Genetic Fine Structure as Revealed in Pollen Assays

Abstract

The sine qua non of attempts to investigate genetic fine structure in any experimental organism is the ability to handle the very large populations necessary to reveal intragenic recombination. This is the reason that the first reports of intragenic recombination in eukaryotic organisms came from studies of the filamentous fungi, Aspergillus and Neurospora, using auxotrophic mutants in the laboratories of Pontecorvo and Giles, respectively. These reports raised the question of whether such recombinational events occurred in the higher organisms that had been so important in the development of genetic theory over the previous half-century. An attempt to test the occurrence of intragenic recombination in maize, the best characterized higher plant genetically, targeted the waxy locus as the most suitable at which to explore the question (Nelson 1957). Brink and MacGillvray (1924) and Demerec (1924) had reported that 50% of the pollen grains produced by a Wx/wx plant were Wx in phenotype (stained black with a I₂/KI stain) indicating that the phenotype of a pollen grain depended on its own genotype at the wx locus rather than that of the plant. Thus, each pollen grain can be a unit of genetic observation, and the more than 2 × 10⁷ pollen grains produced by a vigorous plant can potentially be sampled. The F₁ hybrid between two wx mutants of independent origin where the mutational lesions affected different segments of the coding sequence should produce pollen with a few Wx (black-staining) pollen grains scattered among the numerous wx (tan-staining) pollen grains if recombination occurs between the mutant sites.