Centromeres off the Hook: Investigation of Centromere Size and Structure in Fabeae Species in the Context of CenH3 Evolution

The centromere is a functional chromosome domain that is essential for faithful chromosome segregation during cell division. It can be reliably identified by the presence of CenH3 histone. In most diploid plant genomes, CenH3 is encoded by a single copy gene. A chromosome region containing CenH3 protein constitutes a single compact domain within a morphologically distinct primary constriction. This region usually spans up to a few Mbp of DNA which is composed mainly of one or few families of centromere-specific satellite DNA.

Fabeae tribe of Fabaceae family emerged 16 - 23 million years ago and now comprises ca. 380 species from five genera: Pisum, Lathyrus, Vicia, Lens and Vavilovia. We discovered that Pisum and Lathyrus spp. possess two variants of CenH3, designated as CenH3-1 and CenH3-2, while Lens and Vicia spp. possess just CenH3-2. Phylogenetic analysis revealed that the duplication of the ancestral CenH3 preceded the split of Fabeae species, indicating that the CenH3-1 variant was lost in the lineage leading to Lens and Vicia spp.. Strikingly, Fabeae species with two CenH3 variants had longer centromeres than those spesies that only have CenH3-2. In some of them, the centromeres were very large and displayed multiple domain structure that we referred to as metapolycentric. These results demonstrate that centromere size and organization of CenH3-containing domains are more varied than previously thought. However, further investigation is needed to find if there is a causal link between the presence of two CenH3 paralogs and centromere size or if it is just a coincidence.