Decision letter | Symbiont-induced odorant binding proteins mediate insect host hematopoiesis

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Symbiont-induced odorant binding proteins mediate insect host hematopoiesis

Decision letter

Affiliation details

University of Cincinnati, United States; Yale School of Public Health, United States; University of Connecticut, United States; Yale University, United States
Bruno Lemaître, Ecole Polytechnique Fédérale de Lausanne, Switzerland

In the interests of transparency, eLife includes the editorial decision letter and accompanying author responses. A lightly edited version of the letter sent to the authors after peer review is shown, indicating the most substantive concerns; minor comments are not usually included.

Thank you for submitting your article "Symbiont-induced odorant binding proteins mediate insect host hematopoiesis" for consideration by eLife. Your article has been favorably evaluated by K VijayRaghavan (Senior Editor) and three reviewers, one of whom, Bruno Lemaitre, is a member of our Board of Reviewing Editors.

Summary:

We know that resident microorganisms play a very important role in the development of the immune system of mammals, but much of the detail remains to be resolved and the significance of microorganisms for the immune system development in other animals is largely unknown. Indeed, previous research of these authors showing that the bacterial symbiont Wigglesworthia or a cell-free extract of this bacterium is required for normal development of hemocytes in Tsetse flies (Weiss et al. 2012) provides one of the very few studies on the role of bacteria in maturation of the immune system of any invertebrate animal. This study is important because it demonstrates key elements of the molecular mechanism of the Wigglesworthia effect. It is genuinely intriguing that a single insect protein, OBP6 is necessary and sufficient for the effect, and that the effect is restricted to the differentiation of one type of hemocyte, the crystal cell.

Essential revisions:

Although the experimental set-up and work is highly appreciated, the presented data are in the view of the three reviewers not waterproof to fully support the claim that 1) the intrauterine larval recognition of the obligate Wigglesworthia symbiont regulates the expression of the larval OBP6 and 2) this is an evolutionary conserved component of the insect innate system. The three reviewers raised three important issues, each of them should be addressed before acceptance.

1) Specificity

The authors claim that the regulation of the larval OBP6 expression and the subsequent production of the crystal cells are regulated by the obligate symbiont Wigglesworthia. This is based upon the sole observation that the deprived obp6 expression in 1st / 2nd instar larvae of aposymbiotic flies (= offspring of tetracycline-treated females that lack all microbiota i.e. Wigglesworthia, Sodalis and Wolbachia present in tsetse lab colonies) is restored by the supplementation of the blood meal given to the mother flies with a Wigglesworthia extract. This 'extract' is actually a homogenate of the bacteriome of WT flies that is a cocktail of the Wigglesworthia bacterium with cellular/molecular components of the tsetse anterior midgut. This means that the presented experimental work is only suggestive for a possible involvement of Wigglesworthia in the larval obp6 expression but is not conclusive. Moreover, it is not clear how Wigglesworthia or its components that are administered through a blood meal could finally reach the larva that is developing in the fly uterus. In previous work, the authors showed that Wigglesworthia cannot colonize aposymbiotic flies when administered this way (Weiss et al.2012). In previous work the authors used an ampicilline-treatment approach to have tsetse flies that produce offspring that is specifically cleared for Wigglesworthia but not for the other microbiota. The use of use this model for the experimental work could make the link between Wigglesworthia/OBP6/crystal cells more conclusive. Also, an additional experimental group where the blood meal is supplemented with the bacteriome of aposymbiotic tsetse (contain tsetse midgut components but devoid of Wigglesworthia) would be at least a more appropriate control group in experiment 1F.

2) A better characterization of Tsetse crystal cell

The identification of crystal cells in the tsetse fly appears to be based entirely on the number of melanized spots in the cuticle after heat shock. Previous work by different authors used microscopical analysis of crystal cells to develop and validate the reliability of this assay for Drosophila. Microscopical demonstration of crystal cells in untreated tsetse and their depletion in aposymbiotic or RNAi-treated tsetse would give confidence in the reliability of the assay in tsetse.

3) The part on Drosophila should be better documented

The part on Drosophila is really exciting, suggesting a conserved mechanism. Unfortunately, it is too short in the present version, and straightforward experiments should be added to reinforce the conclusion. The impact of microbiota on obp28a and lozenge expression should be repeated in another genetic background. The study will be strongly reinforced by the use of obp28a mutation (a temporally feasible with the advent of CRISPR), or at least the use of another independent RNAi to confirm the first one. In which tissues is obp28a expressed in larvae? The status of crystal cells could be tested by cooking the larvae and by the use of lz-gal4,uas-GFP transgene. Also, the presence of PPO could be analyzed by antibodies to decipher the nature of the defect. Survival of axenic and conventionally raised animal should be added to confirm the model.

DOI: http://dx.doi.org/10.7554/eLife.19535.040