| Under future global warming scenarios,drought is predicted to be more frequent.Drought can directly cause damages to plants,and affect the survival and reproduction of herbivorous insects as well.This is especially true for those insects like aphids that feed on phloem.We established populations in the laboratory for Sitobion avenae collected from different areas(i.e.,moist,semi-arid and arid)in Shaanxi and Gansu provinces.After rearing for 20 generations,30 generations and 40 generations under intermediate water-stress,we compared and analyzed the life-history traits of S.avenae from different regions under three water treatments(well-watered,intermediately stressed and severely stressed).We also investigated the feeding and choice behaviors of the aphid under different water treatment,as well as the underlying mechanisms.The main results are as follows:(1)For aphids sampled from moist areas,the total developmental time of nymphs for the first generation was significantly shorter than that of the 20 th,30th and 40 th generation on well-watered wheat;however there were no significant differences under intermediately stressed and severely stressed treatments;the results indicated that the adaptation of S.avenae to well-watered conditions could be undermined by continuous water-deficit stress.For aphids sampled from semi-arid areas,the total developmental time of nymphs for the 20 th,30th,and 40 th generation was significantly longer than that for the first generation,showing negative effects of continuous water-deficit stress.For arid-area aphids,no significant differences were found among generations for the total developmental time of nymphs on well-watered wheat;the total developmental time of nymphs for the first generation was significantly shorter than that for the 20 th,30th,and 40 th generation,when tested on intermediately stressed or severely stressed wheat;10-d fecundity of S.avenae under severe water stress was significantly lower than that under well watered conditions for 20 th,30th and 40 th generation.The results indicated that the intermediately water-stress had no effect on aphid adaptation to intermediately stressed and severely stressed wheat.For moist areas aphids,the total developmental time of nymphs for the 1st,20 th and 40 th generation on severely stressed wheat was significantly longer than that on well-watered wheat,as well as 10-d fecundity for the 1st,20 th and 30 th generation on severely stressed treatments was significantly lower than that on well-watered treatments.For aphids sampled from semi-arid areas,the total developmental time of nymphs for the first generation on intermediately stressed and severely stressed treatments was significantly longer than that on well-watered wheat,and 10-d fecundity for the 1st,30 th and 40 th generation on severely stressed treatments was significantly lower than on well-watered and intermediately stressed treatments,besides the 20 th generation on intermediately and severely stressed treatments was significantly lower than that on well-watered wheat.For arid areas aphids,the total developmental time of nymphs for the 20 th and 30 th generation on severely stressed wheat was significantly longer than that on well-watered,and the 20 th,30th and 40 th generation aphids on severely stressed treatments was significantly lower than that on well-watered wheat for the 10-d fecundity.In one word,compared to well-watered conditions,the developmental time of S.avenae from all three areas tended to increase under severe water stress,and its 10-d fecundity tended to decrease under severe water stress.(2)We evaluated the source of variation for the S.avenae population.Population origin,treatment and genotype(nested in population sources),showed significant effects on life history traits(i.e.,the total developmental times of nymphs,adult weight,10-d fecundity)for S.avenae.Genotype explained the highest proportion of the total variance for the above mentioned tested traits,followed by water-stress treatment and population origin.Thus,genetic factors might play a key role in S.avenae's responses to continuous water-deficit stress.(3)The electrical penetration graph(EPG)showed that aphid clones collected from arid areas had a significant increase in the number of probes,the number of probes to the 1st E1,the number of C and the total duration of C on the both plants,compared with those from moist areas.The results indicated that aphid clones of arid areas spent more time on probing during the feeding process in order to find an appropriate location for feeding.Arid area aphid clones under the severely stressed treatment showed significantly higher number,and longer duration of G than under the well-watered treatment,showing that aphids could spend more time on obtaining water under drought conditions.Moreover,moist area aphid clones on severely stressed wheat initiated more probes than those on well-watered wheat.Thus,aphids might consume more energy in finding a good feeding site under water-deficit conditions.(4)Dual-choice behavior assays showed that compared to severely stressed wheat,well-watered wheat were more attractive to 20 th generation aphid clones from moist areas during the first two hours.This suggested that the experience of intermediate water-deficit stress for 20 generations could not enhance S.avenae's preference of severely water-stressed wheat.However,after 20 generations of water-deficit stress,aphid clones from arid areas preferred severely-stressed wheat over well-watered wheat,indicating that long-term waterdeficit stress could increase the preference of these aphid clones for water stressed plants.(5)A total of 26 volatile compounds were identified.Nonanal and decanal were only collected and identified in water-stressed wheat,while methyl 2-ethylpentanoate,methoxy-phenyl-oxime and p-cymene were only collected in well-watered wheat.The above mentioned selectivity of the aphids to different water treated wheat may be affected by these specific volatiles under various moisture treatments,which requires further experiments to verify. |
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