The structural role of Skp1 in the synaptonemal complex is conserved in nematodes.

The synaptonemal complex (SC) is a meiotic interface that assembles between parental chromosomes and is essential for gamete formation. While the dimensions and ultrastructure of the SC are conserved across eukaryotes, its protein components are highly divergent. Recently, an unexpected component of the SC has been described in the nematode C. elegans: the Skp1-related proteins SKR-1/2, which are components of the Skp1, Cullin, F-box (SCF) ubiquitin ligase. Here, we find that the role of SKR-1 in the SC is conserved in P. pacificus. The P. pacificus Skp1 ortholog, Ppa-SKR-1, colocalizes with other SC proteins throughout meiotic prophase, where it occupies the middle of the SC. Like in C. elegans, the dimerization interface of Ppa-SKR-1 is required for its SC function. A dimerization mutant, ppa-skr-1F105E, fails to assemble SC and produces almost no progeny. Interestingly, the evolutionary trajectory of SKR-1 contrasts with other SC proteins. Unlike most SC proteins, SKR-1 is highly conserved in nematodes. Our results suggest that the structural role of SKR-1 in the SC has been conserved since the common ancestor of C. elegans and P. pacificus, and that rapidly evolving SC proteins have maintained the ability to interact with SKR-1 for at least 100 million years.