Karyogamy was inhibited by heat stress in zygotes of Scytosiphon lomentaria
(Lyngbye) Link (isogamy), Cutleria cylindrica Okamura (anisogamy), and Fuc
us distichus subsp, evanescens (C. Agardh) Powell (oogamy), Although high t
emperatures did not inhibit migration of the male and female nuclei, nuclea
r envelope fusion was blocked. The ultrastructural stage at which karyogamy
was inhibited varied among these species. In S. lomentaria, the outer memb
ranes 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 fusi
on proceeded gradually, The block to karyogamy in S, lomentaria zygotes was
reversible, and karyogamy proceeded when zygotes were transferred from 22
degrees to 14 degrees C, Experiments using inhibitors suggested that protei
ns that might be formed de novo after fertilization do not participate in k
aryogamy or its inhibition at either 14 degrees or 22 degrees C.