Symmetric Inheritance of Histones H3 in Drosophila Male Germline Stem Cell Divisions

Mitotically-stable "epigenetic" memory requires a mechanism for the maintenance of gene-regulatory information through the cell division cycle. Typically DNA-protein contacts are disrupted by DNA replication, but in some cases locus-specific association between DNA and overlying histones may appear to be maintained, providing a plausible mechanism for the transmission of histone-associated gene-regulatory information to daughter cells. Male Drosophila melanogaster testis germ stem cell divisions seem a clear example of such inheritance, as previously chromatin-bound Histone H3.2 proteins (presumably with their post-translational modifications intact) have been reported to be retained in the germ stem cell nuclei, while newly synthesized histones are incorporated into daughter spermatogonial chromosomes. To investigate the rate of errors in this selective partitioning that may lead to defects in the epigenetic identity of germ stem cells, we employed a photoswitchable Dendra2 moiety as a C-terminal fusion on Histones H3 (Histone H3.2 and Histone H3.3); we could thereby discriminate histones translated before photoswitching and those translated after. We found instead that male germ line stem cell divisions show no evidence of asymmetric histone partitioning, even after a single division, and thus no evidence for locus-specific retention of either Histone H3.2 or Histone H3.3. We considered alternative hypotheses for the appearance of asymmetry and find that previous reports of asymmetric histone distribution in male germ stem cells can be satisfactorily explained by asynchrony between subsequent sister stem cell and spermatogonial divisions.