Sperm morphology and biochemical composition of zebrafish sperm chromatin. (A) Zebrafish sperm have a round head morphology (∼10-μM diameter). (B) Whole-cell extracts were prepared from sperm and fibroblasts (ZF4) by lysing cells in 2× SDS-PAGE sample buffer. SDS-PAGE analysis of the cell extracts reveals a shared canonical histone pattern, with sperm displaying a higher level of linker histones. Histone components in sperm were verified by mass spectrometry (Supplemental Table 1). (C) Internucleosomal distances are longer in zebrafish sperm. Nucleosomes in sperm and fibroblasts (ZF4) were released by MNase digestion, and the DNA was analyzed by agarose gel electrophoresis and staining with EtBr (duplicate loading for each sample). Sperm displayed a longer average internucleosomal distance, 50 bp, than did the fibroblast, 40 bp. M indicates protein marker; Sp, sperm; Fib, fibroblast; and M1 and M2, DNA markers start from 1 kb with 1-kb increments and from 100 bp in 100-bp increments, respectively.
