Comparison of sequence differences in a variable 23S rRNA domain among sets of cryptic species of ciliated protozoa

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 in freshwater environments. Species of Paramecium are morphologically divided into two distinct groups: the "bursaria" subgroup (foot-shaped) and the "aurelia" subgroup (cigar-shaped). Their placement within the class Oligohymenophorea 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 sequences of P. bursaria, P. calkinsi, P. duboscqui, P. jenningsi, P. nephridiatum, P. primaurelia, and P. polycaryum were determined and aligned. Trees constructed using distance-matrix, maximum-likelihood, and maximum-parsimony methods all depicted the genus as a monophyletic group, clustering with the other oligohymenophorean taxa. Within the Paramecium clade, P. bursaria branches basal to the other species, although the remaining species of the morphologically 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 group.

Laboratory and evolutionary history of Tetrahymena thermophila

An account is given of the early efforts to domesticate tetrahymenas as laboratory instruments for genetics. The rationale for developing a new organismic technology was the comparative leverage provided by a eukaryotic microorganism at a large evolutionary distance from both prokaryotic microbes and multicellular organisms. The tetrahymenine ciliates were considered more favorable materials than paramecia because of their ability to grow on simple media, though in fact their simpler nutritional needs have never been fully exploited. The first task was to sort the large set of phenotypically similar but evolutionarily and molecularly diverse ciliates referred to at the time as T. pyriformis. Then a species amenable to genetic manipulation was identified and its culture and cytogenetics were brought under control. Fortunately, the very first breeding system investigated--that in the species now called T. thermophila--has proved to be suitable for a wide range of studies. A large factor in the program's success was its use of the foundation previously established by studies on paramecia. However, serious unforeseen difficulties were encountered on the way to "domestication." These included inbreeding deterioration associated with their outbreeding life-style and germinal deterioration (mutational erosion) in the unexpressed micronuclear genome after long maintenance in vegetative culture. Cryogenic preservation was an important means of escaping these organismic limitations, and somatic (macronuclear) assortment has proved a valuable supplement to meiotic recombination.