Affiliations: 1) Fred Hutch; 2) Howard Hughes Medical Institute
Keywords: a. spermatogenesis; a. chromatin structure
During spermatogenesis, many animals replace histones with small positively charged proteins that can tightly package DNA, known as sperm nuclear binding proteins (SNBPs). Unlike highly conserved histones, Drosophila SNBPs recently acquired essential functions for fertility and expanded lineage-specific functional paralogs via duplications.To study how SNBPs acquired sperm-specific function in Drosophila, we focus on two young SNBPs, tHMG1 and tHMG2. tHMG1/2 has originated in the Sophorphora subgenus from the duplication of a ubiquitous transcription factor (HmgD or HmgZ) with a high mobility group box (HMGB), and is further tandemly duplicated in D. melanogaster. The roles of tHMG1 and tHMG2 during spermatogenesis are unknown, but they appear to have non redundant functions. Here we show that tHMG1 is under positive selection using population genetic analyses and its knockdowns show a reduction in progeny. However, we did not detect the same signature of positive selection and phenotypes in tHMG2. Consistent with our data, previous studies also show tHMG1 and tHMG2 have different expression and cytological patterns in sperm. We hypothesized that the highly diverged C-terminus, instead of the conserved HMGB domain, between tHMG1 and tHMG2 determine the functional differences between these two recently diverged duplicates. We are generating CRISPR knockouts and transgenic flies of tHMG-1 and tHMG2 to understand their biological function. Our study will highlight how new SNBPs acquired function during sperm development and further shed light on the evolutionary forces shaping SNBPs evolution.