Germ line-limited and somatic chromosomes of Acricotopus lucidus differ in distribution and timing of alterations of histone modifications in male gonial mitosis and meiosis.

Special chromosomes limited to the germ line (=Ks) and exceptional genetic events such as elimination mitoses and a monopolar migration of the Ks in the last gonial mitosis are specific features of the complex chromosome cycle occurring in the chironomid Acricotopus lucidus. In the male, this unequal differential gonial mitosis results in a regular spermatocyte possessing all the Ks in addition to the somatic chromosomes (=Ss) and an aberrant spermatocyte containing only Ss. During evolution, the Ks have developed from the Ss and are composed of euchromatic S-homologous sections and heterochromatic segments. Less is known about the function and the transcriptional activity of the Ks. Specific post-translational histone modifications are known to be associated with transcriptionally active and inactive states of the chromatin. In an immunofluorescence study, the distribution of the following acetylated (ac), methylated (me) and phosphorylated (ph) amino acids in the histones H3 and H4 was analysed in Ks and Ss in male gonial mitoses and meiosis of A. lucidus, namely H3K18ac and H4K8ac, H3K4me3 and H3K9me3, H3S10ph, H3S28ph and H3T3ph. Ks and Ss clearly differ in the distribution of H3S28ph in gonial and meiotic metaphases. The H3S28ph mark covered the entire Ss, while the Ks showed this label only on their pericentromeric heterochromatin bands containing germ line-specific repetitive DNA sequences. A differential timing in the dephosphorylation of H3S10ph, H3S28ph and H3T3ph between Ks and Ss within the same cell was detected in the last gonial mitosis. The dephosphorylation occurred earlier in the Ks migrating first to the pole, than in the later equally segregating Ss. A programmed rapid histone deacetylation and dephosphorylation happened in the unseparated Ss of the aberrant spermatocyte at metaphase I in the connected primary spermatocyte, which correlated with the beginning of a permanent inactivation of these Ss in a metaphase-like condensed state. In meiosis, phosphorylated H3T3 could be detected only in metaphase II chromosomes at the inner centromeres of the attached sister chromatids. The H3T3ph labelling at this region was recently reported to be essential in mitosis for correct deposition of components of the chromosomal passenger complex and so for proper alignment, sister chromatid cohesion and segregation of chromosomes (Wang et al., Science 330:231-235, 2010; Curr Biol 21:1061-1069, 2011). Importantly, in spermatocytes, the euchromatic sections of the Ks were strongly acetylated at H3K18 and H4K8, and trimethylated at H3K4 during meiosis I and II, while the euchromatin of the meiotic Ss was hypoacetylated and hypomethylated at these sites. This result suggests a silencing of the Ss during spermatocyte meiosis. The high levels of active histone modifications detected in the euchromatic K sections support the idea that the Ks of A. lucidus are transcriptionally active in the germ line.