The Challenge of the New Cytochemistry: Highlighting the Obscure Meaning of Genome Composition-Organization. The Case of Eulemur Heterochromatin.

Abstract

In the present report we examine the composition and organization of the heterochromatic fraction of the genome, making use of cytochemical techniques, in a few species of primates of the genus Eulemur, a homogeneous group of lemuroidea from Madagascar. Among the species examined, Eulemur coronatus shows large blocks of heterochromatin distributed in nearly all the large metacentric chromosomes, in the large and small acrocentrics and in the microchromosomes. According to our measurements, such heterochromatin represents about 35% of the genome of the species, probably originated through amplification processes after the splitting off of the species from the common trunk. It must be recalled that in the genus, Robertsonian fusion represented the major mechanism acting in chromosome evolution. E. coronatus heterochromatin shows peculiar FRET (fluorescence resonance energy transfer) characteristics both before and after dehistonisation protracted to the destruction of the nucleosomal structure. Its characteristic FRET pattern differs greatly from that of E. macaco, supposed to have a longer history of evolution. Furthermore, quantitative histochemical analysis and imaging measurements highlighted a peculiar structure of the E. coronatus heterochromatin. Finally, the comparison of the data obtained with in situ hybridization, with probes like (TTAGGG) n and with probes obtained after genomic digestion with some restriction enzymes (Bam HI, Hae III, Pvu II) on E. coronatus, macaco, rubriventer and fulvus chromosomes clearly showed that E. coronatus heterochromatin has a more heterogeneous composition than the other species. Overall the data obtained lead us to reconsider heterochromatin in terms of its potential evolutionary plasticity.