A new haplotype-resolved turkey genome to enable turkey genetics and genomics research

  1. Carolina P Barros
  2. Martijn F L Derks
  3. Jeff Mohr
  4. Benjamin J Wood
  5. Richard P M A Crooijmans
  6. Hendrik-Jan Megens
  7. Marco C A M Bink
  8. Martien A M Groenen

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Abstract

Background

The domesticated turkey (Meleagris gallopavo) is a species of significant agricultural importance and is the second largest contributor, behind broiler chickens, to world poultry meat production. The previous genome is of draft quality and partly based on the chicken (Gallus gallus) genome. A high-quality reference genome of M. gallopavo is essential for turkey genomics and genetics research and the breeding industry.

Results

By adopting the trio-binning approach, we were able to assemble a high-quality chromosome-level F1 assembly and 2 parental haplotype assemblies, leveraging long-read technologies and genome-wide chromatin interaction data (Hi-C). From a total of 40 chromosomes (2n = 80), we captured 35 chromosomes in a single scaffold, showing much improved genome completeness and continuity compared to the old assembly build. The 3 assemblies are of higher quality than the previous draft quality assembly and comparable to the chicken assemblies (GRCg7) shown by the largest contig N50 (26.6 Mb) and comparable BUSCO gene set completeness scores (96–97%). Comparative analyses confirm a previously identified large inversion of around 19 Mbp on the Z chromosome not found in other Galliformes. Structural variation between the parent haplotypes was identified, which poses potential new target genes for breeding.

Conclusions

We contribute a new high-quality turkey genome at the chromosome level, benefiting turkey genetics and other avian genomics research as well as the turkey breeding industry.

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  1. GigaScience

    AbstractBackground The domesticated turkey (Meleagris gallopavo) is a species of significant agricultural importance and is the second largest contributor, behind broiler chickens, to world poultry meat production. The previous genome is of draft quality and partly based on the chicken (Gallus gallus) genome. A high-quality reference genome of Meleagris gallopavo is essential for turkey genomics and genetics research and the breeding industry.Results By adopting the trio-binning approach, we were able to assemble a high-quality chromosome-level F1 assembly and two parental haplotype assemblies, leveraging long-read technologies and genomewide chromatin interaction data (Hi-C). These assemblies cover 35 chromosomes in a single scaffold and show improved genome completeness and continuity. The three assemblies are of higher quality than the previous draft quality assembly and comparable to the current chicken assemblies (GRCg6a and GRCg7). Comparative analyses reveal a large inversion of around 19 Mbp on the Z chromosome not found in other Galliformes. Structural variation between the parent haplotypes were identified in genes involved in growth providing new target genes for breeding.Conclusions Collectively, we present a new high quality chromosome level turkey genome, which will significantly contribute to turkey and avian genomics research and benefit the turkey breeding industry.Competing Interest Statement

    **Reviewer 2. Luohao Xu **

    This manuscript by Barros et al. presents a high-quality dipoid turkey genome assembly which shows significant improvement relative to the previous one. This new assembly is timely and will likely be used as the reference turkey genome, but the authors should acknowledge that the W chromosome is absent (because the F1 individual was a male?). This manuscript fits more with "Data Note" than "Research" as I see most results are descriptive and confirmatory. While the chromosomal assembly is relatively complete, I am concerned whether it still contains assembly errors (because of not being polished by long reads?) which led to fewer genes annotated. This assembly metric needs to be taken into accounts if this assembly were to be used as a reference. The authors need to provide the QV value (see the VGP standard), and evaluate indel errors in coding regions. Some of the results are very brief without showing details or a figure, so difficult for assessment, for instance those SVs affecting genes. Page 4, "two most important avian agricultural species", I think duck should be the second most important poultry species? Page 5, I believe the "F1 assembly" refers to the primary assembly or collapsed assembly - please define it more clearly. Page 6, it's unclear how the 36 chromosome models are defined, particularly for small microchromosomes (29-35). According to the karyotype of turkey (2n=80), a few chromosomal models are missing. Page 6, "This captures the chromosome arms in a single contig" does it apply to all chromosomes? This is unlikely, and data is not shown. Page 6, any idea why the coverage of two parents differs (110X vs. 137X)? Page 6, "anchored the assemblies to the F1 assembly using RagTag". This suggests and chromosomal assembly of the two haplotypes was not independent, and replied on the F1 assembly. This can potentially lead to missing structural variations between two haplotypes (inversions, translocations). Page 7, please show more data to support the correct assembly of the chrZ inversion, including Hi-C heatmap, and long-read alignment spanning the inversion breakpoints. Note the Z chromosome inversion has been reported in Zhang et al. 2011 (BMC genomics), which is not cited until in the Discussion. Page 8, it's possible some genes were not annotated because of the presence of indels in coding regions. The genome assembly QV value can be calculated to measure the error frequency (Rhie et al, 2021 Nature). Page 8, please provide a statistical result for gene density comparison. Page 8, at the bottom, please cite the sources of these bird genomes. Page 9, "Gene family contractions and expansions". These analyses were a bit crude. " Orthologous groups" is not equivalent to "gene family". Page 10, the phrase "F1 and parent assemblies" is confusing. Both haploid assemblies are derived from the diploid F1. Consider changing to "paternal and maternal genomes". Also, as I commented above, both parental chromosomal assemblies are based on the same reference (Mgal_WU_HG_1.0), so the contigs were ordered and placed in the same way. This process could mask the potential non-co-linear segments. For a more appreciated way to independently assemble two chromosome-level assemblies, see the marmoset diploid genome paper (Yang et al., 2021 Nature). Page 10, please use a figure to show the SV over the BLB2 gene. Page 11, again, please visualize the result on the MAN2B2, GEMIN8, RIMKLB and RALYL cases. Page 11, "Loss of function variation", I am wondering whether variations mentioned in this part are fixed in the corresponding populations? Page 11, "Knockouts of this gene lead.." reference is needed. Page 12, "Avian genomes are known to…" references are missing. Page 12, "Distinct genomic landscapes of turkey micro and macrochromosomes", some patterns have been described in the literature, for instance, 10.1111/nyas.13295. Please also perform some statistical analyses to support the claims, not just a figure. Page 13, "Conserved synteny within the Galliformes clade", please cite 10.1159/000078570 and 10.1007/s00412-018-0685-6 Page 13, "it is evident that especially the Z chromosome" also observed in 10.1038/s41559-019-0850-1 Page 13, "inversion of around 19 Mbp on the turkey Z" also reported in 10.1186/1471-2164-12-447 Page 14, "tail of the chicken Z chromosome lacks synteny" also reported in 10.1038/nature09172. This means figure S11 does not provide a novel finding. Page 14, "Combining long reads and genome-wide chromatin interaction data (Hi-C) enables the capture of chromosome arms in a single contig", again, is that correct, chromosome arms in a single contig? Page 18, it's known wtdgb2 assembly tends to contain errors, but it looks the authors did not use long reads for polishing, but only used short reads? Page 20, "The corrected reads from TrioCanu were mapped to the Triocanu assembly with Minimap2 v2.17-r941 (Minimap2, RRID:SCR_018550) [45], options -x map-pb", what was is used for? Page 20, "Duplicated sequences were removed." How was this done?

    Re-review The manuscript has been improved. After reading the revised manuscript, I have a few more concerns.

    Chromosome models. I suggest the chromosome naming should follow chicken's, e.g., chr6 can be chr2a, and the microchromosomes should be named according to chicken homology. I then noticed chr32 and chr35 do not have chicken homology which is very concerning. It is either due to novel. chromosomes (very unlikely), or the sequences could be an unlinked contigs. In either scenario, the chromosome models must be clarified. The authors should provide strong evidence to support the chromosome model assembly for chr32 and chr35, e.g. FISH images, Hi-C zoom-in view (Fig. S1 shows the whole genomes where the microchromosome models are not visible), synteny with chicken (note there is a new chicken assembly ASM2420605v1) or zebra finch chromosomes; otherwise, chi32 and chr35 can not be identified as a chromosome. Centromere and telomere. To support complete chromosome assembly, I suggest the authors provide information about the assembly of telomere and centromere sequences, e.g. the presence/absence of TTAGGG at chromosomal ends. Most galliformes microchromosome centromeres are known to contain a 41-bp satellite (10.1139/gen-2022-0012). The authors should investigate whether such centromere satellites are present in the assembly. Data availability. It appears the Hi-C data is not available in NCBI. The raw reads must be provided. In the abstract, there is not such term as "complete scaffold", please remove "complete". Again, I do not see the support for two chromosome models: chr32 and chr35. The chrZ inversion is highlighted in the abstract, but this is not a novel finding - the writing is thus misleading. Instead, the new genome assembly only CONFIRMS this inversion. The subtitle "Lineage specific expansion and contraction of protein-coding gene families" is unrelated to the following text. "a 1.47 Mbp inversion on chromosome 1" I am wondering if this is the centromere? According to chicken chr1 centromere position, it looks like so. In the Table 5, the Parent2 has a much large size of gained copy. Please show more details, e.g. chromosomal distribution "BLB2", is this gene associated with parent2-specific trait? Similarly, what about TRIM36, GRIA2 and MAN2B2, and LRRC41? "The inversion was supported by a normal alignment at the approximate breakpoints (Supplementary File 1: Table S7 - Figure S16) and by the HiC contact map". The writing here is unclear. Hi-c data does not show signal for inversion, instead, it only supports that the assembly is correct. Bellott et al 2020 should be Bellott et al 2017. "Centromeres, however, are too long to traverse reliably in most cases". I do not see any analyses on centromeres. PRJEB42643 does not contain Hi-C data

    Re-re-review A new chicken genome has been published during the revision: https://www.pnas.org/doi/10.1073/pnas.2216641120, I suggest the authors revise some parts of the manuscript: e.g. L66, L78, L83-85 L103, please make it clear only the F1 was sequenced with long-read. L117-142, those results are very interesting, but perhaps the language can be more concise. L231-236, this paragraph is not important, please either move them to supplementary material or remove them. In general, this manuscript can be much more streamlined. L310-315, this part has also been reported by Huang et al. 2023 PNAS, so this is not a novel finding. Please either streamline or remove it. L327, ref 36 is not a "recent" finding.

  2. GigaScience

    AbstractBackground The domesticated turkey (Meleagris gallopavo) is a species of significant agricultural importance and is the second largest contributor, behind broiler chickens, to world poultry meat production. The previous genome is of draft quality and partly based on the chicken (Gallus gallus) genome. A high-quality reference genome of Meleagris gallopavo is essential for turkey genomics and genetics research and the breeding industry.Results By adopting the trio-binning approach, we were able to assemble a high-quality chromosome-level F1 assembly and two parental haplotype assemblies, leveraging long-read technologies and genomewide chromatin interaction data (Hi-C). These assemblies cover 35 chromosomes in a single scaffold and show improved genome completeness and continuity. The three assemblies are of higher quality than the previous draft quality assembly and comparable to the current chicken assemblies (GRCg6a and GRCg7). Comparative analyses reveal a large inversion of around 19 Mbp on the Z chromosome not found in other Galliformes. Structural variation between the parent haplotypes were identified in genes involved in growth providing new target genes for breeding.Conclusions Collectively, we present a new high quality chromosome level turkey genome, which will significantly contribute to turkey and avian genomics research and benefit the turkey breeding industry.

    This work has been published in GigaByte Journal under a CC-BY 4.0 license (https://doi.org/10.1093/gigascience/giad051) and has published the reviews under the same license. These are as follows.

    **Reviewer 1. Yunyun Lv **

    Reviewer Comments to Author: The turkey has importance for agriculture as it is the second contributor to word poultry meat production. This study completes a chromosome-scale genome assembly with long reads sequencing and use trio-binning approach to generate a haplotype-resolved turkey genome, which give scientific significance to further genetic studies within this species. However, I feel the content within this article need improvement. Some parts were unclear and hard to follow, I list some of them as below. After substantial revisions, I will suggest the publication.

    In abstract: The sentence "These assemblies cover 35 chromosomes in a single scaffold and show improved genome completeness and continuity" seems weird and hard to understand directly. Please revise it and make it clear. "The three assemblies are of higher quality than the previous draft quality assembly and comparable to the current chicken assemblies (GRCg6a and GRCg7)." Please indicate the parameters used for comparison clearly and how prove them with a higher quality. "Structural variation between the parent haplotypes were identified in genes involved in growth providing new target genes for breeding." The theoretical context of this sentence is not clear, so I suggest more information added to make it clear.

    Considering no statistic in the conclusion, I suggest the conclusion sentence can be revised as "we contribute a new high-quality turkey genome at chromosome-level, benefiting turkey genetics and other avian genomics research as well as turkey breeding industry."

    In the introduction: "Most of the chromosomes are small microchromosomes, while only a few macrochromosomes are present in the karyotype." Please clearly indicate how many microchormosomes in turkeys and chicken. "most of" is uninformative for readers. "and by current standards would be considered of draft quality". What is the current standards? Please indicate it clearly. "Ongoing efforts in producing high quality assemblies of the microchromosomes in avian genomes have been unsuccessful due to multiple causes" what the multiple causes represent for? Or the features of microchromsomes leads to the unsuccessful assembly as mentioned above? "For instance, improved annotation of (non)-coding genes benefits the functional interpretation of genome wide association studies (GWAS), and aids in identifying targets for gene editing", why are non-coding genes (I understand the non-coding genes are referred as regulatory regions, but actually, they are not real genes.) benefits …? Why protein-coding genes (structural genes) can not undertake the roles? "The genome assemblies of turkey (this paper) and chicken, however, are of considerably higher quality compared to other Galliforme species. This provides opportunities for an in-depth comparison between the two most important avian agricultural species." I cannot follow the logic of why the placement of this sentence is here. Obviously, it should be part of discussion after the comparison of turkey genome with other avian genomes. "In this study we use a relatively new technique, the trio-binning approach, to construct high quality haplotype-resolved turkey assemblies." I feel it is necessary to give an explanation of the term "trio-binning approach" as many readers do not understand what is standard for? And the long-reads sequencing technology within it also connect the former theoretical context closely.

    In results: Have you used other assemblers to complete the genome assembly? Such as flye, or nextdenovo, or mecat2 that may have better performance. Have you ever tried 3D-dna for chromosome-scale assembly? which may be better as my experience. The gene annotation should be assessed by BUSCOs.

    In discussion: "The quality of the assemblies presented in this study confirms the value of this method in not only providing a quality assembly but also in uncovering structural genomic variation." Please indicate which quality index that reflect your genomic assembly. "Thanks to these recent sequencing technologies, we are able to correct a number of wrongly oriented contigs in Turkey_5.1, a phenomenon often observed in short-read based assemblies." I feel this sentence is not formal in writing.

    Re-review: The author has carefully amended the work in response to my prior concerns, and the quality of the new version has greatly improved, hence it is suggested that the manuscript be accepted.

  3. Version published to 10.1093/gigascience/giad051
  4. Version published to 10.1101/2022.08.18.504375v1 on bioRxiv