Overview of life cycles in model species of the genus Dugesia (Platyhelminthes: Tricladida)

An overview of the reproductive patterns of seven model species of triclads belonging to the genus Dugesia from circum- Mediterranean and Afrotropical freshwater is provided. Populations can reproduce exclusively sexually or exclusively asexually by fissiparity but the coexistence of both reproductive modes is displayed by some lineages. The comparison of data on the life cycles as documented in the literature with new data highlights a wide array of potentialities to shift from an asexual to a sexual state and vice versa in species with fissiparous populations. Life cycles are poorly diversified in species reproducing only sexually. Among fissiparous populations, planarians may sexualise, displaying various grades of reproductive functionality under laboratory conditions. Unexpectedly, asexual reproduction by fission occurred spontaneously in two species during the sexual state of ex-fissiparous individuals. The capacity to develop or reduce the reproductive apparatus during the life cycle either in sexual or in ex-fissiparous individuals illustrates the considerable morphogenetic plasticity in planarians. The Dugesia case contributes to the modelling of reproductive patterns and strategies in basal Metazoa as a continuum from sexual to asexual reproduction and vice versa, rather than a simple clear-cut alternative.