Thermal plasticity along a latitudinal gradient in the moor frog (Rana arvalis)

Sammanfattning

In ectotherms, early life stages are particularly sensitive to ambient temperature, and latitudinalgradients impose predictable variation in thermal regimes and season length. Previouscommon-garden work on the moor frog (Rana arvalis) has shown that larval life-history traitssuch as development time, growth rate, and size at metamorphosis vary along a latitudinalgradient across Northern Europe and are associated with distinct post-glacial colonizationroutes. The molecular regulatory basis of these responses, however, remains poorlycharacterized. This thesis asks how liver gene expression of R. arvalis larvae responds to rearingtemperature across populations, and whether transcriptional plasticity is structured by latitudeor geographic region or is instead organized at the population level. Paired-end RNA-seq data from 70 tadpole liver samples were analysed. Individuals from eightpopulations across Sweden, Finland, Estonia, and Latvia were reared at 15°C and 20°C in acommon garden experiment. Reads were aligned to a recently assembled R. arvalis referencegenome with STAR and quantified per gene with featureCounts. After exploratory qualitycontrol, differential expression was modelled in DESeq2 using a negative binomial generalizedlinear model with the design ~ family + stage + population × temperature. A likelihood ratiotest identified genes with significant population-by-temperature interactions, and withinpopulation contrasts between 20°C and 15°C were used to characterize temperature responsesin each population within a genomic reaction norm framework. After low-count filtering, 17,622 genes were retained. The likelihood ratio test identified 5,336genes (30.3% of the tested transcriptome) with significant population-by-temperatureinteractions, indicating that temperature-associated expression patterns depended on populationorigin. No gene reached significance as temperature-responsive across all eight populations.The number of significant temperature-responsive genes per population ranged from 161 to7,176, and populations differed in effect-size distribution, directional bias, and the coherenceof sample-level expression structure. Pairwise correlations of temperature-response profiles didnot align cleanly with the predefined latitude or region groupings, although temperatureassociated expression structure was clearer in northern and eastern populations than in thesouthern Swedish populations. These results indicate that liver transcriptional plasticity in R. arvalis is heterogeneous amongpopulations under ecologically realistic developmental temperatures, and is not cleanlyexplained by latitude or region. The findings should be interpreted as evidence of liver-specifictranscriptional plasticity rather than as direct evidence of adaptive differentiation. By adding apopulation-comparative transcriptomic perspective to a system previously characterized mainlythrough phenotypic and genetic studies, this thesis contributes regulatory-level insight into hownatural amphibian populations differ in their molecular responses to temperature.