A model for heterochromatin dispersion and the evolution of C-band patterns

Summary

Taking many observations from many species an attempt was made to find general regularities and rules governing the karyo-type position of C-bands in order to provide clues to the mechanisms of cytological and molecular evolution of patterns, A model is presented to explain the identified rules.

Within a heterochromatin-rich complement, the shorter chromosomes (or chromosome arms) tend to have large telomeric C-bands whereas the longer chromosomes tend to have intercalary C-bands, The intercalary C-band distribution patterns are in many species a true reflection of the spatial disposition of the telomeres in the mitotic interphase nucleus (Rabl polarisation). Heterochromatin formed at telomeres may from there be transferred to equi local interstital sites in non-homologous chromosome arms. Transfer requires proximity. Proximity is determined by chromosome disposition in the three-dimensional space of the interphase nucleus. Our model assumes, on the basis of the identified rules, that the molecular processes of heterochromatin amplification and transposition act predominantly at mitotic interphase.

Our model further assumes that the mechanisms responsible for conservation of differences between C-bands and for the homogenisation of heterochromatic DNA sequences respectively act predominantly during first meiotic prophase. This notion is based on a comparison of mitotic and meiotic spatial nuclear organisations (Rabl polarisation vs. bouquet polarisation) in Robertsonian systems. Recombination requires proximity, Proximity is determined by spatial chromosome disposition. We suggest that the nuclear organisation at zygotene and pachytene is a major determinant.

In sum it appears that the incipient evolution of C-band patterns is constrained by nucleotypic properties and the chromosome arrangement at mitotic interphase. The evolution of biochemical differences of equilocal C-bands may be constrained by the nuclear organisation at early stages of first meiotic prophase.