| Insect populations feeding on different plant species are under selection pressure to adapt to natural selection and environmental changes, leading to the formation of different specialized host-adapted biotypes. Host specialization is a very common phenotype in sap-sucking insects, the most typical is aphids. The pea aphid, Acyrthosiphon pisum Harris(Hemiptera: Aphididae), is a polyphagous sap-sucking pest insect, whose host-plant range covers dozens of plant genera of the legume family Fabaceae, such as pea and alfalfa, leading to a decrease in agricultural production. To date, this genetic differentiation among host-adapted pea aphid genotypes has generated at least 11 well-distinguished sympatric host races. Aphids feed on the phloem sap of host plants through stylet penetration, during which saliva from salivary gland is secreted in sieve tube. Salivary gland, as the main feeding organ, may play an important role in host-adaptation. However, the role of salivary-gland-expressed genes involved in host alteration are still almost unknown. Thus, exploring the relationship among fitness, feeding behavior and salivary gland gene expression of aphids on different host plants is an important issue to understand how aphids to adapt host plants.In this study, we used a laboratory experience population feeding on Vicia faba through short-term shift to or long-term acclimation on other three new Fabaceae plants(V. faba, Vicia villosa, Medicago truncatula, and Medicago sativa) and compared its life tables, feeding behavior, and salivary gland gene expression of pea aphids feeding on four different preferred degree host palnts, to reveal the molecular mechanism of pea aphid-host adaptation. The main results were as follows:1. After six month’s acclimation, stable colonies were established on V. villosa and M. truncatula, but not on M. sativa. The body sizes of aphids varied when aphids fed on different host plants. Taking the adults as an example, the average body length of V. faba clone was the largest, followed by M. truncatula colony, and the smallest was V. villosa colony.2. From the life table experiment in the short-term shift, when the V. faba clone was temporarily transferred to V. villosa, M. truncatula, or M. sativa, remarkable decreases were observed in the net reproductive rate, intrinsic rate of increase, and finite rate of increase, especially in M. sativa, on which the pea aphids had the lowest net reproductive rate, intrinsic rate of increase, and finite rate of increase. However, the mean generation time(T) and the periods of nymph and adult were significantly increased during the short-term shift. A high nymphal mortality(more than 30%) was observed when the V. faba clone was temporarily transferred to M. sativa. From the life table experiment in the long-term acclimation, the V. faba clone had the highest fitness, followed by the M. truncatula colony, and then the V. villosa colony. The V. faba clone had the shortest longevity(24 d), whereas the V. villosa colony had the longest longevity(41 d), indicating a difference of 1.7-fold, especially an obvious difference was in adult longevity(1.9-fold). According to body size and life tables, the fitness of pea aphids on different host plans for degrees from high to low is V. faba, M. truncatula, V. villosa and M. sativa.3. The electrical penetration graph(EPG) technique was employed to monitor the feeding behavioral performance of the V. faba clone after short–term shifted to or long-term acclimation on V. villosa, M. truncatula and M. sativa. When the V. faba clone was temporarily transferred to other three host plants, the time spent in passive ingestion of phloem sap(E2 wave) decreased dramatically, and the non-probing(np wave) time increased significantly. In addition, when temporarily transferred to V. villosa, the V. faba clone spent more time probing plant cells to seek phloem sap(C wave). When temporarily transferred to the two Medicago plant species, the V. faba clone met penetration difficulties owing to derailed stylet mechanics(F wave). After acclimation on M. truncatula or V. villosa for six months, the feeding behavior of the two colonies improved. In both M. truncatula and V. villosa colony, the time of watery salivation(E1 wave) and passive ingestion of phloem sap(E2 wave) increased, and the non-probing(np wave) time decreased significantly. The time of penetration difficulties(F wave) were lessened remarkably in the M. truncatula colony compared with the V. faba clone temporarily shifted to M. truncatula. Surprisingly, the V. villosa colony spent more time in drinking water from xylem(G wave) compared with the V. faba clone temporarily shifted to V. villosa.4. Microstructure of different host plant leaves varied obviously. The differences in feeding behavior of pea aphids on different host plants could be partially related to the leaf microstructures. Among the four plants, the cells of V. faba were largest, whereas the cells of V. villosa were smallest; the cells of V. villosa were arranged most loosely, whereas the cells of the two Medicago plants were arranged most closely, especially the palisade tissue cells of M. sativa.5. To reveal to the molecular mechanism of pea aphid host adaptation, we conducted transcriptomic analysis in the salivary glands of pea aphids from V. faba clone after short-term shifted to V. villosa, M. truncatula and M. sativa for 5 h or long-term acclimation on V. villosa and M. truncatula. Gene expression in salivary glands of the V. faba clone was compared before and after this clone was transferred to M. truncatula, M. sativa, or V. villosa for feeding 5 h. The expression levels of 411, 438, and 90 genes were changed on M. truncatula, M. sativa, or V. villosa, respectively. Only 8 gene was commonly varied and downregulated during the short-term shift process. The commonly downregulated genes included 5 heat shock proteins. The degree of gene expression change when the V. faba clone was transferred to the two Medicago plants was more similar than when the V. faba clone was transferred to V. villosa. After six months, gene expressions in salivary glands of the M. truncatula and the V. villosa acclimation colonies were compared with those of the V. faba clone. A total of 156 genes and 188 genes were differentially expressed in the M. truncatula and V. villosa colonies, respectively. The commonly varied genes in the two colonies numbered 72, including 50 upregulated genes and 22 downregulated genes. Whatever the short-term period or the long-term period, the number of genes that varied in a specific colony was negatively correlated with the fitness of the aphids on these host plants. However, there were few genes in common on the same host plant between those differentially-expressed after the short-term shift and long-term acclimation periods. Most genes(over 80%) varied only under one state.6. From the quantitative verified differential genes, gene ACYPI27259 was choosed to explore gene function. Compared with dsGFP injection, the survival curves of the M. truncatula colony after ds27259 injection have no significant difference on M. truncatula, but have a significant difference on V. faba. The feeding bahaviors of the M. truncatula colony after injection have no significant difference on these two plants.In conclusion, our study reveals the plasticity, feeding behaviors and molecular mechanisms involved in aphid-host plant alteration from survival and feeding behavior to the standpoint of gene expression in salivary glands. This study has an important theoretical and practical significance in further selection of new insect-resistant plants and development of improved management strategies of piercing-sucking pest insects. |
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