Postembryonic development of tapeworms — source of novel phylogenetic characters for analysis of cestode evolution: comparative TEM studies

Abstract

Ultrastructural features of juvenile cestodes (metacestodes) can provide useful characters for phylogenetic and evolutionary analyses. Until now, however, they have been relatively little utilised (Beveridge 2001, Chervy 2002). The postembryonic development and structure of fully formed metacestodes were examined in two cyclophyllideans: Taenia parva Baer, 1926 (Taeniidae); and Sobolevitaenia verulamii (Mettrick, 1958) Korniushin, 1972 (Dilepididae). In T. parva, three developmental stages were recognized: (1) an early stage of exogenous budding at the surface of the central vesicle; (2) a stage of polycephalic cyst development accompanied by segmentation of the growing metacestode strobila and an obvious decrease in the size of the central vesicle; (3) a fully formed metacestode of the strobilocercus type with 14–24 invaginated scoleces. The tegument, scolex, subtegumental musculature of the strobilar segments, protonephridial system, calcareous corpuscles and medullary parenchyma of larvae exhibit general similarity to the same structures in adults at both LM and TEM levels. The morphogenesis of the metacestode of T. parva is compared with that of polycephalic metacestodes of other Taenia spp. (T. krepkogorski, T. twitchelli and T. endothoracica) and with other asexually multiplying metacestodes (Mesocestoides vogae, hymenolepidids and dilepidids). In S. verulamii, the body of the cysticercoid with invaginated scolex armed with a double crown of rostellar hooks was completely surrounded by the cercomer, which appears to be separated from the cyst and scolex. The surface of the suckers is covered with a thick layer of glycocalyx. Five cell types were distinguished in the sections: (1) perikarya of metacestode tegument; (2) glycolgen-storing parenchymal cells; (3) glandular-type cells with large, electron-dense secretory-like granules; (4) flame cells; and (5) calcareous corpuscle-forming cells. The surface of the cercomer is covered by elongated microvilli, which evidently differ from characteristic microtriches covering all other parts of the metacestode surface. The ultrastructure of S. verulamii evidently differs from that of the other dilepidid cestode examined to date, Lateriporus geographicus, the cyst wall of which more resembles cysticercoids of Hymenolepididae than those of Dilepididae. Concluding remarks: Ultrastructural studies on metacestodes have considerable promise for providing important new characters for phylogenetic analysis. New TEM data on a great variety of cestode species are urgently needed. Until now, this field has not been exploited in a systematic fashion. Until more comprehensive studies become available, the current data are not yet amenable to analysis.

A standardised terminology of the embryonic envelopes and associated developmental stages of tapeworms (Platyhelminthes: Cestoda)

“…In some aquatic groups, the inner envelope also provides motility for the larva by developing as a ciliated free-swimming larval stage, which must emerge from the outer envelope and shell before swimming (Berrada-Rkhami andGabrion 1990, Świderski 1994a). When the oncosphere larva, including its surrounding embryonic envelopes, is ingested by a suitable host, the hexacanth emerges from the envelopes, penetrates host tissues, and begins metamorphosis into the juvenile metacestode stage (Jarecka 1975, Ubelaker 1980, 1983, Conn 2004c, Świderski 2007.…”

Ultrastructure of the cercomer of the metacestodeMicrosomacanthus paraparvulaRegel, 1994 (Cestoda: Hymenolepididae)

“…In order to understand whether it is possible to use the non-cellular exocyst as a systematic character, further ultrastructural studies of uninvestigated cyclophyllid metacestodes are needed. In addition, new data on the ultrastructure of metacestodes can be utilized for the analysis of phylogenetic relationships among cestodes (Beveridge, 2001; Swiderski, 2007).…”

Ultrastructural studies of the tegument and excretory system of the cestode Nematotaenia kashmirensis (Fotedar, 1966) infecting the toad Bufo regularis in Egypt.

“…The ultrastructure of the cestode tegument of most cyclophyllidean tapeworms has been reported by various authors (Smyth and McManus, 1989;Stoitsova et al,2002 andSwiderski, 2007).…”