Under global warming scenarios, rising temperatures can constitute heat stress to which species may respond differentially. Within a described species, knowledge on cryptic diversity is of further relevance, as different lineages/cryptic species may respond differentially to environmental change. The Brachionus calyciflorus species complex (Rotifera), which was recently described using integrative taxonomy, is an essential component of aquatic ecosystems. Here, we tested the hypothesis that these (formerly cryptic) species differ in their heat tolerance. We assigned 47 clones with nuclear ITS1 (nuITS1) and mitochondrial COI (mtCOI) markers to evolutionary lineages, now named B. calyciflorus sensu stricto (s.s.) and B. fernandoi. We selected 15 representative clones and assessed their heat tolerance as a bi-dimensional phenotypic trait affected by both the intensity and duration of heat stress. We found two distinct groups, with B. calyciflorus s.s. clones having higher heat tolerance than the novel species B. fernandoi. This apparent temperature specialization among former cryptic species underscores the necessity of a sound species delimitation and assignment, when organismal responses to environmental changes are investigated.

Examination of sampled material and review of the relevant literature revealed the presence of 299 benthic and 46 pelagic amphipod species from the Aegean Sea. Two of the species identified, Caprella hirsuta Mayer, 1890 and Apohyale crassipes (Heller, 1866), are recorded for the first time in the Aegean Sea and the Levantine Basin respectively. A checklist of the Mediterranean amphipods is given, as well as their distribution in the Mediterranean territorial areas and the Black Sea. The faunal comparison of the Mediterranean areas showed that the number of species decreases from west to east. In terms of zoogeographical categories the Atlanto-Mediterranean species dominate in all Mediterranean areas followed by the endemic species concerning the benthic amphipods while in pelagic amphipods cosmopolitan species dominate and are then followed by Atlanto-Mediterranean. Three species, Bemlos leptocheirus (Walker, 1909) and Linguimarea caesaris Krapp-Schickel, 2003, Photis lamellifera Schellenberg, 1928, are lessepsian migrants and are reported in the Levantine and Central Mediterranean Seas.

Thermal stress response is an essential physiological trait that determines occurrence and temporal succession in nature, including response to climate change. We compared temperature-related demography in closely related heat-tolerant and heat-sensitive Brachionus rotifer species. We found significant differences in heat response, with the heat-sensitive species adopting a strategy of long survival and low population growth, while the heat-tolerant followed the opposite strategy. In both species, we examined the genetic basis of physiological variation by comparing gene expression across increasing temperatures. Comparative transcriptomic analyses identified shared and opposing responses to heat. Interestingly, expression of heat shock proteins ( hsps ) was strikingly different in the two species and mirrored differences in population growth rates, showing that hsp genes are likely a key component of a species’ adaptation to different temperatures. Temperature induction caused opposing patterns of expression in further functional categories including energy, carbohydrate and lipid metabolism, and in genes related to ribosomal proteins. In the heat-sensitive species, elevated temperatures caused up-regulation of genes related to meiosis induction and post-translational histone modifications. This work demonstrates the sweeping reorganizations of biological functions that accompany temperature adaptation in these two species and reveals potential molecular mechanisms that might be activated for adaptation to global warming.

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).

A species' response to thermal stress is an essential physiological trait that can determine occurrence and temporal succession in nature, including response to climate change. Environmental temperature affects zooplankton performance by altering life-spans and population growth rates, but the molecular mechanisms underlying these alterations are largely unknown. To compare temperature-related demography, we performed cross-temperature life-table experiments in closely related heat-tolerant and heat-sensitive Brachionus rotifer species that occur in sympatry. Within these same populations, we examined the genetic basis of physiological variation by comparing gene expression across increasing temperatures. We found significant cross-species and cross-temperature differences in heat response, with the heat-sensitive species adopting a strategy of high survival and low population growth, while the heat-tolerant followed an opposite strategy. Comparative transcriptomic analyses revealed both shared and opposing responses to heat. Most notably, expression of heat shock proteins (hsps) is strikingly different in the two species. In both species, hsp responses mirrored differences in population growth rates, showing that hsp genes are likely a key component of a species' adaptation to different temperatures. Temperature induction caused opposing patterns of expression in further functional categories such as energy, carbohydrate and lipid metabolism, and in genes related to ribosomal proteins. In the heat-sensitive species, elevated temperatures caused up-regulation of genes related to induction of meiotic division as well as genes responsible for post-translational histone modifications. This work demonstrates the sweeping reorganizations of biological functions that accompany temperature adaptation in these two species and reveals potential molecular mechanisms that might be activated for adaptation to global warming.