic genotype should preferably be fully male fertile and self incompatible and reproduce via pseudogamy. Pollen production is essential for the transfer of genes controlling apomixes to normal sexual genotypes. In case of diplospory, chromosomes should not pair or recombine during the first meiotic division.This is because recombination would generate new gene combinations on the various chromosomes, which may give rise to variation among the progeny. Apomixis should be dominant over sexual reproduction. Usually apomixes is governed by a single dominant gene. Expression of apomixis should be little affected by the environment.

Apomixis controlled by a single dominant gene

Apomixis controlled by a recessive gene

Crosses between plants heterozygous for a recessive apomicts gene

3. Balanced Lethal System Balanced lethal system also leads to fixation of heterosis in some plants. For example, in many species of evening primrose (Oenothera spp.) genetic segregation is almost completely suppressed by a balanced lethal system. The homozygous are lethal and hence they die. Only heterozygous survive. This results in the fixation of heterosis in Oenothera. In Oenothera, this balance lethal system has developed due to complex translocations.

The best example of this mechanism is found as multiple reciprocal translocations in Oenothera lamarckiana, the evening primrose. It results into the formation of a large circle of chromosomes at meiosis. Chromosomes derived from one parent alternate with those derived from the other in the chromosomal ring. At metaphase, the orientation of chromosomes is such that all maternal originating chromosomes go to one pole and those of paternal origin go to the other. These two sets of chromosomes kept separate at meiosis are called gaudens and velans. Though this species produces male and female gametes of both the gauden and the velan type, individuals which are gaudens/gaudens or velans/velans do not appear because these two complexes carry different lethal which prevent the development of fertilized eggs when either kind of lethal is homozygous. It, therefore, allows the heterozygotes to breed true.

Balanced lethals and gametic complexes : permanent hybridity in Oenothera. Permanent hybridity in some species of Oenothera is maintained due to operation of a balanced lethal system, which may function due to gametic lethality or zygotic lethality. Since complete rings are formed and alternate disjunction is a rule, only two types of gametes are formed showing complete linkage between 7 chromosomes. The gametic and zygotic lethality leads to survival of only heterozygotes. It may be noticed that in gametic lethality, only one of the two types of gametes will function on the male side, the other type being functional on the female side, thus giving rise to only one type of progeny, which will be heterozygous. In zygotic lethality on the other hand, both the types of gametes will function on male as well as on female side, but the homozygote progeny due to recessive lethal genes will not survive.

Gametic lethality (A), and zygotic lethality (B) showing balanced lethal systems

4. Polyploidy Heterosis can also be fixed by chromosome doubling or polyploidy especially in interspecific and intergeneric hybrids. For example, the heterosis in wheat-rye cross can be conserved in amphidiploids hybrids through chromosome doubling. The F1 of wheat-rye cross is sterile, which becomes fertile after doubling of chromosomes through colchicines treatment. The doubled species hybrids are often fully fertile and their progenies exhibit heterosis due to combination of genes from two parent species.

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