Parallel evolution of X chromosome-specific SMC complexes in two nematode lineages

Area of Science:

• Evolutionary biology
• Genetics
• Molecular biology

Background:

• X chromosome dosage compensation mechanisms are crucial for sexual reproduction.
• Diversity in dosage compensation strategies across different species remains largely unexplored.
• The nematode Caenorhabditis elegans utilizes a specialized structural maintenance of chromosomes (SMC) complex for dosage compensation.

Purpose of the Study:

• To investigate the evolutionary diversity of X chromosome dosage compensation mechanisms in nematodes.
• To determine the evolutionary age of the condensin-mediated dosage compensation in C. elegans.
• To explore the potential for convergent evolution of dosage compensation mechanisms.

Main Methods:

• Phylogenetic analysis of dosage compensation complex genes.
• Epigenomic profiling, including X-specific topologically associating domains (TADs) and H4K20me1 enrichment.
• Messenger RNA sequencing (mRNA-seq) to analyze gene expression patterns.

Main Results:

• The condensin-mediated dosage compensation mechanism in C. elegans evolved recently via an SMC-4 gene duplication.
• Evidence suggests independent evolution of condensin-mediated dosage compensation in Pristionchus pacificus.
• Dosage compensation itself is an ancestral trait in nematodes, with H4K20me1 enrichment indicating an ancestral mechanism in Oscheius tipulae.

Conclusions:

• The C. elegans dosage compensation system is evolutionarily recent, not ancestral.
• Condensin may have been repeatedly co-opted for dosage compensation in nematodes.
• Evolving new chromosome-wide gene regulatory mechanisms for dosage compensation may be constrained.