The Chromosome-Based Rubber Tree Genome Provides New Insights into Spurge Genome Evolution and Rubber Biosynthesis

The rubber tree,Hevea brasiliensis,produces natural rubber that serves as an essential industrial raw material.Here,we present a high-quality reference genome for a rubber tree cultivar GT1 using single-molecule real-time sequencing(SMRT)and Hi-C technologies to anchor the～1.47-Gb genome assembly into 18 pseudochromosomes.The chromosome-based genome analysis enabled us to establish a model of spurge chromosome evolution,since the common paleopolyploid event occurred before the split of Hevea and Manihot.We show recent and rapid bursts of the three Hevea-specific LTR-retrotransposon families during the last 10 million years,leading to the massive expansion by～65.88%(～970 Mbp)of the whole rubber tree genome since the divergence from Manihot.We identify large-scale expansion of genes associated with whole rubber biosynthesis processes,such as basal metabolic processes,ethylene biosynthesis,and the activation of polysaccharide and glycoprotein lectin,which are important properties for latex production.A map of genomic variation between the cultivated and wild rubber trees was obtained,which contains～15.7 million high-quality single-nucleotide polymorphisms.We identified hundreds of candidate domestication genes with drastically lowered genomic diversity in the cultivated but not wild rubber trees despite a relatively short domestication history of rubber tree,some of which are involved in rubber biosynthesis.This genome assembly represents key resources for future rubber tree research and breeding,providing novel targets for improving plant biotic and abiotic tolerance and rubber production.

The actions of neonicotinoid insecticides on nicotinic acetylcholine subunits Ldα1 and Ldα8 of Leptinotarsa decemlineata and assembled receptors

The insect nicotinic acetylcholine receptor(nAChR)is a pentameric channel protein and also a target for neonicotinoids.There are few reported studies on the molecular interactions of Leptinotarsa decemlineata nAChRs with neonicotinoids.In this study,we analyzed the response of acetylcholine and neonicotinoids(thiamethoxam[TMX],imidacloprid[IMI],and clothianidin[CLO])on hybrid receptors constructed by nAChRα1 andα8 subunits of L.decemlineata(Ldα1 and Ldα8)co-expressed with ratβ2 subunit(rβ2)at different capped RNA(cRNA)ratios in Xenopus oocytes.In addition,we evaluated the expression changes of Ldα1 and Ldα8 after median lethal dose of TMX treatment for 72 h by quantitative polymerase chain reaction(qPCR).The resulting functional nAChRs Ldα1/rβ2 and Ldα1/Ldα8/rβ2 showed different pharmacological characteristics.The neonicotinoids tested showed lower agonist affinity on Ldα1/Ldα8/rβ2 compared to Ldα1/rβ2 at same ratios of subunit cRNAs.The sensitivities of neonicotinoids tested for Ldα1/rβ2 and Ldα1/Ldα8/rβ2 at cRNA ratios of 5:1,1:1 and 5:5:1,1:1:1,respectively,were lower than those for nAChRs at ratios of 1:5 and 1:1:5,respectively,whereas the values of maximum response(Imax)varied.For Ldα1/Ldα8/rβ2,a reduction of Lda8 cRNA resulted in increased sensitivity to IMI and decreased sensitivity to TMX.The expression of Ldα1 and Ldα8 significantly decreased in adults by 82.12%and 47.02%,respectively,while Ldα8 was significantly upregulated by 2.44 times in 4th instar larvae after exposure to TMX.We infer that Ldα1 and Ldα8 together play an important role in the sensitivity of L.decemlineata to neonicotinoids.