Patterns and dynamics of rapid local adaptation and sex in varying habitat types in rotifers

Abstract

Local adaptation is an important principle in a world of environmental change and might be critical for species persistence. We tested the hypothesis that replicated populations can attain rapid local adaptation under two varying laboratory environments. Clonal subpopulations of the cyclically parthenogenetic rotifer Brachionus calyciflorus were allowed to adapt to two varying harsh and a benign environment: a high-salt, a food-limited environment and untreated culture medium (no salt addition, high food). In contrast to most previous studies, we re-adjusted rotifer density to a fixed value (two individuals per ml) every 3–4 days of unrestricted population growth, instead of exchanging a fixed proportion of the culture medium. Thus our dilution regime specifically selected for high population growth during the entire experiment and it allowed us to continuously track changes in fitness (i.e., maximum population growth under the prevailing conditions) in each population. After 56 days (43 asexual and eight sexual generations) of selection, the populations in the harsh environments showed a significant increase in fitness over time relative to the beginning compared to the population in untreated culture medium. Furthermore, the high-salt population exhibited a significantly elevated ratio of sexual offspring from the start of the experiment, which suggested that this environment either triggered higher rates of sex or that the untreated medium and the food-limited environment suppressed sex. In a following assay of local adaptation we measured population fitness under “local” versus “foreign” conditions (populations adapted to this environment compared to those of the other environment) for both harsh habitats. We found significantly higher fitness values for the local populations (on average, a 38% higher fitness) compared to the foreign populations. Overall, local adaptation was formed rapidly and it seemed to be more pronounced in the high-salt treatment.

Monogonont rotifers as model systems for the study of micro-evolutionary adaptation and its eco-evolutionary implications

“…In an earlier study, Becks & Agrawal (2010) demonstrated an increased propensity for sex and an increased frequency of occurrence of sexual individuals in spatially heterogeneous compared to homogenous continuous cultures. Adaptation through sustained sexual reproduction may also explain the observation by Scheuerl & Stelzer (2013) who reported undiminished rates of sexual reproduction in B. calyciflorus populations exposed to stressful levels of salinity despite growth conditions selecting for maximum population growth rate. Similarly to Becks & Agrawal (2012), Declerck et al (2015) also observed high rates of sexual reproduction, but only during an episode of population decline at the start of their selection experiment.…”

“…Whereas studies on B. plicatilis consistently checked the identity of cryptic species, this has seldom been done in studies with B. calyciflorus (Table 1). With the exception of one study (Scheuerl & Stelzer, 2013), experimental studies always made use of chemostats or semi-continuous batch cultures when exposing populations to experimental selection conditions. Tests of adaptation were most often based on the evaluation of short-term fitness indicators, mainly the intrinsic population growth rate or surrogate variables (e.g., lifetime fecundity) and, more rarely, the outcome of direct competition experiments (Stelzer, 2011) or the capacity to suppress food levels (C*; Declerck et al, 2015).…”

“…Rapid adaptation to increased salinity was shown in a laboratory evolution study by Scheuerl & Stelzer (2013) who exposed replicate identical multiclonal populations of the freshwater species B. calyciflorus to three selection conditions: two conditions representing stressful environments (high salinity and low food availability) and one condition representing 'benign' growth conditions (i.e., low salinity combined with high food availability). By design, the experimental set-up ensured selection for fast population growth and simultaneously allowed the monitoring of this fitness variable during the course of the experiment.…”

Transcriptomic differences between euryhaline and stenohaline malaria vector sibling species in response to salinity stress

“…). Differences in saltwater tolerance have been implicated in the stable coexistence of sibling species in cryptic species complexes and local adaptation of aquatic invertebrates such as rotifers (Montero‐Pau & Serra ; Scheuerl & Stelzer ). Rapid evolution of enzymes (V‐type H + ATPase and Na + /K + ATPase) in the copepod Eurytema affinis has been linked with the capacity of this invasive species to move from marine to FW systems (Lee et al .…”

“…Within fishes, salinity tolerance differs both among closely related species and within populations of the same species, with killifish and sticklebacks representing two well-studied examples (Jones et al 2012;Kozak et al 2013). Differences in saltwater tolerance have been implicated in the stable coexistence of sibling species in cryptic species complexes and local adaptation of aquatic invertebrates such as rotifers (Montero-Pau & Serra 2011;Scheuerl & Stelzer 2013). Rapid evolution of enzymes (V-type H + ATPase and Na + /K + ATPase) in the copepod Eurytema affinis has been linked with the capacity of this invasive species to move from marine to FW systems (Lee et al 2011).…”