Anomalous Fertilization in Diploid‐Tetraploid Crosses in Maize1

Abstract

Diploids with 10 heterozygous gene markers on 10 chromosomes were crossed reciprocally with two lines of tetraploid maize (Zea mays L.). Analyses of four tetraploids obtained from 54,250 fertilizations from marked 2n ✕ 4n crosses showed that in each case the diploid set from the female originated by exact duplication of a reduced gametophyte, probably by doubling or by fusion of two reduced female nuclei. No 4n sectors or unreducedmegaspore events were found. Up to eight possible cases of megaspore fusion and parthenogenetic development of diploid sporophytes were detected in the same 2n ✕ 4n cross, but no case was detected of diploid female gametes arising without meiotic assortment.Three tetraploids were recovered from 41,660 fertilizations in 4n ✕ marked 2n crosses. Analyses of these showed that they probably arose from fertilization of the diploid egg by a diploid male nucleus originating from restitution in the second division of microsporogenesis or, less likely, from concurrent fertilization by two haploid nuclei from two pollen tubes.The easiest and most efficient procedure to obtain triploids from diploid‐tetraploid intercrosses is to use tetraploid females and to screen for plump kernels.To explain the occurrence of occasional plump kernels in either 2n ✕ 4n or 4n ✕ 2n crosses, we propose specific ploidy level of the endosperm, 3n or multiples of 3n, for proper endosperm development. Earlier hypotheses of a definite ratio between embryo and endosperm ploidy level in seed development are rejected, since seed development is found to be independent of the ploidy level of the embryo.