ENDOSPERM DEVELOPMENT IN ZEA-MAYS - IMPLICATION OF GAMETIC IMPRINTINGAND PATERNAL EXCESS IN REGULATION OF TRANSFER LAYER DEVELOPMENT

Fertilisation in maize (Zea mays), in common with most angiosperms, in volves two fusion events: one of the two sperm nuclei unites with the egg cell nucleus, while the other sperm nucleus fuses with the two cen tral cell nuclei giving rise to the triploid endosperm, Since deviatio n from this nuclear ratio (2:1 maternal/paternal) in the endosperm can result in abortion, it has been suggested that the genomes of the spe rm and/or central cell are differentially imprinted during sexual deve lopment. By crossing a normal diploid maize line as female with its au totetraploid counterpart, an unbalanced genomic ratio (2:2 maternal/pa ternal) is created in the endosperm which often results in the eventua l abortion of the tissue, Detailed structural comparison of these aber rant endosperms with normal endosperms reveals that the formation of t he transfer cell layer, a tissue formed some 8 days after pollination acid responsible for the transport of nutrients into the endosperm, is almost completely suppressed under conditions of paternal genomic exc ess. The first structural analysis of the development of this tissue i n normal and aberrant endosperms is reported, and the implications of regulating the formation of such a tissue by gametically imprinted gen es are discussed in the light of current theories on the consequences of genomic imbalance on early embryonic development.