Les interactions entre la reproduction et la biologie des populations chez les Monogènes Gyrodactylidae : revue

Abstract

Gyrodactylid monogeneans are viviparous, embryos developing within each other inside the mothers uterus. First- and second-born daughters develop without self- or crossfertilisation, and only the 3rd and subsequent daughters can develop sexually. The relative importance of sexual reproduction depends on population age structure and mortality. In Macrogyrodactylus polypteri, post-natal development of the female reproductive system facilitates cross-insemination in ail post-second birth individuals. Asexual reproduction maximises population growth, but sex occurs regularly. Gyrdicotylus gallieni, from the mouth of Xenopus laevis, has unusually slow population growth, and infections are composed largely of older, sexually mature individuals. Isancistrum subulatae, from the squid Alloteuthis subulata, forms very large populations, but growth is probably slow and the âge structure suggests that sex is common. Within Gyrodactylus, several strategies exist. Gyrodactylus turnbulli on guppies is short lived, experiencing exponentially increasing age specific mortality which results in less than 1% surviving to give birth a third time. Reproduction is primarily asexual, but at high population densities sex is seen. In the field, asexual reproduction probably predominates, and this species can be regarded as a cyclic parthenogen. Gyrodactylus salaris, on salmon, experiences low mortality and 10-15% of individuals survive to reproduce sexually. Inseminated individuals are found, even in light infections, and sex forms a normal part of the biology of this species. G. gasterostei, from Gasterosteus aculeatus, has a mortality at 15 °C resembling that of G. turnbulli, but at 10 °C mortality increases less rapidly with age. Data from natural populations suggest that sex is rare in this species. Sex may be rare in many Gyrodactylus species, but in a natural population of G. arcuatus a phase of population growth was observed when sex may have occurred. This was very local, lasting less than one month and not taking place at a second site, 3 km downstream. Sexual strategies can be correlated with morphological variation and host specificity. Sexual species may be more variable, and show less heterogeneity between populations than the possible asexual species. The sexual forms also appear more catholic in their host choice. The twin barriers of host specificity and asexual reproduction may be important in preventing hybridisation between genetically distinct gyrodactylid stocks, maintaining diversity. The potential impact of Man on gyrodactylid reproductive biology and evolution is discussed.