Mating type differentiation in tetrahymena thermophila: Characterization of the delayed refeeding effect and its implications concerning intranuclear coordination

Abstract

Mating type determination in Tetrahymena thermophila involves developmentally programmed, heritable alterations of the macronucleus, localized to the rntd locus. This determination can be predictably controlled by the environmental conditions during macronuclear development, eg, temperature and time of refeeding. In this article we have further characterized the effects of delayed refeeding on mating type determination, as revealed by the frequency of mating types among the progeny of a cross. Our results show that 1) the magnitude of this starvation effect decreases with temperature of conjugation and becomes undetectable at 18°C; 2) starvation during the interval 14 to 22 hr (after conjugation is induced at 30°C) is a necessary and sufficient condition for the induction of starvation effects; 3) relative mating type frequencies vary monotonically with nutrient concentration present during this critical period; and 4) sister macronuclei, developing under starvation conditions in the same cytoplasm, differentiate majority mating types characteristic of early or late refeeding; sister macronuclei show no apparent correlation with each other. On the basis of our observations on early and late refed cells, we propose that the composition of the newly developed macronucleus is the outcome of two key events: 1) mating type determination at the rntd locus and 2) differential molecular cloning of generally one or two autonomously replicating fragments (ARFs) of the macronuclear DNA bearing the rntd locus.

A small family of elements with long inverted repeats is located near sites of developmentally regulated DNA rearrangement in Tetrahymena thermophila.

“…In order for them to evolve, this negative selective pressure must be counterbalanced by positive selection conferred either by the transposon itself or by the cellular process which produced and/or maintains them. The selective advantage of DNA rearrangement in the ciliates is not known, but it has been suggested that micronucleus-specific sequences act as negative regulatory sequences in the micronuclear genome of T. thermophila (15,37,57) or that variable DNA rearrangement might provide for genetic diversity (29,43,44) as it does for the mammalian immunoglobulin genes (50) and the trypanosome surface antigens (22).…”

Genetic and Environmental Factors Affecting Mating Type Frequency in Natural Isolates of Tetrahymena thermophila

“…Because each new macronucleus (karyonide) is independently determined for mating type, mating type in T. thermophila is said to be karyonidal; macronuclei descended from each karyonide express the same mating type. Environmental factors, particularly temperature and nutritional state, influence the choice of mating type during macronuclear development (Nanney, Meyer, and Chen 1977; Nanney, Meyer, and Portnoy 1980; Orias and Baum 1984, 1985; Portnoy and Nanney 1980). The result is that the different mating types, designated I through VII, appear at different frequencies depending on the genotype and the environment.…”

Amitosis as a strategy of cell division - Insight from the proliferation of Tetrahymena thermophila macronucleus

“…Also notice that regardless of the value of κ, there is no head-start effect when ω = 1 since all existing chromosomes have an equal probability of being selected. Aside from the extreme cases, various degrees of head-start mechanisms 30,33,39 can be achieved by some combinations of κ and ω. In general, the smaller the κ and larger the ω, the stronger the head-start becomes.…”

“…At each of the pre-defined number of cell divisions, a sample of cells was taken from the clonal population, each of which was then cultivated separately to maturity to determine if it is pure for some mts. It should be noted that similar experiments focusing on either very early or late stages of growth of a clonal population were extensively conducted decades ago 32,[38][39][40] . A detailed diagram on the experimental procedure is given in the Methods section.…”

“…Decades of empirical observations show that when mature, the proportion of pure clones derived from a single karyonide is more than 50% or higher. A series of attempts have been made to explain the phenomenon which inevitably involved further specification and hypotheses about the binary fission process 30,38,39 . The latest incarnation of thought is that the phenomenon is in essence due to a combination of head-start replications with intranuclear coordination 30 .…”