Abstract
The presence of chromosomes with diffuse centromeres (holocentric chromosomes) has been reported in several taxa since more than fifty years, but a full understanding of their origin is still lacking. Comparative and functional genomics are nowadays furnishing new data to better understand holocentric chromosome evolution thus opening new perspectives to analyse karyotype rearrangements in species with holocentric chromosomes in particular evidencing unusual common features, such as the uniform GC content and gene distribution along chromosomes.
Highlights
The centromere of eukaryotes was identified as the primary constriction of chromosome visible by light microscopy [1]
Despite the functional importance of centromeres, cytologists have long observed that a primary constriction cannot be observed on chromosomes of all the species and that, in some taxa, spindle microtubules can attach along the entire chromosome length [3]
Starting from the beginning of ’60, it has been necessary to distinguish monocentric chromosomes that attach to microtubules at single region and move toward the pole during anaphase with the centromere leading (Fig. 1a), in contrast to holocentric chromosomes whose kinetochores are diffuse so that they bind to microtubules along their entire length and move broadside to the pole from the metaphase plate (Figs. 1b-e) [3,4,5]
Summary
Erly segregate and be stably maintained since they have microtubule attachments sites along their entire length. Similar effects have been reported by other plant-produced molecules, such as caffeine and ethanol [18, 19] In view of these effects, the presence of holocentric chromosome in phytophagous insects, such as aphids and lepidopteran species, could be a response to the clastogenic effects of some molecules produced by the plant tissue during the insect feeding [20]. Holocentrism in nematodes could avoid the disastrous consequences of unrepaired chromosome breakage events during nematode development [16]. Despite their diffuse presence in protista, plants and animals, holocentric chromosomes have been regarded with a mixture of curiosity and suspicion [2], so that studies concerning chromosome structure have been mainly concentrated upon eukaryotes having monocentric chromosomes, whereas species possessing holocentric /holokinetic chromosomes have been almost neglected. Unlocking Holocentric Chromosomes other ongoing projects (including those of the tick Ixodes scapularis and Varroa destructor), put comparative genomics in a pivotal position to deeply investigate the origin and evolution of holocentric chromosomes with particular regard for Metazoa
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