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QTLs for fiber length mapped in three generations of advanced backcross populations derived from crossing Gossypium hirsutum and Gossypium mustelinum showed opportunities to improve elite cottons by introgression from wild relatives. The molecular basis of cotton fiber length in crosses between Gossypium hirsutum and Gossypium mustelinum was dissected using 21 BC3F2 and 12 corresponding BC3F2:3 and BC3F2:4 families. Sixty-five quantitative trait loci (QTLs) were detected by one-way analysis of variance. The QTL numbers detected for upper-half mean length (UHM), fiber uniformity index (UI), and short fiber content (SFC) were 19, 20, and 26 respectively. Twenty-three of the 65 QTLs could be detected at least twice near adjacent markers in the same family or near the same markers across different families/generations, and 32 QTLs were detected in both one-way variance analyses and mixed model-based composite interval mapping. G. mustelinum alleles increased UHM and UI and decreased SFC for five, one, and one QTLs, respectively. In addition to the main-effect QTLs, 17 epistatic QTLs were detected which helped to elucidate the genetic basis of cotton fiber length. Significant among-family genotypic effects were detected at 18, 16, and 16 loci for UHM, UI, and SFC, respectively. Six, two, and two loci showed genotype × family interaction for UHM, UI and SFC, respectively, illustrating complexities that might be faced in introgression of exotic germplasm into cultivated cotton. Co-location of many QTLs for UHM, UI, and SFC accounted for correlations among these traits, and selection of these QTLs may improve the three traits simultaneously. The simple sequence repeat (SSR) markers associated with G. mustelinum QTLs will assist breeders in transferring and maintaining valuable traits from this exotic source during cultivar development.

Among the seven tetraploid cotton species, little is known about transmission genetics and genome organization in Gossypium mustelinum, the species most distant from the source of most cultivated cotton, G. hirsutum. In this research, an F2 population was developed from an interspecific cross between G. hirsutum and G. mustelinum (HM). A genetic linkage map was constructed mainly using simple sequence repeat (SSRs) and restriction fragment length polymorphism (RFLP) DNA markers. The arrangements of most genetic loci along the HM chromosomes were identical to those of other tetraploid cotton species. However, both major and minor structural rearrangements were also observed, for which we propose a parsimony-based model for structural divergence of tetraploid cottons from common ancestors. Sequences of mapped markers were used for alignment with the 26 scaffolds of the G. hirsutum draft genome, and showed high consistency. Quantitative trait locus (QTL) mapping of fiber elongation in advanced backcross populations derived from the same parents demonstrated the value of the HM map. The HM map will serve as a valuable resource for QTL mapping and introgression of G. mustelinum alleles into G. hirsutum, and help clarify evolutionary relationships between the tetraploid cotton genomes.

The molecular genetic basis of cotton fiber strength and fineness in crosses between Gossypium mustelinum and Gossypium hirsutum (Upland cotton) was dissected using 21 BC3F2 and 12 corresponding BC3F2:3 and BC3F2:4 families. The BC3F2 families were genotyped with simple sequence repeat markers from a G. hirsutum by G. mustelinum linkage map, and the three generations of BC3-derived families were phenotyped for fiber strength (STR) and fineness (Micronaire, MIC). A total of 42 quantitative trait loci (QTLs) were identified through one-way analysis of variance, including 15 QTLs for STR and 27 for MIC, with the percentage of variance explained by individual loci averaging 13.86 and 14.06%, respectively. Eighteen of the 42 QTLs were detected at least twice near the same markers in different generations/families or near linked markers in the same family, and 28 of the 42 QTLs were identified in both mixed model-based composite interval mapping and one-way variance analyses. Alleles from G. mustelinum increased STR for eight of 15 and reduced MIC for 15 of 27 QTLs. Significant among-family genotypic effects (P < 0.001) were detected in 13 and 10 loci for STR and MIC respectively, and five loci showed significant (P < 0.001) genotype × family interaction for MIC. These results support the hypothesis that fiber quality improvement for Upland cotton could be realized by introgressing G. mustelinum alleles although complexities due to the different effects of genetic background on introgressed chromatin might be faced. Building on prior work with G. barbadense, G. tomentosum, and G. darwinii, QTL mapping involving introgression of G. mustelinum alleles offers new allelic variation to Upland cotton germplasm.

This study reports transmission genetics of chromosomal segments into Gossypium hirsutum from its most distant euploid relative, Gossypium mustelinum . Mutilocus interactions and structural rearrangements affect introgression and segregation of donor chromatin. Wild allotetraploid relatives of cotton are a rich source of genetic diversity that can be used in genetic improvement, but linkage drag and non-Mendelian transmission genetics are prevalent in interspecific crosses. These problems necessitate knowledge of transmission patterns of chromatin from wild donor species in cultivated recipient species. From an interspecific cross, Gossypium hirsutum × Gossypium mustelinum, we studied G. mustelinum (the most distant tetraploid relative of Upland cotton) allele retention in 35 BC3F1 plants and segregation patterns in BC3F2 populations totaling 3202 individuals, using 216 DNA marker loci. The average retention of donor alleles across BC3F1 plants was higher than expected and the average frequency of G. mustelinum alleles in BC3F2 segregating families was less than expected. Despite surprisingly high retention of G. mustelinum alleles in BC3F1, 46 genomic regions showed no introgression. Regions on chromosomes 3 and 15 lacking introgression were closely associated with possible small inversions previously reported. Nonlinear two-locus interactions are abundant among loci with single-locus segregation distortion, and among loci originating from one of the two subgenomes. Comparison of the present results with those of prior studies indicates different permeability of Upland cotton for donor chromatin from different allotetraploid relatives. Different contributions of subgenomes to two-locus interactions suggest different fates of subgenomes in the evolution of allotetraploid cottons. Transmission genetics of G. hirsutum × G. mustelinum crosses reveals allelic interactions, constraints on fixation and selection of donor alleles, and challenges with retention of introgressed chromatin for crop improvement.

GA 120R1B3 (Reg. No. GP-931, PI 660980) is a noncommercial breeding line of cotton (Gossypium hirsutum L.) jointly released by the USDA-ARS and the Agricultural Experiment Station at the University of Georgia in 2010. GA 120R1B3 was created using a single-seed descent method in a backcrossing program to combine the resistance to Meloidogyne incognita (southern root-knot nematode) from M-120 RNR with the high-yielding, high fiber-quality characteristics of PD 94042. Two greenhouse evaluations of nematode reproduction were conducted in 2007. Field evaluations for yield and fiber quality were conducted from 2007 to 2009 and included three tests in fields infested with M. incognita and four tests in fields without M. incognita. GA 120R1B3 has a high level of resistance to M. incognita that is equivalent to that found in M-120 RNR, but GA 120R1B3 has significantly better lint percentage and fiber quality than M-120 RNR. GA 120R1B3 has a yield and fiber quality generally comparable to commercial cultivars and a lint percentage that was often greater than that of the commercial standards. GA 120R1B3 had good yield and excellent fiber quality in both the presence and absence of M. incognita.

Seventeen backcross-self families from crosses between two Gossypium hirsutum recurrent parent lines (CA3084, CA3093) and G. tomentosum were used to identify quantitative trait loci (QTLs) controlling fiber quality traits. A total of 28 QTLs for fiber quality traits were identified (P0.001), including four for fiber elongation, eight for fiber fineness, four for fiber length, four for fiber strength, six for fiber uniformity, one for boll weight, and one for boll number. Three statistically significant marker-trait associations for lint yield were found in a single environment, but need further validation. Two-way analysis of variance revealed one locus with significant genotype × family interaction (P0.001) for fiber strength and a second locus with significant genotype × environment interaction (P0.001) in the CA3084 background, and two loci with significant genotype × background interaction (P0.001) for the 28 common markers segregating in both of the two recurrent backgrounds. Co-location of many QTLs for fiber quality traits partially explained correlations among these traits. Some G. tomentosum alleles were associated with multiple favorable effects, offering the possibility of rapid genetic gain by introgression. Many G. tomentosum alleles were recalcitrant to homozygosity, suggesting that they might be most effectively deployed in hybrid cottons. DNA markers linked to G. tomentosum QTLs identified in the present study promise to assist breeders in transferring and maintaining valuable traits from this exotic source during Upland cotton cultivar development. This study also adds further evidence to prior studies indicating that the majority of genetic variation associated with fiber quality in tetraploid cotton traces to the D-subgenome from a diploid ancestor that does not produce spinnable fiber.

Quantitative variation for leaf trichome number is observed within and among Gossypium species, varying from glabrous to densely pubescent phenotypes. Moreover, economically important cotton lint fibers are modified trichomes. Earlier studies have mapped quantitative trait loci (QTLs) affecting leaf pubescence in Gossypium using allotetraploids. In this study, we mapped genes responsible for leaf trichome density in a diploid A genome cross. We were able to map 3 QTLs affecting leaf pubescence based on trichome counts obtained from young leaves (YL) and mature leaves (ML). When the F(2) progeny were classified as pubescent versus glabrous, their ratio did not deviate significantly from a 3:1 model, suggesting that glabrousness is inherited in a simple Mendelian fashion. The glabrous mutation mapped to linkage group A3 at the position of major QTL YL1 and ML1 and appeared orthologous to the t1 locus of the allotetraploids. Interestingly, a fiber mutation, sma-4(ha), observed in the same F(2) population cosegregated with the glabrous marker, which indicates either close linkage or common genetic control of lint fiber and leaf trichomes. Studies of A genome diploids may help to clarify the genetic control of trichomes and fiber in both diploid and tetraploid cottons.

QTL mapping experiments yield heterogeneous results due to the use of different genotypes, environments, and sampling variation. Compilation of QTL mapping results yields a more complete picture of the genetic control of a trait and reveals patterns in organization of trait variation. A total of 432 QTL mapped in one diploid and 10 tetraploid interspecific cotton populations were aligned using a reference map and depicted in a CMap resource. Early demonstrations that genes from the non-fiber-producing diploid ancestor contribute to tetraploid lint fiber genetics gain further support from multiple populations and environments and advanced-generation studies detecting QTL of small phenotypic effect. Both tetraploid subgenomes contribute QTL at largely non-homeologous locations, suggesting divergent selection acting on many corresponding genes before and/or after polyploid formation. QTL correspondence across studies was only modest, suggesting that additional QTL for the target traits remain to be discovered. Crosses between closely-related genotypes differing by single-gene mutants yield profoundly different QTL landscapes, suggesting that fiber variation involves a complex network of interacting genes. Members of the lint fiber development network appear clustered, with cluster members showing heterogeneous phenotypic effects. Meta-analysis linked to synteny-based and expression-based information provides clues about specific genes and families involved in QTL networks.

Root-knot nematodes Meloidogyne incognita (Kofoid and White) can cause severe yield loss in cotton (Gossypium hirsutum L.). The objectives of this study were to determine the inheritance and genomic location of genes conferring root-knot nematode resistance in M-120 RNR, a highly resistant G. hirsutum line with the Auburn 623 RNR source of resistance. Utilizing two interspecific F(2) populations developed from the same M-120 RNR by Gossypium barbadense (cv. Pima S-6) cross, genome-wide scanning with RFLP markers revealed a marker on Chromosome 7 and two on Chromosome 11 showing significant association with the resistant phenotype. The association was confirmed using SSR markers with the detection of a minor and a major dominant QTL on Chromosome 7 and 11, respectively. Combined across the two populations, the major QTL on Chromosome 11 Mi-C11 had a LOD score of 19.21 (9.69 and 9.61 for Pop1 and Pop2, respectively) and accounted for 63.7% (52.6 and 65.56% for Pop1 and Pop2, respectively) of the total phenotypic variation. The minor QTL locus on Chromosome 7 Mi ( 1 ) -C07 had a LOD score of 3.48 and accounted for 7.7% of the total phenotypic variation in the combined dataset but was detected in only one population. The allele from the M-120 RNR parent contributed to increased resistance in the Mi-C11 locus, but surprisingly, the Pima S-6 allele contributed to increased resistance in the Mi-C07 locus. The M-120 RNR allele in the Mi-C11 locus, derived from the Auburn 623 RNR, is likely to have originated from the Clevewilt 6 cultivar. Results from this study indicated that the SSR marker CIR316 may replace the laborious greenhouse screening in breeding programs to identify genotypes resistant to M. incognita.

"The Cotton Diversity Database" (http://cotton.agtec.uga.edu) is a Web resource for cotton (Gossypium spp.) phenotypic and genomic data. A primary goal for this resource is to provide both a useful management tool for breeders and other applied scientists and a research tool for genetic and genomic scientists. The resource contains four interface suites that include displays for each of the available phenotypic or genomic data types. These display suites are accessible via the genotype portal, a search interface that allows users to begin with a cotton accession and obtain all available data. The phenotypic data displays include graphical views of overall cultivar performance with means and between group standard deviations indicated in an easy-to-interpret graphical manner for common trial measures such as lint yield, micronaire, etc. The genomic data displays include interactive graphical views of genetic map and diversity data types. Genetic map data is displayed in both traditional linear and two-dimensional comparative dot plot formats. Genetic diversity data is displayed in an interactive tree-based format showing degrees of similarity among genotypes. The data are stored in Oracle relational database (RDBMS) schemas containing tables and views for data storage, auto-calculated statistics and display parameters. The searchable RDBMS provides flexibility for a wide range of query and search options as well as integration paths amongst the various data types.

The genus Gossypium, which comprises a divergent group of diploid species and several recently formed allotetraploids, offers an excellent opportunity to study polyploid genome evolution. In this study, chromosome structural variation among the A, At, and D genomes of Gossypium was evaluated by comparative genetic linkage mapping. We constructed a fully resolved RFLP linkage map for the diploid A genome consisting of 275 loci using an F2interspecific Gossypium arboreum × Gossypium herbaceum family. The 13 chromosomes of the A genome are represented by 12 large linkage groups in our map, reflecting an expected interchromosomal translocation between G. arboreum and G. herbaceum. The A-genome chromosomes are largely collinear with the D genomes, save for a few small inversions. Although the 2 diploid mapping parents represent the closest living relatives of the allotetraploid At-genome progenitor, 2 translocations and 7 inversions were observed between the A and Atgenomes. The recombination rates are similar between the 2 diploid genomes; however, the Atgenome shows a 93% increase in recombination relative to its diploid progenitors. Elevated recombination in the Dtgenome was reported previously. These data on the Atgenome thus indicate that elevated recombination was a general property of allotetraploidy in cotton.Key words: comparative mapping, polyploidy, genome evolution, inversions, translocations, RFLP.

Improvement in somatic embryogenesis has been achieved in several cotton lines (Gossypium hirsutumL.) from the Georgia and Pee Dee germplasm with culture media containing various Putrescine concentrations. The best results were obtained with the α-naphthalene acetic acid (NAA)-based treatments, S15 g.05 NAA and EMMS2, as compared to the 2,4-dichlorophenoxyacetic acid (2,4-D)-based culture medium, EMMS4. Inclusion of 0.5 mg l−1 Putrescine improved somatic embryo (SE) induction for most treatments and lines tested. An 8-and 2-fold improvement was achieved in SE production on the EMMS2-0.5 Putrescine treatment as compared to EMMS2 alone for cotton lines PD 97019 and GA 98033, respectively. A significant increase in SE number (53-fold) was obtained with the addition of 0.5 mg l−1Putrescine to EMMS2 for PD 97021, which was essentially recalcitrant without Putrescine treatment. Conversion of SEs into plants was both genotype- and culture medium-dependent.

Official Cultivar Trials (OCTs) evaluate transgenic, glyphosate [N-(phosphonomethyl)-glycine]-resistant and nonresistant cotton (Gossypium hirsutum L.) cultivars with a nonglyphosate herbicide regime. Thus, yields of glyphosate-resistant cotton cultivars in OCTs may not reflect cultivar variation in glyphosate resistance or the intended production system because all cultivars are produced under a common herbicide regime. Our objective was to assess in field trials main effects and interaction among glyphosate-resistant cotton cultivars and herbicide systems with and without glyphosate for lint yield. Fourteen glyphosate-resistant cultivars were tested in 1998 while 10 were tested in 1999. Herbicide systems were (i) Standard, preplant soil-applied and postdirect-applied herbicides potentially used in OCTs but no glyphosate; (ii) Residual + Glyphosate, preplant soil-applied and postdirect-applied herbicides including a four-leaf-stage topical glyphosate application; and (iii) Glyphosate Only, a four-leaf-stage topical glyphosate application followed by an eight-leaf directed spray. We found significant (P < 0.05 and P < 0.10, earlier- and later-maturity trials, respectively) herbicide system main effects for lint yield but nonsignificant cultivar x herbicide system interactions. Averaged over years and cultivars, the Glyphosate Only herbicide system produced significantly (P < 0.05) greater yields (597 and 839 kg ha -1 for earlier- and later-maturity trials, respectively) than the Standard herbicide system (419 and 644 kg ha -1 for earlier- and later-maturity trials, respectively). The lack of cultivar × herbicide system interactions for yield suggests OCTs determine relative yield potential among glyphosate-resistant cultivars. The lower yield in the Standard system suggests OCTs can impose a yield penalty on glyphosate-resistant cultivars; thus, comparisons with non-glyphosate-resistant cultivars may not be valid.

Quality of cotton can be defined through seed or fiber properties, but is most often associated with fiber properties that influence processing into yarn and textile products. Global competition in the production and consumption of cotton fiber combined with technological evolution of yarn manufacturing has spurred renewed efforts to enhance cotton fiber quality. Cotton fiber quality can be improved through genetics, crop management, and postharvest processing. Knowledge of the effects of fiber properties on processing and their inheritance, relationships, and environmental influences is necessary to formulate improvement strategies. Breeding to improve fiber quality has traditionally focused on enhancing measures of the longest fibers or fiber strength for ring yarn manufacturing systems. With the technological evolution of yarn manufacturing from solely ring-based spinning to predominately rotor and potentially in the near future air-jet spinning, needs for fiber profiles have been revised for ...

New cotton (Gossypium hirsutum L.) seeds that can produce fiber with enhanced strength would increase efficiency of the rotor form of open end yarn manufacture. We report results of 3-yr of breeding transgenic ‘Deltapine 50’ (DP 50) germplasm expressing enhanced fiber strength. The objective was to identify germplasm that could consistently express and transmit enhanced fiber strength to their progeny. Deltapine 50 cotton was transformed via particle bombardment of meristems by Monsanto. The intent of transformation was to insert a -glucuronidase (GUS) marker gene and a proprietary gene driven by a fiber specific promoter. The R4-R6 generations descendent from three GUS-positive, enhanced strength, R3 greenhouse plants were evaluated in the field from 1996-1998 near Florence, SC. Unusual frequencies of GUS-positive progeny were found upon self-pollination of GUS-positive plants, possibly reflecting altered fitness of transgenic gametes. Fiber strength of most GUS-positive plants was 30-70% greate...

New cotton seeds capable of producing fiber with the properties needed by yarn and textile industries in the process of technological advance are needed to maintain market share for Nature's renewable fiber resource. Conventional breeding, marker-assisted selection, and transgenic technologies are possible strategies to achieve improved fiber properties. Breeding has a documented record of improving fiber length and strength to benefit ring yarn manufacture. Open-end yarn manufacturing is supplanting older ring spinning systems and is demanding improved fiber strength, along with finer fiber, and less short and immature fiber. Breeding can achieve these goals, but may be supplemented by marker-assisted selection and transgenic technologies. Marker assisted selection would have particular application to the simultaneous improvement of yield and fiber quality. For modification of fiber traits with narrow sense heritability of 0.5 or higher (0-1.0 scale), marker assisted selection would likely not r...

Cotton production has increased dramatically on the Coastal Plain of the southeastern USA since eradication of the boll weevil (Anthonomus grandis [Boh.]) in the late 1980s. Most of the cotton production in this area occurs on soils possessing subsoil pans. Soil pans limit root growth, requiring mechanical disruption to increase the root zone and facilitate cotton growth. Mechanical amelioration of a soil pan has several disadvantages including expenses for equipment and energy, equipment to break up the pan may not be available, and the effects of mechanical disruption are temporary and contribute to soil erosion. The need for soil pan disruption might be eliminated or reduced with cotton germplasm capable of rooting through high strength soil. Our objective was to identify germplasm with superior rooting ability and thereby initiate breeding efforts incorporating this trait into cultivars. Cultivars and Pee Dee germplasm lines were evaluated at Florence, SC for 2-yr without irrigation for root ...

Late planting causes cotton (Gossypium hirsutum L.) to flower later in the season so boll development occurs at lower temperatures. Potassium deficiency elicits some of the same responses in cotton that delayed planting does. Together, these stresses may affect the yield and fiber quality beyond the individual effects of late planting and K deficiency. Our objective was to determine the effect of K fertilization on normal- and late-planted cotton. In 1991, 1992, and 1993, three cotton genotypes (PD 5286, PD 5358, and PD 5472) were seeded in late April and late May on a Hornsville loamy sand (clayey, kaolinitic, thermic Aquic Hapludult) soil with and without annual applications of 100 Ib K 2 O/acre. Soil K in the upper 6-in. was 95 Iblacre at the initiation of the experiment. Soil K did not differ between the two K fertilizer treatments after the 1991 growing season. After the 1992 and 1993 seasons, K in the 0- to 6- and 6- to 12-in. soil layers was greater in the 100 Ib K 2 O/acre treatment than in the 0 Ib K 2 O/acre treatment, but no differences occurred at greater depths. Potassium fertilization did not affect flower production in either planting date in any year. Potassium deficiency reduced the lint yield and fiber length of PD 5286 and PD 5358 but not that of PD 5472. Besides yield and fiber length, fiber perimeter was the only parameter reduced by K deficiency. Late planting caused reduced yield in only I of the 3 yr, and for only two of the three genotypes. In general, cotton planted in late May had higher yarn strength, elongation, and fiber length, but lower micronaire and fiber maturity than cotton planted in late April. No interactions that included both K and planting date were significant for lint yield, yarn strength, or any of the fiber properties. This indicates that additional attention to K fertility is not needed in production systems where the boll maturation period is delayed.

Stronger cotton yams are needed by the textile industry as knit and woven fabric manufacturing speeds increase, thus placing greater strain on cotton yarns. We compare fiber properties as selection criteria for the genetic improvement of yarn tenacity in a breeding population consisting of twenty-five cotton germplasm lines grown for two years at two locations near Florence, SC. Fiber properties measured with the fibrograph, Stelometer, fibronaire, and Arealometer are obtained on lint from each experimental line in this population, along with ring spun yam with a tenacity of 27 tex (22 Ne). Among individual fiber properties, selection for 50% fiber span length results in the greatest gain in yarn tenacity. Simultaneous selection for low Micronaire reading, long 50% fiber span length, and high fiber tenacity improve yam tenacity more so than selection for individual fiber properties. Selecting for fiber properties measured with the Arealometer generally results in lower gains in yarn tenacity. We show individual and combinations of fiber properties that are useful as selection criteria for yarn tenacity improvement.

Management of the bollworm (Helicoverpa zea [Boddie]) and tobacco budworm (Heliothis virescens [F.]) complex in cotton (Gossypium hirsutum L.) is a significant production cost and a large contributor of insecticides to the environment. Genetic resistance or tolerance could help alleviate these problems. We conducted a trial to identify and exploit sources of bollworm/tobacco budworm resistance in the Pee Dee (PD) cotton germplasm. The experiment consisted of selecting phenotypically desirable cotton plants in segregating populations when produced with limited control of bollworm/tobacco budworm. Although this methodology had previously been successful in identifying resistant types, we were unable to demonstrate a response to selection. The study did find that PD 0762, a germplasm released as possessing resistance to bollworm/tobacco budworm based on lint yield in limited control experiments, likely is tolerant of square loss as its reaction to this insect complex does not suggest resistance. We ...

Despite concern about genetic vulnerability, little is known about the recent changes in the genetic diversity of upland cotton (Gossypium hirsutum L.). The objectives of this study were to determine the trends in the genetic diversity of upland cotton in the USA during the last 25 yr and to investigate probable causes for these changes. Two estimates of genetic diversity, coefficient of parentage (r p ) and field uniformity (r f ; r p weighted by the proportion of the hectarage occupied), were made for cultivars occupying over 1% of the hectarage within a region (Southeast, South-central, Southwest, and West) from 1970 to 1995 at 5-yr intervals. An average of 17 (range = 8-46) cultivars accounted for 97% of the cotton hectarage within a region. Regional r p values were relatively stable at 0.12 to 0.15 from 1970 to 1990 and then sharply increased to 0.20 in 1995. Higher r p values in commonly grown cultivars than in released cultivars indicated that much of the genetic diversity in cotton remains unused by growers. Field uniformity (r f ) remained at about 0.30 for all regions during the past 25 yr because increases in r p were matched with an increase in the number of cultivars grown and/or a decline in the proportion of the area planted to any single cultivar. The frequent use of several parents for the creation of new cultivars and the planting of only a small portion of the available cultivars has led to a high level of genetic uniformity.

We report a second major QTL for root-knot nematode resistance in the highly resistant Upland cotton line M-120RNR and show epistasis between two resistant QTLs with different mechanisms conferring resistance. In an earlier study, we identified a major QTL on Chromosome 11 associated with resistance to root-knot nematode in the M-120 RNR Upland cotton line (Gossypium hirsutum L.) of the Auburn 623 RNR source. Herein, we re-evaluated the genetics of the resistance to root-knot nematode in the M-120 RNR × Pima S-6 population by linkage mapping using recently published SSR markers. The QTL analysis detected two regions significantly associated with the resistance phenotype. In addition to the QTL previously identified on Chromosome 11 (qMi-C11), a major QTL was identified on Chromosome 14 (qMi-C14). The resistance locus on qMi-C11 originated from the Clevewilt parent, while the qMi-C14 locus originated from the other resistant parent, Mexico Wild Jack Jones. The qMi-C14 locus had logarithms of odds score of 17 and accounted for 45 % of the total phenotype variation in egg production. It was also associated with galling index, but the percent variation explained was only 6 %, suggesting that the qMi-C11 locus had a much stronger effect on root gall suppression than egg production, while the qMi-C14 locus had a stronger effect on egg production than galling. The results also suggest that the transgressive segregation observed in the development of Auburn 623 RNR was due to the pyramiding of at least two main effect QTLs as well as an additive-by-additive epistatic effects between the two resistant loci. The SSRs markers tightly linked to the qMi-C11 and qMi-C14 loci will greatly facilitate the improvement of RKN resistance in cotton via marker-assisted breeding.

Most cotton (Gossypium spp.) breeders today, without recourse to critical data, assume that the genetic base in modern New World cotton cultivars is narrow. The objectives of this study were to : (i) determine the average coefficient of parentage for 260 upland cotton (G. hirsutum L.) cultivars released between 1970 and 1990 ; and (ii) determine the contributions of a number of public and private breeding programs and of various ancestral lines to the genetic diversity of those cultivars. Coefficients of parentage among 260 cultivars showed an average value of 0.07. This estimate suggests substantial remaining diversity. This conclusion must take into account possible bias from widespread reselection of cotton cultivars and the accompanying assumption of a genetic correlation of 0.75 between generations. The most influential breeding programs, in terms of genetic contributions to cultivar development, were Stoneville Pedigreed Seed Company, Coker's Pedigreed Seed Company, and New Mexico Agricultural Experiment Station. Historically, the most influential cultivar is Stoneville 2. The genetic contribution of 54 ancestral lines, including nine introductions, accounted for less than 25% of the total genetic variation among the 260 cultivars. This low value is thought to result from the loss of genetic information through the process of reselection. The genetic base in modern cotton cultivars is not particularly narrow and continue to offer opportunities for cultivar improvement.

Sustained genetic advance requires that genetically diverse parents be mated to form segregating populations for selection. Genetic diversity of U.S. cotton (Gossypium hirsutum L.) cultivars has not been extensively quantified. We assessed diversity among 126 Upland cotton cultivars released between 1980 and 1990 by use of coefficient of parentage (CP). In computing CP, we utilized assumptions appropriate for self-pollinated crops. Mean CP among the 126 cultivars was 0.07, implying a genetically diverse group. However, cluster analysis revealed 12 distinct gene pools, with mean within-cluster CP = 0.25 and between-cluster CP = 0.04. Overall, clusters corresponded to area of cultivar origin. The CP analysis indicates that Acala-type cultivars are more diverse than those bred in the Mississippi Delta or southeastern USA. A trend in germplasm usage in the late 1980s was the repeated mating of genetically related material, or reselection within germplasm, to develop proprietary cultivars. To ensure continued progress in cotton improvement, we suggest that cotton breeders consider the pedigree of parents prior to population synthesis.

Cotton (Gossypium hirsutum L.) growers in South Carolina are replacing some of their soybean [Glycine max (L.) Merr.] hectareage planted after wheat (Triticum aestivum L.) with cotton. A breeder must decide whether a small but expanding hectareage of late-planted (LP) cotton will require a breeding scheme separate from one for the conventional (CN) full-season production system. The objective of this study was to assess selection strategies that will result in cottons with improved lint yield and fiber quality for CN and LP production systems. Twenty-five unselected Pee Dee cotton genotypes, along with a full-season and an early maturing cultivar, were evaluated in a 2-yr experiment conducted at Morence, SC, in two production systems: CN, planted approximately the first week of May, and LP, planted 8 June. Lint yield, fiber, and spinning properties were determined. Significant (P < 0.05) genotypic variation existed for all traits, but there was no significant genotype × production system interaction. Heritability of each trait plus the genetic correlation between production systems was used to compare predicted and observed direct and correlated response in each production system. Predicted gain in LP yield when selection was conducted in the CN production system was a 2.1% increase in the population mean compared with a 3.1% observed response. Selection for 2.5% span length and yarn strength in the CN system resulted in gains in both production systems. Antagonistic genetic correlations between some of the fiber traits and lint yield will make concurrent improvements in both production systems difficult. Use of a low selection intensity in the CN production system in the initial round of replicated yield testing was found to be an efficient method to improve lint yield or yarn strength

Knowledge of heritability and type fo genetic variation invlved in the expresion of fiber traits would facilitate further improvement of coton fiber properties. A 4 × 4 Design II mating was employed to estimate magnitude and tpe of genetic variation controlling fiber traits in populations derived from crossing elite Pee Dee cotton (L.) parents. Significant genetic variation was found for 2.5 and 50% span length (SL), fiber length uniformity ration (UR), fiber strength (T), fiber elongation, and micronaire (MIC). Mating design variances from the F, F, and F generations of the Design II were resolved into estimates of additive, dominance, and additive epistatic variance for the fiber properties. Dominance genetic variance was greater than additive genetic variance for all of the fiber traits. Additive epistatic variance was detected for 2.5% SL, UR, and MIC. Low single-plant heritability for all fiber traits suggested that alternatives to a pedigree generation advance beginning with F plant selection be considered. Evaluation of F bulk populations with a low selection intensity was adequate to identify populations with superior fiber traits. Less than half of the offspring of the top 10% of F lines with highest T were in the 10% of F lines with highest yarn strength (YS). Additionally, the correlation betwen T and YS among 283 unselected F lines was only 0.25 (< 0.05). Maximum progress in improving YS may require selection for traits in addition to T or for components of YS not measured by the standard fiber properties.

Mapping of genes that play major roles in cotton fiber development is an important step toward their cloning and manipulation, and provides a test of their relationships (if any) to agriculturally-important QTLs. Seven previously identified fiber mutants, four dominant (Li 1, Li 2, N 1 and Fbl) and three recessive (n 2, sma-4(h a), and sma-4(fz)), were genetically mapped in six F2 populations comprising 124 or more plants each. For those mutants previously assigned to chromosomes by using aneuploids or by linkage to other morphological markers, all map locations were concordant except n 2, which mapped to the homoeolog of the chromosome previously reported. Three mutations with primary effects on fuzz fibers (N 1, Fbl, n 2) mapped near the likelihood peaks for QTLs that affected lint fiber productivity in the same populations, perhaps suggesting pleiotropic effects on both fiber types. However, only Li 1 mapped within the likelihood interval for 191 previously detected lint fiber QTLs discovered in non-mutant crosses, suggesting that these mutations may occur in genes that played early roles in cotton fiber evolution, and for which new allelic variants are quickly eliminated from improved germplasm. A close positional association between sma-4(h a ), two leaf and stem-borne trichome mutants (t 1 , t 2), and a gene previously implicated in fiber development, sucrose synthase, raises questions about the possibility that these genes may be functionally related. Increasing knowledge of the correspondence of the cotton and Arabidopsis genomes provides several avenues by which genetic dissection of cotton fiber development may be accelerated.

The US cotton production system exemplifiesthe complex challenges that must be met in orderto reduce the genetic vulnerability of a majorcrop.Genetic vulnerability results from acombination of a crop’s evolutionary history,trends in breeding and biotechnology practices,and grower Decisions based on inadequateinformation being available,all in response tothe inevitable pressures imposed by processor