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a USDA-ARS, 2765 F&B Rd, College Station, TX 77845
b Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
c Cotton Incorporated, 6399 Weston Pkwy., Cary, NC 27513. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture
* Corresponding author (frobinson{at}cpru.usda.gov).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: DPL, Delta and Pine Land • RNR, root-knot nematode resistant
a USDA-ARS, 2765 F&B Rd, College Station, TX 77845
b Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
c Cotton Incorporated, 6399 Weston Pkwy., Cary, NC 27513. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture
* Corresponding author (frobinson{at}cpru.usda.gov).
Absence of sources of resistance to the reniform nematode, Rotylenchulus reniformis Linford & Oliveira, 1940, is a major impediment to the production of upland cotton (Gossypium hirsutum L.) in the USA. In this study, two trispecies hybrids of G. hirsutum, G. longicalyx J.B. Hutch. & B.J.S. Lee, and either G. armourianum Kearney or G. herbaceum L. were used as bridges to introgress high resistance to the nematode from G. longicalyx into G. hirsutum. Introgression was accomplished by recurrent backcrosses to G. hirsutum with cytogenetic analysis of early backcross generations to assess progress toward the euploid state (2n = 52), selection for nematode resistance at each generation, and examination of self progeny at the first, third, sixth, and seventh backcross to identify and eliminate lineages with undesired recessive traits. Altogether, 689 BC1 progeny were generated from the two male-sterile hybrids. Introgression was pursued from 28 resistant BC1 plants, each of which was backcrossed four to seven times to G. hirsutum to derive agronomically suitable types. The resistance trait segregated (resistant/susceptible) 1:1 in backcross progeny and 3:1 in self progeny. There was no obvious diminution of the resistance across backcross generations. Advanced backcross plants were indistinguishable from agronomic cotton under greenhouse conditions, and comparisons of 240 homozygous resistant BC6S2 plants with heterozygous, susceptible, and recurrent parent plants in field plantings in 2006 showed normal lint quality and quantity. The upcoming release of seed from this project is expected to provide the cotton industry with a major new tool for managing the reniform nematode in cotton, which costs U.S. producers about $100 million annually.
Abbreviations: DPL, Delta and Pine Land • RNR, root-knot nematode resistant
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Eleven tolerant breeding lines of cotton have been released (Jones et al., 1988; Cook et al., 1997a; Cook and Robinson, 2005) that may find a place in management of the reniform nematode. They yield well in infested fields in the production regions where they were developed; however, they may not perform competitively in other areas (Koenning et al., 2000), and appear to suppress nematode populations by 60 or 70% at best (Beasley, 1985; Jones et al., 1988; Cook et al., 1997a, 1997b; Cook and Robinson, 2005), or not at all (Robinson, unpublished data, 2002), compared with 95 to 99% suppression with corn (Zea mays L.) and resistant soybean (Gazaway et al., 1998, 2000; Lawrence and McLean, 2001; Robbins et al., 2001, 2002; Davis et al., 2003; Westphal et al., 2004). Rotation studies show that populations of the reniform nematode in a cotton field typically rebound immediately after a year in non-host corn. Thus, after growing a tolerant cultivar that might be developed from the tolerant breeding lines available, it would be expected that the following year, heavily infested fields would remain heavily infested and unsuitable for growing susceptible cultivars.
Evaluation of 2000 primitive accessions of G. hirsutum in the search for sources of resistance to the reniform nematode (Robinson and Percival, 1997; Robinson et al., 2004) identified only six with "moderate resistance" and none with "high resistance," defined in two key germplasm screens as <10% of the reproduction on susceptible cv. Deltapine 16 (Yik and Birchfield, 1984; Robinson et al., 2004). High resistance occurs in one or more accessions of G. longicalyx J.B. Hutch. & B.J.S. Lee, G. somalense (Gürke) J.B. Hutch., G. stocksii Mast., G. arboreum L., and G. barbadense L. (Yik and Birchfield, 1984; Stewart and Robbins, 1995). Four G. longicalyx accessions failed to support nematode reproduction and were uniquely classified as immune (Yik and Birchfield, 1984; Stewart and Robbins, 1996). Thus, development of highly resistant cotton cultivars probably will require transfer of resistance from other species or de novo biotechnological synthesis. Gossypium longicalyx offers the highest resistance known within the Gossypium genus but poses several challenges as a source of resistance, including small bolls, scandent growth habit, adaptation to mesic environments, diploidy, and an alien (F) genome (Hutchinson, 1959; Percival et al., 1999).
Transfer of reniform nematode resistance from G. longicalyx (2n = 26) into G. hirsutum (2n = 52) requires introgression of genes from the unique diploid F genome of G. longicalyx into either the A or D subgenome of allotetraploid G. hirsutum. As bridges, two synthetic tetraploid triple-species hybrids, referred to as HLA and HHL, were developed (Bell and Robinson, 2004). The hybrids both have G. hirsutum and G. longicalyx in their backgrounds and differ in having either G. armourianum Kearney (HLA = hirsutum + longicalyx + armourianum) or G. herbaceum L. (HHL = hirsutum + herbaceum + longicalyx) as the third parent. The specific G. hirsutum parents of HLA and HHL were cv. Deltapine 14 (TM-1) and cv. Tamcot CAMD-E, respectively. The accessions of G. longicalyx, G. armourianum, and G. herbaceum used to create the hybrids are not known but were from the USDA Germplasm Collection. Gossypium herbaceum (2n = 26) is a cultivated A genome Old World diploid and G. armourianum (2n = 26) is a D2–1 genome New World diploid wild species that has served as a source for bacterial blight resistance as well as the dominant D2 smooth leaf trait in agronomic cotton (Percival et al., 1999).
Anatomical traits of the HLA and HHL hybrids were given by Bell and Robinson (2004). Both hybrids are male sterile but have sufficient female fertility when pollinated by G. hirsutum to develop single seeds in some bolls if pollinated during the spring and summer months (Bell and Robinson, 2004). Seeds from HLA have green lint, while those from HHL have tan lint. Comparisons of reniform nematode reproduction on the hybrids with that on G. longicalyx and G. hirsutum cv. Tamcot CAMD-E revealed that both hybrids were virtually immune to the reniform nematode (Agudelo et al., 2005).
The objective of this research was to introgress, via backcrossing, reniform nematode resistance from the HLA and HHL hybrids into G. hirsutum, and recover plants with the resistance trait fixed in the absence of any obvious undesirable traits when grown under greenhouse and field conditions.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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50%. All plants in growth chambers and the greenhouse were watered daily with water purified by reverse osmosis, and were fertilized and sprayed to control insects as needed. Seeds for field tests in 2005 and 2006 were germinated individually in peat pellets in the greenhouse and transplanted to the field in late April or early May. Progeny rows included 20 plants plot–1 at 45-cm intervals on 100-cm beds and were not replicated. The field was free of reniform nematodes, with a Lufkin fine sandy loam soil (fine, smectitic, thermic Oxyaquic Vertic Paleustalfs), located at College Station, TX, and irrigated when needed.
Direct Nematode Resistance Assay
Reniform nematode resistance was evaluated with either a direct assay that measured development and egg laying by nematodes feeding on roots, or an indirect assay that measured the number of their progeny in the soil. The direct assay was used previously to compare G. longicalyx and the HLA and HHL hybrids (Agudelo et al., 2005). It was used in this study to examine 69 progeny from the first backcrosses to G. hirsutum, and three groups of plants from the sixth backcross generation that were known to be homozygous resistant, homozygous susceptible, or heterozygous for the resistance trait based on marker and progeny phenotype data.
Plants to be evaluated were grown nematode free in the greenhouse 8 to 12 wk until pot bound within 500-mL plastic pots containing sand or a peat–sand potting medium. Each root ball was then lifted, slipped into a closely fitting pot-shaped sleeve fashioned from 2-mm-mesh fiberglass screen, and transplanted into a 2-L pot containing sand that had been uniformly infested with R. reniformis by mixing infested silt loam soil with nematode-free fine sand (<400 µm) at a ratio varying from 1:1 to 1:25, depending on the availability and concentration of nematodes in the silt loam soil.
After transplanting, the pot was placed in the controlled-environment chamber for 3 wk, then root balls were lifted and roots that had grown out through the screen from the root ball into infested soil were collected, placed in FAA (40:20:6:1 water/formalin/ethanol/acetic acid) and examined microscopically to evaluate female nematode development and the presence or absence of associated eggs. Root balls were then repotted and maintained in the greenhouse for an additional 4 mo, when nematode populations in the soil were measured. There were eight or more replicates of controls, and of each genotype, where applicable, in the direct assays.
In both direct assays, we used a nematode population from Baton Rouge, LA, that was maintained throughout the study in the greenhouse in silty loam soil on tomato (Lycopersicon esculentum Mill.) cv. Rutgers and cotton cv. Fibermax 832. In the assay evaluating the first 65 BC1 plants obtained from the HLA hybrid, however, a split-root configuration was used that allowed all plants to also be tested against the root-knot nematode, Meloidogyne incognita (Chitwood) Kofoid and White. At the time of root ball transplanting, a U-shaped cardboard barrier was installed in each pot to divide the soil volume outside the root ball into two halves. In one half of the pot, soil infested with reniform nematodes was substituted with soil infested with Meloidogyne incognita race 3.
The assay comparing reproduction on the three groups of plants at the sixth backcross included a contrast between the greenhouse population of the reniform nematode and a freshly collected reniform nematode population from the Baton Rouge field. Different nematode populations in this case were put in separate pots, rather than in separated parts of the same pot, as in the earlier root-knot nematode evaluation.
Indirect Nematode Resistance Assay
The indirect assay was modified from Robinson et al. (2004). In this study, it was conducted 19 times to phenotype 2561 progeny from backcrosses, test crosses, and self pollinations between 2003 and 2006. In 17 assays, plants were grown in 500-mL cups in a controlled-environment chamber and the other two times, they were grown in 2.5-L pots in the greenhouse. The standard number of plants included 10 progeny per backcross, five progeny per test cross, and 30 to 45 progeny per self pollination, always compared with 12 replicate plants of G. hirsutum cv. Deltapine 16 as a susceptible control and 12 replicate plants of G. barbadense GB-713 as a resistant control. In the larger of the two greenhouse indirect assays, which evaluated the first set of 164 BC2 plants, vigorous pot-bound G. longicalyx that had been generated from stem cuttings were included as a control to ensure that the primary inoculum diminished to an undetectable level during the course of the assay.
Seed were scarified by nicking the seed coat, germinated in moistened, rolled Whatman no. 3 chromatography paper at 30°C for 24 h and 22°C for an additional 24 h, and transplanted individually into 500-mL pots (or in 2.5-L pots if to be tested in the greenhouse), which were held in a greenhouse for 1 to 2 wk. Nematodes were obtained from soil by Baermann funnels held at 30°C the night before inoculation (Robinson and Heald, 1991), gently aerated for 1 to 3 h before being introduced to pots, and were 95 to 99% motile. A minimum of two separate extractions and inoculations were done for each test between Days 7 and 14 with a total of 4000 nematodes per plant. Pots were placed in random order on the greenhouse bench before inoculating, and nematodes were introduced to pots in aqueous suspension by inserting a hypodermic syringe needle into the bottom of the pot at five points halfway between the plant and the pot wall and injecting 0.6 mL as the needle was withdrawn. The needle tip was modified to force water to spray in all directions. Thus, each pot ultimately was inoculated with 6 mL of nematode suspension placed at 10 points in the pot. If insufficient nematodes were extracted due to a low concentration in the soil, a third extraction and inoculation was done. When 2.5-L pots in the greenhouse were substituted for 500-mL pots in the chamber, the total nematode population per plant was increased proportionally to the soil volume and nematodes were introduced by mixing sand with infested soil rather than by injecting nematodes with a syringe.
The inoculated pots were placed in controlled-environment chambers or on greenhouse benches in a randomized complete block array with one resistant and one susceptible control per block, and held for 7 wk, at which time three soil cores from the top to the bottom of the pot and weighing 40 g total were removed from each pot and weighed. Soil samples from 500-mL and 2.5-L pots weighed 40 and 100 g, respectively, and were collected and processed in random order. Active, vermiform stages were extracted by Baermann funnel and counted. Counts were taken from 500-mL control pots with no plants or non-host plants on numerous occasions to determine nematode survival and confirm that conditions within pots did not allow sufficient nematode survival for primary inoculum to be extractable at the end of the assay.
In both the direct and indirect type of assay, selected plants were retained for breeding or for flower buds for cytogenetic analysis and leaf tissue for DNA studies by transplanting to 10-L pots containing nematode-free potting medium.
Cytogenetic Analysis
Floral buds were slit, fixed in Carnoy's fluid-I fixative (3:1 95% ethanol/glacial acetic acid) and held in fixative until examined, with fixative changed after the first 3 d. Fixed buds were rinsed with water, soaked in running water for 1 h, and several anthers per bud dissected out and transferred to a glass slide. A drop of acetocarmine was applied, anthers were gently popped, and cells checked microscopically. If cells were at meiosis metaphase I, somatic anther tissue was removed, and a clean cover slip was applied and heated mildly on a hot plate to differentiate chromosome stain and free protoplasts. Once free from cell walls and stain differentiated, protoplasts were squashed and classified for chromosome number and configuration under bright-field illumination. Ten to 15 unambiguous cells were analyzed to characterize each of 203 plants representing BC1F1, BC2F1, BC3F1, BC4F1, and BC5F1 generations (Table 1
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The HLA-A group was started during an exploratory phase of the project and there are a few uncertainties of minor importance in this group regarding early backcross parents. The agronomic male parent of HLA-A BC1 plants 1 through 7 were known root-knot nematode resistant genotypes, and included the G. barbadense accession TX-1348, the G. hirsutum breeding lines Auburn M-315 and Auburn 623 root-knot nematode resistant (RNR), and the cultivars Acala NemX and Stoneville LA887. Male parents for crosses producing the remaining 58 BC1 plants in group HLA-A were selected arbitrarily as available and were not recorded, but in all cases were one of four cultivars: Paymaster 1220 RR, Stoneville 373, Suregrow 125, or Tamcot Sphinx. The 65 BC1 progeny of group HLA-A were grown and evaluated for 18 mo (2000–2002) to examine morphology and fertility and backcrossed a second time to G. hirsutum. Some resistant plants were maintained 4 more yr, during which time additional crosses were made with Delta and Pine Land (DPL) 458 B/RR, DPL 5415, Fibermax 958, and Acala NemX as the susceptible agronomic parent, and ultimately those plants were used for DNA marker discoveries and mapping of the resistance gene (Dighe et al., unpublished data, 2006). A number of the second backcrosses of HLA-A to G. hirsutum were to root-knot nematode resistant parents, including Acala NemX, Auburn M-315, Auburn 623 RNR, Stoneville LA887, Pima S2, and several experimental breeding lines descended from sibling lines released by Jones et al. (1988).
All parents of plants in groups HLA-B and HHL are known. The male parent for all 603 HLA-B BC1 and all 21 HHL BC1 plants was Acala NemX. The latter seed were produced by overpollinating all flowers of 40 HLA and 40 HHL vegetatively cloned hybrid plants every day for 4 mo. Recurrent parents for subsequent crosses included Acala NemX, Coker 312, DPL 458 B/RR and 5415, Fibermax 958, and Stoneville 474. Each of these cultivars was the final recurrent parent of one or more seed lots of selfed seed retained in storage. Parents of additional backcross progeny sets, which were descended from a resistant parent but not phenotyped, include the following cultivars: Acala 1517–99; DPL 50, 51, and 493, Acala 90, Deltapearl, and NuCot 33B; Paymaster HS26, HS200, HS1220 BR, and 1218 BG; Fibermax 989 and 5013; Stoneville 373; Suregrow 125 and 747; and the Tamcot cultivars Sphinx and CAMD-E.
Crossing Techniques and Selection Strategy
All crosses, except where noted, were made in bee-proof greenhouses in the absence of natural pollinators. As a standard practice, the calyx, corolla, and staminal column were removed in one piece the evening before anthesis, and pollen was applied to the stigma the next morning with a new cotton swab (Calhoun and Bowman, 1999). Male-sterile BC1 plants of group HLA-A were pollinated without emasculation. Self pollinations in field evaluations in 2006 were made by tying petal tips together with a fine wire.
The primary selection criterion was a nematode population level <5% of the susceptible control, and in most cases the cutoff point used was 3%. In some cases, cytogenetic analyses, DNA markers, and resistance data for progeny from test crosses were utilized to reject or confirm selections. Progeny families that were targeted for seed releases and had a transgenic cultivar as a parent were screened via RR QS Kit, Cotton Leaf (Item AS-011-LT) and Cry1Ac QS Kit (Item AS-003-CTLS, Envirologix, Portland, ME) to eliminate proprietary transgenes. Growth habit, leaf shape, internode length, and flowering behavior were also used subjectively to select plants but only if the quantitative nematode resistance criterion was met.
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RESULTS AND DISCUSSION |
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Flower and leaf morphology was highly variable among the HLA-A plants and, in some cases, the pistil extended 1 cm or more beyond the anthers. Self pollination was not attempted by manually transferring pollen from anthers to pistil, but we speculate that some of the plants classified as male infertile based on absence of seed set may have had good pollen but were functionally autosterile due to elongated pistils.
Most importantly, comparison of fruiting and resistance data for HLA-A BC1 plants during the first 1
yr indicated that nematode resistance was inherited independently of the ability to flower in the greenhouse and independently of self fertility. Thirty-two (49%) of the 65 BC1 plants were classified as resistant, 30 (46%) as susceptible, and three were not classified. Twenty-six (81%) of 32 resistant and 23 (77%) of 30 susceptible plants produced flowers, and the proportion producing flowers was not significantly affected by resistance, by 
2 test (2 = 0.20 with 1 df; P = 0.66). Eight (31%) of the resistant and nine (39%) of the susceptible plants that flowered produced self seed, and the proportion producing self seed was not significantly affected by resistance (2 = 0.17 with 1 df; P = 0.68). Overall, 25% of all resistant plants and 30% of all susceptible plants produced self seed.
Information from the HLA-A group was the basis of the strategy taken with the HLA-B and HHL groups to screen for fertility first and resistance second. Of the 603 HLA-B BC1 plants evaluated, 107 (17%) were self-fertile, 104 of these were bioassayed, and of those, 50 (48%) were classified as resistant. The HHL hybrid had lower fertility than the HLA hybrid, with only 21 seed obtained by overpollinating all flowers of 40 HHL plants daily for 4 mo. Of these 21 plants, four (19%) were self fertile and eight were classified resistant.
Cytogenetics
Our primary concerns were the recovery of the euploid complement of 52 chromosomes of G. hirsutum, normal pairing at meiosis metaphase I, and recombination rates that were normal or at least sufficient to allow differential introgression of subchromosomal segments. These were evaluated by counting and characterizing each meiotic configuration and the incidence of observable crossovers during meiosis metaphase I. Almost half (55 of 128) of the plants in the first two backcross generations had one to three extra or missing chromosomes, whereas all of the 40 plants observed at the BC3F1, BC4F1, and BC5F1 generations had exactly 52 chromosomes (Table 1, Fig. 1
). In 38 of the 57 BC2 plants with 52 chromosomes, some chromosomes in microsporocyte spreads failed to pair normally, resulting in fewer than 26 bivalents at meiosis metaphase I. Sixteen of the 19 plants had normal 26 bivalent chromosome pairing and three plants had a 24II + IV chromosomal configuration. Crossovers involving an alien or partially alien chromosome were observed in two of the 20 resistant BC2 plants, estimating approximately 10% recombination. All of the plants examined at the BC3 and later generations appeared normal with regard to number and behavior of chromosomes.
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2 = 0.67 with 1 df; P = 0.41) test. Corresponding proportions for specific progeny sets of n 20, were 15:5, 23:25, 29:1, 40:5, and 28:6 for HLA-A103 BC1S1, HLA-A77 BC3S1, HLA-A84 BC3S1, HLA-A84 BC3S1 and HLA-A84 BC6S1, respectively.
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During DNA marker discovery (Dighe et al., unpublished data, 2006), a total of 90 segregating plants from selfed resistant BC1S1, BC3S1, and BC6S1 progeny (Fig. 4 and 5) were genotyped with BNL 3279 and phenotyped. In 42 of the 90 cases, testcross progeny were also phenotyped. Genotype and phenotype assignments agreed for 87 of these 90 plants (97%). Thus, the two rates of recombinants calculated from backcross and self progeny were similar. The 3% departure from unity could be attributed to recombination, phenotypic or genotypic misclassification, or some combination of these.
Application of the median plant threshold obtained from backcross progeny to the control data for Deltapine 16 and GB-713 indicated that, if each control were assumed to be genetically uniform with respect to resistance, then the observed rates of misassignment of the 221 Deltapine 16 plants as resistant and the 196 GB-713 plants as susceptible were 5.4 and 3.6%, respectively (Fig. 7 ). We also note that G. longicalyx values clustered tightly near zero and none were above the 32.5% threshold. Thus, the probability that an immune plant would be classified as susceptible with our assay was near zero. Generally, then, data from the controls strongly suggest that the incidence of misassignment was highest for susceptible types and decreased to zero as resistance increased, because the observed incidence of misassignment for Deltapine 16, GB-713, and G. longicalyx was 5.4, 3.6, and 0.0%, respectively. This effect is also apparent in the results from BC6S1 test crosses made for family HLA-A84 (Fig. 6), where plants that were homozygous susceptible based on BNL 3279 marker data occasionally were scored as resistant, contrasting with plants that were homozygous resistant based on the BNL 3279 marker, which were unerringly scored as resistant (Fig. 6). These data indicate that the rate of phenotype misclassification was very close to the observed 3% rate of noncorrespondence between resistance phenotype and BNL 3279 genotype, and lead us to speculate that the incidence of recombinants in these materials between the resistance gene or haplotype with the BNL 3279 marker may be <1%.
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2 test (2 = 0.08 with 1 df; P = 0.77). Thus, both genotype and phenotype data indicated an inheritance pattern consistent with a single dominant gene.
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4.0. Seven HLA-A BC1 plants, two of which were reniform nematode resistant, had root-knot nematode resistant parents. The scores of those seven plants (with parents in parentheses) were: 0.5 and 2.5 (Auburn M-315); 3.0 (Auburn 623); 2.5 and 3.5 (TX-1348); and 4.0 and 4.5 (Stoneville LA887). The unique BC1 plant with the 0.5 root-knot score, HLA-A4, was scored reniform nematode resistant.
Development, Morphology, and Reproduction of Resistant Plants
The morphology and fruiting behavior of all resistant backcross plants beyond the fifth backcross were very similar to current cultivars under our greenhouse conditions. In greenhouse-grown resistant plants at or beyond the third backcross, abnormal morphology was observed only in three sets of BC3S1 progeny. All heterozygous BC3F1 progeny of HLA-A84 planted in field plots in 2005 looked normal. Our most encouraging results came from the 34 BC6S2 progeny rows planted in 2006. The HLA-A84 BC6S1 parent of each row was genotyped by the BNL 3279 marker, phenotyped by the indirect assay, and confirmed by evaluating resistance of test-cross progeny. There were 12 rows (20 plants each) of progeny from homozygous resistant parents, 14 rows of progeny from heterozygous parents, and eight rows of progeny from homozygous susceptible parents, plus one row of the recurrent parent DPL 458 B/RR. Marker analysis of tissue from selected plants in the field confirmed that progeny from heterozygous plants were segregating, whereas progeny from homozygous resistant and homozygous susceptible parents were uniformly homozygous (Fig. 4). Fiber analysis data from each plant indicated no adverse effects of the resistance gene on fiber quality (Table 3
), and the homozygous resistant plants had better mean values for micronaire and strength than the homozygous susceptible and the recurrent parent DPL 458 B/RR. Although the experiment was not designed for statistical yield comparisons, total seed cotton from 5-m row lengths of representative plots confirmed that yields of all plots were in the range of agronomic types, with extrapolated yields ranging from 1300 to 1800 kg ha–1, and resistant plots similar to or exceeding the recurrent parent.
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At the beginning of our study, nothing was known regarding inheritance of reniform nematode resistance from G. longicalyx other than that resistance in the HLA and HHL hybrids appeared slightly lower than in G. longicalyx. Nematode females that invaded roots of the hybrids occasionally elicited the development of normal feeding syncytia by the plant, and occasionally produced one or two eggs, in contrast to G. longicalyx, where no nematode development whatsoever occurred (Agudelo et al., 2005). Throughout, we have been guarded when considering that resistance from G. longicalyx in our 28 advanced families is inherited like a single dominant gene, due to the recognition that the same inheritance pattern would result given multiple resistance genes co-inherited as a haplotype, e.g., if united within a large region of an alien chromosome segment having low or no recombination with the G. hirsutum homeolog. Our marker results indicate introgression into chromosome 11 of the A subgenome (Dighe et al., unpublished data, 2006). Our data do not preclude the possibility that the alien segments present in advanced backcrosses are large, although existing phenotypic and marker data suggest not. Thus, a determination of the size of the alien segment and fine mapping of the nematode resistance gene(s) from G. longicalyx are expected to provide highly complementary information regarding the R gene(s) involved, the stability of the reniform nematode resistance trait, compatibility with other resistance traits in cotton, and the ease of combining it with other resistance traits for breeding purposes.
Prospects
We consider this study to be only a start toward reaching the objective of cost-efficient reniform nematode management in cotton with G. longicalyx resistance. Extremely important questions remain to be answered regarding: (i) yield potential under high nematode pressure; (ii) the ability of the gene(s) to suppress established nematode populations, considering the notorious ability of this nematode to survive (Gaur and Perry, 1991) and failure of some resistance sources to suppress field populations (Robinson et al., 2006); (iii) durability and development of resistance-breaking nematode populations, as has been observed for cyst and root-knot nematode species (Roberts, 2002; Starr et al., 2002); (iv) the ability to combine with other R genes in G. hirsutum; and (v) unpredicted vulnerabilities that could remain linked to this new trait in cotton. The forthcoming release of resistant breeding lines to the public will expedite research on these and other questions.
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ACKNOWLEDGMENTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication December 8, 2006.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  N. D. Dighe, A. F. Robinson, A. A. Bell, M. A. Menz, R. G. Cantrell, and D. M. Stelly Linkage Mapping of Resistance to Reniform Nematode in Cotton following Introgression from Gossypium longicalyx (Hutch. & Lee) Crop Sci., June 26, 2009; 49(4): 1151 - 1164. [Abstract] [Full Text] [PDF]
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