The Mechanisms Of Nonrandom Distribution Of Mitochondrial DNA Variation In The Small Brown Plant-Hoppe (SBPH),Laodelphax Striatellus

The small brown planthopper(SBPH),Laodelphax striatellus(Fallen),which belongs to Hemiptera,Delphacidae,is an important agricultural pest and widely distributed in the world.Besides the direct sucking damage by nymphs and adults,the agricultural significance of SBPH lies in its ability to spread the plant disease.SBPH is able to overwinter in temperate climates,including all regions of China,and no seasonal migration is required between climatic areas of the species range.Although the domestic and international studies have proved that rice planthopper has long-distance dispersal ability,the population genetic structure and genetic diversity of SBPH were little known.The population genetic structure and diversity of 1328 SBPH samples from 22 regions of China were studied,based on the concatenated mitochondrial sequence 1368bp(including 769bp CO?,599bp CO?).The 22 sampling locations are along a gradient covering four climatic zones(moderate temperate zone,MT;warm-temperate zone,WT;northern subtropi-cal zone,NS;and southern subtropical zone,SS)in China.The results are as follow:(1)Population genetic diversity:genetic diversity of SBPH was high in China except populations from the moderate temperate zone,both the levels of haplotype diversity(Hd)and the levels of nucleotide diversity(?)were significantly lower in the moderate temperate zone(MT)than in the three remaining climatic zones:WT,SS,NS,furthermore,the levels of nucleotide diversity(?)were also significantly lower in SS than in WT and NS.(2)Pairwise FST:values computed from mtDNA data ranged from-0.078 to 0.442,with an average FST of 0.075.The average FST value between populations from the MT zone and populations from the three remaining climatic zones was 0.176.In comparison,the FST values for other between-zone population pairs were very low,with an average FST of 0.016.The principal coordinate analysis(PCoA)also showed that the six populations(MDJ,HRB10,HRB12,SJ,YJ11 and YJ12)from the MT zone were separated from the other populations.(3)The Bayesian inference(BI)phylogenetic tree and haplotype network derived from 107 concatenated mitochondrial DNA sequences revealed two well-supported haplogroups.These haplogroups,named HGI and HGII,had non-random distributions across the sampling localities.In general,the frequency of HGI(61.3%)was higher than that of HGII(38.7%)over all the sampled populations,and had an absolute dominance in the MT zone.We hypothesized that temperature plays an important role in shaping the non-random distributions of mitochondrial haplogroups,HGI having absolute dominance in the MT zone may indicate that the HGI mitochondrial haplotype is more favored under cold climates than the HGII mitochondrial haplotype.To elucidate the cold hardness of SBPH from different populations,the temperatures of supercooling point(SCP)of 4th SBPH from different population were evaluated.The results indicated that the average temperatures of SCP of 4th SBPH nymphs from Chuxiong,Nanjing,Mudanjiang were all below-23? and there was no significant difference among populations.To elucidate the cold hardness of different haplotype in 4th SBPH nymphs from the same population,the chill coma recovery time of different haplotype SBPH,after 2h exposure to-1? was measured.In general,nymphs having haplotype HGI recovered considerably faster from the chill coma than nymphs having haplotype HGII;the results indicated that nymphs having haplotype HGI had a stronger cold hardness.Therefore,SBPH having haplotype HGI is more favored under cold climates.The relative mtDNA copy levels between the females having haplotype HGI and HGII from Yunnan and Nanjing populations were compared.The relative expression levels of 12 protein-encoding mitochondrial genes after low-temperature treatment(-1?,2h)were also compared between the females having haplotype HGI and HGII,the main results are as follow:(1)The data indicated that the relative mtDNA copy levels of the females having HGI were significant higher than females having haplotype T at the room temperature.(2)The levels of 12 protein-encoding genes expression of the females of SBPH having HGI increased after low temperature treatment(-1 ?,2h),the relative expression levels of four genes(ndl,nd2,nd3,nd4),which encode NADH dehydrogenase,in the females having HGII decreased after low temperature treatment(-1?,2h),the relative expression levels of remaining genes all increased.(3)In control group,the relative expression levels of 12 mitochondrial genes were generally higher in females having HGII than that in females having HGI,but the difference was not significant.In low temperature group,only three genes(atp6,atp8,nd6)relative expression of the females of SBPH having HGII was higher than that of HGI,while the remaining nine gene expression quantity is lower than that of SBPH females having HGI,further more,the difference of cox2,ndl,nd2,nd4,nd5 genes expression reached significant level.The whole mitochondrial genomes of the two haplotype HGI and HGII were sequenced,three non-synonymous mutations were found,which located on the ND2,ND5 and CYTB.The treeSAAP software was used to analysis the selection presure of the three genes(ND2,ND5 and CYTB).The results showed that many significant physico-chemical properties of amino acid were changed becaused of the non-synonymous mutations.These changes suggested that mitochondrial DNA have occurred adaptive evolution during evolution history.