Production of micronucleoli and onset of cotton fiber growth

Experiments focused on the early development of fiber cells of cotton (Gossypium hirsutum L. cv. MD51 ne) ovules produced two novel findings: one biological, the other methodological. The first concerns a micronucleolus in the nucleus of fibers. This developmental marker appears at or a little before 4 days postanthesis (dpa) in about 10% of the fibers and increases thereafter to nearly 80% provided the fibers are growing on fertilized ovules. Micronucleoli are neither seen in nuclei of fibers at 0-2 dpa nor in nuclei of non-fiber cells. Consequently, it is postulated that they are the product of specific developmental genes associated with fiber growth. The second, methodological finding, involves a cytological means of directly counting the number of fibers produced on young ovules at 1-4 dpa. The method provides quantitative data unavailable in the past. We used this method to show that emasculation caused a temporary 24-h delay in the initiation of fibers, that 30% of the fibers are affected, and that at 3 dpa both fertilized and unfertilized ovules have about 14500 fibers. These data indicate that the fibers on fertilized and unfertilized ovules represent the same cell populations, a finding heretofore unknown.