To clarify phylogenetic relationships among ubiquinone 7 (Q7)-forming species of the genus
Candida, we analyzed the nearly complete sequences of 18S ribosomal RNA genes (18S rDNAs) from fifty strains (including 46 type strains) of
Candida species, and from 8 type strains of species/varieties of the genera
Issatchenkia,
Pichia and
Saturnispora. Q7-forming
Candida species were divided into three major groups (Group I, II, and III) and were phylogenetically distant from a group that includes the type species of the genus
Candida. Group I included four clusters with basal branches that were weakly supported. The first cluster comprised
C. vartiovaarae,
C. maritima,
C. utilis,
C. freyschussii,
C. odintsovae,
C. melinii,
C. quercuum,
Williopsis saturnus var.
saturnus, and
W. mucosa. The second cluster comprised
C. norvegica,
C. montana,
C. stellimalicola,
C. solani,
C. berthetii, and
C. dendrica. Williopsis pratensis,
W. californica,
Pichia opuntiae and 2 related species,
P. amethionina (two varieties), and
P. caribaea were also included in this cluster. The third cluster comprised
C. pelliculosa (anamorph of
P. anomala),
C. nitrativorans, and
C. silvicultrix. The fourth cluster comprised
C. wickerhamii and
C. peltata, which were placed in the
P. holstii-C. ernobii clade with Q8-containing species. Group II comprised
C. pignaliae,
C. nemodendra,
C. methanolovescens,
C. maris,
C. sonorensis,
C. pini,
C. llanquihuensis,
C. cariosilignicola,
C. ovalis,
C. succiphila (including its two synonyms),
C. methanosorbosa,
C. nitratophila,
C. nanaspora,
C. boidinii (including its two synonyms),
W. salicorniae, and
P. methanolica. Group III was composed of four clusters with strong bootstrap support. The first cluster comprised
C. valida (anamorph of
P. membranifaciens),
C. ethanolica,
C. pseudolambica,
C. citrea,
C. inconspicua,
C. norvegensis,
C. rugopelliculosa, and
C. lambica. Three species and two varieties of the genus
Issatchenkia were also included in this cluster. The second cluster comprised
C. diversa,
C. silvae, 4
Saturnispora species, and
P. besseyi. The third comprised
C. sorboxylosa, and the fourth comprised
C. vini. Based on this 18S rDNA sequence analysis, it is evident that Q7-forming
Candida species and the genera
Pichia and
Williopsis are polyphyletic. The genus
Issatchenkia is suggested to be congeneric with the genus
Pichia. The genus
Saturnispora is phylogenetically definable.
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