Within species expressed genetic variability and gene expression response to different temperatures in the rotifer Brachionus calyciflorus sensu stricto

Abstract

Genetic divergence is impacted by many factors, including phylogenetic history, gene flow, genetic drift, and divergent selection. Rotifers are an important component of aquatic ecosystems, and genetic variation is essential to their ongoing adaptive diversification and local adaptation. In addition to coding sequence divergence, variation in gene expression may relate to variable heat tolerance, and can impose ecological barriers within species. Temperature plays a significant role in aquatic ecosystems by affecting species abundance, spatio-temporal distribution, and habitat colonization. Recently described (formerly cryptic) species of the Brachionus calyciflorus complex exhibit different temperature tolerance both in natural and in laboratory studies, and show that B. calyciflorus sensu stricto (s.s.) is a thermotolerant species. Even within B. calyciflorus s.s., there is a tendency for further temperature specializations. Comparison of expressed genes allows us to assess the impact of stressors on both expression and sequence divergence among disparate populations within a single species. Here, we have used RNA-seq to explore expressed genetic diversity in B. calyciflorus s.s. in two mitochondrial DNA lineages with different phylogenetic histories and differences in thermotolerance. We identify a suite of candidate genes that may underlie local adaptation, with a particular focus on the response to sustained high or low temperatures. We do not find adaptive divergence in established candidate genes for thermal adaptation. Rather, we detect divergent selection among our two lineages in genes related to metabolism (lipid metabolism, metabolism of xenobiotics).

Temperature-dependent life history and transcriptomic responses in heat-tolerant versus heat-sensitive Brachionus rotifers

“…Among the differentially expressed genes were NADH dehydrogenase and glutathione S-transferase ( GST ). For the B. calyciflorus s.s., a significant induction (7 × fold change) of NADH dehydrogonase has been reported after sustained cold stress on 14 °C for 30 days 51 . Transcriptional regulation of GST genes also differed in two congeneric copepod species of the genus Tigriopus .…”

“…Samples in TRIzol LS, after having been homogenized using a Tissue Lyzer (4 min, 50 Hz), were incubated overnight at room temperature. A total of 500 μl of chloroform was added to each sample and samples were centrifuged for 15 min at 4 °C to facilitate phase separation 51 . The colorless, upper aqueous phase was transferred into an RNeasy Mini Kit column (Qiagen, Germany) and proceeded to RNA precipitation according to the manufacturer’s instructions.…”

“…Strand-specific libraries were constructed using NEXTflex Rapid Directional RNA-Seq Kit (Bioo Scientific, USA) according to manufacturer’s instructions. Final elution was performed in 16 μl of elution buffer and a PCR amplification of 14 cycles was performed 51 . Libraries were quantified using Qubit dsDNA HS Assay Kit (Invitrogen, Germany) and quality control was performed using Agilent Bioanalyzer 2100 (Agilent Technologies, USA).…”

Fitness response variation within and among consumer species can be co-mediated by food quantity and biochemical quality

“…This might involve adaptations in physiological traits (e.g. nutrient absorption and bioconversion capacities) and requirements, which eventually manifest genetically 30 .…”

“…Recently, four species of Brachionus calyciflorus have been (re)described based on phylogenetic and morphometric differences among strains within this species complex 27 . These species have been shown already to differ in their life history 28 and in their heat stress tolerance 29 , potentially driving natural selection and thus the evolution of new genotypes 30 .…”