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Origin of Parthenogenetic Diploids in Maize and Its Implications for the Production of Homozygous Lines¹

To study the production of diploid gametes in diploid maize (Zea mays L.) and their subsequent parthenogenetic development, we crossed males with dominant markers onto a female parent that was marked for identification of maternal embryos by scutellum color and a seedling marker, and for identification of chromosome reduction by heterozygosity for nine unlinked markers. The genotypes of maternal exceptions were determined by progeny tests; all derived from single fertilization. Heterozygosity at all loci, contributed by the female, would result frown failure of reduction at meiosis I; homozygosity at all loci except when gene-centromere crossingover had taken place from failure of chromatid separation at meiosis II or from fusion of two adjoining megaspores; and complete homozygosity from doubling of reduced egg or fusion of two members of the egg apparatus. An analysis of about 50,000 kernels from the marked crosses failed to detect any cases of unreduced megaspores. However, it yielded two cases in which the diploid female contribution had complete homozygosity for all markers and two cases in which there was a high degree of homozygosity (possibly of megaspore fusion origin). Derivation of an unconventional parthenogenetic diploid individual from a diploid female does not always assure complete homozygosity. Hence, completely homozygous lines cannot be secured by this route.

Key Words: Fertilization • Unreduced gametes • Isogenic lines

² Formerly Assistant in Field Crops, University of Missourie, (now Geneticist, Division of Genetics, Indian Agricultural Research Institute, New Delhi-12, India); and Geneticist, Plant Science Research Division, Agricultural Research Service, U. S. Department of Agriculture, Columbia, Mo. 65201.

Received for publication January 6, 1971.