Scarce Evidence of Heterosis for Growth Traits in Peruvian Guinea Pigs

Round 1

Reviewer 1 Report

This work examines the heterosis for growth traits in two crosses of cuy. The research is well designed, the results correctly analyzed and the paper is well written. The main problem is the originality of the research. It is well known, as the authors recognize in the discussion, that heterosis for growth traits is low in other prolific species (pigs and rabbits, lines 226-228).

Moreover, the interest of the research is limited, since the heterosis for growth that would apply in cuy production is the one resulting from the cross of a crossbred female with a crossbred or pure line male, not the heterosis from the crosses that the authors study in this paper. The main useful conclusion of the paper is that  heterosis for growth traits is low and it is expected that it will be also low in commercial three or four way crosses. 

Additionally, the authors say that two way crosses are the most common in rabbit production (line 83), but the more common scheme in rabbit production is the three way cross, as the authors can verify reading their quotation  [14] or Baselga (2004, 8th World Rabbit Congress, http://world-rabbit-science.com/WRSA-Proceedings/Congress-2004-Puebla/Papers/Genetics/G0-Baselga.pdf).

Incidentally, I am very sceptic about the adaptation of guinea pigs to climatic conditions (lines 77-78, no quotation support this utterance).  This species has always breed inside the houses or in good conditions, no genotype per environment interactions are expected, as there are no such interactions in rabbits. 

Author Response

Reply for the reviewers of the manuscript

“Scarce evidence of heterosis for growth traits in Peruvian guinea pigs”

 

We appreciate the time taken by the reviewers to read and comment on our manuscript. We would like to thank them for their valuable comments to improve the article.

Our reply for the specific comments is presented below (in blue).

 

 

REVIEWER 1

 

Comments and Suggestions for Authors

This work examines the heterosis for growth traits in two crosses of cuy. The research is well designed, the results correctly analyzed and the paper is well written. The main problem is the originality of the research. It is well known, as the authors recognize in the discussion, that heterosis for growth traits is low in other prolific species (pigs and rabbits, lines 226-228).

We think that our study is original research because, while it is true that on other prolific species the heterosis for growth traits is low, studies in guinea pigs investigating the heterosis for growth traits are limited. Therefore, we would like to fill the research gap with our manuscript.

Moreover, the interest of the research is limited, since the heterosis for growth that would apply in cuy production is the one resulting from the cross of a crossbred female with a crossbred or pure line male, not the heterosis from the crosses that the authors study in this paper. The main useful conclusion of the paper is that heterosis for growth traits is low and it is expected that it will be also low in commercial three- or four-way crosses. 

We agree with the observation of the reviewer that the study is limited due to missing data on the final crossbreds (data set was too small for analysis). Nevertheless, we still consider these results important to provide guidance on the continuation of the crossbreeding scheme.

Additionally, the authors say that two way crosses are the most common in rabbit production (line 83), but the more common scheme in rabbit production is the three way cross, as the authors can verify reading their quotation [14] or Baselga (2004, 8th World Rabbit Congress, http://world-rabbit-science.com/WRSA-Proceedings/Congress-2004-Puebla/Papers/Genetics/G0-Baselga.pdf).

Line 86-87: Information was added (highlighted in yellow).

Incidentally, I am very sceptic about the adaptation of guinea pigs to climatic conditions (lines 77-78, no quotation support this utterance).  This species has always breed inside the houses or in good conditions, no genotype per environment interactions are expected, as there are no such interactions in rabbits. 

Currently, in Peru there are different breeds and genetic lines of guinea pigs. Some of them were developed in the Andes and others in the coastal regions of Peru, and it is well known that Peru has different types of climates according to the altitude of the area. In Peru, guinea pigs are still often raised in “traditional farms”, which do not have equipment to control the internal environment of the farms such as in rabbit farms. This is the reason why we mention some genetic lines and the capacity to adapt to the climate condition where they were developed.

 

Reviewer 2 Report

General comments

This “Communication” reports heterosis estimates for growth traits from an experiment involving two paternal and two maternal Peruvian guinea pig lines. There is no question on originality because it was conducted at high altitude (3,320 m above sea level) and involved a species for which just a few breeding papers were published. It is in the most part well written, statistical analyses and data presentation are appropriate, however it prompts a few questions on the choices of genetic materials and experimental design. Important background information on selection procedures is missing. Some parts of the text in Results are not in agreement with data in Tables.

 

Specific comments

Abstract

Lines 22-23: the selection criteria for M2 is litter size at birth, but in the Material and Methods section (lines 113-119) it is reported that M2 was based on the Andina line, selected for litter size at weaning. Which one is correct? Please clarify.

Material and methods

Line 110: change “one” to “a”.

Lines 112-121: Regarding the origin and background of the guinea pig lines involved in this study, only the selection criteria were mentioned. What about selection intensity and the number of generations (or approximate time span) of selection in each line? These data are critical for the evaluation of heterosis estimates because they can give an idea on how different from each other are the paternal lines. The same is valid for the maternal lines. Heterosis (hybrid vigor) results from the crossbreeding of (inbred) lines and requires that genetic distance between these lines exits to manifest.

I expect that this experiment will continue with another generation of crossbreeding by crossing the F1s from each parental cross to obtain the F2s and perhaps obtain additional heterosis.

Table 2: it came to my attention the small sample size in the M1M2 cross. What happened there? The number of records for all traits were too low in that cross and may have influenced your results. Large standard errors were generated (Table 4). Sample sizes were limited overall.

I suggest that the number of litters represented in each crossing are provided, as is usual for polytocous species, instead of giving only the number of individuals.

What was the age at weaning? Please add this information to Table 2 or to the text.

Results

Lines 171-172: “However, all paternal crosses were similar for birth weight and weight at 60 days (p-value > 0.05)”. This is not in accordance with the data presented in Table 3 where birth weight of P2P1 [142 (3.9)^a] > P2 [127 (4.3)^b].

Lines 173-174: “On the other hand, the performance for all maternal crosses was similar for all traits (p-value > 0.05). Again, this is not in accordance with the data presented in Table 4, where birth weight of M1M2 [182 (9.0)^a] was higher than for the other three genetic groups.

Discussion

Lines 194-196: yes. But why? Could you elaborate on that?

Line 198-199: the small sample size for M1M2 cross can have limited your power to detect possible differences.

Lines 203-207: The target weight was not achieved. Does high-altitude help explain any of your results?

Lines 226-229: higher levels of heterosis would be expected for reproductive (litter size) and fitness (survival) traits than for growth traits. Higher levels of heterosis would be expected when you cross paternal and maternal lines (three or four-way crosses).

Lines 230-236: You may have to start from a more diverse genetic basis and/or practice selection within lines for longer term before crossbreeding. This is the reason why I inquired about the selection procedures.

Little editing needed.

Author Response

Reply for the reviewers of the manuscript

“Scarce evidence of heterosis for growth traits in Peruvian guinea pigs”

 

We appreciate the time taken by the reviewers to read and comment on our manuscript. We would like to thank them for their valuable comments to improve the article.

Our reply for the specific comments is presented below (in blue).

 

 

REVIEWER 2

 

General comments

This “Communication” reports heterosis estimates for growth traits from an experiment involving two paternal and two maternal Peruvian guinea pig lines. There is no question on originality because it was conducted at high altitude (3,320 m above sea level) and involved a species for which just a few breeding papers were published. It is in the most part well written, statistical analyses and data presentation are appropriate, however it prompts a few questions on the choices of genetic materials and experimental design. Important background information on selection procedures is missing. Some parts of the text in Results are not in agreement with data in Tables.

 

Specific comments

Abstract

Lines 22-23: the selection criteria for M2 is litter size at birth, but in the Material and Methods section (lines 113-119) it is reported that M2 was based on the Andina line, selected for litter size at weaning. Which one is correct? Please clarify.

Line 121-123: We clarified this point.

Material and methods

Line 110: change “one” to “a”.

Line 110: corrected

Lines 112-121: Regarding the origin and background of the guinea pig lines involved in this study, only the selection criteria were mentioned. What about selection intensity and the number of generations (or approximate time span) of selection in each line? These data are critical for the evaluation of heterosis estimates because they can give an idea on how different from each other are the paternal lines. The same is valid for the maternal lines. Heterosis (hybrid vigor) results from the crossbreeding of (inbred) lines and requires that genetic distance between these lines exits to manifest.

Line 125-127. We added the number of generations of selection. We don´t have information about the selection intensity. Given that the lines used in crossing are not originating from a single line selected in different directions, we think the selection intensity and number of generations of selection do not have an effect on heterosis.

 

I expect that this experiment will continue with another generation of crossbreeding by crossing the F1s from each parental cross to obtain the F2s and perhaps obtain additional heterosis.

Table 2: it came to my attention the small sample size in the M1M2 cross. What happened there? The number of records for all traits were too low in that cross and may have influenced your results. Large standard errors were generated (Table 4). Sample sizes were limited overall.

Line 140-141. We added an explanation why there are few records.

I suggest that the number of litters represented in each crossing are provided, as is usual for polytocous species, instead of giving only the number of individuals.

The traits of interest in this study are individual body weights. For this reason numbers of individuals per cross were reported.

What was the age at weaning? Please add this information to Table 2 or to the text.

We added the age at weaning as a footnote for Table 1 and Table 2.

 

Results

Lines 171-172: “However, all paternal crosses were similar for birth weight and weight at 60 days (p-value > 0.05)”. This is not in accordance with the data presented in Table 3 where birth weight of P2P1 [142 (3.9)^a] > P2 [127 (4.3)^b].

Line 184: We corrected the sentence.

Lines 173-174: “On the other hand, the performance for all maternal crosses was similar for all traits (p-value > 0.05). Again, this is not in accordance with the data presented in Table 4, where birth weight of M1M2 [182 (9.0)^a] was higher than for the other three genetic groups.

Line 186-188: We modified the sentence.

 

Discussion

Lines 194-196: yes. But why? Could you elaborate on that?

Line 210-211: We added an explanation. “These results suggest that the pure maternal lines give birth to smaller pups than the M1M2 crossbred, due to the positive effect of crossing maternal lines. These results are also reflected in the significant heterosis effect of the maternal lines (Table 5).”

Line 198-199: the small sample size for M1M2 cross can have limited your power to detect possible differences.

Line 218-220: We added information about the number of records.

 

Lines 203-207: The target weight was not achieved. Does high-altitude help explain any of your results?

Line 226-229: We added information to explain that.

 

Lines 226-229: higher levels of heterosis would be expected for reproductive (litter size) and fitness (survival) traits than for growth traits. Higher levels of heterosis would be expected when you cross paternal and maternal lines (three or four-way crosses).

Line 259ff: information added in conclusions.

Lines 230-236: You may have to start from a more diverse genetic basis and/or practice selection within lines for longer term before crossbreeding. This is the reason why I inquired about the selection procedures.

Line 155-257: information was already in the text.

 

Comments on the Quality of English Language

Little editing needed.

 

 

Author Response File: Author Response.pdf