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Fiber Evolution

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Fiber Diversity

Cotton fiber diversity and evolution. To understand developmental differences that account for fiber length variation and to place these differences in a phylogenetic context, Applequist et al. conducted SEM of ovules at and near the time of flowering, and generated growth curves. They showed that variation in mature morphologies reflects diversity during expansion, secondary wall synthesis, and maturation. Developmental profiles of the fibers of most wild species are similar, with fiber elongation terminating at ~14 days post-anthesis. In contrast, growth is extended to ~21 days in the A-genome and F-genome diploids. Prolonged elongation is thus phylogenetically revealed as a key evolutionary step in the origin of spinnable fiber, prior to domestication in Africa. Our recent comparative expression profiling work suggests that the evolutionary transition leading to spinnable A-genome fiber involved a prolongation of an ancestral developmental program, as well as a novel metabolism involving enhanced regulation of reactive oxygen species. This work emphasizes the power of combining expression analysis, phylogenetics, and comparative morphology, an integration central to the present proposal.

Variation in seed trichome (fiber) morphology in wild and domesticated cottons. Gossypium seeds exhibits remarkable variation among the ~50 wild and domesticated species. Illustrated are examples from wild species at both the diploid and allopolyploid levels. Early stages in fiber initiation are similar in all species, but developmental variation during primary and secondary wall deposition lead to radically different mature morphologies. Taxa studied here include domesticated and wild G. hirsutum (Cult. and Wild AD1, respectively) and G. barbadense (not shown).

Domestication itself has been associated with further elongation at both the diploid and allopolyploid levels. This provokes speculation that the effects of parallel selection for long fiber in the cultivated species resulted in a genetically convergent or parallel transformation in the developmental program. Ongoing work in Wendel's lab, using a microarray platform capable of distinguishing homoeologous transcripts, is testing this notion. Analyses also indicate a high level of novel expression of D-genome genes during fiber development in allopolyploid cotton. It may be that allopolyploidization provided the raw material necessary for the evolution of novel gene expression patterns, which subsequently were exploited by the aboriginal domesticators (and perhaps modern plant breeders) of G. hirsutum and G. barbadense.

Superimposing the morphological/ developmental variation and the multiple, parallel domestications on the organismal framework identifies the key transformations in cotton fiber evolution and improvement. This perspective provides the foundation for the proposed expression profiling experiments on phenotypically selected introgression lines. These experiments will permit an assessment of the expression changes that accompany cotton fiber evolution and domestication, and will provide insight into developmental processes and genetic regulatory networks involved in fiber growth and maturation. The data also will address fundamental questions regarding the "recruitment of duplicated genes" following polyploid formation, and will provide insight into the novel and exciting question of parallel versus convergent genetic change accompanying repeated domestication of divergent wild species.

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