Bioscience/5. Molecular biology

Candidate of Agriculture Beyshova I.S.

Postgraduate student of natural sciences

Yesmagambetova E.T.

Kostanay State University named after A.Baytursynov,

Republic of Kazakhstan

Intraspecific variability and differentiation of Septoria fungi isolates by cultural and morphological properties on nutrient medium

Septoria fungi grow well and sporulate on artificial nutrient media. This opportunity is used for species identification and study of their cultural and morphological features. N.A.Naumov writes: "There is no single biological method for studying fungi that would be more significant in its accuracy, efficiency and accessibility than the method of axenic cultures.

Artificial cultivation is an additional mean to differentiate species which cannot always be distinguished by symptoms of lesion and morphology. When differentiating the species of Septoria authors usually pay attention to the appearance of colonies, nature and rate of growth of the mycelium, number of pycnidia formed and rate of their formation, ability to develop microconidium and sexual stage in the axenic culture. These criteria, for example, allowed to reveal clear differences between S.nodorum and S.avenae f.sp.triticea, which are sometimes difficult to distinguish on the basis of spores size because of transitional forms. Due to the great diversity of cultural and morphological (CM) signs the differences between the isolates of one septoria species are often evident.

According to various information, S.nodorum forms colonies in culture from white to dark color with aerial mycelium. It is able to quickly form pycnidia in many nutrient media. Being a heterothallic species S.nodorum does not form ascigerous stage in monospore culture. There are data on formation of fungus microspores on Czapek-Dox medium V-8.

Some cultures of S.nodorum have sparse mycelium and many pycnidia; others have dense mycelium and few pycnidia. In accordance with the said above Kurpinskiy and his co-authors identified three major culture and morphological type of fungus: 1 - pycnidial 2 - mycelium type with pycnidia; 3 - mycelium. High level of differentiation can be observed in cultures S.nodorum on Czapek - Dox V-8 medium; the formers had raised colonies from white to olive-green color, while the latters had more spread pink colonies.

By culture features S.tritci is significantly different from the preceding species. Mainly it forms yeast-like colonies of skin color consisting of small number of thin-wall hyphae secondary spores. Then a dark stroma-like mycelium forms on them as a dense mass. Pycnidia S.tritica are difficult to obtain on nutrient media, they form individually or in dense groups and they are not always detectable. Some isolates of the fungus form microspores which are free or inside pycnidia.

According to Parello and others monospore isolates S.tritici are classified into: 1 - mycelium; 2 - stromapycnidial; 3 - powdery white; 4 - yeast-like; 5 - mixed. A.A.Sanina (1991), in turn, identified three morphotypes of this type of fungus (yeast-like, mixed and mycelium) and distinguished 10 phenotypes that differ in colony structure and color.

According to the descriptions available S.avenae f.sp.triticea more often form colonies from pink to light-gray color with a dense mycelium. They include mycelium culture without pycnidia, cultures forming pycnidia under the mycelium and cultures forming pycnidia and pseudothecia simultaneously.

This species is homothallic and easily forms pseudothecia on many nutrient media. Microconidial stage of this fungus was not observed in the culture under any temperature and humidity conditions.

Cultural signs of fungi may serve as taxonomic ones to a certain extent if their features are stable under certain conditions, i.e. all monospore colonies after several reinoculations will have the same cultural characteristics. However, in practice, the monotony of morphotypes is rarely observed since the most fungi are characterized by high variability. Thus, according to Rafti and his co-authors, the posterity of S.nodorum monospore isolates had no cultures with identical CM features observable. According to other sources the stability of the morphotypes was observed only after 4-5 reinoculations or after 3-7 reinoculations starting from microconidial fruiting. Sometimes the first and subsequent generations had colonies with other type sector besides the parent sector. Furthermore, genetically homogeneous culture can convert into a heterogeneous population when stored for a long time in test tubes. Thus, according to Zh.Zh. Kuzhantayeva's observations the cultures after a year of storage were more variable than after three month storage. The species with rare sexual fruiting on nutrient medium are considered to have such variability due to mutation.

There is evidence of S.nodorum mutants using UV-radiation and chemical mutagens. Other causes of Septoria species variability, according to researchers, can be heterokaryosis and parasexual recombination.

Thus, the study of various authors on the differentiation of septoria species and isolates on nutrient medium indicates the possibility of applying the method of axenic cultures for studying the intraspecific structure of pathogen. According to the information available Septoria fungi are characterized by high variability which genetic causes are poorly studied today. Intraspecific structure of the causative agents has no accepted classification yet, especially for species S.nodorum and S.avenae f.sp.triticea.

Reference list

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2 Nicolas Feau, Jerry E. Weiland, Glen R. Stanosz, Louis Bernier, Specific and sensitive PCR-based detection of Septoria musiva, S. populicola and S. populi the causes of leaf spot and stem canker on poplars/Mycological Research, Volume 109, Issue 9, September 2005, Pages 1015-1028

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4 Fraaije B. A. – Hollomon, D. W. 1997. Novel DNA diagnostic technology in plant disease control using Septoria tritici as a model. In: Project report no. 245, IACR-Long Ashton Research Station, Bristol 1997, 27 p.