Program Book - 27th Fungal Genetics Conference

PLENARY SESSION ABSTRACTSFriday, March 15 8:30 AM–12:00 NOONMerrill Hall and ChapelPlenary Session III: Sensing, Cell Biology and DevelopmentChair: Michelle MomanyThe illuminated Spitzenkörper of Neurospora crassa: tracking and tracing secretory vesicles. Meritxell Riquelme 1 , Eddy Sánchez-León 1 , Rosa Fajardo-Somera 1 , Erin L. Bredeweg 2 , Olga Callejas-Negrete 1 , Robert W. Roberson 3 , Salomon Bartnicki-García 1 , Michael Freitag 2 . 1) Dept Microbiology, Center forScientific Research and Higher Education of Ensenada CICESE, Ensenada, Baja California 22860, Mexico; 2) Center for Genome Research and Biocomputing,Department of Biochemistry and Biophysics, Oregon State University, Corvallis, OR 97331, U.S.A; 3) School of Life Sciences,Arizona State University,Tempe, AZ 85287, U.S.A.Tip growth in fungal hyphae is maintained by the vectorial traffic of secretory vesicles to the apex, where they accumulate at the Spitzenkörper (Spk),before fusing with the apical plasma membrane (PM) to provide the enzymatic machinery necessary for cell expansion. Confocal microscopy ofNeurospora crassa strains expressing fluorescently tagged proteins that are predicted to participate in cell wall synthesis revealed that the Spk vesiclescontain different enzymatic activities. Microvesicles (chitosomes) at the core of the Spk contained chitin synthases CHS-1, -3, -4, -5 and -6, whereasmacrovesicles at the outer layer carried glucan synthase (GS). The coordinated action of coats, tethers, motors, Rabs and SNAREs allows the multiplevesicular carriers and their cargoes to traffic between organelles and to be delivered at their final destinations. While it remains to be elucidated whatregulates the spatial stratification of the Spk, our most recent analyses show the differential co-localization of Rab GTPases YPT-1 and SEC-4 with microand macrovesicles, respectively; suggesting different biogenesis for these vesicles. Prior to v-SNARE and t-SNARE recognition and fusion with the PM,secretory vesicles are presumably tethered to their target acceptor membrane in a process mediated by the exocyst, an octameric complex. In N. crassa anintact exocyst complex is required for formation of the Spk and the maintenance of regular hyphal growth. Two distinct localization patterns of the exocystsubunits were observed at the hyphal tip. EXO-70 and EXO-84 accumulated at the frontal part of the Spk external layer, coinciding partially with themacrovesicular layer, whereas the exocyst components SEC-3, 5, 6, 8 and 15 formed a delimited crescent at the apical PM. This suggests the formation oftwo distinct exocyst subcomplexes that may unite during vesicle tethering in post-Spk traffic steps. Collectively our results prove the direct involvement ofthe Spk in cell wall synthesis and confirm that the region of exocyst-mediated vesicle fusion in the hyphal apex coincides with the exocytotic gradientpredicted by the Vesicle Supply Center (VSC) model for fungal morphogenesis, with a maximum at the pole and vanishing gradually in the subapex.Evolution of sexual reproduction: A view from the Fungal Kingdom. Joseph Heitman. Department of Molecular Genetics and Microbiology, DukeUniversity, heitm001@duke.edu.Sex is nearly universal in eukaryotes, and thought to have evolved once. Sex promotes genetic diversity and evolution, yet also confers costs. Bothmechanisms of sex determination and mechanics of sexual reproduction are remarkably diverse. Fungi are exceptional models to analyze these processes,and their study reveals surprising insight into both sex and its impact. We focus on how mating-type identity is specified and modes and roles of sexualreproduction in generating diversity. Many fungi have bipolar sexual cycles with two opposite mating types and a bi-allelic mating type locus. In theBasidiomycota many species have a more complex tetrapolar sexual cycle with two unlinked multi-allelic mating type loci, resulting in thousands of matingtypes and enhanced outcrossing but restricted inbreeding. Our studies reveal how transitions from ancestral tetrapolar to derived bipolar systems haveoccurred in pathogenic species embedded within saprobic sibling taxa. The tetrapolar-bipolar transition has occurred repeatedly in pathogens of plantsand animals, suggesting it might be selected during host adaptation. Pathogenic Cryptococcus species have taken this transition further to a unipolarsexual cycle. These species have global largely unisexual populations and reproduce via an unusual homothallic unipolar sexual cycle involving only onemating type (same-sex mating, unisexual reproduction). Like a-a opposite sex mating, a-a unisexual mating can admix parental diversity in the progeny.However, in other cases solo a-a unisex involves selfing of identical genomes with no genetic diversity to exchange. Why organisms engage in selfingchallenges conventional views on the roles of sex. We find unisex generates genetic diversity de novo, preserving well-adapted genomic configurationswhile generating more limited genetic diversity for selection to act upon. Discovery that other fungi and eukaryotic parasite pathogens also reproduceunisexually generalizes these findings, and suggests unisex may have evolved because it mitigates costs of sex. Studies of fungal sex and its evolution andimpact illustrate general principles by which diversity is generated and maintained with implications for saprobic and pathogenic microbes andmulticellular eukaryotes.Metabolic compensation of the Neurospora clock by a glucose-dependent feedback of the circadian repressor CSP1 on the core oscillator. GencerSancar, Cigdem Sancar, Francois Cesbron, Michael Brunner. Dpet Biochemistry, Univ Heidelberg, Heidelberg, Germany.CSP1 is a global morning specific transcription repressor of Neurospora that modulates expression of about 800 genes. Expression of CSP1 is stimulatedby glucose and under circadian control of the white-collar complex (WCC). In csp1 mutant strains the circadian period length decreases with increasingglucose concentrations due to increased expression of WC1. In contrast, in wild-type strains the period is compensated for changes in glucoseconcentration and WC1 levels are independent of glucose. CSP1 contributes to metabolic compensation of the circadian clock by glucose-dependentrepression of wc1 transcription, which counterbalances the glucose-dependent translation efficiency of wc1 RNA. Forced over-expression of CSP1 reducesof WC1 expression and results in dampening of the circadian clock. Many target genes of CSP1 are rhythmically expressed with an evening specific phasewhile target genes of the WCC are morning specific.Integration of the fungal cell cycle with growth and development. Meera Govindaraghavan, Kuo-Fang Shen, Stephen Osmani. Dept Molec Gen, OhioState Univ, Columbus, OH.A universally important aspect of growth and development is the integration of mitosis with cell division. This helps ensure that cells maintain theirnormal size, shape and nuclear number, which in the fungi can vary considerably. For example, the highly polarized mode of growth of the filamentousfungi is subject to complex developmental regulation yielding diverse cell types containing from one to dozens of nuclei. How fungi integrate theregulation of the developmental axis involving mitosis, cytokinesis, and morphogenesis to maintain their defined cellular shapes, with distinctive numbers27th Fungal Genetics Conference | 25