Abstract
The southern root-knot nematode (RKN, Meloidogyne incognita) is a major soil-inhabiting plant parasite that causes significant yield losses in cotton (Gossypium spp.). Progeny from crosses between cotton genotypes susceptible to RKN produced segregants in subsequent populations which were highly resistant to this parasite. A recombinant inbred line (RIL) population of 138 lines developed from a cross between Upland cotton TM-1 (G. hirsutum L.) and Pima 3–79 (G. barbadense L.), both susceptible to RKN, was used to identify quantitative trait loci (QTLs) determining responses to RKN in greenhouse infection assays with simple sequence repeat (SSR) markers. Compared to both parents, 53.6% and 52.1% of RILs showed less (P<0.05) root-galling index (GI) and had lower (P<0.05) nematode egg production (eggs per gram root, EGR). Highly resistant lines (transgressive segregants) were identified in this RIL population for GI and/or EGR in two greenhouse experiments. QTLs were identified using the single-marker analysis nonparametric mapping Kruskal-Wallis test. Four major QTLs located on chromosomes 3, 4, 11, and 17 were identified to account for 8.0 to 12.3% of the phenotypic variance (R2) in root-galling. Two major QTLs accounting for 9.7% and 10.6% of EGR variance were identified on chromosomes 14 and 23 (P<0.005), respectively. In addition, 19 putative QTLs (P<0.05) accounted for 4.5–7.7% of phenotypic variance (R2) in GI, and 15 QTLs accounted for 4.2–7.3% of phenotypic variance in EGR. In lines with alleles positive for resistance contributed by both parents in combinations of two to four QTLs, dramatic reductions of >50% in both GI and EGR were observed. The transgressive segregants with epistatic effects derived from susceptible parents indicate that high levels of nematode resistance in cotton may be attained by pyramiding positive alleles using a QTL mapping approach.
Highlights
Host-plant resistance is highly effective in controlling crop loss from root-knot nematode (RKN) Meloidogyne incognita Kofoid and White (Chitwood) infection
Seventy-four of 138 lines had lower (P,0.05) galling index (GI) than the parent TM-1 (5.860.25 SE) indicating 53.6% of the lines exhibited transgressive resistance (Figure 2A), and 72 lines had fewer (P,0.05) eggs per gram root (EGR) (839862025SE) indicating 52.1% of the lines showed transgressive resistance (Figure 2B). These observations indicated that multiple genes from both parents are involved in determining the resistance phenotype in these transgressive segregants because both parents are in the typical range of the susceptible phenotype for both GI and EGR
A low correlation (R2 = 0.42) between GI and EGR in the recombinant inbred line (RIL) population indicated that different genes might control the GI and nematode reproduction responses
Summary
Host-plant resistance is highly effective in controlling crop loss from root-knot nematode (RKN) Meloidogyne incognita Kofoid and White (Chitwood) infection. One source of enhanced resistance is that generated through transgressive segregation. Transgressive segregation is the phenomenon in which segregating hybrids exhibit phenotypes that are extreme or novel relative to the parental lines [1,2] and result from epistatic interactions between the genes assembled in novel combinations. A highly susceptible parent can contribute to nematode resistance via transgressive segregation [5]. These crosses can derive highly resistant lines, even when both parents have a susceptible phenotype. Such transgressive segregants can be used as improved resistance sources in crop breeding [5,6]
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