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Dear Dr. Russell and colleagues:
Thank you for revising your manuscript and submitting again in a timely fashion. I firmly believe that you have adequately addressed all of the concerns raised by the reviewers, and thus I find your work ready for publication in PeerJ. Congratulations! This is a great contribution to insect-host research, and particularly studies deciphering the factors mediating Wolbachia symbioses. I very much look forward to seeing this in print. Well done!
-joe
# PeerJ Staff Note - this decision was reviewed and approved by a PeerJ Section Editor covering this Section #
Dear Dr. Russell and colleagues:
Thanks again for submitting your work to PeerJ. I have now received reviews of your manuscript, and I am happy to report that the general consensus is very positive. Well done! As you will see, the reviewers provided minor concerns, which if addressed, will greatly improve your work. I anticipate such a revised manuscript being suitable for publication in PeerJ. Thus, I hope you will entertain these reviews and return your manuscript to us in the very near future.
Best,
-joe
It would be useful to have some more specific and relevant background information on this Wolbachia-Trichogramma system (see comments to author).
Original primary research. Question is well-defined, and methods are clearly described.
It would have been useful to include a Wolbachia-free treatment.
Data is sound. Results are difficult to follow because of how they are presented (hard to pick out main/important results - see comments below).
This study examines the degree to which the interests of symbionts are aligned with those of their hosts. In a detailed and labor-intensive factorial experiment, the authors measured fecundity, pupal survival, and offspring sex ratio in different host and symbiont genotype combinations. The symbiont in question, Wolbachia, is maternally inherited, and transforms Trichogramma wasps into parthenogenetic females. As a result, it is predicted that fecundity and survival, but not offspring sex ratio, will be aligned. The authors found variation in all traits, but this was largely driven by host and not symbiont genetic background, with the exception of sex ratio.
The interaction between parthenogenesis-inducing Wolbachia and Trichogramma wasps is one of the best studied insect symbiont systems - this serves to increase interest in this study.
I have a few comments/suggestions that I hope will strengthen the manuscript.
- The results are difficult to follow. It is hard to pick out what is the pertinent/important/main result, as well as whether differences are biologically meaningful (even if they are sometimes statistically significant).
- It would have been stronger to include treatments with uninfected (Wolbachia-free) combinations, as this would give a sense of the baseline sex ratio in the absence of the symbiont, as well as quantify host genotype fitness differences without the symbiont. While this is not possible now that the experiment is complete, it would at least be useful to include information about what is known about sex ratio and fitness in Wolbachia-free T. kaykai.
- As the authors state, this system is unusual because it is one of the only ones in which parthenogenesis-inducing Wolbachia infection is not 100%. So it would be useful to add some information about why this is the case. Is it because of inefficient transmission or fitness costs of infection? Either of these would affect interpretations of the results of the current study. Are males due to incomplete transmission or penetrance in this system? What other studies have examined this? (Paragraph starting on line 267 could be clearer and more strongly connected to the literature.)
- Did the authors have any reason to expect that there would be variation in Wolbachia genotypes? (I wasn't surprised, as I would not have expected much Wolbachia variation.)
- How does the sex ratio and fitness variation compare to other Wolbachia-T. kaykai studies?
Minor comments:
- line 72: would be useful to state here what mechanism is used in the T. kaykai system
- lines 80, 104: 'reproductive survival' is confusing
- line 306: There are many useful and relevant papers on fitness effects and colonization of new hosts. For example, it might be useful to mention fitness costs (and immune activation) of Wolbachia transfers from flies to mosquitoes.
- paragraph starting line 309 is confusing
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See "general comments" - overall this is fine, but see comments.
See "general comments" - the research question is well-defined, relevant and meaningful. The methods are well described and the statistical analysis is excellent.
See "general comments" - data are robust, statistically sound and controlled.
This is a great contribution that systematically investigates the roles of host (parasitic wasp) and symbiont (Wolbachia) genotypes on host fitness and sex ratio parameters. This is an interesting topic because of the mixture of conflict (Wolbachia causes all female offspring, the host would prefer both sexes) and cooperation (Wolbachia is vertically transmitted, thus benefits alongside the host when hosts produce more offspring) that this relationship entails. This particular study takes advantage of the fact that the parthenogenetic-inducing Wolbachia is unusually not fixed in Trichogramma kaykai, and can be transferred experimentally via co-incubation of infected and uninfected eggs of the wasp in their own host - a moth egg. I found many parts of this paper inspirational (one of the best abstracts and introductions I've read in quite a while), but it needs improvement in execution.
The title "Cyto-nuclear fitness interactions in a Trichogramma-Wolbachia symbiosis" is not precise, because in this study, the Wolbachia is acquired by an intact host genotype in which the cytoplasmic and the nuclear genes have not been rearranged by introgression. I know the idea is that the Wolbachia is the cytoplasmic component, but of course mitochondria are as well, and mitochondrial-nuclear interactions are not addressed here. Suggest something like "Host and symbiont genetic contributions to fitness in a Trichogramm-Wolbachia symbiosis"
Methods
I assume that the Wolbachia in the 4 lines are identical with respect to MLST typing, but the identity is never mentioned - please provide the MLST strain (s) name(s).
l 182. "first born progeny of first-born females..." Please explain - given multiple females were used, I don't know what this means.
l 193 "Analysis" or "Statistical analysis" rather than "Statistics."
l. 198 Please give a reference for the Fligner-Kileen test
Results
In general, the results read as if they are only a few steps away from statistical test output. The finding should be foremost and the statistical details, while present, should be subservient to the findings. Starting the results section with the word "Nonhomogeneity" is not helpful. This first paragraph boils down to a result that host-Wolbachia interactions were important for offspring sex ratios and for pupal survival but not for fecundity, but it takes a couple of readings to come up with that result. It is fine to include that GLMS were a better analysis tool for these data, but that's the thing that should be farther down in the paragraph.
Figures 2-5 are very poorly explained and need significant improvement. There is a general interpretation of the figure as the figure caption, but no description of the four panels, nor even sufficient labeling. Each figure should have a-d denoting the panels, the y axis should be on the left, and an explanation of each panel should be in the figure caption. Included should be what "T1-T4" means, and the meaning of all the pieces of the box-plot (e.g. do the dotted lines show range or some other measure?).
L. 248-249 Not sure how horizontal transfers can show higher fecundity (first part of the sentence), while there is no significant effect of "horizontal transfer on fecundity."
The discussion doesn't reach very deep into the literature for context. Many of the references in this paper are from the authors' labs, and yet the influences of host genes, symbiont genes, and host-symbiont interactions on phenotype is a topic that there's quite an interesting literature on. I would particularly recommend a few papers and papers cited therein - Jaenike 2007 Evolution, Weeks et al. 2007 PloS Biol, Newton and Sheehan 2015 AEM, and Hunter et al. 2016 JEB. Interestingly, the last found, like the current contribution, an interaction between symbiont and host genes in the area of conflict - sex ratios. I'm not asking that you cite these in particular, but that a revision includes a deeper read of the literature to place the work in context.
Small comments on the discussion.
l. 262 and later - I don't understand the meaning of "good mixer" here. I do understand the next sentence in which it is suggested that Wolbachia "blend into" the "fitness landscape of the host environment." If this "mixer" term is a simple restatement of the second statement, I get it better in the second sentence. I think the authors are saying that Wolbachia should have no influence on host genes that are presumably optimized for fecundity.
L. 309, and referring back to the title. I suggest "cyto-nuclear" is too broad because the mitochondria are not involved here; the authors are really referring to one cytoplasmically inherited lineage - the symbiont . I agree with the authors' suggestion that it would be interesting to look at interactions when both horizontal transmission and nuclear introgression are applied to produce infected and uninfected lineages (in the first both genetic components are intact after infection, in the second, the mitochondrial genome associated with the symbiont ends up in a new nuclear background, and makes clear why they should be precise about their language throughout.
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