| Alternating sporophytic and gametophytic generations characterize the life history of plants. Sporophytes are typically diploid, hence gametophytes are typically haploid. In angiosperms, gametophytes are diminutive, simplified organisms. Nevertheless, angiosperm gametophytes express fundamental aspects of plant development and are the primary agents and sites of crucial events of sexual reproduction. This research characterizes gametophyte genetics, as related specifically to understanding the genetic basis of and gaining insight into development and function of the female gametophyte. With these goals, a directed, systematic approach was developed to generate and recover female gametophyte mutations in maize.; Overlapping, reciprocal chromosome translocations created female gametophytes deficient for small regions of chromosome arms 1L and 3L. Developmental morphological analysis of deficient female gametophytes demonstrated the existence of genes that were required for a variety of genetically separable developmental processes. This analysis suggested that single genes involved in these processes might be identified by mutations that reduce female gametophyte function.; To generate mutations, chemical and transposable element mutagenesis targeted two specific regions, on 1L and 3L, that were also analyzed in the overlapping translocation experiments. Seven new, recessive mutations and one new chromosomal deficiency were isolated. All seven mgl{dollar}{bsol}sp*{dollar} (megagametophyte lethal) mutations interfered sufficiently with female gametophyte function that mutants rarely completed sexual reproduction. Analysis indicated lines collected in this research contained approximately 100 additional leaky gametophyte lethals. Based on genetic and RFLP studies relating cytologically defined target segments to gene-based recombination maps, I estimated maize contains 1200 essential, non-redundant female gametophyte loci.; Morphological analysis of mutant female gametophytes showed that the mutations affected different gametophytic functions. Individual mgl{dollar}{bsol}sp*{dollar}-3L phenotypes represented elements of the deficiency phenotype. mgl{dollar}{bsol}sp*{dollar}-3L1 mutants were initially defective in forming antipodal tissue, and mutant gametophytes failed after fertilization. This phenotype suggested mgl{dollar}{bsol}sp*{dollar}-3L1 conferred an antipodal tissue differentiation defect rather than a general cell division defect, implying that antipodal tissue function was required after fertilization in maize. mgl{dollar}{bsol}sp*{dollar}-3L2 mutants maintained egg apparatus structure poorly; mgl{dollar}{bsol}sp*{dollar}-3L5 mutants showed a similar phenotype suggesting these mutations were allelic. mgl{dollar}{bsol}sp*{dollar}-3L4 mutants grew slowly. Finally, mgl{dollar}{bsol}sp*{dollar}-1L2 mutants had apparently normal mature embryo sac morphology but were defective in the fertilization process. |
