ABSTRACT
Guignardia citricarpa is an ascomycete of extreme importance for the brazilian citrus
culture, since it´s the causal agent of Citrus Black Spot. Some isolates of this species
are infected with a double stranded RNA virus (dsRNA), which causes unknown
phenotypical symptons, yet. Seven isolates from this species previously identified as
bearing dsRNA, were studied. These isolates were obtained from 1996 to 2008, one
from the state of Paraná, five from the state of São Paulo and one from South Africa.
Different dsRNA purification protocols were investigated and the best dsRNA yield
was made with the use of acid phenol (pH 4,7). One of the ways to analyze the
influence of dsRNA is through isogenic strains, which differ from one another by the
presence or absence of this genetic material. Two cure and one transmission
protocol were used for the obtainment of isogenic strains. Regarding the cure
experiment, cycloheximide was added to the medium where mycelial plugs were
placed, as described in the literature. This method wasn´t efficent for G. citricarpa
because the fungal growth was completely inhibited, even when 0,5 µg/mL of the
compound was used. The other cure protocol consisted of growth of hyphal tip
sections at 35 ºC, instead of 28 ºC, which is a stressing condition. From the seven
isolates submitted to this treatment, only three managed to grow at these conditions.
From these, only two produced eleven sectors which were investigated regarding the
presence of dsRNA. In nine, no dsRNA bands were observed in agarosis gel after
electrophoresis. At this moment they were considered cured from the dsRNA. For the
transmission of dsRNA, it was necessary to select G. citricarpa isolates with some
characteristics that would allow their differentiation from other strains, asides from the
presence/absence of dsRNA. Isolates were investigated regarding their RAPD
banding pattern, however, they were all the same. It was opted to use a
morphological characteristic, the presence or absence of a yellow halo in oatmeal
medium. All isolates with dsRNA are positive for this characteristic, and twelve
isolates with no dsRNA and negative for the halo were chosen. After hyphal
compatibility was assessed on these isolates, by pairing a dsRNA bearing isolate
with a dsRNA-free one, transmission experiments were performed. Twenty seven
colonies that didn´t present the yellow halo in oatmeal medium, all of which came
from conidia from the contact zone between isolates, were investigated for the
presence of dsRNA and seven of them (28,92%) received this molecule. Isogenic
strains were then tested for colony morphology and it was possible to verify that
dsRNA bearing strains had a significant smaller size after fifteen days of growth.
Statistical analysis corroborated this trait. Aiming to sequence this viral genome, the
cDNA production protocol was optimized. This molecule was yielded and in the next
phases of this research, should be sequenced. This sequence will allow completing
the phylogenetic analysis of viruses from the Totiviridae family performed in this
study. The respective fungal hosts of these viruses were also phylogenetically
grouped by their ITS1-5,8S-ITS2 region sequences, in order to identify a co-evolutive
trait among these viruses and their hosts. However, the analysis of this region didn´t
provide evidences of this relation.
KEYWORDS: Guignardia citricarpa, double stranded RNA, isogeny, cure,
transmission