Chun-Xiao Jiang

Comparative genomics of plant chromosomes

Comparative genomics, the study of similarities and differences in structure and function of the hereditary information in different taxa, uses molecular tools to investigate notions that far preceded the discovery that DNA was the hereditary molecule. Vavilov’s (1922) “law of homologous series in variation” was an early suggestion of the possibility of underlying commonality in the genetic blueprints of different (plant) species. In plants, genetic analysis based upon morphological and isozyme markers provided early hints that the arrangements of genes along the chromosomes of various taxa may have retained parallels since their divergence from common ancestors. DNA-level investigations in diverse taxa point to two broad messages: (1) The small but essential portion of most plant genomes encoding genes evolves relatively slowly, with corresponding genes retaining recognizable DNA sequences and similar order along the chromosomes of taxa that have been reproductively-isolated for millions of years. (2) A wide range of factors, such as DNA sequence mobility, gene deletion, and localized rearrangements, are superimposed on the relatively slow tempo of chromosomal evolution and cause many deviations from co-linearity. (3) Genetic loci that account for common phenotypes in different taxa are often at corresponding genomic locations, and may represent orthologous genes or members of orthologous clusters of genes.

QTL analysis of leaf morphology in tetraploid Gossypium (cotton)

Abstract Molecular markers were used to map and characterize quantitative trait loci (QTLs) determining cotton leaf morphology and other traits, in 180 F2 plants from an interspecific cross between a Gossypium hirsutum genotype carrying four morphological mutants, and a wild-type Gossypium barbadense. The prominent effects of a single region of chromosome 15, presumably the classical ”Okra-leaf” locus, were modified by QTLs on several other chromosomes affecting leaf size and shape. For most traits, each parent contained some alleles with positive effects and others with negative effects, suggesting a large potential for adapting leaf size and shape to the needs of particular production regimes. Twenty one QTLs/loci were found for the morphological traits at LOD≥3.0 and P≤0.001, among which 14 (63.6%) mapped to D-subgenome chromosomes. Forty one more possible QTLs/loci were suggested with 2.0≤LOD<3.0 and 0.001<P≤0.01. Among all of the 62 possible QTLs (found at LOD≥2.0 and P≤0.01) for the 14 morphological traits in this study, 38 (61.3%) mapped to D-subgenome chromosomes. This reinforces the findings of several other studies in suggesting that the D-subgenome of tetraploid cotton has been subject to a relatively greater rate of evolution than the A-subgenome, subsequent to polyploid formation.

Molecular dissection of interspecific variation between Gossypium hirsutum and G. barbadense (cotton) by a backcross-self approach: II. Fiber fineness

A backcross-self population from a cross between Gossypium hirsutum and G. barbadense was used to dissect the molecular basis of genetic variation governing two parameters reflecting lint fiber fineness and to compare the precision of these two measurements. By applying a detailed restriction fragment length polymorphism (RFLP) map to 3,662 BC3F2 plants from 24 independently derived BC3 families, we were able to detect 32 and nine quantitative trait loci (QTLs) for fiber fineness and micronaire (MIC), respectively. The discovery of larger numbers of QTLs in this study than previously found in other studies based on F2 populations grown in favorable environments reflects the ability of the backcross-self design to resolve smaller QTL effects. Although the two measurements differed dramatically in the number of QTLs detected, seven of the nine MIC QTLs were also associated with fiber fineness. This supports other data in suggesting that fiber fineness more accurately reflects the underlying physical properties of cotton fibers and, consequently, is a preferable trait for selection. “Negative transgression,” with the majority of BC3F2 families showing average phenotypes that were poorer than that of the inferior parent, suggests that many of the new gene combinations formed by interspecific hybridization are maladaptive and may contribute to the lack of progress in utilizing G. barbadense in conventional breeding programs to improve upland cotton.