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Zvyšování konkurenceschopnosti studentů oboru botanika a učitelství biologie CZ.1.07/2.2.00/15.0316 © 2011 B. Mieslerová, A. Lebeda, (KB PřF UP v Olomouci) GENETICS AND GENETIC VARIABILITY OF FUNGI STRUCTURE AND ORGANIZATION OF THE FUNGAL GENOME The fungal genome has four components - nuclear DNA, mitochondrial DNA, plasmids and fungal virus genes NUCLEAR DNA Forming chromosomes – in different numbers and ploidy (the number of single sets of chromosomes) The eukaryotic plants and animals are basically diploid. The vegetative hyphae of the majority of fungi are haploid (all Zygomycotina, Ascomycotina and most of Deuteromycotina have a prolonged and dominant vegetative haplophase). In the Oomycota diploidy and polyploidy is common. Some yeasts are predominantly diploid. A few fungi can alternate between haploid and diploid somatic phases (Chytridiomycota) Part of life cycle of some fungi (Ascomycotina, Basidiomycotina) is dikaryotic – two compatible nuclei form a pair – dikaryon. It means plasmogamy is not immediatelly followed by karyogamy IMPORTANCE OF HAPLOID PHASE: Mutation arise are easy to induce and are immediatelly expressed. The natural selection are more rigorous and more immediate than in most diploid organisms. IMPORTANCE OF DIKARYOTIC PHASE: Dikaryon may have selective advantage over the diploid in terms of the establishment of dikaryotic mosaic. This involves the direct exchange of one nucleus of the pair between adjacent dikaryons after hyphal fusion to establish a new dikaryon of different genotype - this provides immediate variation in genotype. Life cycle of Podosphaera leucotricha Ascomycotina The nuclear genome size of fungi is very small in comparison with other eukaryotes (x 104 kbp – kilobase pairs). Haploid chromosome number in fungi and fungi-like organisms Group and species Chromosome number Slime moulds Dictyostelium discoideum 7 Physarum polycephalum 40 Ascomycotina Neurospora crassa 7 Aspergillus nidulans 8 Saccharomyces cerevisiae 16 Basidiomycotina Schizophyllum commune 6 Ustilago maydis 20 MITOCHONDRIAL DNA Mitochondria contain a small circular molecule of DNA. The size of mitochondrial genome is 19-121 kbp. All mitochondrial DNAs code for the same things: some components of the electron transport chain (ATPase subunits), some structural RNAs and mitochondrial transfer RNAs. Mitochondrial DNA PLASMIDS AND TRANSPORTABLE ELEMENTS Plasmids usually are closed-circular molecules of DNA with the ability to replicate autonomously in a cell. Plasmids or plasmid-like DNAs have been found in several fungi. It has no known function, but it has major practical applications in the construction of vectors for gene cloning in yeast. Most other plasmids of fungi are found in the mitochondria. Incubate these two pieces of DNA together VIRUSES AND VIRAL GENES Fungal viruses (virus-like particles) were discovered in some fungi, including representatives of all the major fungal groups Viruses consist of double-stranded RNA and a capsid composed of one major polypeptide The presence of viruses was not associated with any obvious disorder, so most fungal viruses seem to be symptomless. Fungal viruses OYSTER MUSHROOM • • oyster mushroom virus I oyster mushroom virus II Zdravá Healthy Infected Virózní Oyster mushroom on cultivated media Healthy Infected BUTTON MUSHROOM La France isometric virus • Mushroom baciliform virus • Vesicle virus Healthy Infected GENETIC VARIATION IN FUNGI Sources of Genetic Variability NON-SEXUAL VARIATION: MUTATION Main source of variability is mutation, which is spontaneous heritable changes in the genetic information of an organism (additions, deletions, substitutions, inversions..) The haploid organisms cannot accumulate mutations that are not of immediate value: they cannot store variation. Any mutation can lead to increase of the fitness or loss of fitness. Diploid organisms: mutations are often recessive to the wild type and so they are not immediatelly expressed, instead they accumulate and can be recombined in various ways during sexual crossing Selection and mutation pressure has a role on stability of mutations NON-SEXUAL VARIATION: HETEROKARYOSIS HETEROKARYOSIS The hyphae of closely related fungi can fuse with one another (anastomose) locally during normal assimilative growth, exchanging nuclei and thereby producing heterokaryons (mycelia containing genetically different nuclei). The heterokaryotic condition confers great flexibility on many conidial fungi, helping them to cope with different substrates and conditions. The degree of heterokaryosis in natural environments is largely unknown There is a major difference between dikaryon and heterokaryon: In the dikaryon the nuclei eventually fuse and undergo meiosis, as part of the sexual process. In heterokaryons, this is not usually so; there heterokaryon breaks down. Heterokaryosis can arise in two ways: 1/ When hyphae of any two strains fuse at points of contact (anastomosis) so that their nuclei are present in the common cytoplasm. 2/ When mutation occurs in one of the nuclei in a hypha and a mutated nucleus proliferates along with the wild-type of nuclei. In both cases the nuclei would need to proliferate in the apical cells to form a stable heterokaryon. NON-SEXUAL VARIATION: PARASEXUALITY PARASEXUALITY: alternation of sexual recombination without benefit of sex. Typical for some of Deuteromycotina The parasexual cycle has four stages: 1/ Fusion (anastomosis) of adjacent somatic hyphae, and exchange of nuclei, establishing a heterokaryon. 2/ Fusion of different nuclei in the vegetative hyphae, to form somatic diploids. 3/ Somatic recombination (mitotic crossing-over). Result is a creation of chromosomes that are hybrids of the parental chromosomes. 4/ Non-meiotic reduction of the altered nuclei via aneuploidy (loss of individual chromosomes) to the haploid condition. THE WHOLE PROCESS IS VERY RARE THIS IS RESULT OF THE RANDOM CHANGES This sequence of events is rare, happening in fewer than one conidium in a million, but the number of conidia produced by most conidial anamorphs is astronomical, so parasexuality is a practical means for producing genetic variation. Stages of the parasexual cycle are numbered as follows (1) Hyphal conjugation (plasmogamy). (2) Heterokaryosis. (3) Nuclear fusion (karyogamy). (4) Mitotic recombination and nondisjunction. (5) Haploidization and nuclear segregation leading to homokaryosis. SIGNIFICANCE OF PARASEXUALITY Significance in nature is largerly unknown It is not clear why Deuteromycotina abandoned a efficient sexual mechanism of genetic recombination in favour of a more random and seemingly less efficient process. The answer may be that the parasexual events can occur at any time during normal somatic growth and with no preconditions like those needed for the production of sexual stages. Each of this events is relatively rare and they do not constitute a regulated cycle like sexual cycle involving meiosis COMPARING SEXUALITY AND PARASEXUALITY Sexual reproduction Parasexuality Highly organized, often precisely timed process, which is genetically programmed Involves a sequence of uncommon events which seems to operate by chance, rather than by design Nuclear fusion is often mediated by genetic factors, expressed as 'mating types,' happens in highly specific structures, and often involves many pairs of compatible nuclei Nuclear fusion is an isolated event, not mediated by mating-type factors, not found in specialized structures, and involving only individual nuclei During meiosis, crossing-over probably takes place in every homologous pair of chromosomes, and multiple crossovers are common During somatic recombination, crossing-over commonly involves only one or a few chromosomes, and never happens as often as during meiosis In meiosis, segregation happens in a highly organized way during two specialized nuclear divisions Somatic haploidization probably occurs as a result of successive chromosome losses from an aneuploid nucleus (2n-1) over several mitotic divisions until the stable haploid is reached SEXUAL VARIATION Sex is the major mechanism for producing recombinants genetic The pairing of parental chromosomes in meiosis leads to multiple crossing over (chiasma formation). The independent assortment of homologous chromosomes (segregation) from the two parents will mean that the individual daughter haploid nuclei might have different chromosomes from different parent. HOMOTHALLIC - describes fungi in which a single strain can undertake sexual reproduction; self-compatible HETEROTHALLIC - describes fungi in which two genetically distinct but compatible mycelia must meet before sexual reproduction can take place CROSSING-OVER Each chromosome is composed of two parallel strands or chromatids. In the simplest crossover, shown in (b), a break occurs at the same place in one of the 'white' chromatids and one of the 'black' chromatids. The ends rejoin, but in a new arrangement: the part of the 'black' chromatid carrying the dark ascospore gene is now joined to part of a 'white' chromatid and vice versa. Crossing-over is one of the main mechanisms involved in providing the pool of variability on which natural selection acts. Crossing-over: dark-spored 'wild-type' strain is crossed with a pale-spored mutant. MATING-TYPES OF BASIDIOMYCOTINA If the alleles occur at a single locus (A1,A2), the offspring of a single basidioma will be of TWO DIFFERENT MATING TYPES; the mating system of the fungus is called BIPOLAR (most smuts, some Gasteromycetes, Coprinus comatus) If the alleles are at two loci (A1, A2, B1, B2), offspring of a single basidioma will be of FOUR MATING TYPES; the mating system is called TETRAPOLAR (most Aphyllophorales, Agaricales and Gasteromycetes) A1B1; A1B2; A2B1; A2 B 2 GENETICS AND PLANT PATHOLOGY PLANT NON-HOST RESISTANCE – plant could not by any means support the propagation of an otherwise pathogenic microbe. (unsuitable conditions for survival and propagation of most of fungi and bacteria). Plants are completely immune to infection. HOST-RESISTANCE - plant could support the propagation of pathogenic microbe VERTICAL RESISTANCE – RACE-SPECIFIC RESISTANCE- can be conferred by a single resistance gene, which is often controlled by „yes“ or „no“ response. Plants showed very high levels of resistance to a particular physiological race of pathogen. Often showed HR (hypersensitive response) HORIZONTAL RESISTANCE – RACE-NONSPECIFIC RESISTANCE is usually polygenic resulting from cumulative effect of many genes. Equal resistance is shown to all pathogen races; but these resistance may not totally protect plants from infection Horizontal and vertical resistance of potato to Phytophthora infestans PATHOGEN PATHOGENICITY – term referring to the potentiality of particular strains of a microorganism to induce disease in certain individuals of plant species. Genetic determinants of pathogenicity are pathogenicity genes. VIRULENCE - is a qualitative trait referring to the degree of pathogenicity – contolled by single gene AGGRESSIVENESS - is a quantitative trait referring to the degree of pathogenicity controlled by polygenes Whithin a fungal species some individuals may be pathogenic on particular host species. Such fungi are often grouped as FORMAE SPECIALIS Whithin such FORMAE SPECIALIS those individuals which are specifically pathogenic on a particular host variety or cultivar are grouped in to RACES. Term RACE is used to designate genetically distinct mating groups or to describe different genotypes within a species. RACES orr FORMAE SPECIALIS can be recognized by testing each fungal strain against standard set of host varieties. Interactions of races of Cladosporium fulvum with 3 tomato varieties Tomato with resistance gene Cladosporium races 0 1 2 3 1+2 1+3 1 R S R R S S 2 R R S R S R 3 R R R S R S Examples of L. serriola responses to B. lactucae (race BL18) Resistant Moderately susceptible Susceptible GENE-FOR-GENE HYPOTHESIS (Flor, 1971) This suggests that the evolutionary paths of host and pathogen have been so closely linked for so long, that for every gene in the host that is capable of mutating to give resistance, there is a corresponding gene in the pathogen which can mutate to overcome that resistance. Breeding experiments usually shows host resistance to be dominant (R ) and susceptibility recessive (r ). The genes for avirulence is hence classifiable as dominant (Av) and for virulence as recessive (av). Three combinations: R-a, r-A and r-a give rise to compatible reactions and infections are successful. One combination, R-A results in a incompatible reaction and no infection occurs (resistant) It is though that a resistance gene acts by controlling the production of protein receptor molecules at the plant cell surface capable of interacting with glycoprotein molecules (elicitors) located at the fungal surface. Basic interaction of genes avirulence/virulence and resistance/susceptibility Product of resistance gene is RECEPTOR on surface of plant leaf Product of avirulence gene is ELICITOR on surface of fungus