Abstract
Recently we applied randomly amplified polymorphic DNA (RAPD) fingerprinting to detect clonal variability among individual cercariae within daughter sporocysts and rediae of 10 digenean trematodes (Platyhelminthes: Trematoda). The most variable RAPD patterns were obtained for Schistosomatidae representative-avian schistosome Trichobilharzia szidati. In this work, 50 polymorphic DNA fragments of approximately 300-1500 bp from RAPD patterns of individual T. szidati cercariae were cloned and sequenced. As a result genomic DNA sequences (total length of approximately 41,000 bp) revealing clonal variability in T. szidati cercariae were obtained and analyzed. The analysis indicated that these sequences contained tandem, inverted and dispersed repeats as well as regions homological to retroelements of two human parasites, Schistosoma mansoni and S. japonicum. Tandem and inverted repeats constituted 8.9% and 22.1% respectively, while the percentage of dispersed repeats was 21.0%. The average content of these components was 41.7% with the average AT content being 59.0%. About 40% of sequences included regions ranging in length from 96 to 1005 bp which displayed amino acid homology with open reading frame pol products of S. mansoni and S. japonicum retroelements: non-long terminal repeat retrotransposons (nLTRs, 76%), long terminal repeat retrotransposons (LTRs, 14%), and Penelope-like elements (PLEs, 10%). Most of these regions (86.4%) contained frameshifts, gaps, and stop-codons. The largest portion of them was homological to nLTRs of the RTE clade (67%). The number of sequences homologous to the members of CR1 lineage was 7 times smaller (9%). Homology with LTRs of Gypsy/Ty3 and BEL clades was revealed in 5% and 9% of cases respectively. We assume that the repetitive elements including retroelement-like sequences described in the current study may serve as the source of clonal variability detected previously in T. szidati and other digenean trematodes. Such genome regions rapidly accumulate mutations and thus may play an important functional role in the life history of the species.
Highlights
The vast majority of eukaryotic species reproduces bisexually, yet approximately one out of every 1000 multicellular eukaryotic taxa is unisexual or asexual
The analysis indicated that these sequences contained tandem, inverted and dispersed repeats as well as regions homological to retroelements of two human parasites, Schistosoma mansoni and S. japonicum
To detect clonal variability among individual cercariae within sporocysts of T. szidati we selected three random primers (P29, OPA09, OPA10) which previously determined the largest number of polymorphic markers in cercariae randomly amplified polymorphic DNA (RAPD) profiles [15]
Summary
The vast majority of eukaryotic species reproduces bisexually, yet approximately one out of every 1000 multicellular eukaryotic taxa is unisexual (parthenogenetic) or asexual. There are some stages in digenean life cycle (parthenitae) reproducing via diploid parthenogenesis—mother sporocyst and either daughter sporocysts or rediae [2,3]. Numerous free-swimming larvae (cercariae) are formed in daughter sporocysts or rediae after undergoing parthenogenetic reproduction. Since parthenitae are the result of diploid parthenogenesis with only one parent involved, all cercariae forming within sporocyst or redia might have been expected to represent a group of genetically identical individuals—the clone. Diploid parthenogenesis can be viewed as a simple cell division. Acquirement of such type of reproduction is an essential adaptation developed by trematodes which allowed them to be evolutionary successful [2,3]
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