Ancient Coretention of Paralogs of Cid Centromeric Histones and Cal1 Chaperones in Mosquito Species

Despite their essential role in chromosome segregation in most eukaryotes, centromeric histones (CenH3s) evolve rapidly and are subject to gene turnover. We previously identified four instances of gene duplication and specialization of Cid, which encodes for the CenH3 in Drosophila. We hypothesized that retention of specialized Cid paralogs could be selectively advantageous to resolve the intralocus conflict that occurs on essential genes like Cid, which are subject to divergent selective pressures to perform multiple functions. We proposed that intralocus conflict could be a widespread phenomenon that drives evolutionary innovation in centromeric proteins. If this were the case, we might expect to find other instances of coretention and specialization of centromeric proteins during animal evolution. Consistent with this hypothesis, we find that most mosquito species encode two CenH3 (mosqCid) genes, mosqCid1 and mosqCid2, which have been coretained for over 150 My. In addition, Aedes species encode a third mosqCid3 gene, which arose from an independent gene duplication of mosqCid1. Like Drosophila Cid paralogs, mosqCid paralogs evolve under different selective constraints and show tissue-specific expression patterns. Analysis of mosqCid N-terminal protein motifs further supports the model that mosqCid paralogs have functionally diverged. Extending our survey to other centromeric proteins, we find that all Anopheles mosquitoes encode two CAL1 paralogs, which are the chaperones that deposit CenH3 proteins at centromeres in Diptera, but a single CENP-C paralog. The ancient coretention of paralogs of centromeric proteins adds further support to the hypothesis that intralocus conflict can drive their coretention and functional specialization.

Possible identification of CENP-C in fish and the presence of the CENP-C motif in M18BP1 of vertebrates

The centromeric protein CENP-C is a base component of the kinetochore. This protein, along with CENP-A has been shown to adaptively evolve in a number of animal and plant species. In order to determine if CENP-C also evolves in fish species, I attempted to retrieve fish CENP-C sequences from GenBank. No Teleostei CENP-C sequences were found either by name or by BLASTP searches with the vertebrate CENP-C motif sequence. A number of putative Teleostei protein sequences were identified in GenBank that have homology to the C-terminal cupin domain of vertebrate CENP-C. These proteins only have partial homology to the CENP-C motif, but evidence is presented that makes it likely that these fish proteins are orthologs of CENP-C. Interestingly, it was also discovered that the CENP-C motif sequence is also mostly present in M18BP1 proteins of fish and some other vertebrates but not in mammals. This finding may have implications for CENP-C and M18BP1 assembly in centromeric regions of different vertebrate taxa.