Warwick N. Stiller

Integrated mapping and characterization of the gene underlying the okra leaf trait in Gossypium hirsutum L

Highlight Characterization of GhOKRA suggests the involvement of protein activity and transcription of GhOKRA in regulating cotton leaf shape and a possible origin of the okra leaf trait by gene conversion.

Molecular mapping of a new source of Fusarium wilt resistance in tetraploid cotton (Gossypium hirsutum L.)

Fusarium wilt (FW) disease is an economically important disease of cotton worldwide and a major cause of crop losses in Australia and many other cotton-producing countries. Symptoms include wilting, vascular browning and death. Australian races of the causal agent Fusarium oxysporum f. sp. vasinfectum (Fov) are genetically distinct from those in other countries and are thought to have evolved from indigenous races. New sources of resistance for breeding are rare, as cotton cultivars with significant FW resistance against Fov isolates from other cotton-producing regions are usually susceptible to Australian Fov races. MCU-5, an Upland Indian cotton cultivar, has been identified as having improved resistance to Australian Fov and is being used to breed new commercial cultivars with higher resistance to FW. To investigate the genetic basis of the FW resistance in MCU-5, QTL analysis was performed on 244 F3 and 244 F4 families derived from an intraspecific cross between MCU-5 and Siokra 1-4, a cultivar highly sensitive to Australian Fov races. Resistance, as measured by leaf symptoms, vascular browning and survival, showed low to moderate heritability between generations. MCU-5 resistance to FW was found to be complex with three quantitative trait loci (QTL) identified in the F3, and eight in the F4, that explained between 9 and 41% of the phenotypic variation. The QTL were located on four linkage groups including chromosomes A6 (Chr 6), D4 (Chr 22) and D6 (Chr 25), with two QTL located in similar regions to previously identified FW resistance from the Sea Island cultivar Pima 3-79. The QTL identified in this study represent the first targets for marker-assisted selection of FW resistance in Australia.

Jasmonic acid is associated with resistance to twospotted spider mites in diploid cotton (Gossypium arboreum)

The twospotted spider mite (Tetranychus urticae Koch) is capable of dramatically reducing the yield of cotton crops and is often difficult and expensive to control. This study investigated and compared two important plant hormones, jasmonic acid (JA) and salicylic acid (SA), as constitutive and/or induced defence response components in a mite susceptible commercial cotton cultivar, Sicot 71 (Gossypium hirsutum L.) and a resistant diploid cotton BM13H (Gossypium arboreum L.). Foliar application of JA and methyl jasmonate (MeJA) reduced the mite population and leaf damage but application of other potential elicitors, SA and methyl salicylate (MeSA) did not. The concentrations of JA and SA in leaf tissues of induced and non-induced Sicot 71 and BM13H were quantified by liquid chromatography coupled to electrospray ionisation tandem mass spectrometry (LC-ESI-MS/MS). The JA content was constitutively higher in BM13H than Sicot 71 and also highly induced by mite infestation in BM13H but not in Sicot 71. However, SA was not significantly induced in either BM13H or Sicot 71. The expression levels of JA related genes, LOX, AOS and OPR were measured by quantitative PCR and elevated expression levels of JA related genes were detected in mite-infested BM13H. Therefore, JA and MeJA were implicated as key biochemical components in both the constitutive and induced defence responses of BM13H to spider mites.

Fitness of twospotted spider mites is more affected by constitutive than induced resistance traits in cotton (Gossypium spp.)

BACKGROUND Life history parameters are useful tools for comparing the fitness of pests on different host plants. This study compared life history parameters of twospotted spider mites (Tetranychus urticae Koch) on two resistant cotton Gossypium genotypes (BM13H and Sipima 280) and one susceptible genotype (Sicot 71). The effects of both constitutive and induced defences were assessed. RESULTS Mites reared on the resistant genotypes had longer immature development times, lower immature survival and reduced adult fecundity. Mites reared on BM13H that had been induced by prior exposure to mites had a small additional decrease in adult fecundity. The contribution to mite resistance of constitutive resistance mechanisms was much greater than induced responses. The effect of morphological constitutive defences was minor, implicating biochemical defences as the major mite-resistance mechanism. Sensitivity analysis and a population development study using life history parameters of mites showed that a lower immature survival rate on resistant genotypes had the greatest effect on mite fitness and population development. CONCLUSION Use of life history parameters provided valuable insight into the mite-resistance mechanisms of these Gossypium genotypes. Further, the results largely explained mite population development on these genotypes in the field.

Heritability and predicted selection response of yield components and fibre properties in an inter-specific derived RIL population of cotton

Exploiting genetic variation through inter-specific breeding has improved cotton yield, fibre properties and adaptability. The objectives of this study were to examine heritability and predicted selection response of yield components and fibre properties in a recombinant inbred line (RIL) population from an inter-specific cross between Gossypium hirsutum (Gh) variety Guazuncho 2, and G. barbadense (Gb) line VH8-4602. A population of 93 and 82 RILs was tested in two seasons, with two parents and local controls, Sicot 75 (Gh) and Sipima 280 (Gb) in field experiments. Seed cotton samples hand harvested before and after defoliation were used to measure lint percent, boll weight, 100 seed weight and the lint to measure fibre length, uniformity, short fibre index (SFI), elongation, strength, micronaire, maturity ratio (MR), percent of maturity (PM) and fineness. There was large phenotypic variation for individual traits and transgressive segregation occurred in lint percent, lint weight/seed, fibre no./seed, uniformity, SFI, elongation, MR and PM. Narrow sense heritabilities were moderate for yield components (34.3–41.2%) and for key fibre properties, length, strength, micronaire and fineness (38.3–42.1%), which led to a predicted selection response of 6.7–24.0% for yield components and 3.9–10.9% for key fibre properties under a selection intensity of 10%. Favourable associations were found between key fibre properties, but an adverse association between lint percent and each of these fibre properties. Only five RILs were identified with desirable combinations. The results demonstrated the value of exploiting inter-specific variation to develop cotton germplasm and how breeding strategies can be improved.

Enhancing Integrated Pest Management in GM Cotton Systems Using Host Plant Resistance

Cotton has lost many ancestral defensive traits against key invertebrate pests. This is suggested by the levels of resistance to some pests found in wild cotton genotypes as well as in cultivated landraces and is a result of domestication and a long history of targeted breeding for yield and fiber quality, along with the capacity to control pests with pesticides. Genetic modification (GM) allowed integration of toxins from a bacteria into cotton to control key Lepidopteran pests. Since the mid-1990s, use of GM cotton cultivars has greatly reduced the amount of pesticides used in many cotton systems. However, pests not controlled by the GM traits have usually emerged as problems, especially the sucking bug complex. Control of this complex with pesticides often causes a reduction in beneficial invertebrate populations, allowing other secondary pests to increase rapidly and require control. Control of both sucking bug complex and secondary pests is problematic due to the cost of pesticides and/or high risk of selecting for pesticide resistance. Deployment of host plant resistance (HPR) provides an opportunity to manage these issues in GM cotton systems. Cotton cultivars resistant to the sucking bug complex and/or secondary pests would require fewer pesticide applications, reducing costs and risks to beneficial invertebrate populations and pesticide resistance. Incorporation of HPR traits into elite cotton cultivars with high yield and fiber quality offers the potential to further reduce pesticide use and increase the durability of pest management in GM cotton systems. We review the challenges that the identification and use of HPR against invertebrate pests brings to cotton breeding. We explore sources of resistance to the sucking bug complex and secondary pests, the mechanisms that control them and the approaches to incorporate these defense traits to commercial cultivars.

Molecular mapping of bunchy top disease resistance in Gossypium hirsutum L.

Cotton bunchy top (CBT) is an aphid transmitted Polerovirus disease and a significant threat to the Australian cotton industry. Symptoms include stunted plant growth, and leaves often display pale green angular patterns at the leaf margins and dark green centers with a leathery texture. Resistance to CBT was evaluated in 206 F2 plants and 76 F2:3 families derived from the resistant cultivar ‘Delta Opal’ crossed to the susceptible cultivar ‘Sicot 70’, and in 25 other cultivars the majority susceptible to CBT. CBT resistance in ‘Delta Opal’ was shown to be controlled by a single dominant locus designated Cbt. A combination of AFLP and single nucleotide polymorphism markers located Cbt on chromosome A10, close to the mapped resistance locus in ‘Delta Opal’ to another Polerovirus disease of cotton; cotton blue disease. The markers identified flanking CBT resistance will provide useful tools for breeders for marker-assisted selection to alleviate the impact of this disease on cotton production.

Genetic dissection of the fuzzless seed trait in Gossypium barbadense

Five genetic loci were found to be associated with the fuzzless seed trait in Gossypium barbadense, one of them containing MYB25-like_Dt, the best candidate for the N2 gene.

Sources of plant resistance to thrips: a potential core component in cotton IPM

Thrips (Thysanoptera: Thripidae) are important pests of seedling cotton and their damage can delay crop maturity and/or reduce yield. Plant resistance to thrips in cotton would reduce the need to treat crops with insecticides for their control. This would support integrated pest management strategies by reducing the risk of selecting insecticide resistance in concomitant pests and of disrupting the natural enemy complex. Traits that reduce thrips abundance in cotton are poorly understood, but dense leaf hairs and high gossypol content are implicated to negatively affect thrips. Furthermore, some diploid cottons are resistant against thrips. Thrips abundance and damage on a range of Gossypium (Malvaceae) genotypes including hairy leaf, smooth leaf (glabrous), okra leaf shape, and diploid species was evaluated over four crop seasons. Thrips were significantly less abundant and they caused less damage on diploid cotton genotypes from Gossypium arboreum L. (BM13H, Roseum A256), Gossypium thurberi Tod. (GOS5310), and Gossypium trilobum (Moc. & Sess. ex DC.) Skov. (GOS5332) than on the standard commercial Gossypium hirsutum L. cv. Sicot 71. There was no significant relationship between thrips abundance or damage and leaf hairiness, leaf hardness, or leaf shape, though conclusions about the value of these traits must be made cautiously as they are confounded across genotypes that differ in a range of traits simultaneously. The diploid cottons had a lower damage per thrips ratio, indicating that they are more tolerant of thrips feeding damage and thus could be valuable sources of host plant resistance to thrips.

Historical Datasets Support Genomic Selection Models for the Prediction of Cotton Fiber Quality Phenotypes Across Multiple Environments

Genomic selection (GS) has successfully been used in plant breeding to improve selection efficiency and reduce breeding time and cost. However, there has not been a study to evaluate GS prediction models that may be used for predicting cotton breeding lines across multiple environments. In this study, we evaluated the performance of Bayes Ridge Regression, BayesA, BayesB, BayesC and Reproducing Kernel Hilbert Spaces regression models. We then extended the single-site GS model to accommodate genotype × environment interaction (G×E) in order to assess the merits of multi- over single-environment models in a practical breeding and selection context in cotton, a crop for which this has not previously been evaluated. Our study was based on a population of 215 upland cotton (Gossypium hirsutum) breeding lines which were evaluated for fiber length and strength at multiple locations in Australia and genotyped with 13,330 single nucleotide polymorphic (SNP) markers. BayesB, which assumes unique variance for each marker and a proportion of markers to have large effects, while most other markers have zero effect, was the preferred model. GS accuracy for fiber length based on a single-site model varied across sites, ranging from 0.27 to 0.77 (mean = 0.38), while that of fiber strength ranged from 0.19 to 0.58 (mean = 0.35) using randomly selected sub-populations as the training population. Prediction accuracies from the M×E model were higher than those for single-site and across-site models, with an average accuracy of 0.71 and 0.59 for fiber length and strength, respectively. The use of the M×E model could therefore identify which breeding lines have effects that are stable across environments and which ones are responsible for G×E and so reduce the amount of phenotypic screening required in cotton breeding programs to identify adaptable genotypes.