Limits...
Meiotic silencing and fragmentation of the male germline restricted chromosome in zebra finch.

Schoenmakers S, Wassenaar E, Laven JS, Grootegoed JA, Baarends WM - Chromosoma (2010)

Bottom Line: We found that the GRC in the male germline is silenced from early leptotene onwards, similar to the W chromosome in avian oocytes.Subsequently, the GRC forms a micronucleus in which the DNA is fragmented.We conclude that in contrast to MSUC in mammals, meiotic silencing of this single chromosome in the avian germline occurs prior to, and independent of DNA double strand breaks and chromosome pairing, hence we have named this phenomenon meiotic silencing prior to synapsis (MSPS).

View Article: PubMed Central - PubMed

Affiliation: Department of Obstetrics and Gynaecology, Erasmus MC-University Medical Center, PO BOX 2040, 3000 CA, Rotterdam, The Netherlands.

ABSTRACT
During male meiotic prophase in mammals, X and Y are in a largely unsynapsed configuration, which is thought to trigger meiotic sex chromosome inactivation (MSCI). In avian species, females are ZW, and males ZZ. Although Z and W in chicken oocytes show complete, largely heterologous synapsis, they too undergo MSCI, albeit only transiently. The W chromosome is already inactive in early meiotic prophase, and inactive chromatin marks may spread on to the Z upon synapsis. Mammalian MSCI is considered as a specialised form of the general meiotic silencing mechanism, named meiotic silencing of unsynapsed chromatin (MSUC). Herein, we studied the avian form of MSUC, by analysing the behaviour of the peculiar germline restricted chromosome (GRC) that is present as a single copy in zebra finch spermatocytes. In the female germline, this chromosome is present in two copies, which normally synapse and recombine. In contrast, during male meiosis, the single GRC is always eliminated. We found that the GRC in the male germline is silenced from early leptotene onwards, similar to the W chromosome in avian oocytes. The GRC remains largely unsynapsed throughout meiotic prophase I, although patches of SYCP1 staining indicate that part of the GRC may self-synapse. In addition, the GRC is largely devoid of meiotic double strand breaks. We observed a lack of the inner centromere protein INCENP on the GRC and elimination of the GRC following metaphase I. Subsequently, the GRC forms a micronucleus in which the DNA is fragmented. We conclude that in contrast to MSUC in mammals, meiotic silencing of this single chromosome in the avian germline occurs prior to, and independent of DNA double strand breaks and chromosome pairing, hence we have named this phenomenon meiotic silencing prior to synapsis (MSPS).

Show MeSH

Related in: MedlinePlus

Schematic overview of the behaviour of the GRC during male meiosis of the zebra finch. In the transition phase between spermatogonia and the start of the male meiotic prophase, the GRC is identified and from preleptotene, it becomes heterochromatised and gains specific chromatin modifications such as H3K9me3, macroH2A, sumoylation and H4K16ac. With progression through meiotic prophase, all chromatin of the GRC is drawn into a heterochromatic body at the periphery of the nucleus. Few DSBs are induced on the GRC, and these remain unrepaired. Also, H2A on its centromere is not ubiquitylated. Sumoylation of chromatin and acetylation at H4K16 accumulate on the GRC and reach a maximum around mid pachytene. At diplotene, the GRC becomes acetylated at H3K9 and starts to decondense. During metaphase I, the nuclear membrane is degraded, the centromere and kinetochore of the GRC are aberrant and proper attachment to the spindles fails. This may cause DNA degradation of the GRC, and the chromatin to become phosphorylated, ubiquitylated and again de-ubiquitylated. In addition, the GRC forms a micronucleus with ongoing DNA fragmentation in the cytoplasm of the secondary spermatocyte
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2875885&req=5

Fig7: Schematic overview of the behaviour of the GRC during male meiosis of the zebra finch. In the transition phase between spermatogonia and the start of the male meiotic prophase, the GRC is identified and from preleptotene, it becomes heterochromatised and gains specific chromatin modifications such as H3K9me3, macroH2A, sumoylation and H4K16ac. With progression through meiotic prophase, all chromatin of the GRC is drawn into a heterochromatic body at the periphery of the nucleus. Few DSBs are induced on the GRC, and these remain unrepaired. Also, H2A on its centromere is not ubiquitylated. Sumoylation of chromatin and acetylation at H4K16 accumulate on the GRC and reach a maximum around mid pachytene. At diplotene, the GRC becomes acetylated at H3K9 and starts to decondense. During metaphase I, the nuclear membrane is degraded, the centromere and kinetochore of the GRC are aberrant and proper attachment to the spindles fails. This may cause DNA degradation of the GRC, and the chromatin to become phosphorylated, ubiquitylated and again de-ubiquitylated. In addition, the GRC forms a micronucleus with ongoing DNA fragmentation in the cytoplasm of the secondary spermatocyte

Mentions: The appearance of the DAPI dense GRC body during preleptotene indicates that it acquires a heterochromatic configuration upon entrance into meiotic prophase. To confirm this, we immunostained for a well-known marker of inactivation and heterochromatin, H3K9me3. We identified preleptotene nuclei as the stage in which SYPC3 foci start to appear (Fig. 3a, b). Furthermore, the combined staining for SYPC3 and DAPI allowed us to distinguish between the different substages (early, mid and late) of preleptotene based on the appearance of a DDB (Fig. 3a–e). In the earlier preleptotene nuclei, we never observed a distinct subnuclear region containing such a strong H3K9me3 signal as was observed for the GRC body in leptotene spermatocyes (Fig. 3c, e). However, at the end of preleptotene, we observed colocalisation of the DDB and H3K9me3 (Fig 3c, e). For macroH2A, another heterochromatin marker, we observed a similar staining pattern as for H3K9me3 (Figs. 3d and 7). This indicates that the GRC becomes heterochromatised upon entry into meiotic prophase.Fig. 3


Meiotic silencing and fragmentation of the male germline restricted chromosome in zebra finch.

Schoenmakers S, Wassenaar E, Laven JS, Grootegoed JA, Baarends WM - Chromosoma (2010)

Schematic overview of the behaviour of the GRC during male meiosis of the zebra finch. In the transition phase between spermatogonia and the start of the male meiotic prophase, the GRC is identified and from preleptotene, it becomes heterochromatised and gains specific chromatin modifications such as H3K9me3, macroH2A, sumoylation and H4K16ac. With progression through meiotic prophase, all chromatin of the GRC is drawn into a heterochromatic body at the periphery of the nucleus. Few DSBs are induced on the GRC, and these remain unrepaired. Also, H2A on its centromere is not ubiquitylated. Sumoylation of chromatin and acetylation at H4K16 accumulate on the GRC and reach a maximum around mid pachytene. At diplotene, the GRC becomes acetylated at H3K9 and starts to decondense. During metaphase I, the nuclear membrane is degraded, the centromere and kinetochore of the GRC are aberrant and proper attachment to the spindles fails. This may cause DNA degradation of the GRC, and the chromatin to become phosphorylated, ubiquitylated and again de-ubiquitylated. In addition, the GRC forms a micronucleus with ongoing DNA fragmentation in the cytoplasm of the secondary spermatocyte
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2875885&req=5

Fig7: Schematic overview of the behaviour of the GRC during male meiosis of the zebra finch. In the transition phase between spermatogonia and the start of the male meiotic prophase, the GRC is identified and from preleptotene, it becomes heterochromatised and gains specific chromatin modifications such as H3K9me3, macroH2A, sumoylation and H4K16ac. With progression through meiotic prophase, all chromatin of the GRC is drawn into a heterochromatic body at the periphery of the nucleus. Few DSBs are induced on the GRC, and these remain unrepaired. Also, H2A on its centromere is not ubiquitylated. Sumoylation of chromatin and acetylation at H4K16 accumulate on the GRC and reach a maximum around mid pachytene. At diplotene, the GRC becomes acetylated at H3K9 and starts to decondense. During metaphase I, the nuclear membrane is degraded, the centromere and kinetochore of the GRC are aberrant and proper attachment to the spindles fails. This may cause DNA degradation of the GRC, and the chromatin to become phosphorylated, ubiquitylated and again de-ubiquitylated. In addition, the GRC forms a micronucleus with ongoing DNA fragmentation in the cytoplasm of the secondary spermatocyte
Mentions: The appearance of the DAPI dense GRC body during preleptotene indicates that it acquires a heterochromatic configuration upon entrance into meiotic prophase. To confirm this, we immunostained for a well-known marker of inactivation and heterochromatin, H3K9me3. We identified preleptotene nuclei as the stage in which SYPC3 foci start to appear (Fig. 3a, b). Furthermore, the combined staining for SYPC3 and DAPI allowed us to distinguish between the different substages (early, mid and late) of preleptotene based on the appearance of a DDB (Fig. 3a–e). In the earlier preleptotene nuclei, we never observed a distinct subnuclear region containing such a strong H3K9me3 signal as was observed for the GRC body in leptotene spermatocyes (Fig. 3c, e). However, at the end of preleptotene, we observed colocalisation of the DDB and H3K9me3 (Fig 3c, e). For macroH2A, another heterochromatin marker, we observed a similar staining pattern as for H3K9me3 (Figs. 3d and 7). This indicates that the GRC becomes heterochromatised upon entry into meiotic prophase.Fig. 3

Bottom Line: We found that the GRC in the male germline is silenced from early leptotene onwards, similar to the W chromosome in avian oocytes.Subsequently, the GRC forms a micronucleus in which the DNA is fragmented.We conclude that in contrast to MSUC in mammals, meiotic silencing of this single chromosome in the avian germline occurs prior to, and independent of DNA double strand breaks and chromosome pairing, hence we have named this phenomenon meiotic silencing prior to synapsis (MSPS).

View Article: PubMed Central - PubMed

Affiliation: Department of Obstetrics and Gynaecology, Erasmus MC-University Medical Center, PO BOX 2040, 3000 CA, Rotterdam, The Netherlands.

ABSTRACT
During male meiotic prophase in mammals, X and Y are in a largely unsynapsed configuration, which is thought to trigger meiotic sex chromosome inactivation (MSCI). In avian species, females are ZW, and males ZZ. Although Z and W in chicken oocytes show complete, largely heterologous synapsis, they too undergo MSCI, albeit only transiently. The W chromosome is already inactive in early meiotic prophase, and inactive chromatin marks may spread on to the Z upon synapsis. Mammalian MSCI is considered as a specialised form of the general meiotic silencing mechanism, named meiotic silencing of unsynapsed chromatin (MSUC). Herein, we studied the avian form of MSUC, by analysing the behaviour of the peculiar germline restricted chromosome (GRC) that is present as a single copy in zebra finch spermatocytes. In the female germline, this chromosome is present in two copies, which normally synapse and recombine. In contrast, during male meiosis, the single GRC is always eliminated. We found that the GRC in the male germline is silenced from early leptotene onwards, similar to the W chromosome in avian oocytes. The GRC remains largely unsynapsed throughout meiotic prophase I, although patches of SYCP1 staining indicate that part of the GRC may self-synapse. In addition, the GRC is largely devoid of meiotic double strand breaks. We observed a lack of the inner centromere protein INCENP on the GRC and elimination of the GRC following metaphase I. Subsequently, the GRC forms a micronucleus in which the DNA is fragmented. We conclude that in contrast to MSUC in mammals, meiotic silencing of this single chromosome in the avian germline occurs prior to, and independent of DNA double strand breaks and chromosome pairing, hence we have named this phenomenon meiotic silencing prior to synapsis (MSPS).

Show MeSH
Related in: MedlinePlus