Molecular identification of yeasts
The chromosomal DNA was purified from cultures of
each yeast isolate and the D1/D2 region of 26S rDNA
and the ITS1-5.8S- ITS2 (hereafter designated the ITS
region for simplicity) regions of the rDNA were amplified
by PCR. The amplicons obtained were purified
from gels and sequenced on both strands. Isolates
showing 100% identity in both rDNA sequences were
grouped and their DNA sequences were submitted to
GenBank under the accession numbers listed in
Table 1. Species identification was performed by comparison
with the GenBank references, using as criterion
the Blast-hits with ≤ 0.5% difference with the
query [14]. In 84% of the isolates the closest Blast-hits
obtained for both rDNA sequences were coincident.
When this was not the case, the D1/D2 results were
used for identification because they yielded higher
identity percentages than did the ITS (see Additional
file 1). 76% of the isolates could be identified to species
level by this molecular analysis. 22 species belonging
to12 genera were identified, of which 80 and 20%
were Basidiomycetes and Ascomycetes, respectively.
The genera containing the highest number of species
were Mrakia (5 species) and Cryptococcus (4 species).
However, the species Sporidiobolus salmonicolor was
the most abundant, being identified in 24 isolates from
13 different sampling sites. Mrakia gelida was the only
yeast species present in both water and soil samples.
Of the three isolates identified as Leuconeurospora sp.,
two of them (T11Cd2 and T27Cd2) possessed identical
D1/D2 and ITS sequences, both of which differed from
the third (T17Cd1) by 0.7%. However, the macromorphological
characteristics of the three isolates, including
pigmentation, differed markedly under identical
culture conditions (see Additional file 2). Because of
these discrepancies, the molecular and morphological
analyses were repeated several times, but the results
were highly consistent. The carbon source assimilation
pattern also differed between the isolates, as will be
discussed later.