miR-252 targeting temperature receptor CcTRPM to mediate the transition from summer-form to winter-form of Cacopsylla chinensis

Reviewer #1 (Public Review):

Here, the authors describe, in detail, the transition between the summer form and the winter form of the pear psyllid, Cacopsylla chinensis. While the authors explore many components of this transition, the central hypotheses they seek to test are (i) that a protein they deem CcTRPM is a cold-sensitive Transient Receptor Potential Melastatin (TRPM) channel, and (ii) that this channel is involved in the summer-to-winter transition, in response to cold.

The authors demonstrate that: both cold and menthol can initiate the summer-to-winter transition; that the protein of interest is required for the summer-to-winter transition (in vivo); that the protein of interest is involved in menthol-dependent Ca2+ transients (in vitro); that miR-252 expression is temperature-dependent, modulates the seasonal transition, and affects the expression of the transcript of interest; and finally, somewhat separately, that the chitin biosynthesis pathway is linked to the summer-to-winter transition.

Although I generally found the evidence to be convincing, I note a few critical weaknesses in the manuscript as it is currently presented. Firstly, there is insufficient methodological detail to understand how the genes/transcripts/proteins in this work were identified. Further, the structural and phylogenetic analyses are incompletely described and the results are inconsistent with our previous understanding of the structure and evolution of TRPMs. It is thus possible (although unlikely) that this protein has been misidentified. Alternatively, this could be a structurally aberrant TRPM from a lineage previously presumed to be lost in insects, but there is not sufficient evidence to conclude this. Perhaps more importantly, the authors conclude that the protein of interest is cold sensitive (i.e., a "temperature receptor") primarily based on menthol sensitivity. Although menthol and cold activate the same receptors in other species, there is no demonstrated reason to think that menthol sensitivity necessitates cold sensitivity, or vice-versa. Thus, the authors' conclusions are, in my opinion, incomplete and overstated. Below are specific comments giving further context to the criticisms summarized above:

1. The method used to identify the various genes/proteins described herein is not described. Relatedly, the alignment in Figure S1 lacks Trpms from non-hemipteran taxa, making it difficult to judge sequence similarity to other more well-characterized Trpms (e.g., from human, mouse, fly, nematode, etc.), and thus difficult to assess homology from the manuscript alone.

2. The authors suggest that the CcTrpm has ankyrin repeats. To my knowledge, this would be the first description of ankyrin repeats in TRPM. It's not stated how the authors identified these putative ankyrin repeats. There's also no description of the absence or presence of previously identified Melastatin Homology Regions (MHRs), a C-terminal coiled-coil that is typically present, other C-terminal domain motifs, or the TRP domain. In the absence of methodological detail, and given the proposed presence of ankyrin repeats, it seems possible that this may not be TRPM.

3. The authors suggest that, because mRNA abundance for CcTRPM is increased in response to cold, it is cold-sensitive. However, this says nothing as to whether cold actually activates the ion channel -- a critical distinction. The authors finally conclude that CcTRPM encodes a cold-sensitive ion channel because menthol elicits Ca2+ activity in vitro. However, this experiment only demonstrates that the protein is likely menthol sensitive. This experiment does not support the authors' conclusion that this is a cold-sensitive receptor (although their later knockdown experiments do, albeit indirectly).

4. The lack of taxonomic representation in the phylogenetic analysis makes it difficult to interpret, especially in the context of methodological detail concerning the initial identification of the gene/transcript/protein of interest. Further, it's not stated if the tree is rooted (if it is, the rooting methodology is not described), the branch lengths are not shown, and the branch support methodology is not described. Many previous phylogenetic analyses have concluded--implicitly or explicitly--that there are at least two ancestral animal TRPM paralogs. From the perspective of vertebrates, one ancestral copy went on to diversify into TRPMs 1, 3, 6, and 7, and the other ancestral copy went on to diversify into TRPMs 2, 8, 4, and 5. The insect Trpms are generally thought to be more closely related to vertebrate TRPMs 1,3,6, and 7. If this phylogeny is rooted, it implies that the hemipteran Trpms are more closely related to vertebrates 2, 8, 4, and 5 (or at least 8, since that is all that is present here), and quite distantly related to other insect Trpms (and presumably, to vertebrates 1,3,6, and 7, which are not present). To my knowledge, this would be the first description of this Trpm subfamily in insects, but there is insufficient evidence or phylogenetic rigor here to conclude that. The most likely explanation is that the tree is unrooted, incorrectly rooted, or that the protein of interest is not TRPM.

Reviewer #2 (Public Review):

The pear psylla Cacopsylla chinensis has two morphologically different forms, winter- and summer-forms depending on the temperatures. The authors provided solid data showing that the cold sensor CcTRPM is responsible for switching summer- to winter forms, which is in turn regulated by the miRNA miR-252. This finding is interesting and novel.