Sexual reproduction Ascomycota

ascus of hypocrea virens 8 two-celled ascospores

sexual reproduction in ascomycota leads formation of ascus, structure defines fungal group , distinguishes other fungal phyla. ascus tube-shaped vessel, meiosporangium, contains sexual spores produced meiosis , called ascospores.

apart few exceptions, such candida albicans, ascomycetes haploid, i.e., contain 1 set of chromosomes per nucleus. during sexual reproduction there diploid phase, commonly short, , meiosis restores haploid state. sexual cycle of 1 well-studied representative species of ascomycota described in greater detail in neurospora crassa.

formation of sexual spores

the sexual part of life cycle commences when 2 hyphal structures mate. in case of homothallic species, mating enabled between hyphae of same fungal clone, whereas in heterothallic species, 2 hyphae must originate fungal clones differ genetically, i.e., of different mating type. mating types typical of fungi , correspond sexes in plants , animals; 1 species may have more 2 mating types, resulting in complex vegetative incompatibility systems. adaptive function of mating type discussed in neurospora crassa.

gametangia sexual structures formed hyphae, , generative cells. fine hypha, called trichogyne emerges 1 gametangium, ascogonium, , merges gametangium (the antheridium) of other fungal isolate. nuclei in antheridium migrate ascogonium, , plasmogamy—the mixing of cytoplasm—occurs. unlike in animals , plants, plasmogamy not followed merging of nuclei (called karyogamy). instead, nuclei 2 hyphae form pairs, initiating dikaryophase of sexual cycle, during time pairs of nuclei synchronously divide. fusion of paired nuclei leads mixing of genetic material , recombination , followed meiosis. similar sexual cycle present in blue green algae (rhodophyta). discarded hypothesis held second karyogamy event occurred in ascogonium prior ascogeny, resulting in tetraploid nucleus divided 4 diploid nuclei meiosis , 8 haploid nuclei supposed process called brachymeiosis, hypothesis disproven in 1950s.

unitunicate-inoperculate asci of hypomyces chrysospermus

from fertilized ascogonium, dinucleate hyphae emerge in each cell contains 2 nuclei. these hyphae called ascogenous or fertile hyphae. supported vegetative mycelium containing uni– (or mono–) nucleate hyphae, sterile. mycelium containing both sterile , fertile hyphae may grow fruiting body, ascocarp, may contain millions of fertile hyphae.

the sexual structures formed in fruiting layer of ascocarp, hymenium. @ 1 end of ascogenous hyphae, characteristic u-shaped hooks develop, curve opposite growth direction of hyphae. 2 nuclei contained in apical part of each hypha divide in such way threads of mitotic spindles run parallel, creating 2 pairs of genetically different nuclei. 1 daughter nucleus migrates close hook, while other daughter nucleus locates basal part of hypha. formation of 2 parallel cross-walls divides hypha 3 sections: 1 @ hook 1 nucleus, 1 @ basal of original hypha contains 1 nucleus, , 1 separates u-shaped part, contains other 2 nuclei.

diagram of apothecium (the typical cup-like reproductive structure of ascomycetes) showing sterile tissues developing , mature asci.

fusion of nuclei (karyogamy) takes place in u-shaped cells in hymenium, , results in formation of diploid zygote. zygote grows ascus, elongated tube-shaped or cylinder-shaped capsule. meiosis gives rise 4 haploid nuclei, followed further mitotic division results in 8 nuclei in each ascus. nuclei along cytoplasma become enclosed within membranes , cell wall give rise ascospores aligned inside ascus peas in pod. (for general description of meiosis , adaptive function see meiosis , bernstein , bernstein).

upon opening of ascus, ascospores may dispersed wind, while in cases spores forcibly ejected form ascus; species have evolved spore cannons, can eject ascospores 30 cm. away. when spores reach suitable substrate, germinate, form new hyphae, restarts fungal life cycle.

the form of ascus important classification , divided 4 basic types: unitunicate-operculate, unitunicate-inoperculate, bitunicate, or prototunicate. see article on asci further details.