Evidence for Micronuclear Function During Vegetative Growth and Reproduction of the Ciliate, Tetrahymena pyrijormis

An amicronucleate clone of Tetrahymena pyrijormis has been found among the asexual progeny of irradiated cells of strain EU 6000 (variety 6, mating type I). Log-phase cells of this clone, designated EU 6525, have a mean generation time (6.0 hr) longer than that of the micronucleate strain, EU 6000 (2.9 hr). Further irradiation studies of strain EU 6000 indicate that the recovery of viable amicronucleate populations is rare although many amicronucleate cells are found among surviving progeny.¹ Attempts to introduce micronuclei into amicronucleate cells of strain EU 6525 by conjugation have been made. Micronucleate lines are obtained from amicronu create pair members only in low frequency. These results, considered together with those of other workers, suggest that some change in the state of the cell, additional to the physical loss (or gain) of the micronucleus, must occur before viable amicronucleate clones can be obtained from micronucleate cells, or before amicronucleate cells can produce viable micronucleate lineages. An alteration in mean generation time may be a reflection of this change, or it may simply be a direct consequence of micronuclear removal. The results further imply that the ciliate micronucleus unquestionably contributes information to the cell during asexual growth and reproduction.     

THE RELATIONSHIP BETWEEN ENVIRONMENTAL VARIABLES AND THE DENSITY OF THE MUD SNAIL HYDROBIA TOTTENI IN A NOVA SCOTIA SALT MARSH

The relationship between environmental variables and the densityof Hydrobia totteni was examined in a restricted area of LawrencetownMarsh, Nova Scotia. Densities averaged 50/m² in Spartina alterniflora,9800/m² in Zostera marina, and 19,000/m² in sand. Of 8 environmentalfactors considered, depth and sediment grain size were the mostimportant in explaining variations in snail density. Since thesefactors together explained only 42.0% of the total variationno single physical factor controls the density of Hydrobia ina restricted area, though salinity does control densities overa wide area. Salinity and temperature tolerances and the abilityof the snails to select appropriate sediment grain sizes weredetermined experimentally and related to the distribution ofthe snails in the marsh. ^*Present address: Western Australian Museum, Francis Street,Perth, Western Australia, 6000 (Received 20 June 1976;