STRUCTURAL MODIFICATIONS OF THE FEMALE GAMETOPHYTE ASSOCIATED WITH THE INDETERMINATE GAMETOPHYTE (ig) MUTANT IN MAIZE

The indeterminate gametophyte (ig) mutation, when crossed as female, conditions polyembryony, heterofertilization and seed defectiveness (Kermicle, 1971). The structure of the mature embryo sacs of ig/ig plants differs from that found in normal (Ig/Ig) plants in several respects: (1) Normal plants have one micropylar cell (egg) whereas 0 to 5 structurally similar cells occur in ig embryo sacs. (2) The central cell of ig embryo sacs sometimes is accompanied by one or two smaller central cells which may be dinucleate also but more commonly are uninucleate; Ig/Ig plants invariably produce embryo sacs with only one central cell. (3) The primary central cell of ig embryo sacs may have from one to six polar nuclei; normal sacs uniformly have two polar nuclei. (4) Some polar nuclei of ig are found scattered about the cell, whereas some are situated at the standard position near the apex of the egg opposite the micropyle. During double fertilization, only portion of micropylar and polar nuclei are functional. The unfertilized micropylar nuclei averaged 0.88 per ovule and unfertilized polar nuclei 0.75 per ovule. The latter include polar nuclei of extra central cells and those of primary central cell situated in regions other than the standard position.