Germline X Chromosomes Exhibit Contrasting Patterns of Histone H3 Methylation in Caenorhabditis elegans

Abstract

In mammals, one of the two somatic X chromosomes in the female is inactivated, thereby equalizing X chromosome-derived transcription in the two sexes, a process known as dosage compensation. In the germline, however, the situation is quite different. Both X chromosomes are transcriptionally active during female oogenesis, whereas the X and Y chromosomes are transcriptionally silent during male spermatogenesis. Previous studies suggest that Caenorhabditis elegans germline X chromosomes might have different transcriptional activity in the two sexes in a manner similar to that in mammals. Using antibodies specific to H3 methylated at either lysine 4 or lysine 9, we show that the pattern of site-specific H3 methylation is different between X chromosomes and autosomes as well as between germline X chromosomes from the two sexes in C. elegans. We show that the pachytene germline X chromosomes in both sexes lack Me(K4)H3 when compared with autosomes, consistent with their being transcriptionally inactive. This transcriptional inactivity of germline X chromosomes is apparently transient in hermaphrodites because both X chromosomes stain brightly for Me(K4)H3 after germ nuclei exit pachytene. The male single X chromosome, on the other hand, remains devoid of Me(K4)H3 staining throughout the germline. Instead, the male germline X chromosome exhibits a high level of Me(K9)H3 that is not detected on any other chromosomes in either sex, consistent with stable silencing of this chromosome. Using mutants defective in the sex determination pathway, we show that X-chromosomal Me(K9)H3 staining is determined by the sexual phenotype, and not karyotype, of the animal. We detect a similar high level of Me(K9)H3 in male mouse XY bodies, suggesting an evolutionarily conserved mechanism for silencing the X chromosome specifically in the male germline.

Meiotic pairing and imprinted X chromatin assembly in Caenorhabditis elegans

“…Repetitive transgene arrays, which do not pair during meiosis and are transcriptionally silenced in germ cells, also accumulate methylated H3-Lys9 in C. elegans 12 . There was also a general, low-level accumulation of methylated H3-Lys9 on all chromosomes as the homologs de-synapse at the pachynema-diplonema transition 12,16 ( Fig. 3a).…”

“…Antibodies specific to trimethylated H3-Lys4 showed a pattern of staining similar to that of antibodies to dimethylated H3-Lys4 (data not shown). Methylated H3-Lys9 is present at low levels on all chromosomes in early C. elegans embryos and is mostly enriched at telomeres 16 . The exceptional chromosome is the sperm-derived X chromosome (X p ), as determined by combined histone antibody staining and DNA fluorescence in situ hybridization (FISH).…”

Marking Xs, together and separately

“…These complementary approaches are necessary to overcome the various limitations in sensitivity and specificity of each method. Recent studies using such approaches in yeast are consistent with an estimate of 800-1,000 different mitochondrial proteins [3][4][5][6] .…”

“…XX individuals are hermaphrodites and produce both sperm and eggs, whereas XO individuals are committed males, producing only sperm. Previous work 3,4 showed dimorphism in the behavior of the X chromosome in these two germ lines. In both germ lines, the X chromosome is inactive through most of meiotic prophase.…”

“…7) and many other proteins. What probably set the authors down their current path of research is the knowledge that transgene repeat arrays in C. elegans are silenced in the germ line and accumulate methylated 4). Meiotic silencing by unpaired DNA (MSUD) is best studied in Neurospora crassa 8 , where genetic analysis has implicated an RNA interference-mediated silencing 9 .…”