| As an important cereal pest worldwide, the English grain aphid, Sitobion avenae(Fabricius), is also widely distributed in China. Sitobion avenae can feed on various cereals,as well as on a wide range of wild grass hosts in the Poaceae. It has been shown that hostplants can be closely related to population differentiation of aphids, but their effects onpopulation differentiation of S. avenae remain elusive. In order to clarify host-associateddifferentiation or specialization of S. avenae, we studied the effects of host plants on S.avenae’s population differentiation and genetic structure. The main results are as follows:1. There were significant differences identified in nymphal developmental duration,mean relative growth rate and adult fecundity of S. avenae on different host plants underlaboratory conditions. Clones of S. avenae had lower developmental times, faster meanrelative growth rate and higher fecundities on wheat compared to other test plants. Thelongest developmental time and lowest fecundity for S. avenae were found when tested onryegrass. Life table parameters showed that S. avenae had higher intrinsic and finite rates ofincrease, and higher net reproductive rates on wheat than on barley or oat. The growth rates ofS. avenae were lower on ryegrass and cocksfoot than on cereals. Overall, the fitnessparameters of the tested S. avenae population on cereals were higher than those on grasses.2. The effects of host switch on life history traits were compared for S. avenae clonesfrom different host plants, and the results showed that wheat or oat clones had shorterdevelopmental time and adult lifespan, higher survival rate, higher fecundity and higherpopulation growth rates than barley clones when tested on the original host plant. Wheatclones showed lower lifespan and reproductive time after transferred to oat. When they weretransferred to wheat or oat, S. avenae clones from barley showed shorter developmental timeof nymph and higher net reproductive rate than on the original host. After transferred toalternative hosts, S. avenae clones showed significant differences in adult survival rate andfecundity. The fitness traits of S. avenae clones before and after host switch showed that theypresented significant differentiation on tested host plants.3. The fitness traits of barley clones and oat clones were compared in detail in a commonenvironment in the laboratory. The results demonstrated that clones from oat and barley had diverged to a certain extent in terms of fecundity and developmental time of the nymphs. Theseparation of barley clones and oat clones of S. avenae was also evident in a principalcomponent analysis. Barley clones tended to have higher broad-sense heritabilities for fitnesstraits than oat clones, indicating the genetic basis of differentiation between them. Barleyclones showed significantly higher extent of specialization compared to oat clones from twomeasures of specialization (i.e., Xsp and Ysp). Therefore, barley clones were specialized to acertain extent, but oat clones appeared to be generalized. The fitness of S. avenae clonestended to increase with higher extent of specialization.4. Three main secondary symbionts (Hamiltonella defensa, Regiella insecticola andRickettsia sp.) were detected in S. avenae by using16S rDNA diagnostic PCR. All threesymbionts were detected in Qinghai populations, and the infection of S. avenae by symbiontsH. defensa and R. insecticola was more common, especially on ryegrass and wildrye. Theinfection rate of R. insecticola on ryegrass and wildrye was68.8%and83.3%, respectively.Rickettsia sp. showed relatively higher infection rates in Shaanxi populations, and its infectionrate on oat and ryegrass in Shaanxi was86.7%and69%, respectively. Infections of H.defensa and R. insecticola were found only sporadically in individual populations. Host plantand geographical location both had significant influence on symbiotic infection of S. avenae.Logistic regression showed that infection rates for R. insecticola were significantly affectedby host plant, but not for the other two symbionts.5. Five SSR loci were employed to investigate the genetic variation and genetic structurefor populations of S. avenae from different host plants. The results showed that the five SSRloci were high polymorphic. The S. avenae populations from different host plants in Qinghaidisplayed a higher level of genetic diversity compared with those in Shaanxi. The values ofpairwise Fst ranged from0.0166to0.2690, and tested populations showed certain degree ofgenetic differentiation. Genetic differentiation between wheat and barley clones from Qinghaiprovince was low, but it was very high between wheat clones from Yangling and oat clonesfrom Qinghai. AMOVA results indicated that21.5%of overall variation occurred amongpopulations and78.5%within populations. Bayesian clustering analysis showed thatpopulations from the same geographic location had similar genetic structure, and the geneticdistance between wheat and barley clones was smaller than that between wheat and oat clones.It appeared that the divergence between wheat and barley clones was low, but it was highbetween oat and wheat (or barley) clones.In summary, S. avenae showed differential adaptation on different host plants, and it wasbetter adapted to crops than to uncultivated grasses (suggested by higher fitness parameters oncrops). Some S. avenae clones presented certain degree of specialization on particular host plants. Host plant showed significant effects on the infection rates of R. insecticola,suggesting a potenial role of secondary symbionts on population divergence of S. avenae.Populations of S. avenae from the same geographic location had similar genetic structurebased on SSR markers, and it appeared that the divergence between oat and wheat (or barley)clones was high. Therefore, host plant and secondary endosymbionts can both play importantroles in the differentiation of S. avenae populations, and further studies are needed to clarifythe specific causes and mechanism for the divergence patterns identified. |
PDF Full Text Request