During spermatogenesis, chromatin is tightly compacted for efficient delivery of the paternal genome to the embryo. Small basic proteins called protamines facilitate this process. Though many organisms use protamine-like proteins to compact their paternal genome, features of these proteins are unique to each species. We identified three proteins in C. elegans, SPCH-1/2/3, that are small, basic, enriched in sperm chromatin and have high arginine content. Due to these features we hypothesize that SPCH-1/2/3 are C. elegans protamines. Immunostaining revealed SPCH delivery to the 1-cell embryo and its removal from the decondensing paternal pronucleus, similar to mammalian protamine removal after fertilization. However, mammalian protamines incorporate with chromatin after meiosis, but we find SPCH incorporation during sperm meiosis. This incorporation is a unique feature to C. elegans spermatogenesis. Parallel to protamines, we find that SPCH-2 is the primary contributor to SPCH function in male fertility.
spch-2 hermaphrodites produce 70% fewer progeny than wild type. After mating
spch-2;
him-8 males to the self-sterile
spe-8;
dpy-4, we found that mutant males produced 65% fewer cross-progeny than
him-8 males, revealing that the fertility defect is due to faulty sperm.
spch-1 mutants only show a 16% reduction in progeny compared to wild type, thus SPCH-1 is the secondary contributor to SPCH function in fertility.
spch-3 hermaphrodites have no reduction in progeny, though the
spch-3;
spch-1 mutant produces 50% fewer progeny than wild type. These data suggest that SPCH-1 and -2 have distinct roles and partially compensate for each other, while SPCH-3 is functionally redundant. SPCH-2 has a main role in male fertility, but the exact cause of the fertility defect is unknown.
spch-2 mutants do not have obvious defects in sperm production, transfer to hermaphrodites, or abnormal sperm utilization patterns.
spch-2 mutants do not produce large quantities of dead embryos, precluding the idea that sperm are anucleate or have significant DNA damage. Thus it is possible that the reduced fertility is due to a fertilization defect. Taken together this data indicate that SPCH are functioning as protamines. Interestingly, though SPCH-1 and -2 are 88% identical in amino acid sequence, mass spec analysis found SPCH-1 and -2 peptides differentially phosphorylated in a region of non-identity. This provides further evidence that SPCH-1 and -2 have distinct roles in fertility. While SPCH-1/2/3 have some similarities to protamines in other species, their distinct features highlight unique aspects of C. elegans spermatogenesis and male fertility.