| Pea aphid, Acyrthosiphon pisum Harris (Homoptera: Aphididae), is one of most devastating insect pests of leguminous forage, which affect the quality and quantity of herbage production. Intraspecies differentiation of pea aphid has already caused the theoretical and practical attention. The exploration of this pest population genetic structure and quantity control mechanism helps formulating appropriate control strategy and effective prevention measures. Genetic diversity of A. pisum populations from different geographic regions in China was studied by SSR technology. From functional response of two coccinellid predators to A. pisum and the resistant evaluation and mechanism of four alfalfa varieties to A. pisum, the mechanism of the population regulation was explored. All results show as follows:1. Genetic diversity of A. pisum populations from different geographic regions in ChinaThe genetic diversity among ten geographic populations of A. pisum in China was investigated using Simple Sequence Repeat (SSR) markers in this study. The result showed that 45 polymorphic locus were detected using 19 pairs of SSR primers in the 300 individuals of 10 populations,percentage of polymorphic bands were 100%. The number of observed alleles was 1.5889, and that of effective alleles was1.35333, the Nei's genetic diversity and Shannon-Wiener diversity indexes were respectively 0.20496 and 0.30538, percentage of polymorphic locus were 58.89%. The population genetic diversity of Hutubi, Yangling and Zhengzhou were higher, Huhehaote, Jinan and Minhe was lower. A cluster analysis showed that genetic similarity coefficient around 0.60, the populations could be divided into two groups, one containing Jinan and Zhengzhou populations, and one containing the rest of the populations. Analysis of AMOVA showed that there were about 49% of the total variations among populations, 51% presented within populations. Mantel-test showed that genetic distances had no significant correlation with geographic distance and also with the gap of elevation among populations.2. Functional response of two coccinellid predators, Coccinella septempunctata and Propylaea japonica, to A. pisumPlotting prey density against prey killed by four larval instars, and adult males and females of C. septempunctata fit well with the type-II. The adult females had the highest attack rate and shortest handling time. The prey consumption differences of C. septempunctata in the same stage under various prey densities were significant. Based on the functional response data, the model predicted a maximum of 5.7, 11.5, 32.1, 59.5, 57.1, and 76.9 nymphs to be consumed per day by individual first, second, third, fourth instars, adult male, and adult female, respectively.Plotting prey density against prey killed by four larval instars, and adult males and females of P. japonica fit well with the typeâ…¡model of Holling's disc equation. Adult females consumed the highest number of prey, followed by fourth instars and Adult males. Based on the functional response data, the model predicted a maximum of 5.5, 14.1, 29.2, 45.1, 42.3 and 51.5 nymphs to be consumed per day by and individual first, second, third, fourth instars, adult male and adult female, respectively.Rate of attack, handling time and the daily maximum amount were differences between C. septempunctata and P. japonica in the same instar. The daily maximum amount of C. septempunctata were higher than P. japonica, while the handling time of C. septempunctata were lower than P. japonica, when two ladybirds were in the same stages.3. Evaluation on resistance of 4 Medicago cultivars to A. pisumResistance of 4 alfalfa cultivars to A. pisum was evaluated by mass-infesting young seedlings in field and lab, in order to select high resistant material to pea aphid. The result showed that % of resistant plant of Gannong No.5(HA-3)was significantly higher than the others in field and lab, 50.3%,48.9%, respectively. However, % of resistant plant of Hunter River was low in field and lab, 4.0%, 4.3%, respectively. Resistant class of 4 alfalfa cultivars in field were Gannong No.5 (High Resistance, HR), Gannong No.3(Susceptible, S), Golden Empress (Susceptible, S), Hunter River (Susceptible, S), while resistant class of 4 alfalfa cultivars in lab were Gannong No.5 (Resistance, R), Gannong No.3(Low Resistance, LR), Golden Empress(Low Resistance, LR), Hunter River(Susceptible, S).4. Antibiosis of alfalfa cultivars to two color morphs of A. pisumBiology of two color morphs of A. pisum was studied on four cultivars of alfalfa (Medicago sativa L.) in order to identified antibiosis of alfalfa cultivars to two color morphs of pea aphids. The results showed that two color morphs of pea aphids were different in mean development time, net reproductive rate, generation time on JH, in intrinsic rate of increase, population doubling time, infinite rate of increase on G5. Using the innate capacity of increase (r_m) as the index of alfalfa antibiosis to the aphid, antibiosis of four alfalfa cultivars were different to two color morphs of pea aphid, resistance of JH to green aphid was higher and resistance of G5 to pink aphid was higher, while resistance of Hu was lower to two morphs of pea aphids. The level of alfalfa resistance to green aphid and to pink aphid were in order of Hu<G3<G5<JH, and Hu<JH<G3<G5, respectively.5. Physiological and biochemical resistance mechanism of M. sativa to A. pisumThe SOD and POD activities in the susceptible variety (Hu) appeared lower than that of the resistant variety (G5) and low resistant varieties (G3, JH) when their leaves were piercing-sucked by the aphid. The activities of PPO of resistant variety (G5) were higher than that of the susceptible variety (Hu) and low resistant varieties (G3, JH) before invaded by pea aphids, whereas the activities of PPO of resistant variety (G5) decreased rapidly, were lower than that of the susceptible variety (Hu) after damaged by pea aphids. In the period of the aphid attacks, the MDA contents of resistant variety (G5) were lower than that of the susceptible variety (Hu) and low resistant varieties (G3), the total phenolic content and CAT activity of resistant variety (G5) were higher than that of the susceptible variety (Hu) and low resistant varieties (G3, JH), the chlorophyll content of the susceptible variety (Hu)was higher than that of low resistant varieties (G3, JH), lower than that of resistant variety (G5), and that the soluble protein content of resistant variety (G5) and the susceptible variety (Hu) were lower than that of low resistant cultivars (G3, JH), except on the third day were higher than soluble protein content of G3, the Tannin contents of resistant variety (G5) and the susceptible variety (Hu) displayed lower than that of the low resistant ones (G3 and JH). Throughout the period of the aphid attacks, the activities of SOD and POD of all varieties rose firstly and then declined, while PPO of all varieties rose firstly and then declined. The chlorophyll and soluble protein contents and the activities of CAT displayed a decreasing trend in all of the varieties, while total phenolic content appeared increasing trend. The dynamics of MDA and Tannin contents of four alfalfa cultivars showed irregular. It indicated that the SOD, POD, CAT, MDA and total phenolic content were positive correlation with the resistance of M. sativa to pea aphid, while the chlorophyll, soluble protein contents, Tannin contents, PPO were not explicit correlation with that. |
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