-Author to whom correspondence should be addressed. E-mail: cpb@dmu.dk J. theor. Biol. (2001) 210, 407}410 doi:10.1006/jtbi.2001.2317, available online at http://www.idealibrary.com on A New Method for Estimating Environmental Variability for Clonal Organisms, and the Use of Fluctuating Asymmetry as an Indicator of Developmental Instability CINO PERTOLDI*-,TORSTEN NYGAARD KRISTENSEN? AND VOLKER LOESCHCKE? *Department of ¸andscape Ecology, National Environmental Research Institute, Kal~ Grena  vej 14, DK-8410 R~nde, Denmark and ?Department of Ecology and Genetics, ;niversity of Aarhus, Building 540, Ny Munkegade, DK-8000 Aarhus C, Denmark (Received on 15 September 2000, Accepted in revised form on 20 March 2001) Environmental science has yet to devise a com- prehensive, ecologically relevant, in situ bio- monitoring system for assessing ecosystem health. Such a system needs to be su$ciently sensitive to provide an early warning of stress. Given the fact that full life history table analyses for all threatened species will be extremely time and money consuming and given the urgent need of obtaining as much information as possible about the stress responses of the largest number of species available, we have to consider alterna- tive methods. Here we advocate a biomonitoring technique completely di!erent from traditional physiological or demographical methods. The measure suggested is based upon the concept of developmental instability (DI). DI refers to the incapacity of producing a speci"c phenotype under a given set of environmental conditions (Markow, 1994). DI is the product of stress, im- pinging on the individual bu!ering capacity (Palmer, 1996) and has been suggested as having potential as a stress indicator allowing investiga- tors to identify areas in which ecosystems are stressed (Parsons, 1992). Furthermore, it also allows to estimate the degree of stress, not only in a qualitative, but also in a quantitative way. In this way it is possible to examine populations over time, as investigators will be able to deter- mine temporal changes in the degree of stress experienced. As for other toxicological tests DI as a measure of stress su!ers from low reproducibility and large variation in response within populations and between experiments (Woods et al., 1999). Among other things this is caused by the fact that natural populations generally comprise large numbers of genetically diverse individuals in- habiting environments, which are both spatially and temporally heterogeneous (Stearns, 1992). This means that the genetic basis for stress toler- ance can only be properly assessed by separating and quantifying the e!ect of both genes and envi- ronment and, most importantly, their interaction on the expression of tolerance. However, when studying stress tolerance, individuals, even from the same population, may have di!erent phenotypic responses. Therefore, investigations should be conducted employing monoclonal strains in order to remove genotypic variability. In this paper we present ideas based on DI that show that the use of monoclonal organisms in toxicological tests (and as a tool as a general stress indicator) might have a huge potential, which, until now, has not been utilized. By using monoclonal strains in stress studies we remove 0022}5193/01/120407#04 $35.00/0  2001 Academic Press