Phylogenetic relationships of the genus Paramecium inferred from small subunit rRNA gene sequences

The genus Paramecium includes species that are well known and very common i n freshwater environments. Species of Paramecium are morphologically divide d into two distinct groups: the "bursaria" subgroup (foot-shaped) and the " aurelia" subgroup (cigar-shaped). Their placement within the class Oligohym enophorea has been supported by the analysis of the small subunit rRNA gene sequence of P. tetraurelia. To confirm the stability of this placement and to resolve relationships within the genus, small subunit rRNA gene sequenc es of P, bursaria, P. calkinsi, P. duboscqui, P, jenningsi, P, nephridiatum , P, primaurelia, and P, polycaryum were determined and aligned. Trees cons tructed using distance-matrix, maximum-likelihood, and maximum-parsimony me thods all depicted the genus as a monophyletic group, clustering with the o ther oligohymenophorean taxa, Within the Paramecium clade, P. bursaria bran ches basal to the other species, although the remaining species of the morp hologically defined "bursaria" subgroup do not group with P. bursaria, nor do they form a monophyletic subgroup. However, the species of the "aurelia" subgroup are closely related and strongly supported as a monophyletic grou p. (C) 2000 Academic Press.