Analisi della parassitofauna delle rane verdi del complesso Rana esculenta: un metodo di valutazione dell’eterogeneità e dello stato ambientale dell’ecosistema lago di Vico
Author(s)
Cipriani, Paolo
Date Issued
March 15, 2010
Type
Doctoral Thesis
Abstract
Amphibians are generally considered as “sentinel” organisms of environmental degradation. As part of an overall biodiversity crisis, many amphibians populations are in decline throughout the world. Several causes have been invoked to explain these declines. These include: habitat disrupted by human activity, global warming, increasing levels of ultraviolet radiation, environmental contamination, and the introduction of non-native species and diseases. As a consequence to this decline, genetic erosion phenomena in the gene pools of amphibians population may lead also a reduction of their adaptive capacity of defense against pathogens and parasites.
AIM: The aim of this study was: i) to analyze green frogs of the Rana esculenta complex as a environmental quality index (Semlitsch et al., 1995; Fioramonti et al., 1997; Scialanca, 1999; Boncompagni et al., 2002; Di Rosa et al., 2002; Andreani et al., 2003; Venanzi, 2003) from Vico’s lake area (Central Italy); and ii) to add new data about the parasitic fauna of these frogs.
Final aim of the study has been to investigate the correlation between parasitic fauna composition and environmental quality of the Vico lake, and if there is a correspondence between parasite loads and genetic variability values of the hybrid frog, Rana esculenta, and the parental one (R. lessonae).
METHODS: The frogs were identified to species level as R. esculenta and R. lessonae, by means of diagnostic allozymes markers. Parasite infection levels have been estimated according to the parameters of Prevalence (P), mean Intensity (Im) and its range, according to Rosza L et al.(2000) using the software Quantitative Parasitology 3.0. The parasites have been identified based on morphology and molecular genetic methodologies. In particular, molecular systematic studies have been performed on the nematodes belonging to the genus Rhabdias. Indeed the taxonomy of the species reported in Europe in amphibians is a in a state of flux. The sequences of mtDNA cox-1 gene have been thus investigated on specimens of Rhabdias sp. collected from both R. esculenta and R. lessonae, in comparison with specimens of other Rhabdias spp. recovered from other species of frogs and toads (Rana italica, Bufo bufo, Bufo viridis) collected either in sympatry of the green frogs here studied, either from other localities of Italy.
RESULTS: 4 species of digeneans, 5 species of nematodes, and one species of cestodes were identified in the two green frog species. The sequence analysis of the mtDNA cox-1 (677bps) evidenced that the same species of Rhabdias is parasitizing both R. esculenta and R. lessonae: this species was here indicated as Rhabdias sp. A. It is, on the other hand, genetically well distinct from the other Rhabdias spp. so far collected and genetically studied for the same gene.
The relative frequencies of the two frogs of the R. esculenta complex obtained in the different sampling stations were found to be in agreement with the environmental quality(chemical) indexes values recorded in the same collecting localities sites. This result assesses the validity of this frog complex as an environmental quality index. An overall degradation of some of the sampling sites of the lake, in comparison to results of similar studies conducted in 1999 and 2003 has been observed. This has been confirmed by both chemicals and biological indexes used. The relative frequency of the hybrid species (R. esculenta) seems to be higher than that of R. esculenta previously observed from the same collecting localities.
Parasitic load from various parasites taxa has been found to be statistically significant different with respect to the sampling localities, at different level of anthropogenic impact. This seems to reveal a particular sensibility of some species of endoparasites to anthropogenic stressors.
As regarding the correlation between parasite load and frog species, a statistically significant higher prevalence and intensity values of the infection by Rhabdias sp. A was observed in the parental frog, R. lessonae, in comparison to the infection values so far found in the hybrid species, R. esculenta. This seems to suggest the hypothesis that the hybrid species has a higher resistance and adaptability to pathogens, probably thanks to its higher heterotic status.
Molecular analysis of the lung nematodes of the genus Rhabdias, obtained from the two green frogs species in comparison with those from different amphibian hosts (Rana italica, Bufo bufo, Bufo viridis) has evidenced a host preference of different species of Rhabdias, genetically distinct, with respect to different amphibians. This result seems to suggest possible host-parasite co-evolutionary processes in this group of endoparasitic nematodes and their definitive hosts.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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