Sources and distribution of the complementary genes for hybrid necrosis in wheat

Hybrid necrosis in wheat is based on 2 complementary genes Ne ₁ and Ne ₂. Unitl now among the 509 varieties and selections tested 89 Ne ₁-and 207 Ne ₂-carriers were found. By tracing the descendence of these carriers several sources of the necrosis genes (mostly land varieties) could be detected. At the same time it was demonstrated that notably the frequent use of certain carrier-varieties as parents in crosses has promoted the distribution of the necrosis genes.     

Geographical distribution of genes causing hybrid necrosis in wheat

The distribution of Ne ₁- and Ne ₂-genes over varieties of the breeder's stock and over land varieties is discussed. It is preliminarily concluded that the area south and east of the line Mediterranean - Black Sea - Lake Baykal is an Ne ₁-area and the area north and west of this line is an Ne ₂-area.     

Fourth supplementary list of wheat varieties classified according to their genotype for hybrid necrosis

Summary The previous conclusion about the geographical distribution of Ne-genes is supported by new evidence. Since now more data become available it is possible to divide regions within the Ne ₁- and Ne ₂-areas where certain alleles of these genes are common.The fourth supplement contains 685 varieties and selections of which the genotype for hybrid necrosis is given. The total number of tested varieties etc. is now 1835.     

Eighth supplementary list of wheat varieties classified according to their genotype for hybrid necrosis

Summary The eight supplement contains 901 varieties etc. viz. 163 (18.1%) Ne ₁-, 153 (17.0%) Ne ₂- and 585 (64.9%) non-carriers. The total number of varieties etc. tested is 1461 (26.4%) Ne ₁-, 1184 (21.4%) Ne ₂- and 2885 (52.2%) non-carriers, together 5530.Various aspects of hybrid necrosis like the geographical distribution of the Ne-genes and their alleles, linkage, Green Revolution, durum wheat and the cause have been discussed. Some information is given on some of the listed varieties etc.This is my last supplement.     

Hybrid necrosis as a problem for the wheat breeder

Hybrid necrosis is the premature gradual death of leaves and leaf sheaths in certain wheat hybrids. It is based on two complementary genes Ne ₁ and Ne ₂. Hitherto 89 Ne ₁-carriers and 207 Ne ₂-carriers from many countries are known (table 1 and 2). This corresponds with more than 18,000 different necrotic cross combinations.The degree of necrosis in the F₁'s varies greatly, due to multiple allelism of Ne ₁ and Ne ₂. Nine necrosis grades are described in table 3. Grades 6–8 are called severe necrosis (no seed is produced), 3–6 moderate necrosis (premature seed), 0–3 weak necrosis (normal seed). Most severely necrotic F₁'s could be brought to seed production by raising them under extra favourable conditions in a small growth-chamber. Growing moderately necrotic F₁'s in pots outside, at a high fertility level of the soil, resulted in a striking increase of the seed production.On the basis of the above mentioned data and results some practical advice is given to the breeders about the handling of necrotic crosses. In addition some ways to preclude hybrid necrosis are indicated, though the wheat breeder is advised against a systematic avoiding of necrosis because many combinations of excellent varieties then remain untried.     

Hybrid necrosis and hybrid chlorosis in Indian varieties of Triticum dicoccum Schubl.

Summary Forty-four varieties of Triticum dicoccum ex India were crossed to two T. aestivum testers, namely, C 306 (Ne ₁ ne ₂ ch ₁ Ch ₂) and Sonalika (ne ₁ Ne ₂ ch ₁ Ch ₂), to determine their necrosis and chlorosis genes. Thirty-six (81%) varieties were found to be carriers of the Ch ₁gene; the genotype of their necrosis gene (Ne ₁or ne ₁) could not be determined for want of suitable testers with the authors. Six varieties were found to be noncarriers for necrosis and chlorosis genes and two varieties were found to be carriers of the Ne ₁gene and non-carriers for the chlorosis gene. Allelic variation at the Ch ₂locus was observed; two alleles, Ch ₂ ^s and Ch ₂ ^m, are suggested.     

Seventh supplementary list of wheat varieties classified according to their genotype for hybrid necrosis and geographical distribution of Ne-genes

Summary The seventh supplement of the genotype hybrid necrosis in Triticum is presented. This list contains 414 Ne ₁-, 179 Ne ₂- and 662 non-carriers. The total number of varieties tested is 4629 viz. 1298 (28.0%) Ne-, 1031 (22.3%) Ne ₂- and 2300 (49.7%) non-carriers.The various aspects reported in literature and those studied by me are discussed.The geographical distribution of Ne-genes and their alleles indicates two Ne ₁ ^w -areas, one Ne ₁ ^m -area, probably two Ne ₂ ^ms.s -areas and four non-carrier-areas.     

Sixth supplementary list of wheat varieties classified according to their genotype for hybrid necrosis and geographical distribution of Ne-genes

Summary During 1971 and 1972 further progress in the identification of the Ne-genotype of wheat varieties has been made. Many data have been published in literature. Especially Russian workers identified many varieties. The picture formerly given for the distribution of Ne-genes remained unaltered.The sixth supplement consists of 287 Ne-carriers, 198 Ne ₂-carriers and 376 non-carriers, together 861 varieties etc. The total number of varieties is 3374 being 884 (26.2%) Ne _1-, 852 (25.2%) Ne ₂-and 1638 (48.5%) non-carriers.     

Fifth supplementary list of wheat varieties classified according to their genotype for hybrid necrosis and geographical distribution of Ne-genes

Summary An account is given about the progress in the work of the last two years. Further work is being done on the geographical distribution of both the Ne-genes.The fifth supplement consists of 144 Ne ₁-carriers, 208 Ne ₂-carriers and 326 noncarriers, together 678 varieties. The total number of varieties is 2513 being 597 Ne ₁-(23.8%), 654 Ne ₂-(26.0%) and 1262 (50.2%) non-carriers.     

Hybrid necrosis in wheat: evolutionary significance or potential barrier for gene flow?

One hundred and four released varieties of bread wheat (Triticum aestivum L ssp. aestivum) in India were crossed to two T. aestivum L ssp. aestivum testers, namely, C306 (Ne ₁ Ne ₁ ne ₂ ne ₂ ) and HD2329 (ne ₁ ne ₁ Ne ₂ Ne ₂ ) to determine the frequency and distribution of genes for hybrid necrosis present in them. Sixty-seven varieties (65.4 %) showed the presence of Ne ₂ gene and only eight varieties (7.7 %) had Ne ₁ gene in their background. Twenty-nine varieties (27.9 %) were non carrier (ne ₁ ne ₁ ne ₂ ne ₂ ) for both the genes. Most of the Ne ₁-carriers are of Indian origin and their pedigree revealed the involvement of landraces and old varieties as parents. Predominance of Ne ₂ gene in Indian varieties happened after the introduction of semi-dwarf Mexican wheat varieties, which are mostly Ne ₂ carriers and also due to the extensive and continuous use of germplasm from Mexican and European origin in the hybridization programme. Moreover varieties with Ne ₂ gene is selected for their linked beneficial traits mainly rust resistance genes. The phenomenon of hybrid necrosis is one among the post zygotic barrier speciation process which acts as a barrier for either intra or inter specific gene flow. The genetic architecture of hybrid necrosis in wheat is simple following the minimal predictions of the Bateson–Dobzhansky–Muller model. Widespread occurrence of dominant genes for hybrid necrosis in Indian varieties is of great concern to wheat breeders as it often interferes in the choice of elite parents and imposes restrictions on the productivity of crosses.     

Second supplementary list of genotypes of hybrid necrosis of wheat varieties

Male sterile “lines” are being made carrying either Ne ₁ ^sor Ne ₂ ^sin homozygous condition. The “lines” will be used to determine the necrosis genotype of wheat varieties and at the same time to trace genes restoring fertility in cytoplasmic sterile “lines”. It is suggested that hybrid necrosis could be used in hybrid seed production. The article is concluded with a list of 100 varieties and selections of which the genotype for hybrid necrosis is given.     

Distribution and allelic expressivity of genes for hybrid necrosis in some elite winter and spring wheat ecotypes

The distribution and allelic expressivity of hybrid necrosis genes (Ne ₁ and Ne ₂) were studied in 21 winter (mostly exotic) and 43 spring type elite wheat genotypes, by crossing them with two known testers, C 306 (Ne ₁-carrier) and HD 2380 (Ne ₂-carrier).Ne ₁ gene was present in one north-west Himalayan winter wheat landrace, Shoure Local, but absent in the other winter as well as spring wheats. Ne ₂ gene was prevalent to a much lower extent in the exotic winter wheat germplasm (31.57%) as compared to the recently developed Indian and Mexican spring wheat semidwarfs (69.80%). This may suggest that breeders have tried to preclude hybrid necrosis by selecting for non-carrier genotypes in the development of exotic winter wheats in contrast to the situation in spring wheats. Based on the degree of expression of hybrid necrosis genes in the F₁ hybrids, the carrier genotypes were characterized with respect to the allelic strength of the hybrid necrosis genes. The 27 non-carrier genotypes of the two ecotypes identified in the present study have a greater potential use in future hybridization programmes so as to overcome the problem of hybrid necrosis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distribution and Origin of Hybrid Necrosis Genes in German Winter Wheat (Triticum aestivum L.) Cultivars

Hybrid necrosis in Triticum is known to be caused by the interaction of two complementary dominant genes. In the present paper, the genotypes for hybrid necrosis of 64 winter wheat cultivars are presented. 41 cultivars were found to possess the Ne₂ necrosis gene, whereas 23 cultivars were non-carriers. The Ne₁ gene was not found in any of the cultivars analyzed.     

A gene for hybrid necrosis in an inbred line of rye (Secale cereale L.)

Summary An inbred line of rye (Secale cereale L.) has been found to carry a gene for hybrid necrosis. This gene was detected in crosses with a highly crossable wheat (Triticum aestivum L.) genotype which carries the gene Ne₂. This appears to be the first report of a gene for hybrid necrosis being present in the rye genome.     

Hybrid dwarfness in wheat

Hybrid dwarfness is the phenomenon that after crossing of normal genotypes dwarfs are obtained in the F₁ or not before the F₂-generation. The literature on hybrid dwarfness in wheat is critically discussed. A new hypothesis on its genetic basis is given, taking McMillan's (1937) as a starting point. Dwarfness is assumed to be determined by the additive interaction of three genes D ₁, D ₂ and D ₃, differing in dominance relations and in quantitative contribution to the dwarf phenotype.Three dwarf types are described. Type 1-dwarfs are dwarf during their whole life cycle and normally do not produce seeds. Type 2-dwarfs start as normal seedlings, become dwarfs while tillering and die dwarfs; some produce seeds, others do not. Type 3-dwarfs emerge as normal seedlings, become dwarfs during the tillering stage, but after some time they start to shoot and develop into nearly or even completely normal plants; in the F₂ the proportion of dwarfs decreases during the growing season. The occurrence and genetic basis of the three dwarf types is discussed.On the basis of their genotype 315 varieties and lines are divided into six genotypeclasses. Over 1000 intra- and inter-class crosses were made and F₁, F₂, F₃, and BC studied. Also some triple crosses and crosses with pure breeding dwarfs were investigated. In general the results obtained fit the hypothesis. Unstable ratios involving type 3-dwarfs are discussed separately.Linkage of the dwarf gene D ₂ and the necrosis gene Ne ₂ (both on chromosome 2B (XIII) was apparent from F₂-data and from results of a triple cross in which both forms of hybrid weakness occurred. Crossing-over between D ₂ and Ne ₂ is calculated to be 34%.Methods are outlined to use hybrid dwarfness in a wheat breeding programme. The possible incompleteness of the three-gene hypothesis and the variability of dwarfness are discussed and finally some suggestions are made for future research.     

Geographical distribution of genes causing hybrid dwarfness in hexaploid wheat of the old world

Summary The distribution of the three different dominant genes for hybrid dwarfness in the Old World is given. It appears thatD ₁ is found in southern Europe, Africa and Asia, whileD ₃ is located in Europe.D ₂ is common in all wheat-growing areas. The distribution ofD ₁ coincides with that ofNe ₁, while the distribution ofD ₃ is probably similar to that ofNe ₂.TheD ₁-gene is still present inAegilops squarrosa. The sources ofD ₂ andD ₃ have not yet been determined with certainty.     

Transgene introgression from Brassica napus to different varieties of Brassica juncea

Transgene introgression from transgenic rapeseed (Brassica napus) to different varieties of B. juncea was assessed in this study. Crossability between a transgenic rapeseed line Z7B10 (pollen donor) and 80 cultivars of 16 B. juncea varieties (including two wild accessions) was estimated by artificial pollination in a greenhouse. As a result, interspecific crossability between the transgenic Z7B10 line and the 80 B. juncea cultivars varied considerably, with seeds per flower from 0.00–10.67. Seed germination rates of the interspecific F₁ hybrids ranged from 49.0%–89.3%. The estimated frequencies of natural gene flow from the transgenic Z7B10 line to 10 B. juncea cultivars with different uses in the experiment field varied from 0.08% to 0.93%. The natural F₁ hybrids were highly sterile, with seeds per silique ranging from 0.27 to 1.03. In addition, seeds per flower of hybrid descendants varied from 0.02 to 0.22 when F₁ hybrids were self‐pollinated, and those ranged from 0.03 to 0.30 when F₁ hybrids were backcrossed with their corresponding B. juncea parents. Results of this study suggest a low level of transgene introgression from transgenic rapeseed to different B. juncea varieties, which provides a sound scientific basis for the safety management of coexisting transgenic B. napus and B. juncea varieties in China.     

A quinclorac sensitive lethality rice mutant: its discovery,genetics and potential application

In this study, we reported a quinclorac‐sensitive rice natural mutant M4017, derived from an indica restorer line 4017 of three‐line hybrid. The mutant could be killed at three‐leaf stage by quinclorac at 175 mg/l or higher, a dosage that is recommended for monocot herbs control. The mutant at flowering stages can also be effectively killed by quinclorac at 1.25–1.50 g/l or higher, which is safe for its F₁ hybrids and all other normal varieties. Genetic analysis, using the F₁, F₂ and BC₁ populations derived from this mutant and other often‐used varieties, revealed that quinclorac sensitivity/lethality was controlled by a single recessive gene, which was named quin1. This mutant has a potential application in false hybrid seed removal or mechanized production of hybrid seeds.     

Inheritance of insensitivity to culture filtrate of Pyrenophora tritici-repentis, race 2, in wheat (Triticum aestivum L.)

Tan spot of wheat is caused by the fungus Pyrenophora tritici‐repentis. On susceptible hosts, P. tritici‐repentis induces two phenotypically distinct symptoms, tan necrosis and chlorosis. This fungus produces several toxins that induce tan necrosis and chlorosis symptoms in susceptible cultivars. The objectives of this study were to determine the inheritance of insensitivity to necrosis‐inducing culture filtrate of P. tritici‐repentis, race 2, and to establish the relationship between the host reaction to culture filtrate and spore inoculation with respect to the necrosis component. The F₁, F₂, and BC₁F₁ plants and F_2:8 lines of five crosses involving resistant wheat genotypes ‘Erik’, ‘Red Chief’, and line 86ISMN 2137 with susceptible cultivars ‘Glenlea’ and ‘Kenyon’ were studied. Plants were spore‐inoculated at the two‐leaf stage. Four days later, the newly emerged uninoculated third leaf was infiltrated with a culture filtrate of isolate Ptr 92–164 (race 2). Reactions to the spore inoculation and the culture filtrate were recorded 8 days after spore inoculation. The segregation observed in the F₂ and BC₁F₁ generations and the F_2:8 lines of all crosses indicated that a single recessive gene controlled insensitivity to necrosis caused by culture filtrate. This gene also controlled resistance to necrosis induced by spore inoculation.     

Genetic necrosis in Triticum × aegilops pentaploid hybrids

Summary Ten varieties of Triticum aestivum and three synthetic T. durum + Aegilops squarrosa hexaploids were crossed with an Ae. bicornis + Ae. squarrosa amphiploid. 55 hybrid plants involving 9 different combinations developed necrosis and died before maturity. The two plants which survived were both from the F₁ with a synthetic hexaploid, T. durum var. Carleton + Ae. squarrosa.The genetic necrosis developing in these hybrids differed from hybrid necrosis and red hybrid chlorosis both in symptoms and in the genetic basis. The symptoms were similar to those reported for a number of Triticum x Aegilops hybrids in which Ae. bicornis and Ae. squarrosa were involved but the genetic basis was not determined. The survival of 2 of the 13 progenies from one cross is discussed.

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