Mycoviruses: extrachromosomal particles of the fungi. A novel approach in Mycology

Abstract

Mycoviruses or virus-like particles (VLPs) are cytoplasmic factors, spherical or isometric virus-likenucleoprolein particles. which cootain double stranded ribonucleic acid (dsRNA) and exhibit diametersranging from 25 to 40 nm. Most fungal viruses possess genom ell composed of one to three segments ofdsRNA that are se parately encapsidated in isanetric parti cl~. The VLPs are serologically and electrophoreticallyheterogeneous,replicate by duplication and in parallel with the host RNA, DNA and proteinsynthesis mediatrd by an associated RNA polymerasa activity. It has been demostrated that they arepolent inducers of interferon in animals. Viral infe<1ion occurs widely in anamorphic fungi, but withfew exceptions does the host-parasite interaction cuhninate in an altered phenotype of the fungus.Nevertheless, "France disease", the most serious infectious disorder of the common cultivated mush-1'()()0] and the virulence attenuation of the chesnut blight fungus are mediated by mycoviruses and thefungal strain harboring virus exhibits a diverse array or cha.racteristics, tenned bipovirulence-associatedtraits, which distinguish it fran an isogenic virus-free strain. At present, limitcd evidences are availableon the direct influence of viruses on fungal host metabolism and their biological significance.Mycoviruses are latent in fungi ccl1s. probably as a resu lt of gradual coevolution process. Their viralnature can be judged only by their biophysical characterist ics since they do not present the typicalvirulence of true viruses. Mycoviruses or virus-like particles (VLPs) are cytoplasmic factors, spherical or isometric virus-like
nucleoprolein particles. which cootain double stranded ribonucleic acid (dsRNA) and exhibit diameters
ranging from 25 to 40 nm. Most fungal viruses possess genom ell composed of one to three segments of
dsRNA that are se parately encapsidated in isanetric parti cl~. The VLPs are serologically and electrophoreticallyheterogeneous,
replicate by duplication and in parallel with the host RNA, DNA and protein
synthesis mediatrd by an associated RNA polymerasa activity. It has been demostrated that they are
polent inducers of interferon in animals. Viral infe<1ion occurs widely in anamorphic fungi, but with
few exceptions does the host-parasite interaction cuhninate in an altered phenotype of the fungus.
Nevertheless, "France disease", the most serious infectious disorder of the common cultivated mush-
1'()()0] and the virulence attenuation of the chesnut blight fungus are mediated by mycoviruses and the
fungal strain harboring virus exhibits a diverse array or cha.racteristics, tenned bipovirulence-associated
traits, which distinguish it fran an isogenic virus-free strain. At present, limitcd evidences are available
on the direct influence of viruses on fungal host metabolism and their biological significance.
Mycoviruses are latent in fungi ccl1s. probably as a resu lt of gradual coevolution process. Their viral
nature can be judged only by their biophysical characterist ics since they do not present the typical
virulence of true viruses.Tumoroos fonnatioo oo branches of old or young trees and stems of Pu$ea amuicana Mill. and oo
grafted nunety plants has been obse!Ved in the s"'tes of Mexico and Veracruz for mf"y yeoJrs. The
tumors vary in shapes and sizes and may reach up to 8 an in dian~eter. Affected twigs die. Freshly
grafted plants wtder deseased avocado shade trees developed tumors up to 50%. In the wtauended criollo
orchards the infeaions reached 80%, but in attended orchards with Fuene and Hass varieties infectioos
were low or absent. Collected tumours presented black, I 0-15 J1IIl pycnidia on its cracked and varied
surface. From these pycnidia, hyaline, ellipsoid microconidia of 4 x t.S J1m size were isolated and grown
on potato-dextrose-agar medium. After six to seven months, symptoms of !he disease were reproduced
in the greenhouse and Koch's postulates were completed, and the fungus was identified as Sphaeropsis
tumefaciuu Hedges, probably a sl"'cial form on avocado. This fungus was described on lime, producing
C<Jnidia of larger size, but also microconidia o( the same size fowtd on avocado. As far as known, this
is the first report of its kind on avocado in Mexico and in the world. It is suggested that young nursery
t~ees should not be placed under old affeaed trees with tumors and that adequate spraying with copper
compowtds and sanitation measures be obse!Ved in orchards and nurseriesVisual inspection of 500 and 250 g subsamples of wheat grain taken from I kg samples, randomly
collected from vehicles transporting wheat from commercial fields to reception centers in Cd. Obregon,
showed coosistency in their proportion of infected grains with respect to 1 kg. Analysis of 100 g tends
to overestimate the number of samples with low levels of infected grains.Production of fruit bodiesofVo/vari~lla volvocea in a glass chamber, I m loog by 0.6 m wide and 0.85
m high is presented. This is a modification of the proposed ~sign by Li (1984). The factors light,
aeration. temperature and ambient a! humidity were cootrolled. A biological efficiency c:i 14.18±3.46%
was obtained.--