Heritable Phytohormone Profiles of Poplar Genotypes Vary in Resistance to a Galling Aphid

Insect galls are highly specialized structures arising from atypical development of plant tissue induced by insects. Galls provide the insect enhanced nutrition and protection against natural enemies and environmental stresses. Galls are essentially plant organs formed by an intimate biochemical interaction between the gall-inducing insect and its host plant. Because galls are plant organs, their development is likely to be governed by phytohormones involved in normal organogenesis. We characterized concentrations of both growth and defensive phytohormones in ungalled control leaves and galls induced by the aphid Pemphigus betae on narrowleaf cottonwood Populus angustifolia that differ genotypically in resistance to this insect. We found that susceptible trees differed from resistant trees in constitutive concentrations of both growth and defense phytohormones. Susceptible trees were characterized by significantly higher constitutive cytokinin concentrations in leaves, significantly greater ability of aphids to elicit cytokinin increases, and significantly lower constitutive defense phytohormone concentrations than observed in resistant trees. Phytohormone concentrations in both constitutive and induced responses in galled leaves exhibited high broad-sense heritability that, respectively, ranged from 0.39 to 0.93 and from 0.28 to 0.66, suggesting that selection can act upon these traits and that they might vary across the landscape. Increased cytokinin concentrations may facilitate forming strong photosynthate sinks in the galls, a requirement for galling insect success. By characterizing for the first time the changes in 15 phytohormones belonging to five different classes, this study offers a better overview of the signaling alteration occurring in galls that has likely been important for their ecology and evolution. [Formula: see text] Copyright © 2019 The Author(s). This is an open-access article distributed under the CC BY-NC-ND 4.0 International license .

Genetics-based interactions of foundation species affect community diversity, stability and network structure

We examined the hypothesis that genetics-based interactions between strongly interacting foundation species, the tree Populus angustifolia and the aphid Pemphigus betae , affect arthropod community diversity, stability and species interaction networks of which little is known. In a 2-year experimental manipulation of the tree and its aphid herbivore four major findings emerged: (i) the interactions of these two species determined the composition of an arthropod community of 139 species; (ii) both tree genotype and aphid presence significantly predicted community diversity; (iii) the presence of aphids on genetically susceptible trees increased the stability of arthropod communities across years; and (iv) the experimental removal of aphids affected community network structure (network degree, modularity and tree genotype contribution to modularity). These findings demonstrate that the interactions of foundation species are genetically based, which in turn significantly contributes to community diversity, stability and species interaction networks. These experiments provide an important step in understanding the evolution of Darwin's ‘entangled bank’, a metaphor that characterizes the complexity and interconnectedness of communities in the wild.

Molecular evidence for sympatric taxa within Pemphigus betae (Hemiptera: Aphididae: Eriosomatinae)

Mitochondrial DNA cytochrome c oxidase subunit 1 (DNA barcode) and nuclear microsatellite flanking region sequences were used to analyse populations of putative “sugarbeet root aphid”, Pemphigus betae Doane, from sites in Alberta, Canada. Three sympatric genotypes were revealed, identified as P. betae, P. populivenae Fitch, and an undetermined third species. All three genotypes formed morphologically indistinguishable galls on the same set of cottonwood (Populus L., Salicaceae) host species, often on the same tree. Gall morphology is frequently used to identify Pemphigus species. Our results indicate that this practice may be unreliable for these three taxa at least.

REARING METHODS AND DEMOGRAPHIC STATISTICS FOR A SUBTERRANEAN MORPH OF THE SUGARBEET ROOT APHID (HOMOPTERA: APHIDIDAE)

A reliable method for rearing the subterranean summer morph of the sugarbeet root aphid, Pemphigus betae Doane, is described. Field-collected aphids were reared using hydroponic growth pouches that allowed exposed root tissue to be fed on by P. betae. The aphid has been maintained in continuous culture for over 3 years. Open pouches were subsequently modified with cages to provide more successful mass rearing. Cages placed on the pouch were devised to provide an arena that concentrated aphids on one area of the pouch, minimized wandering behavior, and ensured a more vigorous colony. During a 10-week mass rearing experiment, caged pouches consistently averaged 120.8 ± 9.6(se) aphids per cage (fourth instars and adults) following 2-week incubation periods. Caged pouches also were used to isolate individual aphids for development and fecundity studies. Age-specific life tables were developed for P. betae using both open pouches at 20°C, and caged pouches at 24°C. Although the intrinsic rate of increase (rm) was lower at 20°C on a daily time scale (rm = 0.2314 versus 0.2591), rm was significantly greater at 20°C on a degree-day (DD > 7.6°C) time scale (rm = 0.0187 versus 0.0158). The difference on a DD basis resulted primarily from a longer time interval from birth to first reproduction (TFR) at 24°C(TFR = 158.5 versus 147.1 DD at 20°C). These results corroborate previous experience with P. betae, indicating that 20°C appeared to be an optimum temperature for mass rearing. Stable age distributions for P. betae cohorts were only slightly affected by temperature; on average approximately 55, 22, 11, 7, and 5% of the aphids were first, second, third, and fourth instars, and adults, respectively. Though similar studies have not been reported for other Pemphigus spp., all demographic statistics were characteristic of many foliar-feeding aphids held under similar constant temperature conditions.