MATING TYPES IN THE CILIATE DILEPTUS ANSER. INHERITANCE AND GENETIC DETERMINATION
A. L. Yudin, Z. I. Uspenskaya
Institute of Cytology RAS, St. Petersburg;
e-mail: alyudin@mail.ru
Hybridological analysis of mating types (MTs) has been first made for the lower ciliate Dileptus anser.
Clones of an initially unknown genotype belonging to three MTs (MT I, MT II and MT III), characteristic of D.
anser, were isolated from natural reservoirs and further used for crosses. In one group crosses, synclonal
inheritance and typical Mendelian behaviour of the character were observed over sexual generations of ciliates.
The results suggest that MTs in D. anser may be directly controlled by a single mat locus with three
alleles showing peck-order dominance (mat1 > mat2 > mat3). In other words,
cells with mat1/mat1, mat1/mat2 and
mat1/mat3 genotypes belong to MT I, those with mat2/mat2
and mat2/mat3, and the mat3/mat3 belong to MT II and MT
III, respectively. Sexually mature exconjugant clones stably retain their MTs corresponding to their genotypes on
vegetative reproduction. The progeny of other group crosses showed various deviations from typical Mendelian
behaviour of the character. In some cases, standard Mendelian ratios were more or less violated. Most typical was
instability of differentiation for MT in maturing exconjugant clones. Shortly after their maturation, the majority
of clones change their MT, rather frequently more than once, although the finally established MT is stably inherited
afterwards, during vegetative reproduction. When unstable, exconjugant clones can successively express two or even
three MTs characteristic of this species, including MTs that should not have been expected on the basis of parental
genotypes available in a given cross. It looks likely that the mat locus in D. anser is complex and
multipotential; it is inherited as a whole providing for expression of any MT characteristic of the species
(in this respect bearing similarity with Tetrahymena thermophila). Other mechanisms, epigenetic in particular
(Nanney, 1958), determine the final expression of one of the three MT potentialities by a given exconjugant clone.
Stable, persistent functioning of these mechanisms ensures a stable differentiation for MT and Mendelian behaviour
of the character in sexual generations and in crosses. Any disturbances in differentiation control may trigger MT
instability in maturing exconjugant clones and violation of regular Mendelian behaviour.
Back
Contents
Main
