External Actin protects mammalian eggs against chromosome segregation errors Seminar details September 26, 2017, 12:00 pm @ Small Lecture TheatreDr Binyam Mogessie Host: Tomoyuki Tanaka Abstract Binyam Mogessie and Melina Schuh Department of Meiosis, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany Chromosome segregation is driven by a spindle that is made of microtubules, but is generally thought to be independent of actin. Here, we report an unexpected actindependent mechanism that drives the accurate alignment and segregation of chromosomes in mammalian eggs. Prominent actin filaments permeated the microtubule spindle in eggs of several mammalian species, including humans. Disrupting actin in mouse eggs led to significantly increased numbers of misaligned chromosomes as well as lagging chromosomes during meiosis I and II. We found that actin drives accurate chromosome segregation by promoting the formation of functional kinetochore-fibers, the microtubule bundles that align and segregate the chromosomes. Thus, actin is essential to prevent chromosome segregation errors in eggs, which are a leading cause of miscarriages, infertility and Down Syndrome.