Holocentric Chromosomes

Abstract

Holocentric chromosomes possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes [1]. They have been described for the first time in cytogenetic experiments dating from 1935 and, since this first observation, the term holocentric chromosome has referred to chromosomes that: i. lack the primary constriction corresponding to centromere observed in monocentric chromosomes [2]; ii. possess multiple kinetochores dispersed along the chromosomal axis so that microtubules bind to chromosomes along their entire length and move broadside to the pole from the metaphase plate [3]. These chromosomes are also termed holokinetic, because, during cell division, chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes [4][5][6]. Holocentric chromosomes evolved several times during both animal and plant evolution and are currently reported in about eight hundred diverse species, including plants, insects, arachnids and nematodes [7,8]. As a consequence of their diffuse kinetochores, holocentric chromosomes may stabilize chromosomal fragments favouring karyotype rearrangements [9,10]. However, holocentric chromosome may also present limitations to crossing over causing a restriction of the number of chiasma in bivalents [11] and may cause a restructuring of meiotic divisions resulting in an inverted meiosis [12]. Evolution and structure of holocentric chromosomes Evolution of holocentric chromosomesHolocentric chromosomes were described for the first time in 1935 to identify chromosomes with a diffuse kinetochore (or with a diffuse kinetochore activity) making these chromosomes able to bind to microtubules along their entire length. In the last decades, several studies assessed that the same behaviour during mitosis can be observed not only for holocentric/ holokinetic chromosomes, but also for polykinetic chromosomes that contain numerous (but discrete) microtubule-binding sites, but the term "holocentric/holokinetic" is still used for both [1,5,7].Before molecular methods became available, the presence of holocentric chromosomes was evaluated mostly using cytology and, considering that many species are difficult to study cytologically, it can be surmised that the true presence of holocentrism may be underestimated. In addition, there are several taxa, whose chromosomes are still uncharacterized, but their

Absence of positive selection on CenH3 inLuzulasuggests that holokinetic chromosomes may suppress centromere drive

“…2) and, as previously speculated, that holokinetism itself might have evolved as a defence against centromere drive (Talbert et al, 2008;Malik and Henikoff, 2009;Bures et al, 2013). Accordingly, although holokinetism has evolved independently in various lineages of plants and animals (Melters et al, 2012), it appears to have evolved only in lineages with asymmetric meiosis where the need for a defence against centromere drive can be assumed, not in lineages with symmetric meiosis, such as bryophytes, lycophytes, ferns, horsetails or fungi (Bures et al, 2013), where such a defence has no sense. Nevertheless, even if centromere drive is suppressed in holokinetic lineages, holokinetism is likely to provide an opportunity for a different kind of meiotic drive propelling divergence in chromosomal size and number -holokinetic drive (Bures and Zedek, 2014;Escudero et al, 2015).…”

“…Monocentric chromosomes form the kinetochore in a relatively small centromeric region (but see Neumann et al, 2012), while holokinetic chromosomes form the kinetochore along their entire length (Bures et al, 2013;Heckmann et al, 2014). Although a specific DNA sequence is not required for kinetochore formation (HeslopHarrison and Schwarzacher, 2011), centromeric satellites are able to affect CenH3 positioning (Zhang et al, 2013), thereby affecting kinetochore size in monocentrics and switching kinetochore formation from epigenetic to genetic control.…”

Biología reproductiva y citogenética de Distichia muscoides (Juncaceae).

“…La presencia de cromosomas holocéntricos ha sido reportada tanto en Juncaceae como en Cyperaceae (Bozek et al, 2012;Bureš et al, 2013), siendo considerada una sinapomorfía para el clado constituido por ambas familias (Plunkett et al, 1995). Sin embargo, la presencia de esta clase de cromosomas solo ha podido ser demostrada satisfactoriamente en Luzula (Bureš et al, 2013).…”

“…Sin embargo, la presencia de esta clase de cromosomas solo ha podido ser demostrada satisfactoriamente en Luzula (Bureš et al, 2013). A diferencia de lo observado en este género, la existencia de constricción primaria permitió identificar cromosomas monocéntricos en Distichia.…”

“…Los complementos cromosómicos de las especies pertenecientes a Juncaceae usualmente forman series poliploides, siendo los números más comunes correspondientes a especies diploides: 2n = 12 y 2n = 40 (Záveská Drábková, 2013). También se ha reportado aneuploidía en Oxychloe, género más emparentado con Distichia (Balslev, 1996;Záveská Drábková, 2013), y se ha observado la presencia de centrómero difuso en algunos géneros de la familia (Bozek et al, 2012;Bureš et al, 2013). Distichia y Patosia Buchenau son dos géneros de la familia Juncaceae hasta el momento cariológicamente desconocidos.…”