In hypotrich ciliates, the entire silent chromosomal genome of the germinal nucleus (micronucleus) undergoes extensive DNA rearrangements that, during the development of the somatic nucleus (macronucleus) at the beginning a new cell life cycle, eventually result in the production of linear DNA molecules. These molecules represent functional genes, each one consisting of a central coding region flanked by two shorter regions, which apparently lack canonical elements for regulation of replication and transcription. These are amplified to thousands of copies in the "adult" macronucleus of the vegetative cell. We defined the extent of this amplification for allelic codominant genes which, in the macronucleus of Euplotes raikovi, encode polypeptide cell recognition factors (pheromones). This amplification was shown to be allele-specific. The copy numbers of genes coding for pheromones Er-1, Er-2, and Er-10 were determined to be 2.5 -2.9 x lo4, 0.9 -1.2
x lo4, 1.6 -1.85 x 1 O4 respectively, and these numbers did not appreciably vary during the vegetative cell proliferation. This differential amplification of pheromone genes was (i) independent of whether two genes coexisted in the same heterozygous cell or were separated in the corresponding homozygotes, and (ii) directly correlated with quantitative variations in mRNA synthesis and pheromone secretion. O n the basis of these results, it is suggested that a mechanism of gene-specific amplification may be used by hypotrich ciliates to modulate gene expression.