Nuclear envelope-dependent heterochromatin positioning gates the timing of nucleolar assembly in the early embryo

Abstract

The nucleolus is the largest nuclear condensate, yet how its assembly is developmentally timed remains poorly understood. In the Caenorhabditis elegans embryo, nucleoli normally appear at the 6- to 8-cell stage. Here we show that the LINC complex and nuclear lamina restrain premature nucleolar assembly through heterochromatin organization. Depletion of the embryonic LINC complex proteins SUN-1 or ZYG-12 induces precocious FIB-1-positive nucleoli at the 4-cell stage, particularly in the EMS and P2 blastomeres. LMN-1 depletion produces a more severe phenotype combining premature assembly with impaired disassembly. Loss of the heterochromatin anchor CEC-4 alone has modest effects but strongly suppresses precocious nucleolar assembly caused by LINC complex depletion. LINC complex and lamin perturbations alter the heterogeneity of HPL-2-marked chromatin, and FIB-1 condensates occupy locally HPL-2-depleted regions. These findings identify nuclear envelope-dependent heterochromatin organization as a developmental gate for nucleolar condensate assembly.