Holocentric chromosome
Interesting exceptions are represented by insects belonging to the Oligoneoptera and Neoptera, whose monocentric chromosomes probably evolved from an holocentric ancestor in two different and independent events.
However, the hypothesis of holocentrism as an anticlastogenic adaptation needs more systematic testing, including both controlled laboratory experiments and field studies across clastogenic gradients and large-scale phylogenetic analyses.
[8] At the same time, Nagaki et al.[15] proposed that holocentrism can be easily acquired during plant and animal evolution by a slight difference in the kinetochore origin.
As a consequence, chromatids of holocentric chromosomes move apart in parallel and do not form the classical V-shaped figures typical of monocentric ones.
Among arthropods, the presence of holocentric chromosome has been reported in different species belonging to insects (Odonata, Zoraptera, Dermaptera, Psocoptera, Phthiraptera, Thysanoptera, Hemiptera, Trichoptera and Lepidoptera), scorpions (Buthoidea), mites and ticks of the superorder Acariformes and genus Rhipicephalus (Ixodidae), spiders (Dysderidae and Segestridae),[7][14] millipedes[18] and centipedes.
[21][22] Aphids also possess a constitutive expression of the telomerase coding gene so that they can initiate a de novo synthesis of telomere sequences at internal breakpoints, resulting in the stabilization of chromosomal fragments.
[23][24] Among non-polyploid animals, Lepidoptera exhibit the highest variance in chromosome number between species within a genus and notable levels of interspecific and intraspecific karyotype variability.
[12] Comparing the genomes of lepidopteran species it has been also possible to analyse the effect of holocentrism in terms of rate of fixed chromosomal rearrangements.
This approach evidenced in Lepidoptera two chromosome breaks per megabase of DNA per Million of years: a rate that is much higher than what observed in Drosophila and it is a direct consequence of the holocentric nature of the lepidopteran genomes.
[27][28] At a structural level, insect holocentric chromosomes have not been studied in details, but it is interesting to underline the absence of homologues of CENP-C and CENP-A, previously considered essential for kinetochore functioning in eukaryotes.
[30][31][32] Nematode development is typically characterized by fixed lineages and a single inappropriate cell death, therefore, it has been suggested that holocentrism could avoid the disastrous consequences of unrepaired chromosome breakage events.
[36][37] Contrarily to what generally observed in monocentric chromosomes, in holocentric ones the preferential localization of centromeres within heterochromatic areas is missing together with the presence of specific DNA sequences that in C. elegans are not required for the assembly of a functional kinetochore.
[47] Similarly, in plants belonging to the genus Carex, differentiation of the karyotype has been demonstrated to correlate with genetic divergence within species,[49] among populations within species,[50] and within populations,[51] suggesting that, as previously reported in the Lepidoptera,[12] holocentric chromosome rearrangements contribute to genetic differentiation at different evolutionary scales in Carex evolution and speciation.
In plants it has also been suggested that the diffuse kinetochore of holocentric chromosomes may suppress the meiotic drive of centromeric repeats and its negative consequences.
[54] In the late 19th century, van Beneden (1883) and Boveri (1890) described meiosis for the first time through a careful observation of germ cell formation in the nematode Ascaris.
Furthermore, in most cases of inverted meiosis the absence of a canonical kinetochore structure has been observed, together with a restriction of the kinetic activity to the chromosomal ends.
[12] This article was adapted from the following source under a CC BY 4.0 license (2020) (reviewer reports): Mauro Mandrioli; Gian Carlo Manicardi (2020).
Bottom box: If a chromosomal breakage occurs in a monocentric chromosome, acentric chromosome fragments cannot be attached to microtubules during metaphase (M) and they are subsequently lost during anaphase (A). On the contrary, chromosomal breakage of a holocentric chromosome results in chromosomal fragments that retain kinetic activity due to the chromosome-wide centromere extension and can be properly inherited.
