The distribution of actin filaments in all developmental stages of the tapeworm Diphyllobothrium dendriticum was studied. It is the first investigation of the placement of microfilaments during the development of a flatworm, and the results show that actin filaments, in all developmental stages, can be found in the subtegument and the flame cells. Muscle fibers possibly corresponding to the longitudinal, transversal, and dorsoventral muscles of the adult and plerocercoid were already detected in the procercoid. Concerning the adult worm, a new set of longitudinal fibers in the peripheral parts of the adult proglottid was found. The ducts of the protonephridial system and the vitellarias were seen to be surrounded by longitudinally oriented actin filaments, while the uterine ducts and the vagina were encircled by microfilaments. Prominent layers of circular muscle fibers surrounded the cirrus and the seminal vesicle, and radial fibers were also detected. Areas faintly stained with TRITC-phalloidin were found in the developing germ cells, the cells of the genital anlage, vitelline cells, the tegument, and the main nerve cords. None of these structures were autofluorescent, which is also true concerning the intensively labeled oncospheral hooks.
Five cDNAs (pDidact2-pDidact6), representing different actin genes, were isolated from a Diphyllobothrium dendriticum cDNA library, and the DNA as well as the putative amino acid sequences were determined. The corresponding Didact2 and Didact4 genes code for peptides 376 amino acids long, with molecular weights 41,772 and 41,744 Da, respectively, while the deduced Didact3 protein is 377 amino acids long and weighs 41,912 Da. The pDidact5 and -6 cDNAs lack nucleotides corresponding to three to six amino acids at the amino-terminus. Two of the five cDNAs contain the conventional AATAAA as the putative polyadenylation signal, one has the common variant ATTAAA, whereas the hexanucleotide AATAGA is found 15 and 18 nucleotides, respectively, upstream of the poly(A) site in two of the cDNAs. Phylogenetic studies including 102 actin protein sequences revealed that there are at least four different types of cestode actins. In this study three of these types were found to be expressed in the adult D. dendriticum tapeworm. Structurally the cestode actin groupings differ from each other to an extent seen only among the metazoan actins between the vertebrate muscle and cytoplasmic isoforms. In the phylogenetic trees constructed, cestode actins were seen to map to two different regions, one on the border of the metazoan actins and the other within this group. It is, however, difficult to say whether the cestode actins branched off early in the metazoan evolution or if this position in the phylogenetic tree only reflects upon differences in evolutionary rate.
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