FUNCTIONAL ANALYSES OF HISTONE METHYLTRANSFERASES IN SEA LAMPREY EMBRYOS UNDERGOING PROGRAMMED DNA ELIMINATION
During early embryogenesis, the sea lamprey Petromyzon marinus undergoes a dynamic form of genome rearrangement whereby several specific chromosomes are selectively eliminated from somatic progenitor cells. These elimination events effectively silence all genes on these chromosomes in all somatic cells., and Pprevious studies in lamprey and other eliminating species have provided strong evidence that other epigenetic silencing marks (DNA and histone posttranslational modifications) are enriched in the germline-restricted chromosomes and contribute to micronucleus packaging during elimination. These silencing marks include the histone marks H3K9me3 and H4K20me3, which are respectively deposited by KMT1A/SUV39 and KMT5/SUV420 methyltransferases in . To test whether lamprey homologs of methyltransferases contribute to deposition of these marks on elimination micronuclei and whether these marks contribute to the highly coordinated process of DNA elimination in sea lamprey, we leveraged CRISPR/Cas9 gene editing and lightsheet imaging to investigate their potential roles during early development and during the progression of DNA elimination. Analysis of knockout embryos for four methyltransferases show that these KMT genes contribute to the deposition of repressive histone marks on micronuclei, but are not essential for programmed DNA elimination per se. Analyses of later embryogenic stages suggests that these marks may contribute to interim silencing of germline-specific chromosomes, and reveals major impacts on post-blastula survival and developmental patterning. Overall design: RNA-seq profiling of D2 and D3 wildtype embryos and their KMT1 and KMT5A knockout derivatives.