Spatio-temporal and thermal limit differences in two cryptic lineages of Baetis alpinus
The climate is changing at an unprecedented rate, resulting, inter alia, in warmer temperatures and thereby different ecosystem conditions. How organisms will respond to these changes depends on their ability to initially cope with increased temperatures and, in the long term, adapt to them. Traits ensuring high fitness at warmer temperatures should be selected for, and organism with a wide genotypic and phenotypic range may have an advantage as they posses a higher probability to bear those traits. However, cryptic species complexes (i.e. morphologically similar species which do not hybridize) within a presumed species can convey a false picture of a species abundance and trait variation. Hence, obtaining data on cryptic genetic lineages and their characteristics is important for comprehensive conservation of biodiversity.
One crucial trait for understanding the effects of thermal alterations on natural populations is a species thermal tolerance, which is frequently determined with the critical thermal method (CTM). In ectotherms, such as aquatic macroinvertebrates, thermal limits (such as upper thermal limit, CTmax) are thought to be an important link between their physiology and thermal ecology. Consequently, temperature sensitive taxa, such as Ephemeropteran mayflies, can also help to determine the onset of drastic changes in aquatic ecosystems in response to global warming.
In this study, I used microsatellite markers used to first quantify the presence of cryptic genetic lineages within the morphological species of Baetis alpinus (Ephemeroptera) in an alpine stream catchment (Val Roseg, Switzerland). Second, I estimated CTmax in order to explain possible differences in the relative abundance of the cryptic lineages and to gain insight to their putative responses to climate change. To test for environmental affinity of the lineages and spatio-temporal variation in CTmax, I conducted the analyses during two seasons (summer and fall) and in two stream habitat types (main stream and tributary).
I found two different lineages (henceforth, Lineage 1 and Lineage 2) in this system and that the lineages differed in their abundance over the seasons and in their habitat affinity. Overall, I found both lineages at all sites, but Lineage 1 was present at higher frequencies in tributaries in the summer, whereas Lineage 2 was more abundant through seasons. Population genetic analyses (FSTs) further indicated weak genetic structure between populations within the lineages, but that Lineage 2 showed substructure due to divergence of tributary sites. The two lineages also differed in their CTmax values, whereby Lineage 2 showed a shift in CTmax between seasons and CTmax of Lineage 1 remained stable between seasons. Taken together, my results indicate two strongly divergent cryptic lineages within B. alpinus, which likely represent cryptic species. The apparent ecological differences (i.e. habitat/season affinity and CTmax variation) between these cryptic lineages further point to potentially different responses to global warming. The knowledge gained in this study emphasizes the need for further research on cryptic variation and species composition in aquatic ecosystems.