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(American Journal of Botany. 2008;95:664-671.)
doi: 10.3732/ajb.2007388
© 2008 Botanical Society of America, Inc.
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Cell Biology

{gamma}-Tubulin and microtubule organization during meiosis in the liverwort Ricciocarpus natans (Ricciaceae)1

R. C. Brown2 and B. E. Lemmon

Department of Biology, University of Louisiana-Lafayette, Lafayette, Louisiana 70504 USA

ABSTRACT

Extant liverworts are "living fossils" considered sister to all other plants and as such provide clues to the evolution of the microtubule organizing center (MTOC) in anastral cells. This report is the first on microtubule arrays and their {gamma}-tubulin-nucleating sites during meiosis in a member of the Ricciales, a specialized, species-rich group of complex thalloid (marchantioid) liverworts. In meiotic prophase, {gamma}-tubulin becomes concentrated at several sites adjacent to the nuclear envelope. Microtubules organized at these foci give rise to a multipolar prometaphase spindle. By metaphase I, the spindle has matured into a bipolar structure with truncated poles. In both first and second meiosis, {gamma}-tubulin forms box-like caps at the spindle poles. {gamma}-Tubulin moves from spindle poles to the proximal surfaces of telophase chromosomes where interzonal microtubules are nucleated. Although a phragmoplast is organized, no cell plate is deposited, and second division occurs simultaneously in the undivided sporocyte. {gamma}-Tubulin surrounds each of the tetrad nuclei, and phragmoplasts initiated between both sister and nonsister nuclei direct simultaneous cytokinesis. The overall pattern of meiosis (unlobed polyplastidic sporocytes, nuclear envelope MTOC, multipolar spindle origin, spindles with box-like poles, and simultaneous cytokinesis) more closely resembles that of Conocephalum than other marchantiod liverworts.

Key Words: anastral spindle • bryophytes • cytokinesis • evolution • Ricciocarpus natans • Ricciaceae • sporogenesis

Received for publication 28 November 2007. Accepted for publication 9 April 2008.

FOOTNOTES

1 The authors thank Dr. T. Horio for the gift of the G9 antibody (prepared by M. Yamato of Univ. Tokushima), Dr. B. Crandall-Stotler for taxonomic identification, Dr. D. Krayesky for help with phylogeny, and Harold Poole Nursery, Forest Hills, Louisiana for allowing us to collect bryophytes from their greenhouses and growing fields.

2 Author for correspondence (e-mail: rcb{at}louisiana.edu)


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