Dicty News Electronic Edition Volume 19, number 7 September 21, 2002 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu. Back issues of Dicty-News, the Dicty Reference database and other useful information is available at DictyBase--http://dictybase.org. ============= Abstracts ============= Electron microscopy of actin rods and bundles in Dictyostelium discoideum by high-pressure freezing Masazumi Sameshima1*, Yoshiro Kishi1, Masako Osumi2, Dana Mahadeo3, and David A. Cotter3 1Electron Microscopy Center, The Tokyo Metropolitan Institute of Medical Science, Tokyo Metropolitan Organization for Medical Research, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8613, Japan, 2Department of Chemical and Biological Sciences, Faculty of Science, Japan Women's University, Bunkyo-ku, Tokyo 112-8681, Japan, and 3Department of Biological Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada J Electron Microscopy, in press. Abstract A new type of actin rods comprising actin tubules appear in dormant spores of Dictyostelium discoideum. A combination of high-pressure freezing and freeze-substitution (high-pressure freezing) is an effective method for analyzing structures of the actin rods appeared. However, the rods in the nucleus were occasionally observed in an amorphous state using this method. Also in the case of actin bundles formed in the nucleus of vegetative cells exposed to dimethyl sulfoxide, actin filaments seemed to be embedded in matrices. The karyoplasm of spores fixed by high-pressure freezing appeared to be denser than that obtained by other methods. Soluble materials may be efficiently retained in the nucleus, and so actin tubules or actin filaments embedded in those materials may result in hazy images of actin rods and bundles. ----------------------------------------------------------------------------- The costs and benefits of being a chimera Kevin R Foster, Angelo Fortunato, Joan E Strassmann and David C Queller Department of Ecology and Evolutionary Biology, Rice University MS 170 6100 Main, Houston Texas, 77005 USA Proceedings of the Royal Society, Series B, in press. Summary Most multicellular organisms are uniclonal. This is hypothesized to be because uniclonal organisms function better than chimeras (non-clonal organisms), due to reduced levels of internal genetic conflict. We tested this idea using the social amoeba Dictyostelium discoideum. When starving, the normally solitary amoebae aggregate to form a differentiated multicellular slug that migrates towards light and forms a fruiting body, facilitating the dispersal of spores. We added 107 amoebae to petri plates containing 1, 2, 5 or 10 clones mixed together. We found an intrinsic cost to chimerism: chimeric slugs moved significantly less far than uniclonal slugs of the same size. However, in nature joining with other clones to form a chimera should increase slug size, and larger slugs travel farther. We incorporated this size effect into a second experiment by giving chimeras more cells than single clones (uniclonal got 106 cells, 2-clone chimeras got 2x106 cells and so on). The uniclonal treatments then simulated a clone in a mixture that refuses to form chimeras. In this experiment, chimeras moved significantly further than the uniclonal slugs, in spite of the intrinsic cost. Thus, chimerism is costly, which may be why it evolves so seldom, but in D. discoideum the benefits of large size appear to compensate. ----------------------------------------------------------------------------- [End Dicty News, volume 19, number 7]