Pollen mitosis in the slipper orchid Cypripedium fasciculatum

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Pollen mitosis in the slipper orchid Cypripedium fasciculatum was studied using correlated methods of immunofluorescence and transmission electron microscopy. Unlike the more highly evolved orchids, the cypripedioid orchids shed pollen as monosulcate monads. Prior to pollen mitosis, the microspore nucleus migrates to a proximal position opposite the aperture, as is typical of monocotyledons. There is no distinct generative pole microtubule system (GPMS) like that recently reported in development of pollen polarity in the vandoid moth orchid Phalaenopsis. Instead, microtubules in early prophase are concentrated around the nucleus and extend into the cytoplasm toward the future generative pole. Once the nucleus has migrated to the continuous surface opposite the aperture, microtubules surround the nucleus evenly and show no tendency to be more concentrated in the generative domain. The mitotic spindle, which develops from the perinuclear microtubules, is asymmetrically placed in the microspore and is cone-shaped. The generative pole is broad and closely appressed to the continuous spore surface, while the vegetative pole is pointed and located in the interior of the microspore. As the chromosomes move poleward, microtubules proliferate in the interzone and a phragmoplast develops. The phragmoplast expands in a hemispherical path beyond the interzone following an array of microtubules that radiates from the generative nucleus. Data from this study indicate that evolution of pollen in orchids includes a shift in location of the generative cell from proximal to distal and the evolution of a GPMS, in addition in the well-known trend toward increased pollen aggregation and loss of exine.