| The diamondback moth(DBM),Plutella xylostella(L.)(Lepidoptera,Plutellidae),is a Lepidoptera pest that mainly attacks cruciferous plants,including oilseed rape,cauliflower,and cabbage and so on.Because of the abundance of cultivated and weedy host plants,widespread resistance to insecticides,limited natural enemy impact as a consequence of insecticide use,and high fecundity,it has become one of the most widely distributed lepidopteran pest species in the world.The total cost of damage and management worldwide for P.xylostella has reached up to 5 billion per annum.Currently,there are still no effective strategies to control this very prolific insect in China.Previous studies had found that there is an obvious inbreeding depression in P.xylostella population,which is biologically important for conservation and pest management.However,the fitness-related traits involved in inbreeding depression,the correlation with inbreeding history,and its molecular genetic mechanism are currently unknown.In accordance with the above questions,we explored the manifold effects of serially inbreeding on the population fitness of P.xylostella and their molecular genetic mechanisms based on biology and genomics.The main results are as follows:1.The manifold effects of inbreeding on the population fitness in Plutella xylostellaUsing age-stage-specific life tables,we quantified the inbreeding effects on fitness-related traits and demographic parameters of P.xylostella.Egg hatching rate,survival and fecundity of the inbred line significantly declined compared to those of the outbred line over time and these fitness-related traits can be profoundly affected by the duration of inbreeding.The prolonged development duration of egg,larva and pupa were observed in the inbred line,but they varied randomly among the different inbred generations.There were sex differences in the effects of inbreeding on adult longevity.Inbreeding profoundly affected the longevity of male adult,but there was no obvious correlation between the recession degree and inbred generations.However,the shortened longevity of female adult(especially males)were observed only in the inbred F10.The inbred P.xylostella line showed significantly lower intrinsic rate of increase(r),net reproduction rate(R0),and finite increase rate(λ),and increasing generation time(T).In addition,population projection indicated the population quantity of P.xylostella was also remarkable influenced by the duration of inbreeding,which might not sustain the population in inbred F10.Our work provides a foundation for further studies on molecular and genetic bases of the inbreeding depression for P.xylostella.2.The effects of backcross on inbreeding depression of Plutella xylostellaThe backcross was used for the inbred F10 of P.xylostella,whereafter we quantified the effects of backcross on the purging of inbreeding depression of P.xylostella based on the age-stage-specific life tables.The egg hatching rate,survival and fecundity of the inbred F10 of P.xylostella can be significant increased after the backcross,and the prolonged longevity of female and male adult were observed in the backcross line.The population fitness of P.xylostella in the backcross line could get the same or higher level as the outbred line.The backcross P.xylostella line showed significantly higher intrinsic rate of increase(r),net reproduction rate(R0),and finite increase rate(λ)and increasing generation time(T),which showed no significant difference compared to the outbred population.Additionally,population size and stage of P.xylostella did not show any significant different trends in backcross and outbred line,from an initial 10 eggs,there population were predicted to exceed 108 individuals after 60 d in either line.3.Identification and functional analysis of the key genes associated withinbreeding depression of Plutella xylostellaWe performed whole-genome resequencing for both sex of P.xylostella in the initial population(F0),the inbred and outbred line(F1,F4,F7,F10),and the backcross line(BC1,BC2),respectivey,using Illumina HiSeq 2000 platform and produced with an average of 7.62 and 7.87 Gb raw data for female and male in all samples,respectively.The quality of the resequencing was high,with an average Q20 percentages of more than 94%for both sex in all samples,while all of the GC contents were about 38%.In total,we obtained an average depth of 12 X for all the samples.The mean genome mapping rate for female and male was 80%,while the mean genome coverage was also 80%.The total of SNPs of male and female in outbred line was higher,but the number of homozygous SNP was significantly than that in the corresponding inbred line,respectively.The number of homozygous SNP in female and male increased significantly with increasing inbred generations.We identified 8,448,632 and 8,603,538 homozygous SNP in female and male,respectively,relative to the reference genome of P.xylostella,with 231,790 and 233,540 are in CDSs,respectively.The heterozygosity of P.xylostella genome was significantly increased after backcross,and the homozygous SNP and Indel sites recovered rapidly.Phylogenetic tree analysis showed that genetic affinity is positively correlated with inbreeding degree.Interestingly,BC1,BC2,and the outbred F7 and F10 were clustered into a large branch,which indicates that the genetic load of inbreeding for P.xylostella can be eliminated rapidly by the backcross.Additionally,the selective sweeps were analyzed by HpRatio and Fst-pi in the outbred,inbred and backcross lines,and the related genes were annotated with GO,KOG and KEGG database.Finally,a comprehensive analysis was conducted on the homozygous SNP and Indel with non-synonymous mutations and HpRatio and fst-pi selective sweeps sites.Ten and three genes in female and male genome associated with inbreeding depression were identified,which may play important part in food utilization,environmental adaptation,energy metabolism,and development and reproduction of P.xylostella. |
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