Genetic variability in the tolerance of natural populations ofSimocephalus vetulus(Müller, 1776) to lethal levels of sodium chloride

Abstract

-Using several clonal lineages of Simocephalus vetulus (Cladocera, Daphniidae) as a random sample, we investigated the genetic component of the halotolerance of one brackish and two freshwater populations of this littoral filter feeder. We hypothesized that genotypes from the brackish population were more tolerant than freshwater ones, via adaptation to local environmental conditions. Clonal identity was established by a cost-effective molecular fingerprinting technique (microsatellite-primed polymerase chain reaction (MSP-PCR)). Two distinct methodologies were used to assess cladoceran sensitivity to syntheticgrade sodium chloride (NaCl): (i) standard 48-h acute assays and (ii) 12-h survival time (ST) trials. No correlation was found between acute EC 50 and ST values. The sensitivity of brackish and freshwater clones was comparable in terms of acute EC 50 (varied from 2.28 to 3.83 g.L x1). On the contrary, genetically determined differential tolerance to NaCl among populations was found for ST: all brackish genotypes, except one, were more resilient (ST > 120 min) than freshwater clones (ST < 120 min). Bearing in mind that these results were obtained with isolates from the extant population, it is surprising that the range of acute sensitivity of the freshwater and brackish genotypes was similar, and that the only difference between them was the ability of brackish clones to survive longer under high salinity stress (6 g.L x1, in ST trials). We must conclude that the effect of salinity (original environment context) on the selection of genotypes was weaker than we had expected and than other authors have shown for other stressors.

Effects of salinity tolerances on survival and life history of 2 cladocerans

“…Local selective forces play an important role in genetic structure. As a result, even minor salinity increases in freshwaters may cause fine impacts in animal populations, such as the appearance of locally selected races or ecotypes (Loureiro et al, 2012). These data suggest that S. mucronata and S. vetulus have low tolerance, regardless of their habitat of origin.…”

“…Cladocerans, one of the most abundant primary consumer groups, are a key component in lentic ecosystems. They display different physiological tolerances to several ionic compounds (Martínez-Jerónimo and Martínez-Jerónimo, 2007;El-Deeb Ghazy et al, 2009;Loureiro, 2012). The use of cladocerans in toxicity studies provides many advantages: easy handling, asexual reproduction by parthenogenesis, a short lifespan, and great sensitivity to toxic compounds (Martínez-Jerónimo and Martínez-Jerónimo, 2007).…”

Progressive acclimation alters interaction between salinity and temperature in experimental Daphnia populations

“…Culture medium was renewed and organisms were fed -with a Pseudokirchneriella subcapitata ration of 1.5 Â 10 5 cells mL À1 -three times a week (Monday, Wednesday, Friday). For further details on daphniid culture medium and procedures, as well as algal cultures, see , Gonçalves et al (2007), and Loureiro et al (2012). D. galeata belongs to a ubiquitous stenohaline species complex that inhabits European lakes and reservoirs , and this particular genotype is known to be a sensitive ecoreceptor to salinity increase (Loureiro et al, 2013b).…”

“…In freshwater ecosystems, a relevant source of disturbance comes from natural or anthropogenic salinisation (James et al, 2003;Cañedo-Argüelles et al, 2013), as most freshwater taxa are stenohaline. Salinity rise has been shown to have implications in the structure and dynamic of populations (Schallenberg et al, 2003;Sarma et al, 2006;Nielsen et al, 2008;Loureiro et al, 2012) and species richness (Hall and Burns, 2002;Nielsen et al, 2008;Brucet et al, 2010;Duchet et al, 2010). Furthermore, salinisation of freshwaters is likely to be enhanced by climate change via increased evaporation, extended droughts and reduced rainfall (Chopelet et al, 2008;Nielsen and Brock, 2009;IPCC, 2014).…”

“…The alternative hypothesis states that acclimation to culture conditions elicits an adaptive response of daphniids to saline stress or its interaction with temperature. Because previous data (Loureiro et al, 2012(Loureiro et al, , 2013a suggested that acute and chronic responses to NaCl were not coupled, we examined survival time and juvenile growth as measures of halotolerance.…”