Holocentric Chromosome

Holocentric or polycentric chromosomes have centromeric heterochromatin distributed at multiple sites along the length of the chromosome (1). Organisms with holocentric chromosomes include nematodes, such as Ascaris, insects, such as Lepidoptera, and plants in the genus Lazula. Spindle fibers attach along the whole length of holocentric chromosomes (Fig.  1). Sites of attachment of the spindle to centromeric chromatin are interspersed with noncentromeric segments, as in Ascaris, or all the chromatin in the chromosome is competent for attachment, as in the coccid bugs (2). In this latter case, centromeric functions are appropriately described as diffuse. In some organisms, like Lazula, localized centromeric function might be retained in mitotic chromosomes but shift to different positions in meiotic chromosomes. Mammalian tissue culture chromosomes also persist and segregate when they have multiple centromeres, but in this case only one centromere remains functional (4). 

Figure 1. Monocentric and holocentric chromosomes. (a) Monocentric chromosomes have a single site of chromosomal attachment to the spindle. (b) Holocentric chromosomes have multiple sites of attachment to the spindle.

References

1. S. Dimpinelli and C. Coday (1989) Trends Genet. 5, 310–314.  

2. S. Hughes-Schrader (1948) Adv. Genet. 2, 127–203. 

3. J. P. Braselton (1981) Chromosoma 82, 143–153. 

4. B. K. Vig (1984) Chromosoma 90, 39–45.