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Gossyipium hirsutum L. and Gossypium davidsonii Kell. readily cross with each other, but the hybrid embryos become necrotic and abort, giving rise to hollow seeds. This lethal reaction is due to the interactions of alleles Le(1) and Le(2) on homoeologous chromosomes in G. hirsutum with allele Le(day) in G. davidsonii. When the recessive alleles le(1) and le(2) are introduced from Gossypium barbadense L. into G. hirsutum to replace Le(1) and Le(2), the lethal reaction with G. davidsonii is prevented. G. hirsutum x G. barbadense hybrid progeny that have only Le(1) or Le(2) alleles, when crossed with G. davidsonii, form normal seed, but the seedlings develop necrotic flecks on cotyledons and stems, and quickly die. The development of the necrotic flecks is accompanied by spontaneous defense reactions, including phytoalexin synthesis. This observation coupled with the gene-for-gene behavior of the incompatibility reactions indicate that the Le alleles may be involved with regulation of active defense reactions in plants. To test the hypothesis that Le alleles may affect recognition of pathogens (i.e., quickness of active defense reactions), we developed sister plants in the G. hirsutum 'Tamcot CAMD-E' background that were homozygous for Le(1)Le(2) (designated Le-H) from 'Tamcot CAMD-E' or for le(1)le(2) (designated le-1) from breeding lines provided by J. Lee; other sister plants were homozygous for le(1)le(2), (designated le-2) or le(1)Le(day) (designated Le-D) from breeding lines provided by J. Lee. Each gene combination was backcrossed four times into 'Tamcot CAMD-E,' before BC4 progeny were selfed and sister plants were selected. Progeny from sister plants were compared for their reaction to strains of Verticillium dahlias and races of Meloido gyne incognita that varied in virulence to G. hirsutum cultivars. V. dahliae strains TS-2 (from tomato) and pH (from pistachio) belong to vegetative-compatibility group 2, and V44 and V76 (both from cotton) belong to group 1 (defoliating isolates); TS-2 is least virulent and V76 is most virulent to G. hirsutum cultivars. M. incognita race 3 abundantly on most G. hirsutum cultivars. G. davidsonii suffered more weight loss, stunting, and defoliation from all strains of V. dahliae, but especially defoliating strains, than did 'Tamcot CAMD-E'. In contrast, G. davidsonii showed a high level of resistance to reproduction by both race 1 and 3 of M.incognita, while Tamcot CAMD-E' was resistant to race 1 but susceptible to race 3. In some instances progeny of le-1 and le-2 sister plants showed signifiicantly (LSD 0.05) less weight loss and stunting than those from the corresponding Le-H and Le-D sister plants, respectively, when infected with TS-2, pH, or V44 strains of V. dahlias. Each strain caused similar damage to Le-H compared to Le-D progeny. Numbers of eggs/plant and eggs/gm of root from race I or race 3 of M. incognita were not significantly different on progeny from le-1 and Le-H. However, the number of eggs/plant of race 3 were significantly greater on le-1 progeny than on 'Tameot CAMD-E.' The results indicate that the compatibility genes le(1)le(2) may cause slight increases in resistance to Verticillium wilt and decreases in resistance to rootknot nematode, whereas the incompatible gene combination le(1)Le(day) gives reactions to Verticillium wilt that are similar to those obtained with the Le(1)Le(2) genes normally found in G. hirsutum. Observations involving more sister line pairs are needed to confirm these conclusions. |
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©National Cotton Council, Memphis TN |
Document last modified Sunday, Dec 6 1998
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