Karyogamy after Electrofusion of Single Egg and Sperm Cell Protoplasts from Maize: Cytological Evidence and Time Course.

In maize, in vitro fusion of isolated male gametes with isolated egg cell protoplasts can be induced by electric pulses. Until now, karyogamy has not been demonstrated. In this study, we cytologically examined fusion products fixed at different times after electrofusion with phase contrast microscopy and transmission electron microscopy. We obtained a precise timetable from 23 samples studied during the first 3 hr. The sperm nucleus was integrated within the egg cell protoplast, migrated toward the egg cell nucleus, and fused with it within 1 hr, as demonstrated by ultrastructural observations, three-dimensional reconstructions of nuclei, and subsequent nuclear volume estimates. Fusion of nuclei occurred before zygotic mitosis, as is the case in vivo. These findings demonstrate karyogamy during in vitro fertilization of maize.