engleski

Population diversity of Cryphonectria parasitica in Croatia

Ascomycete fungus Cryphonectria parasitica, one of the worst pathogens of sweet chestnut (Castanea sativa) has been destroying chestnut trees in Europe for decades. Asian chestnut species are well adapted to the fungus, but European chestnut succumb to the infection easily. Mycelia enter tree bark through wounds and spread toward cambium, obstructing normal flow of nutrients and water through stem. As a result, the tree more or less rapidly dies. All European chestnut formations have been infected and the canker problem has become a festering wound in European forests. However, few years after the accidental introduction of C. parasitica in Europe, a phenomenon called hypovirulence was observed – some of the trees infected were slowly healing from the disease, and the other were showing much weaker symptoms of infection. It has been determined that for this hypovirulence phenomenon a small double-stranded RNA was responsible. Soon, it was revealed that this dsRNA is a virus which was named Cryphonectria hypovirus (CHV1). Hypovirus can be transmitted from one fungus to the other with the consequence of converting formerly virulent strain of fungus to the hypovirulent one. This conversion occurs between compatible strains of fungus. C. parasitica was introduced in Croatia in 1955., but only recently vegetative compatibility (vc) types of the fungus were determined. This is very important for genetic diversity studies and estimation of the incidence of fungal sexual reproduction which results in recombination and, as a consequence, in new vegetative compatibility types. A total of 18 different vegetative compatibility types were observed in Croatia, more than in other Balkan countries, but less than in Italy and Switzerland. Some of the vc-types observed (EU-1 and EU-2) are dominant in west, while EU-12 is dominant in the south of Europe. EU-12 was also observed only in eastern continental populations in Croatia, while EU-1 and EU-2 are dominant in western and coastal populations. Therefore, in respect to the occurrence of the main vc types, the C. parasitica populations in Croatia combine features of both north-western and the south-eastern European populations. Perithecia and both mating types of C. parasitica in approximately 1:1 ratio were found in all populations suggesting that sexual reproduction of the fungus is common in Croatia. Microsatellite loci of C. parasitica were also analyzed in population studies. Preliminary results on eight microsatellite loci revealed the existence of as much as 37 different haplotypes in Croatia, also much more than in other Balkan countries (Bulgaria, Greece, Macedonia and Romania). Diversity indexes were similar to those determined by vc-typing of fungus. GST estimates showed strong genetic differentiation between coastal and eastern continental populations, and some Croatian mid-continental populations showed mixture of coastal and eastern microsatellite characteristics. When ANOSIM was performed, eastern populations significantly differentiated from the coastal, but mid-Croatian populations showed no significant difference from either coastal or eastern continental populations, also a strong indication of mixing of C. parasitica populations from north-west and south-east in Croatia. Multilocus linkage disequilibrium index also supports the hypothesis that C. parasitica populations in Croatia predominantly reproduce sexually. These data show a potential threat of formation of new vc-types, thus making transmission of hypovirulence harder and natural biological control of disease more difficult then assumed. This problem deserves much attention especially in the coastal region of the country where the incidence of hypovirulence is low and human-mediated biocontrol is needed.

biological control; genetic diversity; population studies

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