Late steps of egg cell differentiation are accelerated by pollination in Zea mays L.

Egg cells were analysed cytologically during the female receptivity period in maize (Zea mays L., line A 188), Three classes of egg cell were distingu ished: type A - small, non-vacuolated cells with a central nucleus; type B - larger cells with small vacuoles surrounding the perinuclear cytoplasm lo cated in the middle of the cell; type C - big cells with a large apical vac uole and the mid-basal perinuclear cytoplasm. The less-dense cytoplasm of t he vacuolated egg cells usually contained numerous cup- or bell-shaped mito chondria. The three egg types appeal to correspond to three late stages of egg cell differentiation. The frequencies of each of the three egg types we re monitored in developing maize ears before and after pollination. In youn g ears, with the silks just extending out of the husks, small A-type cells were found in about 86% of ovules. Their frequency decreased to about 58% a t the optimum silk Length, remained unchanged in non-pollinated ears, and f ell to 16% at the end of the female receptivity period. However, after poll ination and before fertilisation the frequency of these cells decreased to about 33%, and the larger vacuolated egg tells (types B and C) prevailed. A t various stages of the receptivity period, pollination accelerated changes in the egg population, increasing the number of ovules bearing larger, vac uolated egg cells. Experiments with silk removal demonstrated that putative pollination signals act immediately after pollen deposition and are not sp ecies-specific.