Local Adaptation Of Bitter Taste And Ecological Speciation In The Galilee Blind Mole Rat

Ecological speciation refers to the process by which barriers to gene flow evolve between populations as a result of ecologically based divergent selection between environments.In this mode of speciation,differences in ecological factors such as food,temperature,and interaction with other organisms will lead to the divergent selection and local adaptation of different populations,and eventually the occurrence of reproductive isolation between populations.The sensory system,which connects external environmental signals with internal physiological responses,often plays an important role in the process of ecological speciation.Previous studies on the involvement of the sensory system in ecological speciation have focused on visual,auditory,and olfactory senses,while the role of taste perception in ecological speciation has been underappreciated.There are two adjacent habitats in the Upper Galilee Mountains of Israel: a highsunlight hot and arid chalk soil habitat on the one side,and is a low-sunlight,cool and moist basalt soil habitat on the other side.The two habitats are adjacent,but the sunlight intensity,soil physical and chemical properties,temperature and humidity are significantly different.The distribution of surface plants and underground storage organs is different.Different vegetation distribution makes the animals living in these two habitats eat different food.Previous studies found that differences in soil types,food resources,and other ecological factors lead to a series of phenotypic and genetic differentiation between the two Galilee blind mole rat(Spalax galili)populations living in different soils,which are considered to be undergoing the incipient sympatric ecological speciation.Genomic studies revealed the significant genetic divergence of chemosensory genes including bitter taste receptor genes(Tas2rs)between the two populations,indicating that the sensory system may be involved in ecological speciation of the blind mole rats.Bitter taste receptors mediate bitter taste and help animals perceive bitter toxic and harmful substances in nature.The genetic divergence of Tas2 rs between the two populations may have caused by adaptations to different food resources.However,whether the genetic divergence at the sequence level leads to functional divergence is still unknown.This study integrates evolutionary analysis of gene sequences and functional assays to explore the function and adaptive mechanism of bitter taste receptors(Tas2rs)of the blind mole rats living in different soil environments and feeding on different food resources.Firstly,we explored the genetic divergence of Tas2 rs in Galilee blind mole rat at the sequence level of evolutionary analysis.Previous studies have identified 20 intact Tas2 rs in the Galilee blind mole rat,and the Tas2 rs showed significant genetic divergence between the basalt and chalk populations.In this study,we newly identified 12 intact Tas2 rs in the blind mole rat by local TBlast N search against its genome and PCR amplification from its genomic DNA.Among the 32 intact Tas2 rs,by calculating the fixation index of the same Tas2 r between the two populations,and using haplotype phasing to determine the Tas2 r haplotypes in the two populations,we found that there are six Tas2rs(Sg?Tas2r118,Sg?Tas2r119,Sg?Tas2r134,Sg?Tas2r137,Sg?Tas2r143 and Sg?Tas2r582a)with significantly different distributions between the two populations.The sequence analysis indicates that the genetic divergence between Tas2 rs is apparent in the two populations.Secondly,based on the results of evolutionary analysis of gene sequences,this work studied the functional divergence of Tas2 rs in the Galilee blind mole rat.To test whether the six Tas2 rs with significant genetic differentiation lead to functional divergence,we explored the function of the representative haplotypes in different soil types of these Tas2 rs by cell-based functional analyses.The functional assays found that Sg?Tas2r134,Sg?Tas2r137 and Sg?Tas2r582a didn't respond to the bitter compounds in the experiments,while Sg?Tas2r118,Sg?Tas2r119 and Sg?Tas2r143 had different responses to some bitter compounds: the reaction intensity of the basalt-type bitter taste receptor haplotypes to bitter substances was higher than that of the chalk-type bitter taste receptor haplotypes.The Tas2 rs differed functionally between the two populations may due to the difference in food resources.Sg?Tas2r118 has five amino acid substitutions between the two populations.The results of functional assays showed that the replacement of five amino acid sites of Sg?Tas2r118 may affect its sensitivity to bitter compounds.Cellular localization of the receptors based on immunocytochemical experiments showed that different haplotypes of the same bitter receptor had similar expression levels(14.0%-17.9%)and membrane localization,and the mutant receptors of Sg?Tas2r118 also had similar expression levels(14.8%-20.7%)and membrane localization,indicating that the difference in response was independent of expression efficiency and membrane localization.The structure modeling and molecular docking of these three bitter taste receptors with different ligands showed that the residue F249 of Sg?Tas2r118 is directly involved in the interaction of receptors with helicin,salicin and arbutin.The above-mentioned results of evolutionary analysis and functional assays showed that the Tas2 rs between the two populations of the Galilee blind mole rat not only had a certain level of genetic divergence but also had functional divergence.The response intensity and sensitivity of the basalt-type bitter taste receptor haplotypes to bitter substances was higher than that of the chalk-type bitter taste receptor haplotypes.This functional change may help animals in the basalt population with more sensitive bitter taste perception choose foods with less bitterness in the basalt soil with more abundant food types,while the weaker bitter taste perception in the chalk population allows individuals to feed on a wider range of plants in a relatively scarce food environment.These results indicated that the differences in food resources may lead to divergent selection and adaptive evolution of Tas2 rs in different soil types in the two populations.The local adaptation of Tas2 rs makes the two populations prefer different food resources and habitats.The kind of habitat isolation hinders gene flow between the populations and promotes the process of the ecological speciation in the Galilee blind mole rats.This study shows that taste perception,as an important sensory system,can potentially play an important role in ecological speciation.