Testing the extended phenotype hypothesis as phylloxera induce stomata and reorganize metabolism in grapes

Background/Question/Methods

Parasites dramatically alter host behavior, physiology, and morphology to enhance their fitness while reducing the health and productivity of hosts across kingdoms. In plant hosts, viral, bacterial, and insect parasites reduce productivity by altering protodermal cell fate, changing sink/source identity, and reducing defenses. Because these changes are directly linked to parasite fitness, they are hypothesized to serve as an extension of the parasite’s phenotype. We tested this hypothesis by characterizing a novel, insec parasite-induced morphology that physiologically benefits the plant host. The gall forming insect, phylloxera, induces stomatal formation and this provides a unique opportunity to determine if stomata enhance resource availability in response to parasitism (to serve the insect) or in defense of parasitism (to serve the host). We characterized the sink and source strength of these stomata using an open-system infrared gas analyzer and traced assimilation through the stomata to the parasite using ¹³C-labeled CO₂. We determined the ratio of ¹³C/¹²C using isotope ratio mass spectrometry. We then surveyed the transcriptome of the grape-phylloxera system using high-throughput (RNA) sequencing to reveal regulatory networks facilitating the extended phenotype.

Results/Conclusions

In both cultivated and native grapes phylloxera attack elicited functional stomatal formation in a gradient of increasing density toward the parasite. Labeled C was assimilated by the induced stomata, and detected in both the developing plant gall and the parasite living within the gall. Labeled C was not detected in any plant or insect tissues outside the labeled leaf area suggesting the induced stomata fix C locally and transport it to the strongest sink: the nearest developing plant gall and insect. These data suggest the induced stomata function as an extended phenotype of the insect and provide the first account of functional insect-induced stomata in nature. The transcriptomic survey indicated photosynthesis genes were downregulated whereas assimilate transport and storage were upregulated. These expression patterns suggest a shift in gene regulation from source to sink tissue. Genetic regulators of defenses showed strong up and downregulation in a manner that suggests enhanced defense against predators but suppressed defense against the insect in gall tissue. Taken together, these transcriptional data provide genetic evidence that gall tissue is genetically manipulated to enhance fitness of the parasite.