Gametic differentiation in Chlamydomonas reinhardtii. III. Cell wall lysis and microfilament-associated mating structure activation in wild-type and mutant strains.

Abstract

Cell fusion between mating type plus (mr +) and minus (rot-) gametes ofChlamydomonas reinhardtii is analyzed structurally and subjected to experimental manipulation. Cell wall lysis, a necessary prelude to fusion, is shown to require flagellar agglutination between competent gametes; glutaraldehyde-fixed gametes ("corpses") of one mating type will elicit both agglutination and cell wall lysis in the opposite mating type, whereas nonagglutinating impotent (imp) mutant strains are without effect. The fusion process is mediated by a narrow fertilization tubule which extends from the mt+ gamete and establishes contact with the mt-gamete. Formation of the tubule requires the "activation" of a specialized mating structure associated with the mt + cell membrane; activation causes microfilaments to polymerize from the mating structure into the growing fertilization tubule. Mating structure activation is shown to depend on gametic flagellar agglutination; isoagglutination mediated by the lectin concanavalin A has no effect. Gametes carrying the imp-I mt+ mutation are able to agglutinate but not fuse with rotcells; the imp-1 gametes are shown to have structurally defective mating structures that do not generate microfilaments in response to gametic agglutination.The mating reaction of the biflagellate alga Chlamydomonas reinhardtii involves an elaborate sequence of events that can be presumed to depend on precise levels of cellular communication. The reaction is initiated by an agglutination between the flagellar tips of mating type plus (mr +) and minus (mr-) gametes (18), as illustrated in Fig. 1, an interaction that appears to involve surface components of the flagellar membranes (1, 24). Four events follow in quick succession: (a) a lytic activity digests the cell walls so that gametes are converted to naked protoplasts (2): (b) a cytoplasmic protuberance [the fertilization tubule (4)] extends from the mt+ cell surface (Fig. 1, tong arrow), an event mediated by the specialized mating structure first described by Friedmann et al. (4); (c) the fertilization tubule makes contact with an mt-mating structure to form a narrow cytoplasmic bridge (4); and (d) the bridge expands in diameter until the mating pair is transformed into a single quadriflagellate zygote, an event that can be termed zygotic cell fusion (see Fig. I). Quadriflagellate zygotes typically appear within 5 rain after mt § and rot-gametes are mixed; all of these events must therefore occur in a rapid and highly coordinated fashion.

Visualizing the Spatial Arrangement of Flagella-Eyespots-Cell Fusion Sites in Gametes and Planozygotes of Chlamydomonas reinhardtii (Chlorophyceae, Chlorophyta) with High-Resolution FE-SEM

“…In addition to the fertilization tubules and flagella it was possible to clearly visualize the eyespot and their lipid globules using FE-SEM, which have not been observed using conventional SEM (Goodenough and Weiss 1975, Forest 1983, 1987. Since the eyespots were clearly recognized, it becomes possible to directly determine the spatial relationship of flagella-eyespot-cell fusion site in the gamete.…”

“…Recently, it has been found that the cell fusion sites of the gametes occupy different positions between the opposite mating types in several green algae (see Miyamura and Nagumo 2007). This was first indicated in biflagellate isogametes of chlorophycean alga Chlamydomonas reinhardtii Dangeard (Holmes andDutcher 1989, Gaffal et al 1991), in which cell fusion site is differentiated to specialized cell membrane "mating structure" (Goodenough and Weiss 1975). The mating structure of mt Ϫ gamete is located at the cell apex on the same side of the flagellar basal bodies as the eyespot (photoreceptive device), whereas the mt ϩ structure is located in the opposite position (Holmes andDutcher 1989, Gaffal et al 1991, see Fig.…”

“…1 for reference). Both positions are presumably defined by microtubular roots that extend from the basal bodies (Goodenough and Weiss 1978). According to the numbering system for basal bodies (no.…”

La fécondation chez une Ptéridophyte, leMarsilea vestitaLes régions spécialisées dans les processus de reconnaissance des gamètes

“…Un tel processus a déjà été envisagé pour un certain nombre de Ptéridophytes (Tourte, 1971 ;Duckett et Bell, 1972). Le rôle des flagelles dans la reconnaissance a particulièrement été étudié chez diverses espèces de Chamydomonas (Cavalier-Smith, 1975 ;Triemer et Brown, 1975 ;Goodenough et Weiss, 1975 ;Weiss et coll., 1977 ;Wiese et Wiese, 1978 ;Mesland et Van den Ende, 1979 ;Snell et Roseman, 1979). Cependant, le premier point de fusion concerne des zones différenciées du plasmalemme chez Chlamydomonas et Ulva lactuca (Melkonian, 1980) et toute généralisation peut paraître encore préma-turée.…”

Expression of Conventional and Unconventional Actins in Chlamydomonas reinhardtii upon Deflagellation and Sexual Adhesion

“…Various changes during the mating reaction are triggered by the adhesion between flagella of opposite mating types, which induces an increase in the intraflagellar cAMP concentration (6,18). When gametic cells were activated by addition of flagella isolated from gametes of opposite mating types, the level of actin mRNA increased in wild-type cells and the level of NAP mRNA increased in ida5 cells (Fig.…”