Abstract
<jats:title>Abstract</jats:title> <jats:p> Heterochromatin and its core effectors from the Heterochromatin Protein 1 (HP1) family are essential for epigenetic silencing of repetitive regions and genome integrity from yeast to humans. However, how HP1s and associated factors regulate heterochromatin properties <jats:italic>in vivo</jats:italic> to sustain silencing across development, aging, and environmental factors, remains incompletely understood. Here, we identify Clump/CG30403 — a previously uncharacterized MADF-BESS domain protein — as a novel heterochromatin factor required for robust transposon silencing during <jats:italic>Drosophila</jats:italic> oogenesis and sustained fertility with age and temperature stress. Clump/CG30403 interacts with the main HP1-family silencing effector Su(var)205/HP1a through a noncanonical binding motif within its large intrinsically disordered region. Notably, in the absence of Clump/CG30403, HP1a mobility and silencing capacity are compromised despite largely unperturbed genomic distribution, showing that HP1a presence alone is insufficient for repression. We also show that Clump/CG30403 uniquely accumulates at its own promoter to self-repress and prevent ectopic aggregation, revealing a feedback mechanism to constrain protein dosage and phase behavior. Overall, we propose that Clump/CG30403 is a HP1 corepressor that acts as a tightly calibrated safeguard of the heterochromatin environment properties to ensure stable silencing, genome integrity, and persistent reproductive function. </jats:p>