KARYOGAMY BLOCK BY HEAT STRESS IN THE FERTILIZATION OF BROWN ALGAE

Karyogamy was inhibited by heat stress in zygotes of Scytosiphon lomentaria (Lyngbye) Link (isogamy), Cutleria cylindrica Okamura (anisogamy), and Fucus distichus subsp. evanescens (C. Agardh) Powell (oogamy). Although high temperatures did not inhibit migration of the male and female nuclei, nuclear envelope fusion was blocked. The ultrastructural stage at which karyogamy was inhibited varied among these species. In S. lomentaria, the outer membranes fused with each other, but the inner membranes did not fuse. Partial fusion of the nuclear envelope occurred in C. cylindrica. In F. distichus, the block of karyogamy at high temperature was incomplete, and nuclear fusion proceeded gradually. The block to karyogamy in S. lomentaria zygotes was reversible, and karyogamy proceeded when zygotes were transferred from 22° to 14° C. Experiments using inhibitors suggested that proteins that might be formed de novo after fertilization do not participate in karyogamy or its inhibition at either 14° or 22° C.