QTLs mapping for Verticillium wilt resistance at seedling and maturity stages in Gossypium barbadense L.
Introduction
Verticillium wilt is a common fungal disease that causes severe yield and quality losses in many crops, including cotton (Gossypium spp.), tomato (Lycopersicon esculentum Mill.), potato (Solanum tuberosum L.), alfalfa (Medicago sativa L.), strawberry (Fragaria grandiflora Ehrh.), mint (Mentha piperita L.), sunflower (Helianthus annuus L.) and eggplant (Solanum melongena L.) [1]. Vascular wilt diseases such as Verticillium and Fusarium are among the most difficult plant diseases to control. To date, the most effective means of combating these diseases is the use of resistant varieties. Verticillium wilt in cotton is caused by the soil inhabiting fungus Verticillium dahliae Kleb. The fungus has a worldwide distribution and causes serious economical loss over its range. It is regarded as the major threat to cotton production [2]. The pathogen has a broad range of hosts, moves among its hosts using a variety of mechanisms, has a high pathogenicity and is long lived. The fungus has a critical temperature of 27 °C [3] and a warm and humid climate can accelerate the spread of Verticillium wilt [4]. The pathogen can be divided into two types: defoliating and non-defoliating, according to their virulence. Early symptoms of the plant caused by defoliating pathogen were downward curling and epinasty in the terminal leaf followed by epiansty of most leaves, and then the epinasty leaves exhibited general chlorosis and defoliation followed; if plants were inoculated with non-defoliating pathogen, lower leaves exhibited interveinal chlorosis followed by necrosis, there was little or no epinasty and dead leaves usually remained attached to plants.
Studies have demonstrated that breeding for resistant varieties to Verticillium is the most effective and economical disease control method. However, little progress has been made on the breeding of resistance to Verticillium wilt largely because immune or highly resistant germplasm is lacking. Modern Gossypium hirsutum and Gossypium barbadense cultivars possess variation for important economic traits including yield, fiber quality, pest resistance and tolerance to environmental adversities [5], [6]. It has been reported there were Verticillium wilt resistant lines in G. barbadense [7]. These lines can provide important contributions to Verticillium wilt resistance breeding. Genetic studies on Verticillium wilt resistance in cotton have reported different inheritance patterns. The inheritance can be classified into two types: major gene [8], [9], [10], [11] and/or polygene [12], [13], [14] contributing the resistance. Because of this genetic complexity, our understanding of resistance mechanisms is limited. Therefore, advance in Verticillium wilt resistance breeding has been limited and far from meeting the needs of the cotton industry.
Molecular biology provides DNA markers to identify useful polymorphisms in plant breeding programs. The construction of genetic linkage maps with molecular markers is the foundation of gene mapping, cloning and genome structure and function research. At present, several genetic linkage maps have been constructed using molecular markers [15], [16], [17], [18], [19], [20], [21], [22], and the genetic basis of some important agricultural characteristics such as yield, lint quality and insect resistance have been mapped [23], [24], [25], [26], [27], [28]. Recently, Verticillium wilt resistant genes/QTLs have been detected in G. barbadense and G. hirsutum cultivars. Bolek et al. [29] inoculated individuals of a F2 population derived from a cross of a G. barbadense cultivar and a G. hirsutum cultivar with a single isolate. Five parameters were investigated at seedling stage in a greenhouse and three loci (CM12, STS1 and BNL3147-2) on chromosome (chr.) 11(A11) had large effect on resistance to Verticillium wilt. In addition, several QTLs which were resistant to a single isolate were detected [30], [31], [32], [33], [34]. Different markers, isolates and developmental stages were employed in these studies but chromosome tagging data is not available to date. Consequently, comparisons among these data could not be conducted.
In the present study, we detected several resistant QTLs for different isolates of V. dahliae in different cotton growing stages by using molecular markers. The markers linked to the QTLs will provide novel basic information to facilitate cotton breeding for resistance to Verticillium wilt.
Section snippets
Plant materials
G. barbadense var. Hai 7124 is highly tolerant to Verticillium wilt [10], [22]. G. hirsutum var. Junmian 1 had been widely distributed in the Xinjiang municipality but is highly sensitive to Verticillium wilt. In 2004, Hai 7124 and Junmian 1 were crossed and F1 seeds planted in Hainan to produce self-pollinated F2 progeny. This population was comprised of 128 individuals. In 2005, the population was planted in the pathogen nursery in Jiangpu Breeding Station, Nanjing Agricultural University
Resistant performance
In 2005, the average disease grade score at seedling and maturity stages was 1.4 and 1.5 in Hai 7124, while the corresponding score in Junmian 1 was 3.4 and 3.2 in the BP2 nursery. Additionally, in 2006, the average disease grade score of Hai 7124 at seedling and maturity stages was 1.5 and 1.4 in the BP2 nursery, 1.8 and 1.3 in the VD8 nursery and 0.8 and 1.5 in the 592 nursery, whereas corresponding scores 3.9 and 3.0 at the seedling and maturity stages in the BP2 nursery, 3.7 and 4.0 in the
Discussion
In this study, two genetic linkage maps were constructed based on our backbone genetic map [16], [37] and the whole genome was scanned to detect the resistant QTLs to Verticillium wilt. Our study was the first to detect the QTLs for resistance to Verticillium wilt in cotton at different developmental stages with different isolates. Two stable QTLs exist in different populations and in different developmental stages. The QTL qVL-A5-2F2 and qVL-A5-1BC1S2VD8, were both located between NAU2121 and
Acknowledgements
This work was financially supported in part by grants from the State Key Basic Research and Development Plan of China (2006CB101708), the Ministry of Agriculture (NYHYZX07-052), the National Natural Science Foundation of China (30370899, 30270806), the Changjiang Scholars and Innovative Research Team in University of MOE, China (IRT0432) and Jiangsu Natural Science Foundation, China (BK2007719).
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