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Home » Archive » 2008

TDK conference 2008

Study on the taxonomy, biogeography and ecology of terrestrial isopod sibling species (Trichoniscus spp.)
Végh Attila - year 5
Institute of Biology, Department of Ecology
Supervisors: Dr. Hornung Erzsébet associate professor, Dr. Matty Berg associate professor

Abstract:

The aim of our study was to determine the distribution and the hypothetical niche differences of two terrestrial isopods (Isopoda, Oniscidea), Trichoniscus provisorius and T. pusillus, common in The Netherlands. Moreover, we wanted to work out an adequate method for taxonomical identification of the species. The ancestor, T. provisorius is a diploid, sexually reproducing species, while its descendant, T. pusillus is a triploid, parthenogenetic species. Due to the lack of morpological differences, former studies estimated the species composition of a population with measuring the sex ratio, based on the ratio 1:1 of T. provisorius and 1:100~♂:♀ of T. pusillus found in laboratory colonies. Sex ratios of preliminary collections suggested the typical characteristics of the different soil types play a crucial role in the determination of the geographical pattern.

Isopod specimens were collected with standardized methods (1 m2; 45 min) covering the whole area of The Netherlands. Sampling sites (51) were distributed equally between the main soil types (clay, sand, peat), the geological formations (holocene, pleistocene) and the categories of the amounts of annual precipitation and mean temperature. The most important habitat characteristics affecting the distribution of isopods, were recorded on field data sheets. Soil samples were collected for further laboratory analyses (pH, Ca-content, water-holding capacity). For the identification of the individuals a restriction fragment length polymorphism (RFLP) method was developed based on the sequenced 18S rDNS differences.

The study disclaimed the validity of previous methods used for indentification of the species composition of Trichoniscus populations (sex ratio, female size/brood size ratio). However, the molecular method applied presents a valid taxonomical identification. We found the physical and chemical characteristics of the soil to be the most important factors in the species’s geographical distribution. These parameters are strongly related to the geological formations. Our results proved the existance of mixed populations in areas of holocene and pure T. pusillus populations in areas of pleistocene origin.

Based on the spatial occurence of the two species we can state a niche segregation: T. pusillus can be found along wide range of the environmental variables, while T. provisorius survives only between close limits. The ecological comparison supports the hypothesis about the advantage of parthenogenesis and/or polyploidy in colonization and survivorship under extreme conditions.



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