SummaryAn extremely unorthodox method of chromosome segregation is found in sciarid flies (Diptera, Sciaridae), where at male meiosis, the whole paternal complement is eliminated and the maternal X chromosome undergoes non-disjunction. At meiosis I, a monopolar spindle directs the segregation of maternal chromosomes to the single pole, whereas paternal chromosomes are discarded. At meiosis II, although maternal autosomes segregate normally, the X chromosome remains undivided. A cis-acting locus within the heterochromatin proximal to the centromere is known to regulate X centromere activity. By immunofluorescence analysis in spermatocytes from Sciara ocellaris and Sciara coprophila, we investigated histone H3 phosphorylation at Ser10, Ser28, Thr3 and Thr11 during male meiosis. We found that chromosome condensation and H3 phosphorylation patterns differ between chromosomes of different parental origin at the time of paternal set elimination. Importantly, at meiosis II, the maternal X chromosome differs from the rest of the chromosomes in that its centromeric region does not become phosphorylated at the four histone H3 sites. We provide here the first evidence linking the under-phosphorylated H3 status of the X chromosome centromeric region with its meiotic non-disjunction in sciarid flies. Our findings strongly support the idea that the deficiency in local H3 phosphorylation inactivates the X centromere at the transition from meiosis I to meiosis II.

SummaryIn sciarid flies (Diptera, Sciaridae), one or two paternally derived X chromosomes are discarded from the soma at early cleavages to determine the sex of the embryo (XX, females; X0, males). X chromosome(s) elimination is achieved by an abnormal anaphase segregation so that X sister chromatids do not reach the poles and are not included in the daughter nuclei. A cis-acting locus (CE) within the heterochromatin proximal to the centromere is known to regulate X chromosome elimination. By immunofluorescence analysis in early embryos from Sciara ocellaris and Sciara coprophila, we investigated histone H3 phosphorylation at Ser10, Ser28 and Thr3 prior to, and during, the X elimination process. We found that the regular syncytial nuclear divisions are characterized by a gradual loss of H3S10 phosphorylation along the chromosome arms at anaphase. Importantly, the eliminating X chromosomes show a retardation in anaphase chromatid segregation and high levels of H3S10 phosphorylation in the chromosome arms. In the present study, we provide the first evidence linking the hyper-phosphorylated H3 status of the X chromosome with a delay in sister chromatid separation at anaphase. Our findings support the idea that the CE induces a deficiency in H3 dephosphorylation in the paternal X chromosomes to be eliminated.

Sciara coprophila (Diptera, Nematocera) constitutes a classic model to analyze unusual chromosome behavior such as the somatic elimination of paternal X chromosomes, the elimination of the whole paternal, plus non-disjunction of the maternal X chromosome at male meiosis. The molecular organization of the heterochromatin in S. coprophila is mostly unknown except for the ribosomal DNA located in the X chromosome pericentromeric heterochromatin. The characterization of the centromeric regions, thus, is an essential and required step for the establishment of S. coprophila as a model system to study fundamental mechanisms of chromosome segregation. To accomplish such a study, heterochromatic sections of the X chromosome centromeric region from salivary glands polytene chromosomes were microdissected and microcloned. Here, we report the identification and characterization of two tandem repeated DNA sequences from the pericentromeric region of the X chromosome, a pericentromeric RTE element and an AT-rich centromeric satellite. These sequences will be important tools for the cloning of S. coprophila centromeric heterochromatin using libraries of large genomic clones.