Distribution of heterochromatin markers in lampbrush chromosomes in birds

Abstract

Changes in functional gene activity not affecting primary DNA structure can be inherited by a cell and in total represent epigenetic mechanisms of the genome expression regulation. Epigenetic genome modifications include DNA methylation, histone modifications, and binding of the proteins and noncoding RNA. The distribution of heterochromatin markers (such as methylated cytosine, HP1 heterochromatin protein, and modifications of H3, H4, and H2A histones) was analyzed by example of lampbrush chromosomes typical for growing bird oocytes. On the lampbrush chromosomes of domesticated chicken (Gallus gallus domesticus), Japanese quail (Coturnix japonica), and chaffinch (Fringilla coelebs), methylated cytosine was mainly detected in chromomeres, as well as in unextended regions of lateral loop axes. The largest accumulation of methylated cytosine was demonstrated by chromosome W and dense chromomeres of macro- and microchromosomes. The HP1β protein distribution in the lampbrush chromosomes corresponded in general to the distribution of methylated cytosine. Chromomeres of chromosome W as well as centromeric and terminal chromomeres of macro- and microchromosomes accumulated the highest amount of the HP1β protein as compared with the remaining chromomeres. The character of distribution of the H3K9 me3 and H3K27 me3 histone modifications, as well as phosphorylated H4 and H2A histones, was different. Phosphorylated H4 and H2A histones were distributed proportionally to the staining by DNA specific dyes and were accumulated in all chromosomes, while trimethylated H3K9 histone was enriched in the regions of constitutive heterochromatin. We assume that the HP1β heterochromatin protein is involved in compaction and transcriptional inactivation of the chromatin in chromomeres of the lampbrush chromosomes.