Plasma Membrane Fusion Is Specifically Impacted by the Molecular Structure of Membrane Sterols During Vegetative Development of Neurospora crassa

Cell-to-cell fusion is crucial for the development and propagation of most eukaryotic organisms. Despite this importance, the molecular mechanisms mediating this process are only poorly understood in biological systems. In particular, the step of plasma membrane merger and the contributing proteins and physicochemical factors remain mostly unknown. Earlier studies provided the first evidence of a role of membrane sterols in cell-to-cell fusion. By characterizing different ergosterol biosynthesis mutants of the fungus Neurospora crassa, which accumulate different ergosterol precursors, we show that the structure of the sterol ring system specifically affects plasma membrane merger during the fusion of vegetative spore germlings. Genetic analyses pinpoint this defect to an event prior to engagement of the fusion machinery. Strikingly, this effect is not observed during sexual fusion, suggesting that the specific sterol precursors do not generally block membrane merger, but rather impair subcellular processes exclusively mediating fusion of vegetative cells. At a colony-wide level, the altered structure of the sterol ring system affects a subset of differentiation processes, including vegetative sporulation and steps before and after fertilization during sexual propagation. Together, these observations corroborate the notion that the accumulation of particular sterol precursors has very specific effects on defined cellular processes rather than nonspecifically disturbing membrane functioning. Given the phenotypic similarities of the ergosterol biosynthesis mutants of N. crassa during vegetative fusion and of Saccharomyces cerevisiae cells undergoing mating, our data support the idea that yeast mating is evolutionarily and mechanistically more closely related to vegetative than sexual fusion of filamentous fungi.

GENETIC ANALYSIS OF MATING LOCUS LINKED MUTATIONS IN CHLAMYDOMONAS REINHARDII

ABSTRACT The mating-type (mt) locus of Chlamydomonas reinhardii has been analyzed using four mutant strains (imp-1, imp-10, imp-11 and imp-12). All have been shown, or are shown here, to carry mutations linked to either the plus (mt +) or the minus (mt -) locus, and their behavior in complementation tests has allowed us to define several distinct functions for each locus. Specifically, we propose that the mt + locus contains the following genes or regulatory elements: a locus designated sfu, which is necessary for sexual fusion between gametes; a locus designated upp (uniparental plus), which controls aspects of chloroplast gene inheritance and perhaps also zygote maturation; and a locus designated sad, which functions in sexual adhesion. The mt - locus also contains a sad locus as well as a gene or regulatory element designated mid, which is necessary for the minus dominance in mt +/mt - diploids.

A GENE, ALCA, AFFECTING THE LIFE CYCLE FORM EXPRESSED IN PHYSARUM POLYCEPHALUM

ABSTRACT The usual sequence of forms in the Physarum polycephalum life cycle is plasmodium–spore–amoeba–plasmodium. So-called "amoebaless life cycle" or alc mutants of this Myxomycete undergo a simplified plasmodium–spore–plasmodium life cycle. We have analyzed three independently isolated alc mutants and found in each case that the failure of the spores to give rise to amoebae is due to a recessive Mendelian allele. The three mutations are tightly linked to one another and belong to a single complementation group, alcA. The mutations are pleiotropic, not only interfering with the establishment of the amoebal form at spore germination, but also affecting the phenotype of alc amoebae, which occasionally arise from alc spores. The alc amoebae (1) grow more slowly than wild type, particularly at elevated temperatures; (2) tend to transform directly into plasmodia, circumventing the sexual fusion of amoebae that usually accompanies plasmodium formation; and (3) form plasmodia by the sexual mechanism less efficiently than wild-type amoebae. The various effects of an alc mutation seem to derive from mutation of a single gene, since reversion for one effect is always accompanied by reversion for the other effects. Moreover, a mutation, aptA1, that blocks direct plasmodium formation by alcA amoebae, also increases their growth rate to near normal. The manner of plasmodium formation in alcA strains differs significantly from that in another class of mutants, the gad mutants. Unlike gad amoebae, alcA amoebae need not reach a critical density in order to differentiate directly into plasmodia and do not respond to the extracellular inducer of differentiation. In addition, alcA differentiation is not prevented by a mutation, npfA1, that blocks direct differentiation by most gad amoebae.

Generic Circumscription in the Family Ectocarpaceae (Phaeophyceae)

The family Ectocarpaceae currently contains the simplest, and probably also the most primitive, members of the Phaeophyceae. The thallus form of every species is a heterotrichous filament with little or no structural modification. Some species may possess quite extensive cortication of axial filaments by adventitious rhizoids but advanced pseudoparenchyma is never formed. Periclinal cell divisions within filaments have also been reported, particularly in the course of sporangium development, but true parenchyma is unknown. Growth of the erect system of filaments is diffuse, for although intercalary sites of relatively active cell division may sometimes be observed, these lack the permanence of position and sustained activity of true intercalary meristems. The growth of the horizontal branch system is probably mainly apical, but is also too diffuse to be considered meristematic. Sporangia and gametangia are formed from vegetative cells and occur either as intercalary structures within filaments or as modified branch apices. Sexual fusion of gametes may be isogamous or anisogamous.