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. 2006 Jun;38(2):250–257.

Phenotypic Expression of rkn1-Mediated Meloidogyne incognita Resistance in Gossypium hirsutum Populations

C Wang 1, W C Matthews 1, P A Roberts 1
PMCID: PMC2586458  PMID: 19259455

Abstract

The root-knot nematode Meloidogyne incognita is a damaging pest of cotton (Gossypium hirsutum) worldwide. A major gene (rkn1) conferring resistance to M. incognita was previously identified on linkage group A03 in G. hirsutum cv. Acala NemX. To determine the patterns of segregation and phenotypic expression of rkn1, F1, F2, F2:3, BC1F1 and F2:7 recombinant inbred lines (RIL) from intraspecific crosses between Acala NemX and a closely related susceptible cultivar Acala SJ-2 were inoculated in greenhouse tests with M. incognita race 3. The resistance phenotype was determined by the extent of nematode-induced root galling and nematode egg production on roots. Suppression of root galling and egg production was highly correlated among individuals in all tests. Root galling and egg production on heterozygous plants did not differ from the susceptible parent phenotype 125 d or more after inoculation, but were slightly suppressed with shorter screening (60 d), indicating that rkn1 behaved as a recessive gene or an incompletely recessive gene, depending on the screening condition. In the RIL, rkn1 segregated in an expected 1 resistant: 1 susceptible ratio for a major resistance gene. However, within the resistant class, 21 out of 34 RIL were more resistant than the resistant parent Acala NemX, indicating transgressive segregation. These results suggest that rkn1-based resistance in G. hirsutum can be enhanced in progenies of crosses with susceptible genotypes. Allelism tests and molecular genetic analysis are needed to determine the relationship of rkn1 to other M. incognita resistance sources in cotton.

Keywords: cotton, Gossypium hirsutum, Meloidogyne incognita, resistance, rkn1, root-knot nematode, phenotypic expression, transgressive segregation

The southern root-knot nematode Meloidogyne incognita is an important pest of cotton Gossypium hirsutum (Goodell and Montez, 1994) and many other crops worldwide (Sasser, 1977). Nematode infection causes root galling, shoot stunting, and loss of yield. In addition, the presence of root-knot nematodes can increase the incidence, rate of development, and severity of Fusarium wilt (FW) in cotton as a disease complex (Abawi and Chen, 1998). Fusarium wilt symptoms typically are associated with the presence of M. incognita in fields with coarsely textured sandy soils (Jeffers and Roberts, 1993). In the San Joaquin Valley of California, where cotton is grown intensively under irrigation, M. incognita and FW complex infections occur in up to 20% of the cotton planting area (Goodell et al., 1992; Anonymous, 1996). Restrictions on nematicide use and their relatively high cost in cotton production have expedited the development of root-knot resistant cotton cultivars.

The first highly resistant cotton germplasm available for breeding resistance to root-knot nematode was Auburn 623 RNR (G. hirsutum), a transgressive segregant for resistance from a cross of “Clevewilt 6-3-5” and “Mexico Wild” (Shepherd, 1974). Subsequently, Auburn 634 RNR and other derived lines with high levels of resistance, such as the M-line series developed from Auburn 623 RNR and Auburn 56, were released (Shepherd, 1982a; Shepherd et al., 1988, 1996). These lines were not suitable as commercial cultivars but provided advanced breeder line resistant stocks. Early attempts at genetic analysis of root-knot nematode resistance in these materials indicated the presence of multiple genes with dominant or additive effects and the occurrence of transgressive segregation for resistance in some crosses (Shepherd, 1974, 1986). However, no clear understanding of the genetic control of resistance was revealed. McPherson et al. (2004) reported a two-gene model for resistance in M-315 derived from Auburn 623 RNR. Analysis of an F2 population indicated that one recessive gene conferred moderate resistance in Clevewilt 6–1 (Bezawada et al., 2003).

In 1995, the upland cotton cultivar Acala NemX (G. hirsutum) was released, having been developed as a single line selection in a self-pollinated population with high resistance to M. incognita (Oakley, 1995; Ogallo et al., 1997). Acala NemX was developed from the cross Acala B1662 × N-3; N-3 was derived from the nematode resistant line N6072 (Hyer and Jorgenson, 1984). The origin of the M. incognita resistance in Acala NemX is not clear from the existing pedigree reports (Hyer and Jorgenson, 1984; Oakley, 1995; Robinson et al., 2001). The nematode resistance in Acala NemX is highly effective in protecting plants from the effects of root infection. The lint yield of Acala NemX was less than that of susceptible Acala Maxxa in noninfested fields or those with low levels of nematode infestation. However, Acala NemX yields decreased only slightly, whereas Acala Maxxa yields were severely decreased with medium or high levels of nematode infestation (Ogallo et al., 1997). The utilization of Acala NemX also can greatly increase the rotational value of cotton for managing root-knot nematodes (Ogallo et al., 1999). In addition, nematode resistance in cotton can protect the plant from FW disease under field conditions (Shepherd, 1986; DeVay et al., 1997; Ogallo et al., 1999). Variation in virulence among M. incognita isolates to the Acala NemX resistance has been reported (Ogallo et al., 1997). Recently, we identified a major gene, rkn1, in Acala NemX conferring resistance to M. incognita and localized rkn1 to linkage group A03 in the cotton genome using SSR markers (Wang et al., 2006). An understanding of the genetic basis of root-knot nematode resistance in cotton will facilitate breeding of cultivars with improved resistance and indicate possibilities for combining resistance traits to obtain higher or more durable levels of resistance.

The objective of the present work was to determine the phenotypic expression of rkn1-mediated resistance to M. incognita using nematode-induced root galling and egg production phenotypes of progenies generated from an intraspecific G. hirsutum cross between the rkn1 donor Acala NemX and a related susceptible cultivar Acala SJ-2.

Materials and Methods

Plant materials and crosses: Plant genotypes used in this study were susceptible G. hirsutum cv. Acala SJ-2 and resistant G. hirsutum cv. Acala NemX. Two sets of progenies were produced from separate crosses. In the first set (set I), crosses were made between Acala SJ-2 and Acala NemX to generate F1, F2, F2:3, F2:7 (69 RIL) and BC1F1 (32 plants of NemX × F1 and 37 plants of F1 × NemX) populations. The second set (set II) included F1, 99 F2 plants, 100 plants of BC1F1 (NemX × F1) and 50 plants of BC1F1 (SJ-2 × F1). In addition, a resistant sister line of NemX, N901, and another susceptible cultivar, Acala Maxxa, were included in the test for the parental and F1 screening.

Nematode resistance screening: A culture of M. incognita race 3 (isolate Project 77), originating from a San Joaquin Valley, CA, cotton field was maintained and multiplied on the tomato cultivar Tropic. The species and race identity of the culture were confirmed by isozyme phenotyping and a host differential test as described previously (Roberts et al., 1996). Cotton populations were evaluated for nematode resistance under controlled conditions in a greenhouse. Individual cotton seeds were planted into 10-cm-diam. × 17-cm-deep plastic pots filled with steam-sterilized sand. Plants were fertilized with 17–6-10 controlled release fertilizer (Scotts-Sierra Horticultural Products Co, Marysville, OH). Three-wk-old seedlings were inoculated with approximately 50,000 eggs of M. incognita. Inoculum was prepared by extracting eggs from tomato roots with NaOCl (Hussey and Barker, 1973). Pots were dripirrigated to maintain plant growth. Air temperatures in the greenhouse were maintained between 28 and 35°C during the day and at 24°C at night.

Due to the large numbers of plants evaluated and also to the testing of different generations or populations, the phenotyping experiments were done in different tests. In order to collect F2, F2:3 and F2:8 seeds in the set I populations (Tables 1 and 2), F1, F2 and F2:7 RIL plants were phenotyped for resistance reaction 150, 150 and 125 d after inoculation, respectively. The set I BC populations were phenotyped 146 d after inoculation and the set II F1, F2 and BC populations 60 d after inoculation. A 0-to-10 root-gall index (GI) was used to evaluate resistance reaction to nematodes. The GI was modified from the Bridge and Page (1980) root-knot nematode rating chart as follows: 0 = no galls; 1 = few small galls; 2 = small galls with less than 10% of roots infected; 3 = 10% to 30% of roots infected, main roots clean; 4 = 31% to 40% of roots infected; 5 = 51% to 60% of roots infected, galling on parts of main roots; 6 = 61% to 70% of roots infected, galling on main roots; 7 = 71% to 80% of roots infected, majority of main roots galled; 8 = 81% to 100% of roots infected, all main root galled; 9 = all roots severely galled and plant usually dying; 10 = all roots severely galled with diminished root system and plant usually dead. Cotton resistance also was evaluated by the numbers of nematode eggs per gram fresh root. Eggs were extracted from the roots in NaOCl (Hussey and Barker, 1973).

Table 1.

Classification for resistance to Meloidogyne incognita of parental lines and segregating populations derived from crosses between resistant Acala NemX and susceptible Acala SJ-2.

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Table 2.

Segregation data for Meloidogyne incognita resistance in the F2 population and derived F2:3 families of the cross Acala NemX × SJ-2.

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Due to the different lengths of the tests and the influence of time of year in the greenhouse, the GI and number of eggs per gram root varied among tests for the same parental genotypes. Therefore, the criterion for classifying individuals as resistant or susceptible was based on the separation of the parent phenotype scores in each test. The mean and SD for GI and eggs per gram root for each parent were used to determine the threshold for resistance in each test. Plants with a score equal to or less than the resistant parent mean plus 1 SD value were classified as resistant. For the GI, the threshold in the set I tests (Table 1) was ≤ 1.9 (F1), ≤ 3.0 (F2:7) and ≤ 3.5 (F2, F2:3 and BC1), and ≤ 2.0 in the set II tests. For egg production, plants with ≤ 500 eggs/g root were classified as resistant and > 500 as susceptible in set I. On almost all plants, the galling and egg scores provided a matching classification. However, on a few test plants, a galling score slightly above the resistance threshold was matched with a typically resistant egg production score, and these plants were classified as resistant in the segregation analysis. This occurred in the F2:3 segregating progenies, where 13 individuals with a GI of 4.0 were classified as resistant based on low egg production scores (< 500) (Tables 1 and 2). In segregating F2 populations, homozygous resistance was identified when all individual plants in a F2:3 family were resistant, homozygous susceptible when all individual F2:3 plants were susceptible, and heterozygous when plants in a F2:3 family were segregating susceptible and resistant.

Data analysis: Data were subjected to one-way AN OVA analysis. Fisher's Protected LSD test was used to compare the treatment means. Data for the nematode egg production were transformed to log10 (x + 1) for analysis. The data for resistance segregation were tested for goodness-of-fit to predicted Mendelian inheritance ratios by χ2-test.

Results

Phenotype of parents and F1: The resistant sister lines, Acala NemX and N901, had lower (P < 0.05) GI (mean 1.17 and 1.35, respectively, Fig. 1A) and supported fewer (P < 0.05) numbers of nematode eggs per gram of roots (291 and 175, respectively, Fig. 1B) than the two susceptible parent genotypes Acala SJ-2 (GI = 5.58 and 4,129 eggs) and Acala Maxxa (GI = 5.85 and 6,430 eggs). In separate tests with the different segregating populations derived from Acala NemX and Acala SJ-2, the two parents were included in each test. Acala NemX and Acala SJ-2 differed from each other for GI and egg production (P < 0.05) in each test; the parent means are presented in Table 2 and Figures 1–4. The four F1 from resistant × susceptible crosses of the four parents did not differ from two susceptible parents Acala SJ-2 and Maxxa in GI scored at 150 d after inoculation (Fig. 1A). Based on egg production at 150 d, the two F1 from NemX and N901 crossed with susceptible SJ-2 did not differ from SJ-2, and the two F1 from NemX and N901 crossed with susceptible Maxxa did not differ from Maxxa (Fig. 1B). The mean values of eggs per gram root (Fig. 1B) and total eggs per root system (data not shown) of susceptible Maxxa and SJ-2 did not differ. However, the F1 (Maxxa × N901) with 462,000 eggs/root system and 7,784 eggs/g root supported greater nematode reproduction and root galling (P < 0.05) than F1 (SJ-2 × N901) with 147,000 eggs/root system and 2,487 eggs/g root. The resistant x resistant F1 (NemX × N901) supported less egg production (65 eggs/g root) and root galling (0.75) (P < 0.05) than the resistant parents.

Fig. 1.

Fig. 1.

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Root galling (A) and egg production (B) of Meloidogyne incognita on susceptible (Acala SJ-2, Acala Maxxa) and resistant (Acala NemX, N901) cotton cultivars and breeding lines and their F1 from the first set of crosses. Log10 (x + 1) transformed data were used for analysis of eggs per gram of root. Galling Index: 0 to 10 scale; 0 = no galling, and 10 = severe galling. Bars represent 1 standard deviation.

Fig. 4.

Fig. 4.

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The distribution of different classes of resistance reaction to Meloidogyne incognita of F2:7 RIL (NemX × SJ-2) from the first set of crosses based on galling index. Mean values of four plants per line plus standard deviation bar. Score of the resistant (NemX) and susceptible (SJ-2) parents are indicated. Galling Index: 0 to 10 scale; 0 = no galling, and 10 = severe galling.

In a shorter screening 60 d after inoculation, the F1 (NemX × SJ-2) had lower galling (GI = 5.1) (P < 0.05) and numbers of eggs per gram root (7,252 eggs) (P < 0.05) than susceptible parent Acala SJ-2 (GI = 5.6; 12,431 eggs) and higher GI and greater number of nematode eggs than resistant parent Acala NemX (GI = 1.8; 518 eggs/g root).

Galling index and egg production in backcross populations: The combined backcross populations of NemX × F1 and F1 × NemX had 69 individual plants which showed a close fit to a 1:1 segregation between resistance and susceptibility (Table 1). Root-galling index was highly correlated (r = 0.744) with egg production in the back-cross population (Fig. 5), confirming the results from the parent and F1 phenotype reactions (Fig. 1). Based on egg production, 32 plants had < 500 eggs/g root and 37 plants had > 500 eggs/g root, whereas the parents had 374 eggs/g root in Acala NemX and 2,841 eggs/g root in Acala SJ-2. Galling index (Fig. 5) gave the same distribution, with 32 plants having a GI ≤ 3.5 and 37 plants having a GI > 3.5, with parent phenotypes having a mean GI =1.9 (range 1 - 3) for Acala NemX and GI = 5.7 (range 5–6.5) for Acala SJ-2.

Fig. 5.

Fig. 5.

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The relationship between galling index and egg production in the combined segregating backcross populations of F1 × NemX and NemX × F1 from the first set of crosses. Log10 (x + 1) transformed data were used for analysis of eggs per gram of root. Galling Index: 0 to 10 scale; 0 = no galling, and 10 = severe galling.

A second set of progenies developed from separate crosses of NemX × SJ-2 included 100 individual back-cross plants from resistant NemX × F1 and 50 backcross plants from susceptible SJ-2 × F1 (Fig. 2). In the back-cross population to resistant Acala NemX, 54 plants had a GI ≤ 2 and 46 plants had a GI > 2 (Fig. 2A). In the backcross population to susceptible parent Acala SJ-2, 46 out of 50 plants showed moderately to highly susceptible galling phenotypes (GI = 2.5–6.0) (Fig. 2B). Four individuals had resistant responses based on galling and egg production. They were confirmed to be true heterozygotes for the resistance region based on subsequent marker analysis (data not shown) and probably were infection escapes with small root systems. The mean GI of the parents were 1.6 for Acala NemX and 5.0 for Acala SJ-2. Even though the phenotype screens for the second set of progenies had less infection overall compared with the first set, classification of resistance phenotype based on galling index as 54 resistant: 46 susceptible for BC1F1 NemX × F1 (P = 0.424) and 4 resistant: 46 susceptible for BC1F1 SJ-2 × F1 conformed to an expected segregation for the rkn1 gene determining resistance in Acala NemX.

Fig. 2.

Fig. 2.

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The distribution of different classes of resistance reaction to Meloidogyne incognita of backcross plants from the second set of crosses based on root-galling index. Galling Index: 0 to 10 scale; 0 = no galling, and 10 = severe galling. A: NemX × F1 (NemX × SJ-2); B: SJ-2 × F1 (NemX × SJ-2).

F2 and F2:3: In the second set of progenies, 99 F2 plants were tested for resistance based on galling index (Fig. 3) and egg production (data not shown). Twenty plants with a GI ≤ 2 were classified as resistant, and 79 plants with GI > 2 were classified as susceptible (parent mean GI were 1.6 for Acala NemX and 5.0 for Acala SJ-2). This segregation distribution fit a 1 resistant: 3 susceptible ratio (P = 0.270) expected for rkn1 behaving as a recessive gene for resistance.

Fig. 3.

Fig. 3.

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The distribution of different classes of resistance reaction to Meloidogyne incognita of 99 F2 (NemX × SJ-2) plants from the second set of crosses based on galling index. Galling Index: 0 to 10 scale; 0 – no galling, and 10 – severe galling.

In the first set of progenies, 43 families of F2:3 were developed from individual F2 plants that were screened for resistance. Each F2:3 family, represented by 10 to 16 plants/family, was then screened for resistance. The resistance categories of the F2 individuals and the F2:3 families, based on root galling and egg production phenotype screens together with their predicted genotypes, are given in Table 2. The homozygous resistant lines had low GI, low total eggs per root system and low eggs per gram root phenotypes for both individual F2 plants (mean values were 0.6, 12,000, and 350, respectively) and their derived F2:3 families (mean values were 2.2, 16,000, and 470, respectively). The homozygous susceptible lines had correspondingly high GI and egg production per root system and gram root phenotype scores for F2 (mean values were 6, 322,000, and 7,800, respectively) and F2:3 (mean values were 6, 345,000, and 6,350, respectively). In the segregating F2:3 families derived from heterozygous F2 plants, the progenies showed a range of phenotypes for root galling and egg production that included both resistant and susceptible responses (Table 2). Based on the F2 and F2:3 phenotypes, the segregation of the 43 lines was 7 resistant: 29 segregating: 7 susceptible, conforming to a 1:2:1 distribution for homozygous resistant: heterozygous (segregating in F2:3): homozygous susceptible genotypes expected for a single gene determining resistance (Table 1). The 427 individuals pooled from the 29 segregating F2:3 families segregated in a 1 resistant (104 individuals): 3 susceptible (323 individuals) ratio, further confirming the recessive condition of gene rkn1 in Acala NemX (Table 1).

Recombinant inbred lines (RIL): For the phenotypic test of 69 F2.7 RIL, 4 plants/line were screened for nematode resistance. The distribution of mean GI values grouped the 69 lines into two distinct classes, with 34 lines with GI of 0.25 to 2.88 classified as resistant and 35 lines with GI of 4.88 to 6.33 classified as susceptible (Fig. 4). This segregation fit the 1 resistant: 1 susceptible expected distribution for gene rkn1 determining resistance in the RIL population (Table 1), in which lines are either homozygous resistant or homozygous susceptible for rkn1. Within the susceptible group, all lines were similar to Acala SJ-2 (GI = 5.8 ± 0.41). However, within resistant lines, 21 (GI of 0 - 2.0) were more resistant (P < 0.05) than resistant parent Acala NemX (GI = 2.5 ± 0.48). In addition, eggs were extracted from a few lines from the resistant and susceptible RIL groups. Similar to the root-galling reactions, the two groups were differentiated (P < 0.05), e.g., three resistant lines had a mean of 14,111 eggs/root system and 268 eggs/g root, compared to a typical susceptible line with 345,000 eggs/root system and 6,001 eggs/g root.

Discussion

The phenotypic analysis of rkn1-mediated resistance in multiple progenies developed from an intraspecific G. hirsutum cross demonstrated that rkn1 showed a typical bimodal pattern of segregation for a major gene. The rkn1 gene explained the main resistance pheno-type effects in Acala NemX of suppressing both nematode-induced root galling and nematode reproduction. Further, the level of resistance expression in heterozygous plants or populations including the F1 indicated that rkn1 operated as a recessive gene or incompletely recessive gene depending on the screening conditions. Root-galling reactions in heterozygous plants were not different from homozygous susceptible plants in tests that were terminated 125 d or longer after inoculation. However, the heterozygous plants screened for this gene for the short duration of 60 d after inoculation showed slight suppression of nematode egg production and root galling compared to the susceptible parent. This confirmed our finding with SSR markers for rkn1, in which the co-dominant marker CIR316 in the heterozygous condition correlated with plants in the susceptible class that had root-gall indices between 2 and 4, with homozygous susceptible plants having gall ratings mostly greater than 4 in short duration screenings (Wang et al., 2006). Because of the sensitivity of the resistance phenotype to test conditions, markers will be especially helpful in distinguishing heterozygous plants from homozygous susceptible plants. Levels of galling and egg production varied among tests, being lower in the shorter duration screening of the second set of progenies. However, we found that including resistant and susceptible parents in each test and basing the resistance threshold on the mean plus 1 SD of the resistant parent score provided a definitive classification of resistance for each test.

In a preliminary study based on F1 (NemX × Deltapine 90) and 43 F2 individuals, Zhou et al. (1999) suggested that Acala NemX contained a recessive or neutral gene for M. incognita resistance. McPherson et al. (2004) reported a two-gene model for M. incognita resistance in M-315 RNR (G. hirsutum) that was developed by backcrossing Auburn 634 RNR to Deltapine 61 (Shepherd et al., 1996), with one dominant gene (Mi1) and an additive gene (Mi2). However, they hypothesized that Acala NemX may have the same additive gene Mi2 as in M-315 and M78-RNR and that homozygous Mi2 may confer a low level of resistance (McPherson et al., 2004). In our study, rkn1 in Acala NemX acted as a major resistance gene in suppressing root galling and nematode reproduction. Therefore, rkn1 in Acala NemX may be different from Mi1 and Mi2 in M-315.

McPherson et al. (1995) postulated that highly resistant Auburn 623 RNR may carry two genes, with one coming from each parent, Clevewilt 6-3-5 and Mexico Wild Jack Jones. Bezawada et al. (2003) reported that one recessive gene in Clevewilt 6-1 may control root-knot nematode resistance in crosses with Stoneville 213. Assuming Clevewilt 6-3-5 has the same gene as Clevewilt 6-1, Auburn 623 RNR should have one recessive gene controlling M. incognita resistance. Auburn 634 RNR, a highly resistant breeding line, was developed by backcrossing Auburn 623 RNR to Auburn 56, a moderately resistant cultivar (Shepherd, 1982b) with less resistance than Clevewilt 6 (Shepherd, 1983). Therefore, Auburn 634 RNR may carry resistance genes from both Auburn 623 RNR and Auburn 56. If M-315 contains only one dominant gene and one additive gene for resistance, the recessive gene in the pedigree may have been lost during breeding selection. Whether the recessive gene in Clevewilt 6-1 is the same as rkn1 in Acala NemX is not known, but could be determined by allelism tests with the cross NemX × Clevewilt 6. In a study of crosses of root-knot-resistant × susceptible G. barbadense L. breeding stocks (Turcotte et al., 1963), two recessive genes were reported to determine M. incognita resistance in this tetraploid cotton species. Resistance genes found in a related species background should be tested for their relationship to the rkn1 gene in Acala NemX.

In tracing the origin of the resistance in Acala NemX, different accounts of the pedigrees were found (Robinson et al., 2001). The advanced line donor of the Acala NemX resistance was breeding line N6072, which Hyer and Jorgenson (1984) reported that they had developed from the cross of a Missouri line, FBCX-2, with a Shafter AXTE line. FBCX-2 was moderately resistant and developed from the cross of Auburn 56, carrying some resistance, and Sea Island Seabrook 12-B2. Oakley (1998) indicated that N6072 was developed from the cross Acala 1-2302 × Tanguis, with Acala 1-2302 derived from susceptible Acalas SJ-1 and SJ-2. N6072 had greater resistance than Auburn 56 (Hyer et al., 1979), whereas the level of resistance in Acala NemX was similar to that in N6072 (Ogallo et al., 1997). The greater resistance in N6072 or Acala NemX may be due to transgressive inheritance, which is quite common in cotton, such as that reported for Auburn 623RNR, a transgressive segregant for root-knot nematode resistance from the F6 generation of a cross of Clevewilt 6-3-5 and Mexico Wild, and Auburn 61 from an F6 of the cross of Hybrid 257 and Mexico Wild (Shepherd, 1974). Three lines (N9281, N9308, and N9311) also had greater resistance than their resistant parent N6074, one of the sister lines of N6072 (Hyer and Jorgenson, 1984).

In our study, evidence for transgressive segregation involving the rkn1 gene in Acala NemX was found. The F1 between Acala NemX and its resistant sister line N901 had greater resistance than either parent. Further, in the F2:7 RIL population, the significant variation in the level of resistance among the 34 resistant lines, with 21 being more resistant than Acala NemX, indicated transgressive segregation in this G. hirsutum cross. Presumably, a resistance-enhancing factor was contributed from susceptible Acala SJ-2, in which the enhanced resistance was achieved when both the rkn1 gene from Acala NemX and the Acala SJ-2 factor were present in the homozygous condition. The susceptible RIL did not differ from susceptible parent Acala SJ-2, indicating that the Acala SJ-2 factor had no measurable effect on susceptibility in the absence of the rkn1 gene from Acala NemX. However, differences in F1 susceptibility, with those produced from Maxxa as susceptible parent being more susceptible than when Acala SJ-2 was the susceptible parent, may indicate a minor influence of the transgressive factor in the susceptible background. Such minor effects would require more stringent phenotype testing to be more clearly characterized.

In summary, the M. incognita resistance in Acala NemX determined by gene rkn1 was conferred in an incomplete recessive manner in an intraspecific G. hirsutum cross. This gene is effective in suppressing both nematode-induced root galling and nematode reproduction on cotton roots as measured by numbers of eggs produced during two or more months from inoculation. The reduced galling and egg production resistance phenotypes are highly correlated among individuals of different segregating populations. The rkn1-based resistance was found to be enhanced in some F2:7 recombinant inbred lines by a modifying gene or genes contributed by the susceptible parent genotype Acala SJ-2. These results suggest that the Acala NemX resistance level can be improved in G. hirsutum crosses depending on the transgressive interaction with additional genes in susceptible G. hirsutum genotypes. Understanding relationships between resistance sources and development of molecular markers will expedite the transfer of the resistance genes into commercial cotton and determine their value in gene combinations pyramided into cultivars to produce more durable and higher levels of root-knot nematode resistance.

Footnotes

This study was funded in part by a Cooperative Research Agreement from Cotton Incorporated and a grant from the University of California Discovery Grant (BioSTAR) Program. The authors thank Steven Oakley, California Planting Cotton Seed Distributors for providing cotton seed, and Kathie Carter and Teresa Mullens for technical help.

This paper was edited by J. L. Starr.

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