Pleiotropic Effects on Seed Development of the Indeterminate Gametophyte Gene in Maize

Not more than half the seeds borne by maize plants of the W23 inbred strain, homozygous for a mutant gene termed “indeterminate gametophyte” ( ig ), develop normally. The remainder exhibit polyembryony (6 %), heterofertilization (7 %), elevated ploidy level of the endosperm (45 %), or other less frequent anomalies. The ig effects were identified and characterized by chromosome counts and genetic tests. Twins were regularly found to be diploid and concordant for heterozygous marker genes introduced maternally. When the marker genes were introduced paternally, twins frequently were non‐concordant, or the embryo and endosperm phenotypes did not correspond. The proportion of the various types and the absence of conjoined embryos indicated that, although identical in their maternal inheritance, the twins were regularly dizygotic. Those twins identical in paternal inheritance probably originate from the fertilization of two genetically identical eggs by the two sperm of one male gametophyte. When ig ig compared to normal diploid plants were pollinated by tetraploids, the proportion of plump seed formed was markedly increased. Evidently this result is the consequence of elevation of the endosperm ploidy level by the ig gene. The ig gene thus appears to interfere with differentiation of the components of the female gametophyte so that the number of eggs and polar nuclei, instead of being one and two, is indeterminate.