Meiosis and Development of the Embryo Sac in Gunnera insignis (Oersted) D.C. (Halorhagaceae)

Abstract

Gunnera insignis (Oersted) D.C. is a halorhagacean plant from the mountains of Costa Rica, a giant species which can be considered as a potential ornamental. The chromosome relations of the whole genus Gunnera have been practically unknown. The diploid chromosome complement of Gunnera insignis consists of 34 acrocentric chromosomes of quite differ ent size. Both PMC and MMC meiosis are normal and vigorous, showing 17 bivalents in the first division. Absolute procentric localization of the first (or only) chiasma occurs in MMC. In PMC, the same tendency is seen in a couple of big bivalents. The rate of terminalization of chiasmata in the remaining PMC bivalents is low. The mature embryo sac of Gunnera insignis is tetrasporic, 16-nucleate, 2-phasic, and multipolar. It consists of an egg cell, a giant polyploid nucleus formed by fusion of 7 polar nuclei, and 6 antipodal cells. In exceptional cases 2 smaller fusion nuclei may occur instead of 1 large. This is considered a sign of an incomplete repolarization of the embryo sac nuclei.