Comparative development of fiber in wild and cultivated cotton

One of the most striking examples of plant hairs is the single-celled epide rmal seed trichome of cultivated cotton. The developmental morphology of th ese commercial "fibers" has been well-characterized in Gossypium hirsutum, but little is known about the pattern and tempo of fiber development in wil d Gossypium species, all of which have short, agronomically inferior fiber. To identify developmental differences that account for variation in fiber length, and to place these differences in a phylogenetic context, we conduc ted SEM studies of ovules at and near the time of flowering, and generated growth curves for cultivated and wild diploid and tetraploid species. Trich ome initiation was found to be similar in all taxa, with few notable differ ences in trichome density or early growth. Developmental profiles of the fi bers of most wild species are similar, with fiber elongation terminating at about two weeks post-anthesis. In contrast, growth is extended to three we eks in the A- and F-genome diploids. This prolonged elongation period is di agnosed as a key evolutionary event in the origin of long fiber. A second e volutionary innovation is that absolute growth rate is higher in species wi th long fibers. Domestication of species is associated with a further prolo ngation of elongation at both the diploid and allopolyploid levels, suggest ing the effects of parallel artificial selection. Comparative analysis of f iber growth curves lends developmental support to previous quantitative gen etic suggestions that genes for fiber "improvement in tetraploid cotton wer e contributed by the agronomically inferior D-genome diploid parent.