Abstract
The southern root-knot nematode (RKN), Meloidogyne incognita (Kofoid & White) Chitwood, is one of most destructive species of plant parasitic nematodes, causing significant economic losses to numerous crops including cucumber (Cucumis sativus L. 2n = 14). No commercial cultivar is currently available with resistance to RKN, severely hindering the genetic improvement of RKN resistance in cucumber. An introgression line, IL10-1, derived from the interspecific hybridization between the wild species Cucumis hystrix Chakr. (2n = 24, HH) and cucumber, was identified with resistance to RKN. In this study, an ultrahigh-density genetic linkage bin-map, composed of high-quality single-nucleotide polymorphisms (SNPs), was constructed based on low-coverage sequences of the F2:6 recombinant inbred lines derived from the cross between inbred line IL10-1 and cultivar ‘Beijingjietou’ CC3 (hereinafter referred to as CC3). Three QTLs were identified accounting for 13.36% (qRKN1-1), 9.07% and 9.58% (qRKN5-1 and qRKN5-2) of the resistance variation, respectively. Finally, four genes with nonsynonymous SNPs from chromosome 5 were speculated to be the candidate RKN-resistant related genes, with annotation involved in disease resistance. Though several gaps still exist on the bin-map, our results could potentially be used in breeding programs and establish an understanding of the associated mechanisms underlying RKN resistance in cucumber.
Highlights
The southern root-knot nematode (RKN), Meloidogyne incognita (Kofoid & White) Chitwood, can cause severe yield loss of many crop species, including cucumber (Cucumis sativus L.), in China and around the world (Fassuliotis 1970; Ma et al 2014; Milligan et al 1998; Pham et al 2013; Shukla et al 2017; Xu et al 2013)
Twenty-one introgression lines (ILs), together with one cucumber cultivar and one wild cucumber species C. hystrix (Fig. 2, showing the root phenotyping of C. hystrix), were tested for RKN resistance in the summer of 2009 in the greenhouse at Pailou experimental base, Nanjing Agricultural University (Table 1)
A continuous distribution from resistance to susceptible was obtained by calculating the gall index of the F2:3 population, which was the predominant indicator for resistance to RKN, suggesting polygenic control of RKNresistance (Fig. 1a, b, Table S2)
Summary
The southern root-knot nematode (RKN), Meloidogyne incognita (Kofoid & White) Chitwood, can cause severe yield loss of many crop species, including cucumber (Cucumis sativus L.), in China and around the world (Fassuliotis 1970; Ma et al 2014; Milligan et al 1998; Pham et al 2013; Shukla et al 2017; Xu et al 2013). IL10-1 which was identified as resistant to RKN is one of the introgression lines derived from the successful fertile interspecific hybridization between the synthesized allotetraploid species Cucumis × hytivus J.F.Chen & J.H. Kirkbr. Huang et al (2009a, b) developed and applied a high-throughput method, a sliding-window approach instead of single-nucleotide polymorphisms (SNPs), for genotyping of a recombinant inbred line (RIL) population derived from a cross between two sequenced rice varieties. Xu et al (2013) applied the parent-independent method developed in rice into soybean to identify three QTLs to RKN resistance using a 246 RIL population derived from two low coverage sequenced parents. We successfully constructed a linkage binmap containing 1048 bin markers using the parent-independent method for genotyping of a recombinant inbred lines (RIL) population generated from crossing the introgression line IL10-1 with a cucumber cultivar CC3. The results from this study will facilitate breeding for root-knot nematode resistance in cucumber
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