This study aimed at validating backfat thickness as an indicator of fatness in laying hens, estimated its genetic parameters, and examine its relationships with other traits. Overall, this study provided useful information that is of potential interest to egg layer breeders and researchers.  My main concern is that an indirect measurement of fatness (CT scan) rather than a more direct measurement (such as abdominal and mesenteric fat weight or chemical analysis) was used to validate the backfat thickness method . So, I would recommend being less assertive with the conclusion, as suggested below (L325)     

Comments/suggestions/questions:
L35: “extend the career of laying hens” should be reworded, as “career” is typically used in reference to people, and its use for layer hens is uncommon.
L41: Usage of  “…” is uncommon in the scientific literature. Please consider removing these throughout the text. For example, you could use “such as x, y and z” to indicate that there are more items than x, y and z.
L110: change “soon after death”  to “immediately after euthanasia”  
L115: “Fatty components were located between -130 and -90 HU.” Was this established based on prior studies? If so, please provide citations. If the answer is no, please provide explanation as to how this has been established. Some readers like myself may not be familiar with this technique.
L134-135: “A first blood sample was collected from the wing vein at 17 weeks of age and a second blood sample was collected during the neck bleeding at the slaughter process, at 53 weeks of age” since blood from neck bleeding contains a mix of arterial and venous blood, as oppose to  blood samples collected from the wing vein, do the authors expect no impact of blood source on the levels of compounds measured in this study? 
L187-190: this sentence is difficult to understand; it could be revised and split to shorter sentences
L185: “Descriptive statistics of the data suggested a bimodal distribution of backfat.” Was this observed within both R- and R+ lines, or just within one of these lines?
L186: change “Reported heritabilities” to “Reported heritability estimates”
L237: change “consistent whith the findings previously” to “consistent with the findings previously”
L268: change “latter alining with” to ““latter aligning with””
L283: change “incluence in our setup” to “influence in our setup”
L315: change “few studies mentionning” to “few studies mentioning”
L318: change “a targetted neither” to “a targeted neither”
L325: change “backfat thickness is an accurate indicator of the overall fatness of laying hens” to “backfat thickness is a potentially accurate indicator of the overall fatness of laying hens” 
 
 
 
 
 

Review on: Genetic background of body reserves in laying hens through backfat thickness phenotyping
General comments:

The overall purpose of the manuscript is to investigate backfat thickness phenotyping and investigating its heritability and relation to phenotypes in laying hens. This is a well-prepared manuscript and carefully prepared and carried out in opinion of the reviewer. Overall, the authors highlight the potential to utilize backfat thickness for additional genetic information, but given that the association was “moderate and negative”, the authors could potentially shift the discussion to show how backfat thickness could serve as a “more\less/easier/harder” efficient technique to drive selection? The materials and methods section can be improved for repeatability of the research work being presented. Overall a well written and thoroughly cared for manuscript.
 
Need better table descriptions so they are standalone items for text
 
 
 
Specific comments:
 
Lines 46-52: what are the nutritional effects/confounding effects and how could it be adjusted to deal with these issues, is this a management issue or a reality across all production systems?
 
Lines 60-62: Aren’t these techniques developed through time-point slaughter through different slaughter points in different physiological stages? Perhaps making the argument differently could avoid this confusion, but allometric sequential slaughter studies are a good way to determine dynamics of body reserves.
 
Lines 79-81: citing of ideal physiological body conditions from literature
 
Lines 101-103: more details on facilities, diet, operations would allow for reproducibility
 
Lines 101-105: Would suggest creating a table with the diets for the animals, also need more details on housing of the animals and feeding management for reproducibility
 
Line 113: recommend “hens were placed dorsally on the x-ray”
 
Line 137-140: Why were blood samples collected from different locations, would adipokine concentration be distributed in similar concentrations in both tissues?
 
Table 1. Standardize decimal places for all values reported
 
Line 212: Fort “For” ?
 
Line 214: “performed on one batch only”, also can this be fixed with additional sample analyses?
 
Line 221: “leaner”
 
Line 235-237: need to cite the papers you are referring to
 
Line 238: The reviewer would suggest the need to establish and justify highly, and moderately, different fields will have different thresholds on what is a good/high correlation value, list results and p values.
 
Lines 281-283: would ghrelin and adipokine levels shift at different physiological stages? Is the association linear or are there allometric coefficients attributable to different life stages? Diet/management effect on these markers is overlooked if not discussed further, would you believe this could influence the results you observed?
 
Line 308 and throughout: the reviewer would suggest not starting sentences with abbreviations.