| The climate warming trend appears to be evident with an increasing frequency of drought events in Northwestern China, which may become one of the greatest challenges for cereal production under future warming scenarios. The impacts of climate change on insect pest outbreaks have been controversial. Phloem feeders, such as aphids, are often assumed to be sensitive to increased drought events expected under climate change. To explore the potential effects of water-deficit stress on aphid responses, the English grain aphid [Sitobion avenae(Fabricius)] clones from areas of different drought levels were collected. The life histories and behaviors of collected clones were then compared on wheat under well-watered,moderately water-stressed and severely water-stressed conditions in the laboratory. We also characterized their molecular variation using six microsatellite loci and analyzed the differences among populations in genetic structure.For our experiments, we developed initially protocols of creating short term and long term water stress conditions by maintaining specific amounts of soil moisture contents and water potential of host plant leaf under laboratory conditions. In our study, we used three water-stress levels: control(70% ~ 80% of field capacity and water potential range of-0.0025 MPa ~-0.02 MPa), moderate water stress(55% ~ 65% of field capacity and soil water potential range of-0.02 MPa ~-0.035 MPa), and severe water stress(35% ~ 45% of field capacity and soil water potential <-0.045 MPa). The corresponding waterpotentials of wheat leaves in the three treatments were kept at a range of-0.12 MPa to-0.2 Mpa,-0.2 MPa to-0.6MPa, and-0.6 MPa to-0.8 Mpa, respectively. We found that moderate and severe water-deficit stress not only reduced water potentials of wheat leaves, but also prolonged the developmental time, reduced tiller numbers and fresh plant materials above the ground, and increased nitrogen contents and leaf temperatures. Along with the extension of stress level and stress time, these changes are more significant.As to the test aphid, our results demonstrated that semi-arid area clones of S. avenae had longer developmental times, shorter reproductive times, lower fecundities, and lower net reproductive rates compared with moist area clones. Age-specific reproductive rates of moist area clones tended to be higher than those of semi-arid area clones. Significant differences between semi-arid and moist area clones were found for the survival functions when testedunder water-stressed conditions, and semiarid area clones tended to have a lower survival rate than moist area clones throughout their lives. “Population origin”(i.e., semi-arid and moist area clones) and “clone” together explained 62.74% ~ 96.56% of the total variance of tested life-history traits, suggesting the genetic basis for differentiation of clones from both areas.Significant differences in correlations, and selection differentials and gradients of life-history traits were also found between clones from both areas, providing further evidence of genetic basis for the life-history differentiation between them.Choice bioassays showed that winged and wingless adults of first and fifth generation S.avena from moist and arid areas preferred well-watered host plants over severe water-stressed plants. Preference of severe water-stressed plants by fifth generation alatae from arid areas was higher than by the first generation apterous adults. Although all aphids from arid and moist areas were maintained on moderate water-deficit stress conditions, non-significant changes in adaptability or preference were observed after 5 generations. In contrast, both winged and wingless aphids of fifth generation S. avenae from semi-arid areas showed higher preference to severely water-deficit stressed plants, compared to the first generation.Under different water-deficit stresses, feeding behavior of S. avenae was observed using EPG(electrical penetration graph) technique. The results showed that, under severe water-deficit stresses, C and PD waves of S. avenae were significantly increased compared to those under the well-watered treatment, whereas the E2 wave was significantly reduced.Aphids from semi-arid and arid areas showed reduced C wave durations on well-watered plants compared to those from moist areas, but E2 wave durations were significantly increased. Compared to moist area aphid clones, arid area aphid clones spent more time on feeding. Under severe water-deficit treatments, aphids from moist areas showed significantly longer “G” wave durations than under control, while aphids from semi-arid and arid areas showed non- significant differences between the treatments. This indicates that S. avenae from semi-arid and arid areas may have had certain experiences of water-deficit stressesin the field.We established water-stressed S. avenae populations for 5, 10 and 15 generations to compare their life-history parameters with those of the first generation. Our results revealed significantly shortened developmental times of aphid nymphs from moist area. However, 10 d fecundity and weight of adults were significantly reduced at 15 thgeneration, indicating their inadaptability against water-deficit stress. Aphids from semi-arid area presented longer developmental times at 5th generation, but significantly reduced at 10 th and 15 th generation.Weight and 10 d fecundity showed quite similar patterns, and and they were significantly reduced with the increase of generations and stress level. However, aphids from arid areasshowed different responses to the water-deficit stress, and they presented prolonged developmental time. But they showed increases in fecundity and body weight with the increase of generation, indicating their ability to adapt to the water-deficit stress. Overall,potential adaptation could occur in aphid clones from semi-arid and arid areas, since they may have had past experiences of water-deficit stresses(fluctuated intensity and duration).Genetic background, population structure and differentiation of S. avenae populations from different drought levels were studied using 6 microsatellite loci. Our results showed that these loci in different wheat aphid populations showed high genetic polymorphism. We analyzed the “Polymorphism Information Content” of the 6 microsatellite loci, and found their values varied from 0.6276 to 0.7560. They were all greater than 0.5, showing high polymorphism of every locus. For arid area populations, the allele numbers of loci(Na) were3.3333 ~ 10.6667, and the effective numbers of alleles(Ne) were 3.0240 ~ 6.7710, showing also the high polymorphism of these loci. Several S. avenae populations presented high genetic diversity. Most of the test populations showed expected heterozygosity(He) lower than the observed heterozygosity(Ho), inidicating that heterozygosities in the microsatellite loci were high, and excess of heterozygotes had occurred. Inbreeding coefficients of all populations(Fis) were negative, suggesting that these aphid populations mainly had the thelytoky reproductive mode. We compared all locations and found that arid area populations had higher Shannon information index(I) than other areas. As a result, the genetic diversity of S. aveane populations in arid areas was higher than that of semi-arid or moist areas.In this study, 9 different populations(3 from each area) had non-significant genetic structures, and low genetic differentiation between the populations of the same area was found. The populations from moist areas had higher level of genetic differentiation(Fst =0.0578 ~ 0.14231) than those from semi-arid areas and arid areas(Fst =-0.00476 ~ 0.04376and-0.0163 ~ 0.06705, respectively). This shows that the wheat aphid population in the moist area may have higher adaptability to local environments. There was a certain degree of genetic exchanges between wheat aphid populations in most parts of samping areas. When the genetic similarity between populations was higher, and the genetic distance was lower(e.g.,LC-TC and MQXT-MQSL), population genetic exchanges were the most frequent, and the differentiation level between populations was low. Genetic exchanges between LT and MQSL populations were lowest, indicating that there was obvious genetic differentiation between the populations. Aphid population cluster analyses were carried out in this study by using the principal component analysis method and the construction of phylogenetic trees of aphid populations from all areas. Our results showed that there was a correlation between aphid population genetic structure and geographical environments. The results of principalcomponent analysis showed that there were significant differences in the distribution of aphid clones in differentareas. Analyses of molecular variance(AMOVA) showed that aphid individuals accounted for most of the total genetic variation, while populationsor areas explained a small proportion. Populations from moist, semi-arid and arid areas were segregated, and populations from three locations of the same area were clustered together,which further illustrated that S.avenae populations of different areas presented different genetic structures oand significant genetic differentiation among them.Our results demonstrated a genetic basis for the divergence of S. avenae populations among sampling areas. Test aphid clones of 1st generation tended to have lowered fecundity and weight under severe water stress. Semiarid and arid area clones tended to have higher heritabilities for test life-history traits than moist area clones. Moist and semiarid area clones showed higher adaptation to water stress levels of their original environments than arid area clones. After water-deficit stress of five generations, life-history heritability tended to increase for moist area clones, but decreased for semiarid area clones; adult weight declined for moist and semiarid area clones, but enhanced for arid area clones, indicating their changing adaptability. Adaptation potential of test clones from semiarid and arid areas, but not moist areas, tended to be enhanced by exposure to short-term water-deficit stress. Rapid changes in S. avenae's life-history and resulting evolutionary dynamics of this aphid under water-deficit conditions could have significant implications for aphid outbreaks under future climate change scenarios. |
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