Patch size of gall-forming aphids: deme formation revisited

The dynamics and the timeline of speciation in the gall-forming aphid Geoica spp. within and among Pistacia host tree species

Trees of the genus Pistacia serve as obligate hosts for gall-forming aphids (Hemiptera, Aphididae, Fordini). Each aphid species induces a characteristic gall on a single Pistacia host species. The genus Geoica (Fordini) induce similar spherical closed galls on the lower side of the leaflet's midvein, on different Pistacia species. Two species of Pistacia trees that harbor Geoica galls grow naturally in Israel: P. palaestina and P. atlantica. We analyzed the phylogeny and the genetic structure of the Geoica species complex in Israel, and assessed the genetic differentiation and the level of host plant specificity of the aphids between P. atlantica and P. palaestina. We found that the splitting of the genus between P. atlantica and P. palaestina is estimated to have occurred 24-25 Ma (the Oligocene/Miocene boundary). Five different haplotypes suggesting five different species have been further speciating among Geoica spp., galling on P. atlantica, and an additional three species, on P. palaestina.

Transcriptional up-regulation of host-specific terpene metabolism in aphid-induced galls of Pistacia palaestina

Galling insects gain food and shelter by inducing specialized anatomical structures in their plant hosts. Such galls often accumulate plant defensive metabolites protecting the inhabiting insects from predation. We previously found that, despite a marked natural chemopolymorphism in natural populations of Pistacia palaestina, the monoterpene content in Baizongia pistaciae-induced galls is substantially higher than in leaves of their hosts. Here we show a general up-regulation of key structural genes in both the plastidial and cytosolic terpene biosynthetic pathways in galls as compared with non-colonized leaves. Novel prenyltransferases and terpene synthases were functionally expressed in Escherichia coli to reveal their biochemical function. Individual Pistacia trees exhibiting chemopolymorphism in terpene compositions displayed differential up-regulation of selected terpene synthase genes, and the metabolites generated by their gene products in vitro corresponded to the monoterpenes accumulated by each tree. Our results delineate molecular mechanisms responsible for the formation of enhanced monoterpene in galls and the observed intraspecific monoterpene chemodiversity displayed in P. palaestina. We demonstrate that gall-inhabiting aphids transcriptionally reprogram their host terpene pathways by up-regulating tree-specific genes, boosting the accumulation of plant defensive compounds for the protection of colonizing insects.

Differences in Monoterpene Biosynthesis and Accumulation in Pistacia palaestina Leaves and Aphid-Induced Galls

Certain insect species can induce gall formation on numerous plants species. Although the mechanism of gall development is largely unknown, it is clear that insects manipulate their hosts' anatomy, physiology, and chemistry for their own benefit. It is well known that insect-induced galls often contain vast amounts of plant defensive compounds as compared to non-colonized tissues, but it is not clear if defensive compounds can be produced in situ in the galled tissues. To answer this question, we analyzed terpene accumulation patterns and possible independent biosynthetic potential of galls induced by the aphid Baizongia pistaciae L. on the terminal buds of Pistacia palaestina Boiss. We compared monoterpene levels and monoterpene synthase enzyme activity in galls and healthy leaves from individual trees growing in a natural setting. At all developmental stages, monoterpene content and monoterpene synthase activity were consistently (up to 10 fold on a fresh weight basis) higher in galls than in intact non-colonized leaves. A remarkable tree to tree variation in the products produced in vitro from the substrate geranyl diphosphate by soluble protein extracts derived from individual trees was observed. Furthermore, galls and leaves from the same trees displayed enhanced and often distinct biosynthetic capabilities. Our results clearly indicate that galls possess independent metabolic capacities to produce and accumulate monoterpenes as compared to leaves. Our study indicates that galling aphids manipulate the enzymatic machinery of their host plant, intensifying their own defenses against natural enemies.

The mono - and sesquiterpene content of aphid-induced galls on Pistacia palaestina is not a simple reflection of their composition in intact leaves

Pistacia palaestina Boiss. (Anacardiaceae), a sibling species of P. terebinthus also known as turpentine tree or terebinth tree, is common in the Levant region. The aphid Baizongia pistaciae L. manipulates the leaves of the plant to form large galls, which provide both food and protection for its developing offspring. We analyzed the levels and composition of mono-and sesquiterpenes in both leaves and galls of ten naturally growing trees. Our results show that monoterpene hydrocarbons are the main constituents of P. palaestina leaves and galls, but terpene levels and composition vary among trees. Despite this inter-tree variation, terpene levels and compositions in galls from different trees resemble each other more than the patterns displayed by leaves from the same trees. Generally, galls contain 10 to 60 fold higher total terpene amounts than leaves, especially of the monoterpenes α-pinene and limonene. Conversely, the leaves generally accumulate more sesquiterpenes, in particular E-caryophyllene, germacrene D and δ-cadinene, in comparison to galls. Our results clearly show that the terpene pattern in the galls is not a simple reflection of that of the leaves and suggest that aphids have a strong impact on the metabolism of their host plant, possibly for their own defense.