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INTRODUCTION
Precocious egg production in parasites has been known since the report by von Siebold (1835), 'Helminthologische Beiträge', on eggs released from metacercariae found in the crayfish Astacus astacus . The term 'progénèse', i.e. progenesis later in English, was originally proposed by Giard (1887) for sexual maturity of animals that have not yet attained the adult stage. Dollfus (1924) first applied the term to trematodes when he observed egg production by the metacercariae of Pleurogenoides medians (Pleurogenidae). Now, the concept of progenesis is widely employed in palaeontology and botany, and quite commonly reported in many animal taxa (Anura, Urodela, Polychaeta).
At present, the accepted definition is the formal one proposed by Gould (1977): any heterochronic development in which first reproduction occurs at an earlier age, i.e. sexual maturation in an organism still in a morphologically juvenile stage. It is often confused with the term neoteny, but differs from it in that, in neotenic organisms, first reproduction occurs at the same age as in normal organisms, via the retardation of somatic development. Both these processes are thus characterized by the retention of juvenile characters by adult stages (paedomorphic phenotypes; see for example Reilly, Wiley & Meinhardt, 1997). So, basically, progenesis is wrongly applied to trematodes, in which precocious worms are morphologically similar to the adults found in the definitive host. However, the term progenesis is now so widely used in the parasitological literature that it would be pointless and confusing to introduce a more specific term. We must simply use the term keeping in mind that it refers to the attainment of both sexual and somatic development in the non-definitive host. In our view, the most relevant definition for progenesis in trematodes is the production of viable eggs in individuals inhabiting an organism that would normally be considered an intermediate host.
Progenesis in trematodes thus encompasses all cases where a larval stage, a metacercaria or even a cercaria, attains such a degree of maturity that it can produce viable eggs. As a result, the life-cycle may be completed in 2 hosts or even in a single host. Progenesis may manifest itself in various degrees, from sporadic cases of egg production (1st stage of progenesis, according to Buttner, 1955) up to the obligate progenetic life-cycle (4th stage...