SummaryChromosome segregation errors during female meiosis are a leading cause of pregnancy loss and human infertility. The segregation of chromosomes is driven by interactions between spindle microtubules and kinetochores. Kinetochores in mammalian oocytes are subjected to special challenges: they need to withstand microtubule pulling forces over multiple hours and are built on centromeric chromatin that in humans is decades old. In meiosis I, sister kinetochores are paired and oriented toward the same spindle pole. It is well established that they progressively separate from each other with advancing female age. However, whether aging also affects the internal architecture of centromeres and kinetochores is currently unclear. Here, we used super-resolution microscopy to study meiotic centromere and kinetochore organization in metaphase-II-arrested eggs from three mammalian species, including humans. We found that centromeric chromatin decompacts with advancing maternal age. Kinetochores built on decompacted centromeres frequently lost their integrity and fragmented into multiple lobes. Fragmentation extended across inner and outer kinetochore regions and affected over 30% of metaphase-II-arrested (MII) kinetochores in aged women and mice, making the lobular architecture a prominent feature of the female meiotic kinetochore. We demonstrate that a partial cohesin loss, as is known to occur in oocytes with advancing maternal age, is sufficient to trigger centromere decompaction and kinetochore fragmentation. Microtubule pulling forces further enhanced the fragmentation and shaped the arrangement of kinetochore lobes. Fragmented kinetochores were frequently abnormally attached to spindle microtubules, suggesting that kinetochore fragmentation could contribute to the maternal age effect in mammalian eggs.
SUMMARY-Meiosis in three wolf-spiders, Lycosa sp., Hippasa oliracea and Pardosa birmanica belonging to family Lycosidae have been studied. The diploid number as determined from testicular material was 22 in Lycosa sp. while it was 28 in Hippasa oliracea and Pardosa birmanica. In all the three species, chromosomes are acrocentric with X1 X2 0 type of sex determining mechanism in males, the sex elements being unequal. The sex chromosomes are heteropycnotic during meiotic prophase and form an accessory plate in metaphase I lying parallel to each other. During anaphase I and anaphase II the sex chromosomes were reductional and equational, respectively, in characters. Karyomorphometrical analysis of spermatogonial metaphase in these three species have been worked out. Discussion was made with regard to the trend of chromosome evolution in the cytologically worked out congeneric and confamilial species within Lycosidae.
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