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Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
Mating Systems and Sexual Morphogenesis in Ascomycetes, Page 1 of 2
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This chapter provides an up-to-date review of complex cellular transition steps in Pezizomycotina, from the differentiation of reproductive cells to the development of the fructification. Initiation of sexual reproduction in the sordariomycete Arachniotus albicans, as it is for several ascomycetes, is clearly not dependent on a particular pH range. In many fungi, light is one of the prominent physical factors controlling sexual reproduction, either by stimulating or inhibiting the formation of reproductive structures. Although most Pezizomycotina can be classified as self-compatible or self-incompatible, some species present peculiar mating characteristics. There are several studies showing that pheromone and pheromone receptors are essential for the fusion of the trichogyne with the male cell in self-incompatible filamentous ascomycetes. Recent data have begun to shed light on the various molecular processes occurring during sexual reproduction in filamentous Ascomycetes. In addition to the well-known variety of body plan exhibited by Pezizomycotina fruiting bodies, sequencing has uncovered a large set of mating-type structures, all based on a common pattern. Indeed, it is now well established that MAT1-1-1 and MAT1-1-2 genes control fertilization by using pheromone/receptor genes in self-incompatible Pezizomycotina, but little is known about their roles after fertilization. The few indications that come from Podospora anserina, N. crassa, and C. heterostrophus indicate that they control the formation of biparental ascogenous hyphae and meiosis.
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Fruiting body plan evolution in filamentous ascomycetes. (A) Representative fruiting bodies of major evolutionary lineages. Lineages in light gray do not produce multicellular fructifications; those underlined produce mostly apothecia or apothecia-like (e.g., truffles and others) fructifications. Laboulbeniomycetes and Sordariomycetes produce mostly perithecia, Dothideomycetes produce mostly pseudothecia, and Eurotiomycetes produce mostly cleistothecia and gymnothecia. Abbreviations: Ni, Neolecta irregularis; Oc, Orbilia curvatispora; Aa, Aleuria aurantia; Ap, Ascodesmis porcina; Mc, Morchella conica; Ph, Peltigera horizontalis; Xpa, Xanthoria parietina; Lm, Leptosphaeria maculans; Ch, Cochliobolus horizontalis; An, Aspergillus nidulans; Gu, Geoglossum umbratile; Hc, Helotium citrinum; Ll, Leotia lumbrica; Ca, Chlorociboria aeruginascens; Pa, Podospora anserina; Xp, Xylaria polymorpha; Hh, Haematonectria haematococca; Cc, Cordiceps capitata. Aa, Ap, Mc, Hc, Hh, Ll, Pa, Hh, Xpa, Lm, Xp, Cc, Ca, and An photos are by P. Silar; images of Oc and Gu were kindly provided by J. L. Cheype, that of Ch by B. G. Turgeon, and that of Ni by Raymond Boyer. (B) Schematic representation of various Pezizomycotina fruiting bodies.
Life cycle and fruiting body development in Pezizomycotina. (A) Schematic representation of the major stages of the sexual cycle of a heterothallic pezizomycotina. The germinating ascospore (a) gives rise to a mycelium on which both male and female gametes differentiate(b). Fertilization occurs only between male and female gametes of opposite mating types (c). The trichogyne catches a microconidium (c, arrow), and the ascogonium develops into a perithecium(d). Inside the perithecium, dikaryotic ascogenous hyphae emerge from the plurinucleate dikaryotic cells. After one or two rounds of mitosis, the ascogenous hypha gives rise to a crozier (e). Karyogamy takes place in the upper cell (indicated by an asterisk) and is immediately followed by meiosis, a mitosis, and ascospore formation (f and g). (B) Schematic representation of various croziers (adapted from a drawing by M. Chadefaud [ Chadefaud, 1960a ]). All these structures derive from a dikaryotic hypha. The cell in which karyogamy takes place is indicated by an asterisk. (a through d) Tricellular crozier. (a and b) Simple crozier; (c) crozier with a lateral cell which does not fuse with the basal one; (d) prototypical crozier. The lateral cell will fuse with the basal one to give rise to another crozier cell; (e and f) bicellular crozier. The lateral cell does not exist any more.
Mating-type structure in Fungi. α1, genes encoding transcription factors with an α1 domain; PPF, genes encoding proteins with a PPF domain; HMG1 and HMG2, genes encoding transcription factors with an HMG domain (phylogenetically related genes are boxed); HOM1, genes encoding transcription factor with a TALE homeodomain (reviewed by Bürglin, 2005 ); HOM2, genes encoding a transcription factor with a typical homeodomain; other, genes encoding proteins with uncharacterized features. Mating-type structures from S. cerevisiae, Kluyveromyces lactis, Candida albicans, and Yarrowia lipolytica were compiled from works of Butler and others ( Butler et al., 2004 ; Butler, 2007 ). The mating-type structures of S. pombe, Ustilago maydis, and Phycomyces blakesleeanus were obtained from studies by Kelly et al. (1988) , Kahmann et al. (1995 ), and Idnurm et al. (2008) , respectively.
Amino acid alignment of the α1 domain of deduced MAT1-1-1 proteins of selected pezizomycotina with the MATα1 protein of S. cerevisiae: N. crassa (AAC37478), S. macrospora(CAA71623), P. anserina (CAA45519), C. globosum (EAQ89967), M. oryzae (strain 70-6) (BAC65087), C. parasitica (AAK83346), Diaporthe sp. W (BAE93750), Diaporthe sp. G (AB93756), G. fujikuroi (AAC71055), G. zeae (AAG42809), C. takaomontana (BAC67541), P. brassicae(CAA06844), A. fumigatus (AAX83122), A. nidulans (EAA63189), H. capsulatum (AB087596), C. heterostrophus (CAA48465), C. luttrellii (AAD33439), Stemphylium loti (AAR04470), and S. cerevisiae (P01365). Stars indicate the residues of the α1 proteins that may interact with MCM1 (adapted from Yuan et al., 1993 ). Fraction that must agree for shading is 0.7. Alignment was performed with Clustal version 2 ( Larkin et al., 2007 ), and shading was done with BOXSHADE version 3.21.
Amino acid alignment of the PPF domain of deduced MAT1-1-2 proteins of various pezizomycotina: N. crassa (AAC37477), S. macrospora (CAA71626), P. anserina (CAA52052), C.globosum MAT1-1 (EAQ89966), C. globosum MAT1-2 (EAQ91646), M. oryzae (strain 70-6) (BAC65088), C. parasitica (AAK83345), Diaporthe sp. W mat1 (BAE93749), Diaporthe sp. W mat2 (BAE93752), Diaporthe sp. G mat1 (BAE93755), Diaporthe sp. G mat2 (BAE93758), G. fujikuroi(AAC71054), F. oxysporum (( Yun et al., 2000 ), G. zeae (AAG42811), C. takaomontana (BAC67540), and C. purpurea (BAD72603). Fraction that must agree for shading is 0.7. Alignment was performed with Clustal version 2 ( Larkin et al., 2007 ), and shading was done with BOXSHADE version 3.21.
Amino acid alignment of the HMG domain of deduced MAT1-1-3 proteins of selected pezizomycotina. N. crassa (AAC37476), P. anserina (CAA52051), C. globosum(EAQ89965), M. oryzae (BAC65085), C. parasitica (AAK83344), Diaporthe sp. W mat1 (BAE93748), Diaporthe sp. W mat2 (BAE93751), Diaporthe sp. G mat1 (BAE93754), Diaporthe sp. G mat2 (BAE93757), G. fujikuroi (AAC71053), G. zeae (AAG42812), and P. brassicae(CAA06846). Fraction that must agree for shading is 0.7. Alignment was performed with Clustal version 2 ( Larkin et al., 2007 ), and shading was done with BOXSHADE version 3.21.
Amino acid alignment of the HMG domain of deduced MAT1-2-1 proteins of selected pezizomycotina: N. crassa (AAA33598), S. macrospora (CAA71624), P. anserina (CAA45520), C. globosum (EAQ91645), M. oryzae (BAC65090), C. parasitica (AAK83343), Diaporthe sp. W2 (BAE93753), Diaporthe sp. G2 (BAE93759), G. fujikuroi (AAC71056), G. zeae (AAG42810), C. takaomontana (BAC66503), C. eucalypti (AAF00498), P. brassicae (CAA06843), A. fumigatus (EAL92951), A. nidulans (AAQ07985), H. capsulatum (ABO87595), C. heterostrophus(CAA48464), C. luttrellii (AAD33439), Stemphylium loti (S. loti) (AAR04483), M. graminicola(AAL30836). Fraction that must agree for shading is 0.7. Alignment was performed with Clustal version 2 ( Larkin et al., 2007 ), and shading was done with BOXSHADE version 3.21.
Phylogenetic tree of deduced MAT1-1-3 and MAT1-2-1 proteins of selected pezizomycotina. The neighbor-joining tree for the HMG proteins encoded by mating types indicates that MAT1-1-3 and MAT1-2-1 transcription factors form two distinct families. For the accession numbers of proteins, see Fig. 6 and 7 . Phylogenetic analysis was conducted with MEGA 4 ( Tamura et al., 2007 ).
Structure of MAT locus in selected self-incompatible and asexual pezizomycotina. The phylogenic tree has been built upon previous studies by James et al. (2006) . Mating-type locus structures are not to scale.
Structure of MAT locus in self-compatible pezizomycotina. The phylogenic tree has been built upon previous studies by James et al. (2006) . Mating-type locus structures are not to scale.
Crozier formation in P. anserina mutant strains. (A) Wild-type-like ascus development. Dikaryotic hyphae emerge from a plurinucleate cell and form, after one or two divisions, the crozier cells in which the two nuclei divide. This coordinate mitosis gives rise, after septum formation, to three cells: an upper binucleated cell (long arrow) and two uninucleated cells, one lateral (arrowhead) and one basal (arrow). Karyogamy will take place in the upper cells, which contain two nuclei of opposite mating types. Nuclear fusion is followed by meiosis and elongation of this upper cell, now called an ascus (a through d). A young small ascus where karyogamy has just occurred (a) and a midprophase ascus (d) are visible. (B) In a cross between a mat mutant and the wild type, uni-nucleated croziers are formed (arrow); the arrowhead points to an enlarged cell that might evolve into an ascus. (C) Crozier phenotype of a cro1 X cro1 mutant cross. Most croziers are plurinucleated and often form giant croziers (arrow), in which nuclei divide synchronously, but no septa are formed. On the left of this large crozier is a smaller one (arrowhead), still containing an abnormally high number of nuclei but exhibiting a normal size (when compared with the wild-type crozier size in A). (D) Binucleated croziers issued from a ▵pex2 mutant cross. Differentiation of the dikaryotic cells is similar to that in the wild type; however, instead of undergoing karyogamy, the two nuclei isolated in the upper binucleated cell (long arrows) divide again to form another crozier cell. Lateral and basal cells are indicated by an arrowhead and an arrow, respectively. Scale bar, 5 μm. Light micrographs are a kind gift from D. Zickler (IGM; Université Paris Sud-11, France); rosettes of asci are stained by hematoxylin (see Zickler et al., 1995 , for details).
Cross talk between mycelium and perithecium envelope. The model is based on data obtained with P. anserina and N. crassa. After differentiation, the ascogonium sends a signal to neighboring hyphae to form a protective envelope. The signaling pathway containing PaNox1, IDC1, and the PaMpk1 module is necessary to mobilize these hyphae. Then, in the absence of fertilization, development stops. After fertilization, the same signaling pathway is necessary for the development of the fructification. PaNox1 and IDC1 act in the wall of the fructification, whereas the MAPK module acts in the surrounding hyphae. Signaling then results in the transfer of nutrients to the developing fructification. At later stages of development, a neck is formed under the control of the BEK-1 transcription factor and GPR-1 receptor.