Adaptive Evolution Of Sucrose Specific Hydrolase And Non-digestive ?-glucosidase In Lepidoptera

During the long-term co-evolution of herbivorous insects and host plants,the host plants can secrete multiple secondary metabolites to resist the feeding of insects.On the other hand,the herbivorous insect has evolved adaptive phenotypes in physiological structures and metabolic regulations to reduce the activation and release of plant secondary metabolites and get rid of the risk of plant defence.Among these strategies,the high-alkaloid midgut and transcriptional plasticity are the representative defence mechanisms that evolved in lepidopteran insect.a-glucosidase family plays pivotal role in physiological regulations,including carbohydrate metabolism,amino acid transport and glycoprotein modification.However,the transcriptional plasticity and adaptive evolutionary mechanism of a-glucosidase family in Lepidoptera is still unknown.In order to ascertain these questions,we studied the transcriptional plasticity and adaptive evolution of digestive and non-digestive a-glucosidase,including GH13 a-glucosidase and glucosidase ?,in Lepidoptera.Through the analyses of genetic expansion and divergent evolution toward the lepidopteran GH13 ?-glucosidase family,we found a sucrose-specific a-glucosidase of Lepidoptera,SUH,was diverged from the a-glucosidase family during the early evolution of Lepidoptera,then it duplicated into SUH1 and SUH2 in.These two differentiation times were in accordance with the radiative eruption of seed plants and angiosperms,respectively.It reflected that the two outbreak events of plant orders exerted great selective pressures to the survival and feeding habits of Lepidoptera,even caused the duplication and differentiation of digestive enzymes.Moreover,we found SUH underwent significant functional divergence compared with other GH13 ?-glucosidases revealed by functional divergence analysis.Related amino acids for functional divergence were also identified.Selective pression analysis showed that SUH went through significant positive evolution during the evolution process.Nine sites were determined which affected by both of the functional divergence and positive evolution.By homologous modeling,we obtained the spatial structural model of SUH and predicted its sucrose-binding domain.The above nine sites were located around the ligand-binding groove of SUH,and they could be responsible for the ligand-binding preference and hydrolytic specificity of SUH for sucrose,and indicated they are important in the divergence and evolution of SUH from the perspective of spatial structure.In addition,we performed the analysis of the effect of secondary metabolites to the physiological functions except digestion and detoxification in Lepidoptera,and used the adaptation of mulberry-specialist lepidopterans to the mulberry-derived alkaloid DNJ as an example,?-glucosidase II(GII),which is composed of a catalytic GII?subunit and a regulatory GII? subunit,is an evolutionary conserved enzyme that regulates glycoprotein modification in a range of species from bacteria to human.Here we found that the GlI? expression of the mulberry-specialist Lepidoptera,Bombyx mori,was significantly induced after DNJ treatment compared with the control and non-mulberry-specialist Lepidoptera,Samia cynthia ricini.However,GII? transcripts were not significantly changed in the two lepidopteran groups.These results revealed that the catalytic function of GII harbors a more effective plasticity of adaptation in the mulberry-specialist lepidopterans after DNJ treatment.Moreover,positive selections were detected in GII? when mulberry-specialist insects diverged from the lepidopteran order;whereas GII? was mainly subjected to purifying selection,thus indicating an asymmetrically selective pressure of GII subunits,and suggesting divergent evolution of catalytic and regulating functions.Moreover,positively selected sites were enriched in around the 1-DNJ binding sites and in the C-terminal region of the GII? subunit of mulberry-specialist lepidopterans revealed by the structural distribution analysis of positively selected sites.These sites probably affected the DNJ sensibility and conformational stability of GII complex in the mulberry-specialist lepidopterans.Our study revealed the adaptive evolutionary mechanism of digestive and non-digestive glucosidase family in lepidopterans from the perspective of molecular evolution.Our study indicated that positive selection,as an important evolutionary force for the adaptative evolution,drove the expansion and functional differentiation of GH13 a-glucosidases and the exclusive occurrence Lepidoptera-specific sucrase SUH,and promoted the plastic evolution of catalytic function under the DNJ stress of non-digestive a-glucosidase.This study will contribute to the understanding of the evolutionary mechanism of specific digestion and absorption in the insect,and provide background information and theoretical support to the research that related to differentiation,expansion and divergent evolution of functional genes in the insect.