Rumex acetosa Y chromosomes: constitutive or facultative heterochromatin?

Abstract

Condensed Y chromosomes in <i>Rumex acetosa</i> L. root-tip nuclei were studied using 5-azaC treatment and immunohistochemical detection of methylated histones. Although Y chromosomes were decondensed within root meristem in vivo, they became condensed and heteropycnotic in roots cultured <i>in vitro</i>. 5-azacytidine (5-azaC) treatment of cultured roots caused transitional dispersion of their Y chromosome bodies, but 7 days after removal of the drug from the culture medium, Y heterochromatin recondensed and again became visible. The response of <i>Rumex</i> sex chromatin to 5-azaC was compared with that of condensed segments of pericentromeric heterochromatin in <i>Rhoeo spathacea</i> (Sw.) Stearn roots. It was shown that <i>Rhoeo</i> chromocentres, composed of AT-rich constitutive heterochromatin, did not undergo decondensation after 5-azaC treatment. The Y-bodies observed within male nuclei of <i>R. acetosa</i> were globally enriched with H3 histone, demethylated at lysine 4 and methylated at lysine 9. This is the first report of histone tail-modification in condensed sex chromatin in plants. Our results suggest that the interphase condensation of Y chromosomes in <i>Rumex</i> is facultative rather than constitutive. Furthermore, the observed response of Y-bodies to 5-azaC may result indirectly from demethylation and the subsequent altered expression of unknown genes controlling tissue-specific Y-inactivation as opposed to the global demethylation of Y-chromosome DNA.

Keywords

• Y chromosomes
• Heterochromatin
• Methylation
• Histone H3
• Rumex