A unique pollen wall mutation in the family Compositae: Ultrastructure andgenetics

During a routine screening of pollen fertility in the It = 2 chromosome rac e of Haplopappus gracilis, a spineless pollen wall mutation was discovered that renders the otherwise functional pollen grains completely unrecognizab le as Compositae pollen. Normal Haplopappus pollen is characterized by an o uter layer, the ektexine, consisting of large spines supported by a roof (t ectum), which in turn is supported by collumellae that are joined basally. A large cavity (cavea) stretches from aperture to aperture and separates co lumellae bases from the final ektexine unit, the foot layer. The spines, te ctum, columellae, and columellae bases are filled with perforations (intern al 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 clearl y definable unit. The endexine layer is similar to the endexine in normal p ollen. The mutation apparently disrupts only the organization of ektexine u nits, and mutant pollen appears to be without the caveae and foot layer cha racteristic of normal pollen. In genetic tests, the mutant allele is recess ive. There is a simple Mendelian pattern of inheritance of the mutant gene, and its phenotype is under sporophytic control.