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A unique pollen wall mutation in the family Compositae: ultrastructure and genetics¹

² Department of Biological Sciences, Texas Tech University, Lubbock, Texas 79409-3131 USA; ³ Oklahoma Biological Survey and Department of Botany and Microbiology, University of Oklahoma, 770 Van Vleet Oval, Norman, Oklahoma 73019-6131 USA; and ⁴ Samuel Roberts Noble Microscopy Laboratory, University of Oklahoma, Norman, Oklahoma 73019-6131 USA

During a routine screening of pollen fertility in the n = 2 chromosome race of Haplopappus gracilis, a spineless pollen wall mutation was discovered that renders the otherwise functional pollen grains completely unrecognizable as Compositae pollen. Normal Haplopappus pollen is characterized by an outer layer, the ektexine, consisting of large spines supported by a roof (tectum), which in turn is supported by collumellae that are joined basally. A large cavity (cavea) stretches from aperture to aperture and separates columellae bases from the final ektexine unit, the foot layer. The spines, tectum, columellae, and columellae bases are filled with perforations (internal foramina), while the foot layer is without them. Immediately underlying the foot layer is a thickened, lamellate, disrupted, internal foramina-free second exine layer, the endexine. In contrast, the mutant pollen ektexine is a jumble of components with randomly dispersed spines as the only clearly definable unit. The endexine layer is similar to the endexine in normal pollen. The mutation apparently disrupts only the organization of ektexine units, and mutant pollen appears to be without the caveae and foot layer characteristic of normal pollen. In genetic tests, the mutant allele is recessive. There is a simple Mendelian pattern of inheritance of the mutant gene, and its phenotype is under sporophytic control.

Key Words: cavea • Compositae • ektexine • endexine • foot layer • Haplopappus • internal foramina • mutant pollen

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