A genetic analysis of the origin of maternal haploids in maize.

irst case of haploidy in higher plants was reported in Datura stramonium T $ : ~ ~ ~ ~ ~ ~ ~ et al. 1922). Subsequently, haploid sporophytes have been discovered in many plant species; STADLER and RANDOLPH first reported haploidy in maize (1929 unpublished, cited by RANDOLPH 1932). Numerous investigations have been conducted on the occurrence of spontaneous and induced haploids in a number of species, but the mechanism of haploid origin remains elusive. Several comprehensive reviews and discussions on haploidy have appeared (GATES and GOODWIN 1930; IVANOV 1938; KOSTOFF 1942; KIMBER and RILEY 1963; MAGOON and KHANNA 1963). A haploid embryo can arise through sporophytic development of a male gamete or unfertilized egg or other cell of the embryo sac. Androgenesis, development of the embryo from a male nucleus, can be detected easily by the use of suitable cytological or genetic markers, and its occurrence has been demonstrated in several species (MAGOON and KHANNA 1963). Androgenesis occurs very rarely in maize; a frequency of 1 per 80,000 fertilizations has been estimated (GOODSELL 1961 ; CHASE 1963). Exact modes of origin of maternal haploids are difficult to establish by cytological and genetic analysis, but several postulates have been put forward (see IVANOV 1938; KOSTOFF 1942; MAGOON and KHANNA 1963). These include origination from the egg by incomplete fertilization, from other nuclei of the embryo sac by degeneration of the fertilized or unfertilized egg, and from other nuclei or other embryo sacs in connection with polyembryony. Little is known of the fate of male nuclei following entry and the initiation of maternal haploid development. Low frequency of occurrence (0.1% in maize, for example-CHASE 1949) makes critical cytological analyses of the entire process difficult to conduct. Genotypic control of the frequency of maternal haploidy has been observed (see MAGOON and KHANNA 1963 for discussion). In maize, both female (CHASE 1952) and male parents (CHASE 1952; COE 1959) are known to influence haploid frequency. Identification of a high haploidy line in maize, designated “stock 6”