Signalling Development in the model Fungus Podospora
anserina
Philippe Silar
Université Paris Diderot Paris 7 & Université Paris Sud 11
Fungi are found in nearly all biotopes present on earth. Their great success is due
in part to their ability to undergo developmental programs that allow their spores
to germinate when appropriate, their mycelium to invade efficiently their growth
substrate through the differentiation of dedicated cells (e.g., appressorium and
appressorium-like) and to disperse thanks to the production of multicellular
fruiting bodies. We study these programs in the model ascomycete Podospora
anserina. We previously evidenced a role at all stages of development of the ROS
generating NADPH oxidases. We more recently showed that the PaMpk1 and
PaMpk2 MAP kinases also regulate development, while the PaMpk3 MAP kinase
does not. We also identified through a classical genetic approach many mutants
affected for germination, development of the appressorium-like cells and
maturation of fruiting bodies. Often, these mutants are affected at more than one
stage of the life cycle. I will present how a coherent model that accounts for the
maturation of P. anserina fruiting bodies can be obtained through a combination
of genetic and grafting experiments, the same methods that have been applied to
study development in the other multicellular eucaryotes, i.e. animals and plants.
Unlocking the fungal natural products treasure chest
Nancy Keller
University of Wisconsin, Madison, Wisconsin, USA
Sequence of fungal genomes has revealed that these microorganisms have a large
number of secondary metabolite (SM) pathways, likely a reflection of a chemical
arsenal important in niche securement. Most of these SMs and their biosynthesis
pathways are currently unknown despite considerable efforts to induce
production in the laboratory. However, recent insights into SM synthesis through
discovery of global SM regulatory complexes and a linkage of chromatin
modifications with SM cluster activity has dramatically changed the scope of
what is possible in fungal SM studies. Using Aspergilli as a model system, the
mechanism of Velvet complex regulation of SM synthesis, the impact of
chromatin remodeling on SM expression and, ultimately, the consequences of
manipulations of these regulators on fungal biology and technology will be
discussed.
Regulation of Ribosomal RNA Gene Copy Number and Its
Role in Modulating Genome Integrity in Yeast
Takehiko Kobayashi
Division of Cytogenetics, National Institute of Genetics / The Graduate University for
Advanced Studies, SOKENDAI, Mishima, Shizuoka 411-8540, Japan
The genes encoding ribosomal RNA are the most abundant genes in the
eukaryotic genome. They are encoded in tandem repetitive clusters, in some
cases totaling hundreds of copies. Due to their repetitive structure and highly
active transcription, the ribosomal RNA gene repeats are some of the most fragile
sites in the chromosome (1). A unique gene amplification system compensates for
loss of copies, thus maintaining copy number, albeit with some fluctuations.
These unusual properties of ribosomal RNA gene repeats affect cellular functions
such as senescence (2). Moreover, we recently found that the repeat number
determines sensitivity of the cell to DNA damage (3).
In my talk, I would like to introduce a new aspect of the ribosomal RNA gene
repeat as a center of maintenance of genome integrity and discuss its contribution
to evolution (4).
References:
1, T. Kobayashi (2006) Genes Genet Syst 81, 155-161.
2, A.R.D. Ganley, S. Ide, K. Saka, T. Kobayashi (2009) Mol Cell 35, 683-693
3, S. Ide, T. Miyazaki, H. Maki, T. Kobayashi (2010) Science 327, 693–696
4, T. Kobayashi (2008) BioEssay 30, 267-272.
Evolution of Morphogenetic Regulatory Systems in Fungi
Steven D. Harris
University of Nebraksa, Lincoln, Nebraska, USA
Fungal cells display a diverse array of shapes and sizes. The availability of
annotated genome sequences that span the fungal kingdom has revealed that the
morphogenetic machinery involved in localized cell surface expansion and cell
wall deposition is largely conserved across the fungi. Instead, it seems likely that
regulatory modules that specify where and when the machinery is deployed are
the primary determinants of cell shape. The yeast bud site selection system is one
of the best understood examples of such a module in fungi. To determine the
extent to which the function of this module is conserved, we have characterized
homologues of the axial bud site markers Bud3, Bud4, and Axl2 in the
filamentous fungus Aspergillus nidulans. Our results iimplicate these proteins in
the regulation of septum formation during hyphal growth and conidiophore
development. Notably, our characterization of Axl2 suggests the existence of a
phialide-specific morphogenetic program that underlies the formation of asexual
conidiospores. Our results also provide a novel perspective on how the reduction
of morphological complexity (i.e., the loss of hyphal growth and the ability to
form fruiting bodies in yeasts) may have impacted the evolution of regulatory
modules such as the bud site selection system.
Six ways to organize a yeast MAT locus
Ken Wolfe
Smurfit Institute of Genetics, Trinity College Dublin, Ireland
Mating-type switching is an evolutionary innovation that allows a haploid yeast
cell to become diploid, even when no mating partner is available. Being diploid is
advantageous because diploid cells grow faster and are more resistant to DNA
damage. Switching occurs by a programmed DNA rearrangement event in which
the cell deliberately makes a double-strand break at one particular site in its
genome (the MAT locus) and then repairs it by ectopic gene conversion using a
different part of the genome (HML or HMR) as the template, thereby changing the
sequence of the MAT locus from MATa to MATalpha or vice-versa.
The switching process places unique evolutionary constraints on the MAT, HML
and HMR loci. The primary constraint is the need to maintain three identical
copies of each of the two regions (Z and X, ~500 bp each) that enable the MAT
locus to initiate and terminate recombination with HML or HMR. Between
closely-related pairs of species such as S. cerevisiae and S. bayanus, the sequences
of Z and X are absolutely invariant, making them most conserved regions in the
genome. The triplicated structure of Z and X seems to prevent them from
diverging between species by gradual nucleotide substitution. However, the
constraint is for triplication per se and not for triplication of any particular
sequence. Something must be triplicated but it doesn't matter what. When we
compare MAT loci among species in the Saccharomyces/Kluyveromyces clade, we
find that the sequences of the Z and X regions have been replaced continually
during yeast evolution. The MAT locus has been an evolutionary hotspot for
deletions, causing erosion of the chromosomal regions flanking it on each side.
Consequently, the MAT chromosome is getting progressively shorter. We suggest
that the high deletion rate flanking MAT is the result of recovery from errors that
occasionally happen during mating-type switching. The deletion process was
greatly accelerated by the introduction of redundancy into the genome by wholegenome duplication.