Abstract

Ascomyceteous fungi reproduce by asexual and/or sexual manners. In their life cycle, asexual reproduction through mycelial elongation, sclerotial formation, sporal (conidia and/or chlamydospores) formation, or bud cell production is often predominant. Asexual reproduction theoretically generates clonal progenies. When the conditions are appropriate, they may switch their cycle to sexual reproduction. Sexual reproduction in ascomycetes is represented by two manners, the self-incompatible, or heterothallic, manner and the self-compatible, or homothallic, manner. Self-incompatible is the standard manner in ascomycetous sexual reproduction. In the self-incompatible manner, individuals displaying different mating type can mate, but individuals of the identical mating type cannot mate with each other. Mating type of each individual is determined by single genomic locus (MAT1) that contains one or the other of the two forms so called idiomorphs. The cells carrying different mating types recognize each other and the haploid cells fuse. Nuclear fusion, bivalent formation, possible chromosomal cross-over, meiosis, and often two times mitoses occur sequentially, and usually 8 haploid progeny cells so called ascospores are formed. In self-compatible species, the cells are heterokaryons or carry both alleles of MAT1 in the genome, and the cells showing opposite mating types mate with each other by performing all steps of the self-incompatible manner. In ascomycetes, there are species lacking sexual reproduction. Previously, these species were grouped in ‘Fungi imperfecti’, or Deuteromycotina, together with the asexual members of basidiomycetes. However, at present they are understood to be the members of ascomycetes and are called mitosporic ascomycetes, asexual ascomycetes, or imperfect ascomycetes. We can easily find asexual species among phytopathogenic and fermentative ascomycetes. Fusarium oxysporum, Alternaria alternata, Verticillium dahliae, Aspergillus oryzae are examples. If we could cross the isolates in these asexual species, analyses on phytopathogenic and fermentative activities by genetic strategy could be achieved. Then now we are studying to answer the question ‘why are these asexual species asexual?”. At first, we proposed four hypotheses about the causes of their asexuality, 1. they lack MAT1 locus and/or MAT1 genes, 2. even if they have MAT1, function of the MAT1 locus and/or genes are abortive, 3. signal transduction cascades that work downstream of MAT1 are incomplete, 4. they cannot find mating partners in their communities. To study the hypotheses 1 and 2, we compared the structures and functions of MAT1 between asexual and sexual ascomyceteous species. F. oxysporum is an asexual relative of Gibberella fujikuroi (anamorph, F. moniliforme). Asexual A. alernata is rather close to Cochliobolus heterostrophus (anamorph, Biporalis maydis). Isolates of Magnaporthe oryzae (anamorph, Pyricularia oryzae) obtained in Japan are asexual, however, we can find isolates possessing sexuality from some restricted areas such as the Yunnan Province of the People’s Republic of China or from Shan, Kachin States of Union of Myanmar, northeastern and northern regions of Thailand, and several states of India (Mekwatanakarn et al 1999; Dayakar et al 2000; Hayashi and Kaku 2004). From all these asexual fungi we have identified the MAT1 loci, and demonstrated that the genes at the MAT1 locus were expressed (transcripted) similarly to their sexual relatives, respectively (Arie et al. 2000; Yun et al. 2000). Moreover, MAT1-products from asexual A. alternata were functional in C. heterostrophus by heterologous expression (Arie et al. 2000). These results have denied the hypotheses 1 and 2. We have cloned genes encoding pheromone receptors (pre1, the homolog of STE3 of Saccharomyces cerevisiae; pre2, the homolog of STE2 of S. cerevisiae), G-protein β-subunit (gpb1), and MAP kinase (mpk1) from G. fujikuroi. Disruptants of gpb1 or mpk1 in G. fujikuroi showed female sterility, respectively. These suggested that lack or disorder of signal cascade through GPB1 and/or MPK1 could be the cause of asexuality. However, presence and transcription of gpd1 and mpk1 in F. oxysporum equilibrate with those in G. fujikuroi were confirmed. Further investigation on the hypothesis 3 is still under way. Three evolutionary groups (A1-A3) of the tomato wilt pathogen, F. oxysporum f. sp. lycopersici were found among a worldwide sample of isolates based on phylogenetic analysis of the ribosomal DNA intergenic spacer region. Each group consisted of isolates carrying a single mating type (Kawabe et al. 2005). Concretely, group A1 and A2 contained only MAT1-1 isolates and group A3 contained only MAT1-2 isolates. Isolates within each group would then be prevented from crossing within each group. Isolates in the groups A1 and A2, all carrying MAT1-1, were expected to cross with the isolates in group A3, carrying MAT1-2, however, successful crosses were not observed in pairings of any A1 or A2 isolates with any A3 isolates. These results suggested the possibility that the groups have become mating populations and isolates in each group could not find mating partners any more. References Arie T, Kaneko I, Yoshida T, Noguchi M, Nomura Y, Yamaguchi I (2000) Mating type genes from asexual phytopathogenic ascomycetes, Fusarium oxysporum and Alternaria alternata. Mol Plant-Microbe Interact 13:1330-1339 Dayakar BV, Narayanan NN, Gnanamanickam SS (2000) Cross-compatibility and distribution of mating type alleles of the rice blast fungus Magnaporthe grisea in India. Plant Dis 84:700-704 Debuchy R, Turgeon BG (2006) Mating-type structurer, evolution, and fumction in euascomycetes. In The Mycota I. Growth, differentation and sexuality (Kues/Fisher eds.) Springer-Verlag, Berlin Heidelberg, pp. 293-323 Hayashi N, Kaku H (2004) Sexual compatibility and pathogenic race of rice blast fungus in the east region in Myanmar. Jpn J Phytopathol 70:34 (abstract in Japanese) Meckwatanakarn P, Kostitrarana W, Phromraksa T, Zeigler RS (1999) Sexually fertile Magnaporthe grisea rice pathogens in Thailand. Plant Dis 83:939-943 Yun S-H, Arie T, Kaneko I, Yoder OC, Turgeon BG (2000) Molecular organization of mating type loci in heterothallic, homothallic and asexual Gibberella/Fusarium species. Fungal Genet Biol 31:7-20