Fungal Genetics and Biology 45 (2008) S15–S21 Contents lists available at ScienceDirect Fungal Genetics and Biology journal homepage: www.elsevier.com/locate/yfgbi Review Sex in smut fungi: Structure, function and evolution of mating-type complexes Guus Bakkeren a, Jörg Kämper b, Jan Schirawski c,* a Agriculture & Agri-Food Canada, Pacific Agri-Food Research Centre, Summerland, BC, Canada V0H 1Z0 b University of Karlsruhe, Institute for Applied Biosciences, Department of Genetics, 76187 Karlsruhe, Germany c Max-Planck-Institute for Terrestrial Microbiology, Karl-von-Frisch-Strasse, 35043 Marburg, Germany article info abstract Article history: Smut fungi are basidiomycete plant pathogens that pose a threat to many important cereal crops. In order Received 4 February 2008 to be pathogenic on plants, smut fungal cells of compatible mating-type need to fuse. Fusion and path- Accepted 11 April 2008 ogenicity are regulated by two loci, a and b, which harbor conserved genes. The functions of the encoded Available online 22 May 2008 mating-type complexes have been well-studied in the model fungus Ustilago maydis and will be briefly reviewed here. Sequence comparison of the mating-type loci of different smut and related fungi has Keywords: revealed that these loci differ substantially in structure. These structural differences point to an evolution Pheromone receptor from tetrapolar to bipolar mating behavior, which might have occurred several independent times during Homeodomain fungal speciation. Sporisorium reilianum Ustilago hordei Ó 2008 Elsevier Inc. All rights reserved. Malassezia globosa 1. Introduction vanced our understanding of the biology of smut fungi. The U. maydis genome sequence is publicly available through the Smut fungi are important cereal crop pathogens that depend on Broad Institute (http://www.broad.mit.edu/annotation/fungi/usti- sex to cause disease. There are approximately 1200 smut species lago_maydis/) and has been manually annotated by the Munich known that together can infect more than 4000 different plant spe- Institute for Protein Sequences (MIPS; http://MIPS.gsf.de/genre/ cies. The vast majority of plant species serving as hosts are from proj/ustilago/). Genome sequencing efforts are under way for the grass family (Graminaceae) and includes the world’s most S. reilianum (R. Kahmann and J. Schirawski, unpublished) and for important crops: corn, barley, wheat, oats, sorghum, sugarcane U. hordei (R. Kahmann, J. Schirawski and G. Bakkeren, unpublished) and forage grasses. Smut symptoms are characterized by the for- that will allow whole genome comparison of these related smut mation of fruiting structures containing black masses of teliospores species and is expected to lead to insights into the determinants that give the infected tissue a ‘‘sooty” or ‘‘smutted” appearance. of host selection and symptom formation. While most smut fungi develop sexual spores exclusively in the Common to all smut species investigated so far is their need to inflorescence and symptoms are visible only late in the infection undergo a successful mating reaction to form dikaryotic hyphae upon heading, Ustilago maydis, the corn smut pathogen, is a notable before being able to infect a host plant. On the other hand, the exception that can induce tumors, in which the fungal spores de- infectious dikaryon requires a host for proliferation and for the for- velop, on all above-ground parts of the plant. Since fungal biomass mation of sexual spores. Thus, sex and pathogenesis are intimately usually develops in inflorescences of infected plants, considerable intertwined in these species. yield reductions are suffered. Therefore, it is not surprising that For mating to occur, two haploid cells of different mating-type smuts have been the subject of intense study for the last century. need to recognize each other and fuse to form the infectious dikar- However, molecular insight on mating systems and their function yon. Mating is regulated by two loci, a and b, which harbor con- has so far only been obtained for three smut species, the corn served genes. At the a locus, these genes encode pheromones and smuts U. maydis and Sporisorium reilianum and the barley smut pheromone receptors while at the b locus two subunits of a hete- Ustilago hordei. All three species show a close phylogenetic rela- rodimeric transcription factor are encoded. In S. reilianum and tionship (Bakkeren et al., 2000) that is reflected by the ease with U. maydis that have a tetrapolar mating system, these genetic loci which molecular tools developed for one species can be transferred segregate independently, while in bipolar species, such as to the other species. The elucidation of the complete genome se- U. hordei, the a and b loci are linked and MAT segregates as one lo- quence of U. maydis (Kämper et al., 2006) has enormously ad- cus. Despite the similarity in gene function and sequence, in the three smuts that have been analyzed at a molecular level (U. may- * Corresponding author. Fax: +49 6421 178609. dis, U. hordei and S. reilianum), the mating-type loci differ substan- E-mail address: [email protected] (J. Schirawski). tially in locus structure. These differences will be reviewed to cover 1087-1845/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.fgb.2008.04.005 S16 G. Bakkeren et al. / Fungal Genetics and Biology 45 (2008) S15–S21 function, structure and evolution of mating complexes in compar- different mating-type alleles are known (Bakkeren and Kronstad, ison to a few more distantly related fungal basidiomycete species 1994), five different b alleles have been described in S. reilianum for which molecular data exist. (Schirawski et al., 2005) and at least 19 exist in U. maydis (J. Kämper and R. Kahmann, unpublished). Interestingly, both bE 2. Structure of mating-type loci and bW mating-type genes are present in the genome of the Usti- laginomycete Malassezia globosa (Table 1), a human pathogenic The b mating-type genes encode two subunits of a homeodo- fungus involved in dandruff disease, for which no sexual cycle is main (HD) transcription factor, consisting of an HD1 class and an known (Xu et al., 2007). Even the distantly related, opportunistic HD2-class protein. In general, the HD1 and HD2 proteins are not human pathogen, the Tremellomycete Cryptococcus neoformans related to each other in primary sequence; however, they share a (Table 1), carries HD1- and HD2-encoding genes within the mat- common domain organization. The N-terminal regions of the HD ing-type region (Loftus et al., 2005). For C. neoformans, two mating proteins contain the highest degree of variation when different al- types are known but in this species each mating type has retained leles are compared and are thus designated as variable regions, only one of the b mating-type genes, with the HD1 protein being while the C-terminal regions of the proteins, including the homeo- specific for the MATa mating type and the HD2 protein being pres- domains, are highly conserved (Gillissen et al., 1992; Kronstad and ent only in the MATa mating-type region (Fig. 1; Fraser et al., 2004; Leong, 1990; Schulz et al., 1990). The tetrapolar species U. maydis Lengeler et al., 2002). and S. reilianum as well as the bipolar species U. hordei possess In addition to the b mating-type complexes, smut fungi contain one divergently transcribed gene pair encoding the homeodomain genes necessary for cell–cell recognition located in the a mating- proteins bE (HD1) and bW (HD2; Fig. 1). For the b mating-type type loci, encoding pheromones and pheromone receptors. The genes of U. maydis, S. reilianum and the U. hordei MAT-1 locus, gene detailed structure of these loci has been determined for both a order, orientation, as well as the genomic context are conserved alleles of U. maydis, for all three a alleles of S. reilianum and (Fig. 1). The orientation of the divergently transcribed b gene pair for the MAT-1 allele of U. hordei (Fig. 1). For the MAT-2 allele of seems inverted in the U. hordei MAT-2 locus. In the absence of U. hordei only partial information is available (Fig. 1). additional sequence information for the MAT-2 locus of U. hordei, Both U. maydis and U. hordei have two alleles of an a mating sys- it cannot be assessed, whether this gene pair is part of an inversion tem with one pheromone receptor (pra) and one functional phero- covering a large genomic context. While from U. hordei two mone gene (mfa) per locus (Anderson et al., 1999; Bakkeren and Fig. 1. Genetic organization of the mating-type loci of selected basidiomycetes. Genes are indicated by arrows with the arrow denoting direction of transcription. Related genes are denoted by the same color and respective gene functions are explained in the lower part of the Figure. Ã indicates that the relative order and orientation of these genes has not been determined. In the tetrapolar species U. maydis and S. reilianum the a and b specific sequences reside on different chromosomes while they are linked by spacer regions (which are not drawn to scale and whose length is indicated) in the bipolar species U. hordei and C. neoformans, as well as in M. globosa. The black bars on top of the figure indicate the regions of the b locus, which covers the two homeodomain protein genes bE and bW, and the a locus (that expands to different length in the different loci, indicated by a broken line) from the lba gene to the rba gene. Sequence information was obtained from the following Accession Nos. AF043940, AM118080, AAC- P01000083, AACP01000013, AJ884588, AJ884583, AJ884590, AJ884585, AJ884589, AJ884584, U37796, M84182, AF184070, AF184069, Z18531, AAYY01000003, AF542530, and AF542531. G. Bakkeren et al. / Fungal Genetics and Biology 45 (2008) S15–S21 S17 Table 1 Taxonomic information on the basidiomycete fungi compared Class Species Mating system Reference Agaricomycotina Agaricomycetes Coprinopsis cinerea (syn.