Macronuclear Actin copy number variations in single cells of different Pseudokeronopsis (Alveolata, Ciliophora) populations

Macronuclear chromosomes of ciliates, especially those of Spirotrichea, Armophorea and Phyllopharyngea, are extensively fragmented and their copy numbers vary significantly. A recent study suggested that parental RNA molecules regulate macronuclear copy number in offspring cells after conjugation. However, variations in patterns of macronuclear copy number during vegetative growth are not clear. Previous studies have reported macronuclear copy numbers of population averages, potentially masking individual variation. In the present investigation, we studied copy number variations among closely related species of Pseudokeronopsis and among individual cells during vegetative growth. We found that macronuclear copy numbers of Actin I, II in our Pseudokeronopsis populations are in the same range as in other spirotrichean species, but no close relationship is detected among morphologically related Pseudokeronopsis species. Copy numbers of three cells within each Pseudokeronopsis population range from 1.01 to 4.55 fold, suggesting that stochastic influences copy number during vegetative growth. Furthermore, the absence of a relationship between macronuclear copy numbers of Actin I and Actin II within Pseudokeronopsis is consistent with the fact that these genes are located on different gene-sized macronuclear chromosomes. Additionally, Actin II might have disappeared in P. carnea during evolution within the Actin gene family.