The effect of VRN1 genes on important agronomic traits in high‐yielding Canadian soft white spring wheat

For reproductive success, flowering time must synchronize with favourable environmental conditions. Vernalization genes play a major role in accelerating or delaying the time to flowering. We studied how different vernalization (VRN1) gene combinations alter days to flowering and maturity and consequently the effect on grain yield and other agronomic traits. The study focussed on the effect of the VRN1 gene series (Vrn‐A1, Vrn‐B1 and Vrn‐D1) and their combinations. The Vrn gene group Vrn‐A1a, Vrn‐B1, vrn‐D1 was the earliest to flower and mature, while Vrn‐A1b, Vrn‐B1, vrn‐D1 was the latest to flower. Spring wheat lines with vrn‐A1, Vrn‐B1, Vrn‐D1 were the highest yielding and matured at a similar time as those having vernalization genes Vrn‐A1a, Vrn‐B1 and Vrn‐D1. The findings of this study suggest that the presence of Vrn‐D1 has a direct or indirect role in producing higher grain yield. We therefore suggest the introduction of Vrn‐D1 allele into higher‐yielding classes within Canadian spring wheat germplasm.     

Genetic and Molecular Characterization of Vernalization Genes Vrn‐A1, Vrn‐B1, and Vrn‐D1 in Spring Wheat Germplasm from the Pacific Northwest Region of the U.S.A.

The objective of this study was to determine the Vrn‐1 allelic composition of spring wheat germplasm from the Pacific Northwest region of the USA. Individual plants from 56 spring wheat lines were crossed to near‐isogenic tester lines carrying the dominant allele Vrn‐A1, Vrn‐B1 or Vrn‐D1. F₂ progeny were evaluated for growth habit in the field and Vrn‐1 allelic composition was determined through chi‐square analysis. Lines also were analysed with DNA sequence‐based Vrn‐1 allele‐specific markers. A majority of the germplasm carried the dominant allele Vrn‐A1a alone or in combination with Vrn‐B1, Vrn‐D1 or Vrn‐B3 alleles. Vrn‐B1 and Vrn‐D1 were almost always associated with other dominant Vrn‐1 allele(s). Based on DNA sequence analysis, a novel Vrn‐B1 allele referred to as Vrn‐B1b, which carried a single nucleotide polymorphism (SNP) and a 36 bp deletion, was identified in cultivar ‘Alpowa’. These results will be useful to wheat breeders for choosing parents with different Vrn‐1 alleles for crossing to maximize diversity at the Vrn‐1 loci with an expectation of identifying superior Vrn‐1 allelic combinations for cultivar improvement.     

Variation of PGM and IDH isozymes for identification of alfalfa varieties

Growth habit, heading date and Vrn genotypewere examined for wheat landraces cultivated in China,Korea and Japan, to study their ecogeographicaldifferentiation in east Asia. Spring type landracesaccounted for 43.6% of the whole, and the frequencyvaried between the localities, being closely relatedto the degree of winter coldness. Spring typelandraces mainly adapted to north and south Chinawhere average January temperature is under –7 ^°Cand over 4 ^°C, respectively. On the contrary,winter type adapted to areas of average Januarytemperature from –7 ^°C to 4 ^°C. As toheading date, significant difference was not observedbetween spring and winter type landraces but betweenlocalities, and those cultivated in north China weresignificantly later in heading. It is thereforeindicated that spring type mainly adapts to areaswhere wheat is sown in spring to avoid frost injury,and where winter temperature is not low enough tovernalize winter type wheat. Genetic analysis forspring type landraces showed that the relativefrequency of four Vrn genes was different witheach other. Vrn3 was most widely and frequentlyfound among the four genes, followed by Vrn1 andVrn2. Only seven landraces proved to be thecarrier of Vrn4. The frequency was alsodifferent between localities. Genotype with Vrn1plus other dominant gene(s) adapted to spring sowingto avoid severely cold winter in north China, whilegenotype with only Vrn3 adapted to winter sowingin south China and southwest Japan. It is thereforeconcluded that at least three ecotypes, differing ingrowth habit and Vrn genotype, areallopatrically distributed in east Asia, as a resultof adaptation to winter coldness in each locality.     

The detection of allelic variants at the recessive vrn loci of winter wheat

Substitution lines with reciprocal substitutions of chromosomes containing recessive alleles of the homoeologous group 5 chromosomeVrn genes between varieties of winter wheat with high vernalisation requirement (‘Mironovskaya 808’) and low vernalisation requirements (‘Bezostaya 1’) have been created. On this basis the genetic determination of vernalisation requirement was established. Substitution lines Mironovskaya 808 (Bezostaya 1 5A), Mironovskaya 808 (Bezostaya 1 5B), Mironovskaya 808 (Bezostaya 1 5D) and reciprocal substitution lines Bezostaya 1 (Mironovskaya 808 5A), Bezostaya 1 (Mironovskaya 808 5B) and Bezostaya 1 (Mironovskaya 808 5D) were grown under different durations of vernalisation (3, 4, 5, 6, 7 and 8 weeks) and their response was evaluated. Photoperiodic sensitivity of the original parental genotypes was also determined. Reciprocal substitution lines of the same chromosome that carries the same vrn allele responded differently to vernalisation deficit. Differences have been shown between all group 5 reciprocal substitutions. Lines carrying chromosomes 5A and 5D of Mironovskaya 808 had a high vernalisation requirement whereas lines carrying chromosome 5B of Bezostaya 1 (vrn2^B) had a low vernalisation requirement. The reciprocal lines had a reverse requirement. This explains the different vernalisation requirements of the original varieties: Mironovskaya 808 with a high vernalisation requirement carries two alleles (vrn1^M and vrn3^M) in its genotype that increase the vernalisation requirement, whereas Bezostaya 1 with a lower requirement for vernalisation contains only one such allele (vrn2^B). By combination of the alleles in the lines with the substitution of chromosome 5B carrying vrn2 allele that in both original genotypes work inversely to the other alleles, transgressive genotypes have been formed: genotype vrn1^M vrn2^B vrn3^M determines a higher vernalisation requirement than original variety Mironovskaya 808, and genotype vrn1^B vrn2^M vrn3^B determines a lower vernalisation requirement than the original Bezostaya 1. An incomplete vernalisation requirement prolonged the time to heading, with exponential dependence on the vernalisation deficit, or prevented heading altogether. The original varieties further differed in photoperiodic sensitivity (Mironovskaya 808 sensitive, Bezostaya 1 less sensitive) that also influenced the background of substitution lines. The impact of the background on the heading time showed itself by about one week difference between Mironovskaya 808 and Bezostaya 1 grown under 8 weeks vernalisation and normal photoperiod. The difference between the lines with Mironovskaya 808 background and the lines with Bezostaya 1 background was approximately the same and was not significantly changed in different vernalisation variants of the lines. This difference may be caused by different photoperiodic sensitivity of the original varieties, but also by other genes, such as genes of earliness per se. This revised version was published online in July 2006 with corrections to the Cover Date.     

Location of a gene for frost resistance on chromosome 5A of wheat

Summary A gene for frost resistance on chromosome 5A of wheat was located using single chromosome recombinant lines from the cross between the substitution line Hobbit (Triticum spelta 5A) and Hobbit. In this sample of recombinant lines the locus for frost resistance, designated Fr1, is completely linked to the locus Vrn1 controlling vernalisation requirement. The results can be explained by a pleiotropic action of the Vrn1 locus or close genetic linkage between Vrn1 and Fr1. Further detailed study is necessary to resolve these alternative hypotheses.     

Adaptation and ecological differentiation in wheat with special reference to geographical variation of growth habit and Vrn genotype

Geographical variation of growth habit was studied for 749 landraces from various parts of the world, with special reference to their adaptation and ecogeographical differentiation. The total frequency of spring‐type landraces was 49.9%, and varied between localities. Spring‐type landraces were frequent in two distinct areas where the average January temperature was either below ‐7°C or above 4°C, with winter‐type landraces in areas from ‐7°C to 4°C. These results indicated that geographical variation of growth habit is closely related to the degree of winter coldness. An analysis of the Vrn genotype for 216 spring‐type landraces demonstrated the uneven distribution of four Vrn genes, with Vrn4 being the least frequent. The adaptive Vrn genotype was different between localities. Genotypes carrying Vrn‐A1 and additional Vrn gene(s) were frequent in two distinct areas where the average January temperature was either below ‐7°C or over 10°C, while genotypes with any of three Vrn genes, except Vrn‐A1, adapted to areas with temperatures from 4°C to 10°C. Therefore, it was concluded that the adaptability of wheat landraces differed depending on their growth habit and Vrn genotype, and that ecotypes with different Vrn genotypes were allopatrically distributed as a result of adaptation to different winter temperature. However, the differential distribution of Vrn‐B1, Vrn‐D1 and Vrn4 could not be explained by their adaptability, and might reflect the polyphyletic origin of common wheat.     

Mapping of the Vrn-B1 gene in Triticum aestivum using microsatellite markers

An F₂ population segregating for the dominant gene Vrn‐B1 was developed from the cross of the substitution line ‘Diamant/'Miro‐novskaya 808 5A’ and the winter wheat cultivar ‘Bezostaya 1′. Microsatellite markers (Xgwm and Xbarc) with known map locations on chromosome 5B of common wheat were used for mapping the gene Vrn‐B1. Polymorphism between parental varieties was observed for 28 out of 34 microsatellite markers (82%). Applying the quantitative trait loci mapping approach, the target gene was mapped on the long arm of chromosome 5B, closely linked to Xgwm408. The map position of Vrn‐B1 suggests that the gene is homoeologous to other vernalization response genes located on the homoeologous group 5 chromosomes of wheat, rye and barley.     

Classification of GA response,Rht genes and culm length in Japanese varieties and landraces of wheat

Summary The GA response, Rht genes and culm length of 133 Norin varieties, 6 breeding lines and 16 landraces of Japanese wheat were investigated. Out of 133 Norin varieties tested, 103 were GA-insensitive and 30 GA-responsive. The 6 breeding lines were all GA-insensitive. Out of 16 landraces tested, 10 were GA-insensitive and 6 GA-responsive. Among the 10 GA-insensitive landraces, only Daruma had a Rht1 genotype. The other 9 had a Rht2 genotype. None of the landraces tested carried both Rht1 and Rht2 or Rht3. Out of the 103 GA-insensitive Norin varieties, 22 carried only Rht1, another 79 carried only Rht2, and only Norin 10 and Kokeshikomugi carried both Rht1 and Rht2. No tested variety carried Rht3. Some Norin varieties carrying Rht2 showed tall culms comparable to that of the rht tester line Chinese Spring. These results suggest that these varieties had a nullifier or modifier gene(s) or height promoting genes in the background controlling the height-reducing effect of Rht2. Conversely, six GA-responsive Norin varieties were as short as Akakomugi which carries the GA-responsive Rht genes, Rht8 and Rht9. The also seemed to carry a GA-responsive Rht gene or genes, and moreover all but one may carry gene(s) other than the Akakomugi genes. The origin of Rht1 and Rht2 of Norin 10 was speculated on the GA-response and Rht genotypes of its related varieties and landraces.     

Effect of alien 5R(5A) chromosome substitution on ear-emergence time and winter hardiness in wheat-rye substitution lines

Ear emergence time and response to vernalization were investigated in 12 alien substitution lines in which a pair of chromosomes 5A of recipient spring wheat cultivars was replaced by a pair of chromosomes 5R of Siberian spring rye ‘Onokhoiskaya’. The recipients were 12 spring cultivars of common wheat, each carrying different Vrn genes. Spring rye ‘Onokhoiskaya’ had the Sp1 (now called Vrn-R1) gene for spring growth habit located on chromosome 5R, but its expression was weaker. The Vrn-R1 gene had no effect on growth habit, ear emergence time and response to vernalization in wheat-rye substitution lines. Ears emerged significantly later in the 5R(5A) alien substitution lines than in the recipient wheat cultivars with the Vrn-A1/Vrn-B1/vrn-D1 or Vrn-A1/vrn-B1/Vrn-D1 genotypes. No difference in ear emergence time was found between most of the 5R(5A) alien substitution lines and the cultivars carrying the recessive vrn-A1 gene. The presence of the Vrn2a and Vrn2b alleles at the Vrn2 (now called Vrn-B1) locus located on wheat chromosome 5B was confirmed.The replacement of chromosome 5A by chromosome 5R in wheat cultivars ‘Rang’ and ‘Mironovskaya Krupnozernaya’, which carries the single dominant gene Vrn-A1, converted them to winter growth habit. In field studies near Novosibirsk the winter hardiness of 5R(5A) wheat–rye substitution lines of ‘Rang’ and ‘Mironovskaya Krupnozernaya’ was increased by 20–47% and 27–34%, respectively, over the recurrent parents.     

Alien introgression of spring habit dominant genes into bread wheat

Summary Alien dominant genes of spring habit were introgressed into bread wheat. The introgression was undertaken by simple crossing of winter bread wheat to related spring species or genera, followed by backcrossing to winter bread wheat, and did not involve the use of the ph mutants or embryo culture. The introgressed genes were located mostly on chromosomes of homoeologous group 5, and were allelic to the known Vrn genes in bread wheat. Nevertheless three groups of lines were discovered with the genes possibly located on other chromosomes. These genes were non-allelic to each other and to known Vrn genes and were designated Vrn6 ^Sc , Vrn7 ^Sc (introgressed from Secale cereale) and Vrn8 ^Ts (from Triticum sphaerococcum).     

The influence of photoperiod on the Vrn-H2 locus (4H) which is a major determinant of plant development and reproductive fitness traits in a facultative × winter barley (Hordeum vulgare L.) mapping population

To determine the effect of Vrn‐H2 locus on plant developmental and agronomic traits, detailed controlled environment tests involving a factorial set of vernalization and photoperiod treatments were carried out using doubled haploid lines developed from a facultative (Vrn‐H2^?) × winter (Vrn‐H2⁺) barley cross. The allele phase at the Vrn‐H2 locus influenced heading date as well as the developmental and agronomic traits. The performance of Vrn‐H2⁺ lines was significantly influenced by vernalization: reproductive fitness traits showed significant decreases without vernalization. However, the effects of alleles at the Vrn‐H2 locus extended beyond simple satisfaction of the vernalization requirement. Vrn‐H2⁺ lines showed increased reproductive fitness compared with Vrn‐H2^? lines when vernalization was followed by a long photoperiod. The responses of the two Vrn‐H2 allele classes to photoperiod duration were quite different in terms of heading date, developmental and agronomic traits. These results suggest that alleles at the Vrn‐H2 locus – and/or tightly linked gene(s) – respond primarily to the exogenous signal of vernalization (temperature), but when the vernalization requirement has been fulfilled, they also respond to photoperiod duration.     

The effects of whole chromosome substitutions differing in alleles for hybrid dwarfing and photoperiodic sensitivity on tissue culture response (TCR) in wheat

Summary The wheat varieties Chinese Spring, Marquis and Thatcher and five intervarietal Chinese Spring substitution lines for chromosomes 2B (2 lines), 2D (2 lines) and 4A^*, differing from the recipient variety in alleles for hybrid dwarfing genes and/or the photoperiodic response genes were analysed for tissue culture response (TCR). It could be demonstrated that only chromosome 2B has a major effect on TCR. Allelic variation at the hybrid dwarfing loci seems to have no effect on tissue culture performance, even in the combination D1D2D3 which gives the grass dwarf phenotype. Also the allelic constitution at the Ppd loci, gave no indication for a direct major effect of those alleles, however there seems to exist genetical factors for TCR on the homoeologous group 2 chromosomes which may be closely linked to the Ppd loci.     

Evaluation of cold hardiness in two sets of near-isogenic lines of wheat (Triticum aestivum) with polymorphic vernalization alleles

In wheat, variation at the orthologus Vrn‐1 loci, located on each of the three genomes, A, B and D, is responsible for vernalization response. A dominant Vrn‐1a allele on any of the three wheat genomes results in spring habit and the presence of recessive Vrn‐1b alleles on all three genomes results in winter habit. Two sets of near‐isogenic lines (NILs) were evaluated for DNA polymorphisms at their Vrn‐A1, B1 and D1 loci and for cold hardiness. Two winter wheat cultivars, ‘Daws’ and ‘Wanser’ were used as recurrent parents and ‘Triple Dirk’ NILs were used as donor parents for orthologous Vrn‐1 alleles. The NILs were analysed using molecular markers specific for each allele. Only 26 of 32 ‘Daws’ NILs and 23 of 32 ‘Wanser’ NILs had a plant growth habit that corresponded to the marker genotype for the markers used. Freezing tests were conducted in growth chambers programmed to cool to ?21.5°C. Relative area under the death progress curve (AUDPC), with a maximum value of 100 was used as a measure of death due to freezing. The average relative AUDPC of the spring habit ‘Daws’Vrn‐A1a NILs was 86.15; significantly greater than the corresponding winter habit ‘Daws’Vrn‐A1b NILs (42.98). In contrast, all the ‘Daws’Vrn‐A1bVrn‐B1aVrn‐D1b and Vrn‐A1bVrn‐B1bVrn‐D1a NILs (spring habit) had relative AUDPC values equal to those of their ‘Daws’ sister genotypes with Vrn‐A1bVrn‐B1bVrn‐D1b NILs (winter habit). The average AUDPC of spring and winter habit ‘Wanser’ NILs differed at all three Vrn‐A1, Vrn‐B1 and Vrn‐D1 locus comparisons. We conclude that ‘Daws’ and ‘Wanser’ have different background genetic interactions with the Vrn‐1 loci influencing cold hardiness. The marker for Vrn‐A1 is diagnostic for growth habit and cold hardiness but there is no relationship between the Vrn‐B1 and Vrn‐D1 markers and the cold tolerance of the NILs used in this study.     

The transfer of triazine resistance from Brassica napus L. to B. oleracea L. III. First backcross to parental species

Summary Atrazine resistant Brassica napus × B. oleracea F₁ hybrids were backcrossed to both parental species. The backcrosses to B. napus produced seeds in both directions but results were much better when the F₁ hybrid was the pollen parent. Backcrosses to B. oleracea failed completely but BC₁s were rescued by embryo culture both from a tetraploid hybrid (2n = 4x = 37; A₁C₁CC) and sesquidiploid hybrids (2n = 3x = 8; A₁C₁C). Progeny of crosses between the tetraploid hybrid and B. oleracea had between 25 and 28 chromosomes. That of crosses between the sesquidiploid hybrid and B. oleracea had between 21 and 27. A few plants that had chromosome counts outside the expected range may have originated from either diploid parthenogenesis, unreduced gametes or spontaneous chromosome doubling during in vitro culture. Pollen stainability of the BC₁s ranged from 0% to 91.5%. All the BC₁s to B. oleracea were resistant to atrazine.     

Photoperiod insensitivity gene essential to the varieties grown in the northern limit region of paddy rice (Oryza sativa L.) cultivation

Summary The release of extremely early maturing varieties has made it possible to cultivate rice in Hokkaido (NL45-42°) in Japan, the northern limit region of paddy rice cultivation. Until then, rice cultivation in this region has been impracticable due to the climate condition, especially short summer and long-day more than 15 hours during summer. Experiment results confirmed that the success of rice cultivation in this area depends on raising photoperiod insensitivity varieties with short basic vegetative growth period. Moreover, in this study, the genetic factors controlling the photoperiod insensitivity of Hokkaido varieties were analyzed by using 8 kinds of tester lines for three loci, E1, E2, and E3, controlling photoperiod sensitivity. It was found out that all the varieties examined carry el, a photoperiod insensitivity allele of E1 locus, but as for the other loci, E2 and E3, the existence of plural alleles were recognized. We have already clarified that almost all the japonica-type varieties grown in Japan (except Hokkaido) and Taiwan carry E1 bringing about strong photoperiod sensitivity. Accordingly, it can be concluded that el is the gene essential to Hokkaido varieties: the interchange of E1 with e1 enabled rice cultivation under long-day condition.     

Genetic variation for resistance to low-temperature photoinhibition of photosynthesis in maize (Zea mays L.)

Summary In the partially allogamous faba bean, yield and yield stability can be improved significantly by heterozygosity and heterogeneity. The commercial production of hybrid varieties for the full exploitation of heterosis is still unfeasible. Hence, the breeding of synthetic varieties has repeatedly been recommended. The present study aimed at investigating whether and to what extent effects due to heterozygosity and heterogeneity occur in such varieties, particularly in relation to Syn-generation. A sample of 36 inbred lines (Vicia faba L. minor) was used to generate several entries of different population structure, e.g. blends of inbred lines (=Syn-0), experimental synthetics of generations Syn-1 to Syn-4 and polycross progenies. In 1986 to 1991 these entries were evaluated together with their parental lines grown in pure stands in six series of multi-locational field trials in West Germany. The yield of the synthetics increased with successive Syn-generations and asymptotically approached its maximum by as early as generations Syn-2 or Syn-3. The yield increase was mainly caused by heterozygosity. In two synthetics the yield increase corresponded to about one-half to two-thirds of the mid-parent heterosis. Effects due to heterogeneity were small and mostly non-significant. In one out of three expriments significant effects due to seed source were observed which, however, did not alter the yield increase with successive Syn-generations. Hence, Syn-generations Syn-2 or Syn-3 may be sold to the farmer.     

The influence of a spring habit gene, Vrn-D1, on heading time in wheat

The adaptability of wheat cultivars to environmental conditions is known to be associated with a vernalization requirement, that is, spring/winter habit. To clarify the genetic effect of the spring habit gene, Vrn‐D1, on heading time in the field, recombinant inbred lines (RILs) with or without the Vrn‐D1 gene were produced from F₂ plants of the cross between ‘Nanbukomugi’ and ‘Nishikazekomugi’, non‐carrier and carrier cultivars of this gene, respectively. Using growth chambers with a controlled temperature and photoperiod, three components of heading time, i.e. vernalization requirement, photoperiodic sensitivity and narrow‐sense earliness (earliness per se), were evaluated in each RIL. RILs with the Vrn‐D1 gene (E lines) showed greatly reduced vernalization requirements and slightly shorter narrow‐sense earliness than RILs without Vrn‐D1 (L lines), although no difference in photoperiodic sensitivity was observed between the two groups. RILs were planted at four different sites in Japan and examined for their heading time in the field. E lines headed significantly earlier than L lines at all locations, indicating that the earliness of E lines is stable in various environmental conditions. These results indicated that spring habit caused by Vrn‐D1 gene, as well as narrow‐sense earliness, was responsible for heading time in the field.     

An assessment of the potential of 4DS.4DL-4s l L translocation lines as a means of eliminating tall off types in semi-dwarf wheat varieties

Summary Wheat varieties tend to be chromosomally unstable producing on average 2–3% of plants with abnormal chromosome numbers. A number of semi dwarf wheat varieties, carrying the gibberellic acid insensitive dwarfing genes Rht1 or Rht2, have been seen to produce distinct tall off types due to reduction in dosage of the chromosome carrying the dwarfing gene. The UK variety Brigand, carrying Rht2 on chromosome 4D, produced very distinct tall off types when this chromosome was reduced in dosage. The frequency of tall off types was sufficiently high to cause the variety to fail United Kingdom statutory uniformity tests. An attempt to prevent the loss of chromosome 4D was made by constructing translocation chromosomes involving the short arm of chromosome 4D, which carries Rht2, and the long arm of chromosome 4S ^l from Aegilops sharonensis, which carries a gene(s) conferring preferential transmission. The work in this paper describes the field evaluation of two lines carrying 4DS.4DL-4S ^l L translocations, and demonstrates their success in preventing spontaneously occurring monosomy of chromosome 4D in semi-dwarf wheats.     

Resistance to the Barley Yellow Mosaic Virus Complex — Differential Genotypic Reactions and Genetics of BaMMV-Resistance of Barley (Hordeum vulgare L.)

Barley yellow mosaic disease is caused by several viruses, i.e. barley yellow mosaic virus (BaYMV), barley mild mosaic virus (BaMMV) and BaYMV-2. The reaction of different barley germplasms to the barley mosaic viruses was studied in field and greenhouse experiments. The results show a complex situation; some varieties are resistant to all the viruses, while others are resistant to one or two of them only. Crosses between different barley germplasms were earned out in order to test whether genetic diversity of resistance against mosaic viruses does exist, particularly, BaMMV. A total of 45 foreign barley varieties were crossed to German cultivars carrying the resistance gene ym4. In F₂ of 27 crosses, no segregation could be detected, leading to the conclusion that the resistance genes of the foreign parents are allelic with ym4 e.g. Ym1 (‘Mokusekko 3’) and Ym2 (‘Mihori Hadaka 3’). A total of 18 crosses segregated in F₂ indicating that foreign parents, like ‘Chikurin Ibaraki 1’, ‘Iwate Omugi 1’, and “Anson Barley”, carry resistance genes different from the gene of German cultivars, e.g. ‘Asorbia’ or ‘Franka’. By means of statistical evaluation (Chi²-test), the observed segregation ratios were analyzed in order to obtain significant information on the heredity of resistance. All the resistance genes described here as being different from the gene ym4, act recessively. Most of the exotic varieties seem to carry only one resistance gene. In a few cases, more than one gene may be present.