Active chi-like sequences are present in the ITS1 region of polyembryonic adult Collyriclum faba trematodes encysted in pairs.

Collyriclum faba (Plagiochiida: Collyriclidae) adults occur in pairs within subcutaneous cysts. Here, we tested the extensive C. faba infrapopulation for five DNA loci known to display variability among Central European C. faba individuals. The infrapopulation tested shared 100% similarity in four of the five mitochondrial and nuclear DNA loci tested. Contrariwise, the internal transcribed spacer 1 (ITS1) loci in all but one individual differed from each other. We found only 0.0-1.5 base substitutions per 1,000 sites within the cysts, while we found 0.7-9.0 substitutions between the cysts of the single host and 3.0-9.0 substitutions when comparing C. faba individuals isolated from different host individuals. We observed the most of the ITS1 variability within 48 bp repetitive sequences featured by the chi-like sequence 5'-GCTTGTCTGCC-3' at their beginning. Similarly to the extensive C. faba infrapopulation examined, we determined the presence of highly variable number of repetitive sequences within the ITS1 locus of C. faba isolated from multiple host species and from various geographic locations. While similar variability was observed earlier in mutually unrelated specimens of several Schistosomatidae and Microphallidae species, here, we for the first time document it among multiple individuals of a single infracommunity possessing single mitochondrial haplotype. Lower ITS1 evolutionary divergence rates observed between individuals within the cysts when compared to those between the cysts suggest that the recombination occurs at multiple stages of the life cycle. We propose DNA recombination involving chi-like sequences to serve as a general feature shared by multiple families of digenetic trematodes to increase genetic diversity of their polyembryonic populations infecting their definitive hosts.