Abstract
Species of the legume genera Lathyrus and Pisum possess chromosomes that exhibit a unique structure of their centromeric regions, which is clearly apparent during metaphase by the formation of extended primary constrictions which span up to a third of the length of the chromosome. In addition, these species express two different variants of the CenH3 protein which are co-localized in multiple domains along the poleward surface of the primary constrictions. Here, we show that the constrictions represent a distinct type of chromatin differing from the chromosome arms. In metaphase, histone phosphorylation patterns including H3S10ph, H3S28ph, and H3T3ph were observed along the entire constriction, in a way similar to holocentric chromosomes. On the other hand, distribution of phosphorylated H2AT120 was different from that previously reported from either, holocentric and monocentric chromosomes, occurring at chromatin surrounding but not overlapping CenH3 domains. Since some of these phosphorylations play a role in chromatid cohesion, it can be assumed that they facilitate correct chromosome segregation by ensuring that multiple separate CenH3 domains present on the same chromatid are oriented toward the same pole. The constrictions also displayed distinct patterns of histone methylation marks, being enriched in H3K9me2 and depleted in H3K4me3 and H3K27me2 compared to the chromosome arms. Super-resolution fluorescence microscopy revealed that although both CenH3 protein variants are present in all CenH3 domains detected on metaphase chromosomes, they are only partially co-localized while there are chromatin subdomains which are mostly made of only one CenH3 variant. Taken together, these data revealed specific features of extended primary constrictions of Lathyrus and Pisum and support the idea that they may represent an intermediate stage between monocentric and holocentric chromosomes.
Highlights
Centromeres are defined as sites of kinetochore formation, facilitating the attachment of a spindle that drives chromosome segregation during cell division
In the few plant species with holocentric chromosomes investigated so far, no specific repeats have been found to be associated with CenH3loci in Luzula elegans (Heckmann et al, 2013), while CenH3 chromatin in Rhynchospora was shown to be enriched in one family of satellite repeats and several mobile elements (Marques et al, 2015)
Histone phosphorylation patterns were investigated in parallel in Lathyrus sativus and Pisum sativum and were found to display similar distributions on chromosomes of both species
Summary
Centromeres are defined as sites of kinetochore formation, facilitating the attachment of a spindle that drives chromosome segregation during cell division. CenH3 is deposited along almost the entire length of the chromosome, forming a lateral groove on each chromatid of mitotic chromosomes where spindle fibers attach (Cuacos et al, 2015). In terms of their underlying nucleotide sequences, the regional centromeres of monocentric chromosomes are typically enriched with satellite repeats and specific lineages of mobile elements. In the few plant species with holocentric chromosomes investigated so far, no specific repeats have been found to be associated with CenH3loci in Luzula elegans (Heckmann et al, 2013), while CenH3 chromatin in Rhynchospora was shown to be enriched in one family of satellite repeats and several mobile elements (Marques et al, 2015)
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