James McD. Stewart

Molecular evidence for homoploid reticulate evolution among australian species of Gossypium

Interspecific hybridization and introgression are important evolutionary processes in plants, but their full significance with respect to speciation at the diploid level remains unresolved. In this study, molecular markers from the plastid and nuclear genomes were used to document an unusual evolutionary history of Gossypium bickii Prokh. (Malvaceae). This species is one of three morphologically similar Australian cottons (along with G. austrate F. Muell. and G. nelsonii Fryx.) in section Hibiscoidea. In contrast to expectations based on previous morphological data, cladistic analysis of maternally inherited cpDNA restriction site mutations unites G. bickii with G. sturtianum J. H. Willis, a morphologically distant species in a different taxonomic section (Sturtia). Few restriction site mutations distinguish the plastomes of G. bickii and G. sturtianum, but these two cpDNAs are differentiated from those of G. australe and G. nelsonii by a minimum of 33 mutations (out of 640 sites scored). These two highly distinct clades are not supported by phylogenetic analyses of allozyme markers (from 58 populations) and restriction site mutations in nuclear ribosomal DNAs. Rather, phylogenies based on 83 nuclear markers indicate that G. bickii shares a more recent common ancestor with G. australe and G. nelsonii than it does with G. sturtianum. We suggest that the striking discrepancy between independent molecular phylogenies from two different genomes indicates a biphyletic ancestry of G. bickii. Our preferred hypothesis involves an ancient hybridization, in which G. sturtianum, or a similar species, served as the maternal parent with a paternal donor from the lineage leading to G. australe and G. nelsonii. Because we detected no G. sturtianum nuclear genes in G. bickii, we suggest that the nuclear genomic contribution of the maternal parent was subsequently eliminated from the hybrid or its descendent maternal lineage. Several possible mechanisms of cytoplasm transfer are suggested, including repeated backcrossing of the hybrid, as female, into the paternal donor lineage, selection against recombinant nuclear genomes and a form of apomixis known as semigamy. This example, and several others in Gossypium as well as other genera, attest to the evolutionary possibility of interspecific cytoplasmic transfer, and perhaps the origin of diploid species via reticulate speciation. In addition, this study offers an example of natural cytoplasmic introgression without long‐term survival of nuclear genes from the maternal progenitor.

Physiological responses of cotton leaves and roots to water deficit induced by polyethylene glycol

Abstract The characterization of plant water relations is a prerequisite for subsequent selection and genetic manipulation for drought tolerance. To evaluate roots in addition to leaves, a method based on PEG was developed to obtain clean, stressed roots quickly while avoiding toxic effects associated with PEG treatment. A mild water deficit of −0.3 MPa was induced with polyethylene glycol (PEG 6000) around the roots of four cotton ( Gossypium hirsutum ) genotypes with different water deficit tolerances. Treatment with PEG during diurnal dark period avoided toxic effects observed with treatment during the light period. The genotypes examined included: (1) Siokra L-23, a drought-tolerant cultivar; (2) T-1521, a wild-type with significant osmotic adjustment capability; (3) CS-50, a moderately susceptible cultivar; and (4) Stoneville 506, a susceptible cultivar. The water potentials ( ψ w ) and osmotic potentials ( ψ s ) of roots and leaves of stressed and non-stressed plants were determined psychrometrically. In response to the water deficits Siokra L-23 and T-1521 showed 25 and 20% reductions in leaf ψ s , respectively, compared with unstressed controls. At the same time, ψ w of these two genotypes did not change significantly. Conversely, the decrease in the ψ s of CS-50 and Stoneville 506 was not significant, while their ψ w decreased significantly. By maintaining a higher ψ w during stress, the tolerant genotypes were able to maintain photosynthesis, stomatal conductance and relative water content near unstressed control levels. The use of PEG 6000 to maintain a constant ψ w in the root environment provides an efficient method for controlling ψ w while allowing for rapid sampling of clean root tissue for analysis. The results demonstrated that the empirically determined differences in drought tolerance of these cultivars can be related to measurable physiological parameters. These results suggest that physiological monitoring can be an effective tool in germplasm selection and improvement.

Evolution and Natural History of the Cotton Genus

We present an overview of the evolution and diversity in Gossypium (the cotton genus). This framework facilitates insight into fundamental aspects of plant biology, provides the necessary underpinnings for effective utilization of cotton genetic resources, and guides exploration of the genomic basis of morphological diversity in the genus. More than 50 species of Gossypium are distributed in arid to semi-arid regions of the tropics and subtropics. Included are four species that independently have been domesticated for their fiber, two each in Africa-Asia and the Americas. Gossypium species exhibit extraordinary morphological variation, ranging from trailing herbaceous perennials to ∼15 m trees with a diverse array of reproductive and vegetative characteristics. A parallel level of cytogenetic and genomic diversity has arisen during the global radiation of the genus, leading to the evolution of eight groups of diploid (n = 13) species (genome groups A through G, and K). Data implicate an origin for Gossypium about 5–10 million years ago and a rapid early diversification of the major genome groups. Allopolyploid cottons appear to have arisen within the last 1–2 million years, as a consequence of trans-oceanic dispersal of an A-genome taxon to the New World followed by hybridization with an indigenous D-genome diploid. Subsequent to formation, allopolyploids radiated into three modern lineages, two of which contain the commercially important species G. hirsutum and G. barbadense.

Mapping QTLs of traits contributing to yield and analysis of genetic effects in tetraploid cotton

Fiber yield and yield components – including lint index (LI), seed index (SI), lint yield (LY), seed cotton yield (SCY) and number of seeds per boll (NSPB) – were investigated on the farm of Huazhong Agricultural University in a population of 69 F2 individuals and corresponding F2:3 families derived from a cross between high-fiber-yield Gossypium hirsutum CV Handan 208 and a low-fiber-yield Gossypium barbadense CV Pima 90. On the basis of the genetic map constructed previously from the same population by Lin et al. (Plant Breed., 2005), quantitative trait locus (QTL) analysis was performed with the software QTL Cartographer V2.0 using composite interval mapping method (LOD ≥ 3.0). A total of 21 QTLs were identified, which were located in 15 linkage groups. The number of QTLs per trait ranged from one to seven. Of these QTLs detected, one affecting LI explained 24.3% of phenotypic variation (PV), five influencing SI explained 16.15–39.21% of PV, seven controlling LY explained 13.01–28.35% of PV, and two controlling SCY explained 22.76 and 39.97% of PV, respectively. Simultaneously, the detected six QTLs for NSPB were located on five linkage groups, which individually explained 28.01–38.32% of the total phenotypic variation. The results would give breeders further insight into the genetic basis of fiber yield.

Identification of salt responsive genes using comparative microarray analysis in Upland cotton (Gossypium hirsutum L.)

Salinity negatively impacts plant growth and productivity, and little is known about salt responsive genes in cotton. In this study, an intra-specific backcross population of cotton (Gossypium hirsutum L.) was treated with 200 mM NaCl after which differentially expressed genes were identified by comparison between salt tolerant and susceptible segregant bulks using comparative microarray analysis. Microarray analysis identified 720 salt-responsive genes, of which 695 were down-regulated and only 25 were up-regulated in the salt tolerant bulk. Gene ontology of annotated genes revealed that at least some of the identified salt responsive transcripts belong to pathways known to be associated with salt stress including osmolyte and lipid metabolism, cell wall structure, and membrane synthesis. About 48% of all salt-responsive genes were functionally unknown. Quantitative RT-PCR was used to validate 17 selected salt responsive genes. This work represents the first study in employing microarray to investigate the possible mechanisms of the salt response in cotton. Further analysis of salt-responsive genes associated with salt tolerance in cotton will assist in laying a foundation for molecular manipulation in development of new cultivars with improved salt tolerance.

Molecular Systematics of Australian Gossypium section Grandicalyx (Malvaceae)

Australian Gossypium consists of 17 species classified into three sections. The largest and most poorly understood is the group of 12 species in sect. Grandicalyx that occur in the Kimberley and Cobourg regions of NW Australia. These areas are characterized by annual monsoon rains and dry-season fires. Species in sect. Grandicalyx exhibit a suite of morphological and ecological features that are otherwise unknown in the genus and that apparently evolved in response to seasonal fires and ant mutualism. These features include an herbaceous perennial habit, the ability to regenerate from thickened rootstocks following fires and ex-

Genetic diversity in and phylogenetic relationships of the Brazilian endemic cotton,Gossypium mustelinum (Malvaceae)

Gossypium mustelinum, one of five tetraploid species in the cotton genus, is geographically restricted to a few states in NE Brazil. Allozyme analysis was used to assess levels and patterns of genetic diversity inG. mustelinum and its relationship to the other tetraploid species. Genetic variation was low, with only 6 of 50 loci examined being polymorphic, a mean of 1.14 alleles per locus and a mean panmictic heterozygosity of 0.08. These estimates are low relative to other tetraploid cotton species, but are typical of island endemics. Interpopulational genetic identities were uniformly high, lending support to the concept of there being only one wild species of Brazilian cotton. The limited allelic diversity observed was correlated with geographical distribution, although variability is so limited in the species that geographically marginal populations are electrophoretically ordinary. Phylogenetic and phenetic analyses demonstrate thatG. mustelinum is isolated among polyploid cotton species, occupying one of the three basal clades resulting from an early radiation of polyploid taxa subsequent to polyploid formation. We suggest thatG. mustelinum represents a paleoendemic that presently exists as a series of widely scattered, relictual populations. Despite several centuries of sympatric cultivation ofG. barbadense andG. hirsutum, there was little evidence of interspecific introgression of alleles from cultivated cottons intoG. mustelinum.

Cotton genetic resources in the western states of Mexico: in situ conservation status and germplasm collection for ex situ preservation.

The Gossypium hirsutum gene pool from Mexico is one of the primary sources for improvement of most of the Acala and Upland cotton growing in the world today. Mexico is also the center of diversity of the Gossypium genus with 11 of the 13 known diploid Gossypium species of the Western Hemisphere endemic to its boundaries. In 2002 and 2003, the current status of these important genetic resources was surveyed, and germplasm was collected in the western states of Mexico. Information was collected to verify current status and circumscription of the endemic Gossypium species. Sixty years after the first in-depth studies of Gossypium in Mexico, increasing human population, modernization and urbanization have severely reduced the survival of G. hirsutum landraces. Representatives of cotton landraces evidently survive only as curiosities in garden plots or dooryards, or as occasional feral plants. Populations of seven known species, G. aridum, G. barbadense, G. gossypioides, G. hirsutum, G. laxum, G. lobatum, and G. schwendimanii, and one undescribed wild diploid Gossypium taxon were located during the survey. In situ conservation of some of these species is threatened. Samples of the collected germplasm were deposited in the Cotton Collection of the US National Plant Germplasm System where they will be available to scientists worldwide for research, breeding, and education. Additionally, a Gossypium species nursery is being established by the Mexican government for the preservation, and as a working legacy, of this resource. Knowledge of the diversity and, consequently, utilization of the genetic resources in these species cannot be fully realized in situ under current conditions.

Genes for jeans: biotechnological advances in cotton

Abstract Cotton is a crop of global economic importance. The impact of advances in cotton genetic engineering will therefore go beyond just altering the patterns of agronomic practice to have a major effect on both economic and social structures. Although the majority of characteristics currently being engineered into cotton (i.e. insect and herbicide-tolerance) relate to improved crop management, the longer-term goals of modifying fiber are to improve and develop novel properties for the product.

Genotypic variation for drought tolerance in cotton

Increasing scarcity of irrigational water is a major threat to sustainable production of cotton (Gossypium hirsutum L.). It could be resolved by developing drought-tolerant cultivars. Osmotic adjustment and cellular membrane stability are well-documented traits that help to sustain yield under drought in cereals. However, their utility in cotton is not well established. Here, we studied genotypic variability and relationships among osmotic adjustment, cell membrane stability and productivity traits under field-induced water stress at the flowering stage. We evaluated a set of cotton germplasm comprising 32 cotton genotypes under contrasting water regimes for measurements of productivity including seedcotton yield, number of bolls per plant and boll weight, and physiological attributes such as osmotic adjustment and cell membrane stability in two field trials. The mean reduction in seedcotton yield due to water deficit was 20 and 43% in 2003 and 2004, respectively. Genotypes differed considerably for relative yield losses due to water stress ranging from 20 to 74%. Significant association between number of bolls and seedcotton yield under a water-limited regime suggests boll retention as the principal determinant of yield in a water-deficit-stress environment. Cell membrane stability varied significantly among the cotton genotypes; however, its association with productivity measurements was not significant in the water-limited regime. The significant positive correlation found between cell membrane stability and osmotic adjustment implicates the role of osmolytes in the protection of various cellular functions, including those associated with cellular membranes. Moderate but significant differences for osmotic adjustment were found among the genotypes in both years. Osmotic adjustment was positively associated with seedcotton yield under the water-limited regime and inversely correlated with the drought susceptibility index. These results demonstrated the contribution of osmotic adjustment in sustaining yield under water-deficit stress in cotton. Thus, like cereals, osmotic adjustment may be useful as a selection criterion in breeding programs with the objective of improving drought tolerance and yield in cotton under water-limited environments; however, the role of cell membrane stability as a drought-tolerant trait requires further investigation.