Delayed cell fusion during fertilization in actin defective Chlamydomonas mating type minus mutants.
Yen Phan and Michael Misamore
Department of Biology, Texas Christian University, TCU Box 298930, Fort Worth, TX, USA
Actin plays a prominent role in several aspects of fertilization. In particular, actin is frequently associated with the sites of gamete binding, in the incorporation of sperm into the egg, release of polar bodies, and subsequent cleavage of zygotes. In Chlamydomonas, actin is known to play a prominent role in the formation of the fertilization tubule in mating type plus (mt+) gametes. Previously, actin-defective mating type plus (ida5+) gametes were shown to have greatly reduced fertilization rates and no fertilization tubule formation (Kata-Minoura et al. 1997). The role of actin in mating type minus (mt-) gametes is less well known. No distinct labeling of actin is observed in mt- gametes and actin-defective mt- (ida5-) gametes were shown to form zygotes at levels comparable to wild type mt- gametes. Here we reconfirmed that matings between wild type mt+ and ida5- gametes formed zygotes. However, the rate at which gamete fusion occurred was significantly reduced in ida5- gametes relative to wild type mt- gametes. Numerous partially fused, dumbbell-shaped zygotes were observed in mt+ x ida5- gametes. Dumbbell-shaped zygotes are typically seen in wild type matings, however most zytoes complete the fusion process within 5-10 minutes. Here we observed dumbbell-shaped zygotes as late as 30 minutes after mixing. Furthermore, several triple-cell fusions were observed suggesting polygametic fusions. Recently, polygamy was shown in to occur in Chlamydomonas at low levels. The elevated levels observed in ida5- gametes suggest an alteration of the blocks to polygamy. Funding provided the TCU SERC grant.
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