Histone H2B.8 compacts flowering plant sperm through chromatin phase separation

Area of Science:

• Plant biology
• Molecular biology
• Genetics

Background:

• Sperm chromatin condensation is vital for male fertility.
• Flowering plants lack protamines, using an unknown mechanism for sperm nuclear condensation.
• Sperm nuclei in flowering plants are transcriptionally active despite condensation.

Purpose of the Study:

• To elucidate the mechanism of sperm chromatin and nuclear condensation in Arabidopsis thaliana.
• To identify the key factors mediating sperm compaction in flowering plants.

Main Methods:

• Investigated the role of histone variant H2B.8 in Arabidopsis thaliana.
• Conducted loss-of-function and ectopic expression experiments of H2B.8.
• Performed reciprocal crosses to assess the impact on male transmission and fertility.

Main Results:

• The histone variant H2B.8 mediates sperm chromatin and nuclear condensation in Arabidopsis.
• Loss of H2B.8 resulted in enlarged sperm nuclei with dispersed chromatin.
• Ectopic H2B.8 expression led to smaller nuclei with aggregated chromatin, demonstrating its sufficiency for condensation.
• H2B.8 aggregates transcriptionally inactive AT-rich chromatin into phase-separated condensates, facilitating compaction without affecting transcription.
• Mutation of h2b.8 significantly reduced male transmission, highlighting its importance for fertility.

Conclusions:

• H2B.8 is a key factor in achieving sperm nuclear condensation in flowering plants.
• This mechanism allows for nuclear compaction while maintaining transcriptional activity.
• H2B.8 represents an evolutionary innovation in flowering plants for compatible nuclear condensation and transcription.