Fertilization in the cestode Echinococcus multilocularis (Cyclophyllidea, Taeniidae)

The first ultrastructural observations of the egg-forming complex of Calicotyle affinis, an endoparasitic monocotylid monogenean which lacks a uterus

This ultrastructural study of the female reproductive system of Calicotyle affinis, a monogenean without a uterus, demonstrates, for the first time in a monogenean, the presence of accessory cells located in the peripheral region of the ovary and a single large cell whose surface is penetrated by deep invaginations which restrict the passage of oocytes through the lumen of the distal extremity of the ovary. The cytoarchitecture of the epithelial lining of the two vaginae is a syncytium formed by an anucleate epithelial lining with sunken epithelial perykaria. The wall of the seminal receptacle is an enlarged prolongation of the vaginal epithelial lining. Fertilization occurs in the fertilization chamber and fertilized oocytes retain cortical granules within their cytoplasm, a characteristic known to occur in free-living flatworms. Our study also highlights concentrations of two distinct groups of Mehlis' gland cell ducts on either side of the proximal end of the ootype distinguished by ultrastructural characteristics of their secretory granules. The epithelial wall of the ootype is formed by a single layer of regular, columnar, glandular epithelial cells; these cells are closely adjacent, conjoined towards their apical region by septate junctions and produce rounded, electron-dense granules which are discharged into the ootype lumen via a merocrine or holocrine mechanism. Released granules concentrate around the eggshell and form an additional fibrous coat. The morphological diversity observed in the female reproductive system of the Monogenea is commented on it relation to clarifying patterns in monogenean evolution and for understanding the phylogeny of the Neodermata.

Evolutionary developmental biology (evo-devo) of cestodes

Cestodes (tapeworms) have complex adaptations to their obligatory parasitic life-style. Among these adaptations, they show many evolutionary innovations in their development, including complex life-cycles with multiple hosts and life-stages, several independent origins of asexual reproduction, and the evolution of segmentation as a mean to generate massive reproductive output. Therefore, cestodes offer many opportunities for the investigation of the evolutionary origins of developmental novelties (evo-devo). However, cestodes have not been exploited as major models for evo-devo research due to the considerable technical difficulties involved in their study. In this review, a panoramic view is given of classical aspects, methods and hypothesis of cestode development, together with recent advances in phylogenetics, genomics, culture methods, and comparative analysis of cestode gene expression. Together with the availability of powerful models for related free-living flatworms, these developments should encourage the incorporation of these fascinating parasites into the first-line of evo-devo research.