Introduction

The dromedary (single humped camel), with an estimated global population of 26.49 million1, is one of the most popular domestic species in regions with harsh climatic conditions. These animals predominantly inhabit arid and semi-arid areas that are not suitable for most crop and livestock production, mainly due to challenges of unpredictable rainfall and frequent occurrences of drought. Camels are mostly reared for milk, meat, draught, and racing and contribute significantly to the subsistence of many pastoral communities in Africa and Asia. As an adaptation to climate change, pastoralists in Africa who historically depended on cattle are shifting to camels due to their tolerance of severe droughts and ability to contribute to household nutrition and economy during dry periods2,3. Camel milk is fast gaining popularity across markets in many countries, with a good potential to support and improve the resilience of traditional pastoral systems4,5,6. In spite of the opportunities for sustainable camel production, systematic breeding for genetic improvement is constrained by several factors, such as lack of animal identification, performance recording systems, and modern genetic and genomic tools. Genetic resources for camelids (dromedaries and Bactrian camels, alpacas, and llamas) have been limited and developed only recently in the past ten years, lagging behind other livestock species.

Current genomic resources available for camelids include a comparative chromosome map of the dromedary with humans, cattle and pigs7, a whole genome cytogenetic map of the alpaca8,9,10, and genome assemblies at the scaffold level11,12,13. All camelids possess highly similar karyotypes with a rather high diploid number of 2n = 74, which particularly complicates chromosome identification and mapping7,8. There is no fine-scale high resolution mapping resources and/or chromosome level assemblies available for camelid genomes. However, availability of mapping resources will open up the opportunities for whole genome scans to identify selection signatures, perform linkage analysis, genome-wide association studies, comparative evolutionary genomics and development of genomic tools for breeding and improvement of camels. It has also been shown that a combination of RH panels with different levels of resolution produces superior maps14,15. In this paper, we describe the construction and validation of radiation hybrid panels for the dromedary, a useful genomic resource for studies in camelids.

Material and Methods

Animal sampling and ethics statement

A biopsy of ear skin tissue was obtained from a female dromedary camel named ‘Waris’ living at the first Austrian Camel Riding School in Eitental, Austria. The 6 mm biopsy punch was collected commensal during a diagnostic treatment for skin parasites by a veterinary surgeon following standard practices and the owner agreed to the usage of the sample for this study. Therefore, no further license from the “Ethics and animal welfare committee” of the University of Veterinary Medicine, Vienna, Austria was required. The pedigree of Waris showed that she was born to a dam of Canary Islands origin and a sire of North African origin. The whole genome sequence of Waris was previously published13 and the genome assembly is available in NCBI-GenBank (Bioproject: PRJNA269274; Assembly: GCA_008031251; Nucleotide: JWIN01000001-JWIN01035752). A second skin biopsy was collected from a male dromedary named CJ owned by Franklin Safari, Texas, USA. The 6 mm biopsy punch was collected following United States Government Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research and Training. The Informed Owner Consent Form for the procurement of blood and tissue samples from client-owned animals was approved by Texas A&M University Clinical Research Review Committee (CRRC 09–47). The 53X Illumina and 20X PacBio hybrid assembly of the CJ’s genome is being constructed16.

Establishment of donor and recipient cells

The primary fibroblast cell line (CDR3) derived from ear perichondrium of female dromedary was established using standard methods of enzymatic tissue digestion17. The primary fibroblast cell line (CDR2) from male dromedary was established from the skin biopsy following the protocol of preparation of primary cultures using collagenase digestion18. The primary camel fibroblasts were propagated in AlphaMEM media with nucleosides and GlutaMAX supplemented with 15% FBS, 1X penicillin/streptomycin (100 units/mL, 100 µg/mL), amphotericin B (2.5 µg/ml), gentamicin (50 µg/ml), 1.5% AmnioMAX C-100 supplement and bFGF. Fibroblast cells from the third passage (CDR3) and from the sixth passage (CDR2) were used as donors while the thymidine kinase (TK−) deficient Chinese hamster cell line A2319 was used as the recipient. The recipient cell line was propagated in AlphaMEM media with nucleosides and GlutaMAX supplemented with 10% FBS, penicillin/streptomycin (100 units/mL, 100 µg/mL), amphotericin B (2.5 µg/ml) and gentamicin (50 µg/ml). The A23 cell line was also treated with 5-Bromo-2′-Deoxyuridine (BrdU) (0.03 mg/ml) one passage prior to fusion to remove TK− revertants.

Generation of 5000RAD camel-hamster radiation hybrids

The generation of camel-hamster radiation hybrids was performed following the standard irradiation and cell fusion protocol adapted from Page and Murphy20. For the fusion experiment, 2 × 107 female camel fibroblast cells (CDR3) were irradiated at 5000RAD by gamma rays using a Cobalt-60 (60Co) source (Gamma Cell 220). Irradiated camel fibroblast cells were mixed with A23 hamster cells at two different ratios of 1:2 and 1:4 and fused using the incubation with polyethylene glycol (PEG, Cat. P7181, Sigma). Post-fusion, the cells were seeded onto 100 mm2 petri dishes at three densities to produce well-isolated colonies: 5 × 103, 1 × 104, and 5 × 104 cells per dish (66 dishes total). The fused cells were grown in AlphaMEM media with nucleosides and GlutaMAX supplemented with 1 µM Ouabain, HAT, (100 μM hypoxanthine, 0.4 μM aminopterin, 16 μM thymidine), 10% FBS, penicillin/streptomycin (100 units/mL, 100 µg/mL), amphotericin B (2.5 ug/ml) and gentamicin (50 ug/ml). Ouabain (rodent cell line is less sensitive to toxic effect of ouabain) was used only during the first week post-fusion to eliminate intact camel cells that could have survived lethal irradiation. Hybrid colonies appeared after one week and were collected and transferred to 24-well plate using high vacuum grease (Cat. No. 15471977; Dow Corning Corp., MI, USA) and Pyrex® 8 × 8 mm cloning cylinders (Cat. No. 3166-8, Corning Inc, NY, USA). After reaching confluency, the cells were transferred to 25 cm2 (T25) flasks and then passaged to two 150 cm2 (T150) flasks. After achieving confluency in T150 flasks, 4 million viable cells were cryopreserved in two vials, with 0.75 million used for preliminary DNA extraction and screening, while the remaining large pellet was kept for a final extraction of bulk DNA. The DNA from 0.75 million cell pellets was extracted in 96-well plate format using NucleoSpin® 96 Tissue Core Kit (Macherey-Nagel, Düren, Germany) in an automated epMotion® 5075 (Eppendorf, Hamburg, Germany). The bulk DNA was extracted from large cell pellets of selected clones using MasterPure™ DNA Purification Kit (Epicentre-Illumina, CA, USA). The final bulk radiation hybrid DNA was dissolved in Tris-EDTA buffer and stored at −80 °C.

PCR based screening and scoring of radiation hybrids

A total of 48 primer pairs were designed using Primer 321 to verify retention of donor genome in the camel-hamster radiation hybrids. To design the primer pairs, the target regions in camel genome were selected based on alpaca RH markers (Perelman et al.; unpublished). Selected marker sequences were searched against the whole genome shotgun sequence of Waris (NCBI Bioproject: PRJNA269274; NCBI Assembly: GCA_008031251 (Revised-CdRom64 assembly)) using NCBI MEGABLAST22. To ensure the primers were non-specific to hamster DNA, the target camel sequences were searched (blastn) against nucleotide collection (nr/nt) of the order Rodentia (taxid: 9989). In the few cases of hit sequences to Cricetulus griseus, we selected only markers that showed no matches to hamster sequence with camel primers based on Primer3. PCR was performed with 20 ng of DNA and 1.5 mM MgCl2 in 20 µl reactions under following conditions: initial denaturation for 5 min at 95 °C, followed by 35 cycles of denaturation for 30 s at 95 °C, 1 minute at specific annealing temperatures of marker locus, elongation at 72 °C for 1 min, and a final extension at 72 °C for 10 min. The details of primers along with expected amplicon size and annealing temperatures are presented in Supplementary Table ST1. Four out of 48 markers were excluded due to ambiguous PCR results. PCR screening of 186 radiation hybrids was performed twice for each of the 44 markers with the following controls: camel genomic DNA from the same donor cell line as a positive control, genomic DNA from hamster A23 cell line as a negative control, and water as a no template control. The PCR products were electrophoresed on a 2% agarose gel and scored manually as follows: 0 - No PCR product; 1 – Strong PCR product of the expected size; 2 – Weak PCR product of the expected size; 3 - Strong/weak PCR product of a similar but not expected size. Results from duplicate PCR screenings for each hybrid clone were combined to obtain a final score of positive, negative, or discordant as shown in Supplementary Table ST2. PCR repeatability was evaluated based on discordance (e.g. strong positive amplification of expected product during the first PCR, but no amplification during the second PCR) and difference (e.g. strong positive amplification during the first PCR, but weak positive amplification of expected product during the second PCR). The scoring was performed by a single evaluator for all markers.

Generation and screening of high dose (15000RAD) camel-hamster radiation hybrids

Camel fibroblasts derived from the male dromedary CJ (CDR2) were irradiated at 15000RAD using the same Cobalt source (Gamma Cell 220). For fusion, irradiated camel fibroblast cells were mixed with A23 hamster cells at two different ratios of 1:1 and 4:1. The fused cells were seeded onto 100 mm2 petri dishes with 1 × 104, 5 × 104 and 1 × 105 cells per dish (81 dishes total) to produce well-isolated colonies. Subsequent culture and expansion of hybrid cells were performed similar to the 5000RAD panel. PCR screening of 93 hybrids derived from 15000RAD panel was also performed twice with the same set of 44 markers under the conditions mentioned in Supplementary Table ST1. Additionally, 15000RAD hybrids were also screened for two other markers, TR4520 and TR5720, to ascertain the retention of dromedary Y-chromosome. The PCR products were electrophoresed on a 2% agarose gel and scored manually.

Results

For the 5000RAD panel, a total of 245 well isolated camel-hamster radiation hybrid clones were transferred into 24 well plates and cryopreserved, of which 186 were grown to confluent cultures in two 150 cm2 (T150) flasks. DNA extracted from the hybrid clones was used for PCR based screening of 44 marker loci distributed across 33 autosomes and the X chromosome. All of the 186 clones demonstrated strong amplification in at least one marker locus, suggesting retention of camel chromosomes. Among the markers tested, the mean RF per marker ranged from 14.5% (VOLP10) to 94.6% (JMJD6), with a mean of 49.6%. Higher retention frequency for markers located on chromosomal fragments containing dromedary camel scaffold 8666493 (e.g. JMJD6) was expected as it contains the marker gene (thymidine kinase) for post-fusion hybrid selection. Nevertheless, the mean RF per marker observed in the camel-hamster radiation hybrids was higher than reported for other species, including humans23,24. Further, certain marker loci showed higher discordance between duplicate runs, i.e. positive amplification in the first PCR and negative or non-specific amplification in the second PCR. Thirteen marker loci (KITL, GG_435, VOLP10, GNAQ, CMS13, CMS15, CMS9, GG_1032, GG_984, GATA4, GG_498, CREM, VOLP67) that showed discordant PCR results in more than seven out of 186 hybrid clones or high difference rate were excluded from further analysis. The remaining 31 markers were distributed across 25 autosomes and the X chromosome. The mean retention frequencies based on these 31 markers ranged from 3.2% to 93.5% per hybrid clone, with an overall mean of 50.3%. Among the 186 radiation hybrids, seven (3.8%) had a mean RF less than 10%, 17 (9.1%) had a mean RF ranging from 10–20%, 124 (66.7%) had a mean RF ranging from 20–70%, while 38 (20.4%) had a mean RF greater than 70%. Earlier reports based on simulation studies suggested that selection of hybrids having an overall mean RF of around 50% would be optimal, as hybrids retaining most or a few of the loci were equally uninformative for mapping, but radiation dose is an important factor to be considered too25,26,27,28.

In the present work, standard irradiation (5000RAD) was used to break the donor chromosomes and hence the collection of 124 hybrids that showed a mean RF range of 20–70% was considered for selection (Fig. 1). 93 hybrids in the above mentioned range that showed little or no discordance between two independent PCR screenings were selected. Additionally, those hybrids that showed more than six differences (not discordance; See Supplementary Table ST2) between duplicate runs were excluded. A high number of such differences suggest the presence of unstable or low copy fragments in the hybrids. Among the selected hybrids, 12 had a mean RF between 20–30%, 17 had a mean RF between 30–40%, 18 had 40–50%, 29 had 50–60% and 17 had 60–70%. The overall mean RF of the final camel 5000RAD radiation hybrid panel was 47.7%, with a mean RF ranging from 22.6% to 67.7% per hybrid. The mean discordance and difference between repetitive screenings per selected hybrid was 0.007 and 0.082 respectively (Table 1).

Figure 1
figure 1

Frequency distribution of 186 camel radiation hybrids (5000RAD) based on the retention of donor genome.

Full size image
Table 1 Retention frequency, stability of donor fragments, cell and DNA yield for the selected 5000RAD camel-hamster radiation hybrids.
Full size table

Extraction of DNA from the final 93 clone set of 5000RAD camel radiation hybrid panel was performed from the bulk cell pellet collected from two T150 flasks. The estimated yield of harvested cells varied from 6.1 million to 62 million among different hybrids, with a mean of 24.5 million/hybrid. The total DNA yield from these cells ranged from 164.2 µg to 1621.6 µg with a mean of 657.4 µg per hybrid (Table 1). The quality of extracted DNA from the final camel RH panel was high, with mean A260/280 and A260/230 ratios of 1.92 and 2.05 respectively. The total DNA yield from the camel RH panel was estimated to be sufficient for performing at least 4100 PCR reactions in duplicates.

To complement the standard 5000RAD panel, we also generated a high irradiation dose (15000RAD) RH panel for mapping with increased resolution. A total of 238 well isolated hybrid clones were transferred into 24-well plates. 93 of 238 hybrids were further grown and screened with the same set of 44 markers in duplicate. Seven marker loci (CMS121, CMS15, GG_435, GG_1032, HTR3B, ATP6AP1 and AKAP12) that showed higher discordance were excluded and remaining 37 markers were considered for further analysis. Among the 93 high dose radiation hybrids, 20 (21.5%) had a mean RF less than 20%, 42 (45.2%) had a mean RF ranging from 20–50%, 16 (17.2%) had a mean RF ranging from 50–70% while 15 (16.1%) had a mean RF greater than 70%. The mean RF per hybrid ranged from 2.7% to 97.3% with an overall mean of 40.7%. Three hybrid clones with highest and lowest RF were dropped, leaving 90 clones in the panel with a mean RF of 39.9%. Further, screening of Y chromosome specific TR4520 and TR5720 markers showed a mean retention of 35.6% and 40.0% respectively. The mean retention of dromedary Y chromosome was in a similar range reported for bovine Y chromosome29.

Overall, the 5000RAD camel radiation hybrid (RH) panel contains 93 hybrids and controls with an average retention frequency of 47.7%. This resource is immediately available for use to construct a camel RH map, which can assist in assembling camelid genomes to the chromosome level. The high-resolution 15000RAD camel radiation hybrid (RH) panel contains 90 hybrids and controls with an average retention frequency of 39%, and is available to map complex regions such as the major histocompatibility complex (MHC) or the sex chromosomes.

Discussion

With the introduction and subsequent evolution of next generation sequencing (NGS) technologies involving short and long reads, large volumes of genomic data can be produced in a short period of time and at a relatively low cost. However, de novo assembly of genomes to the chromosome level has remained a challenge due to highly fragmented short read assemblies and a lack of inexpensive scaffolding techniques. Radiation hybrid (RH) mapping has been proven to be a reliable approach for producing chromosome level maps for animals. High resolution whole genome RH maps played a pivotal role in obtaining chromosome level assemblies for the genomes of several mammalian and avian species that are currently listed in the ENSEMBL, UCSC and NCBI databases, e.g. human30,31, chicken32, cattle33,34, pig14, horse35 and goat36,37. Other mapping techniques such as interfacing NGS with optical mapping38, chromatin interaction based chromosome-scale (Hi-C) scaffolding39 and reference assisted chromosome assembly40 integrated with physical mapping41 are increasingly used to assemble genomes to chromosome level. However, recent reports indicate that radiation hybrid data is highly valuable when combining different sequencing and mapping procedures to produce highly accurate reference genome assemblies (e.g. goat37). Radiation hybrid data has been useful in resolving the conflicts related to misorientation of contigs within scaffolds, prediction of scaffold placements and orientation before final gap filling and polishing. Bickhart et al.37 reported that they were unable to find any other data set, apart from the RH map, that accurately predicted PGA (Hi-C based proximity guided assembly) scaffolds containing orientation errors to a high degree of accuracy.

Camelids are a group of species with strikingly similar chromosomal complement. The comparative chromosome painting and cytogenetic mapping data indicate that there are little or no inter-chromosomal rearrangements among camelid species7,9. However, as evidenced by comparative genomics of many closely related species, intra-chromosomal rearrangements like inversions and differences in heterochromatin distribution could be expected among the genomes of camelids. Nevertheless, the dromedary RH panel can form the basis for high resolution mapping and chromosome level assembly of all camelid species, once the reference assembly is created. The fibroblast cell line used for radiation hybrid fusion in the present study was purposefully established from an animal that already has a scaffolded whole-genome sequence assembly. Availability of the RH panel and whole-genome sequence from the same animal is expected to simplify the process of achieving a chromosome level assembly. This may be useful if low coverage survey sequencing is applied for characterizing the RH panel42, particularly in the absence of a camel specific SNP microarray for genome-wide typing. Further, metacentric and sub-metacentric chromosomes often represent challenge for the chromosome assembly as many mapping methods struggle with the positioning of centromeric regions. Even more, in case of dromedary, chromosomes 7,9,14,16,34 and X have euchromatin on short arms while the rest of the chromosomes have heterochromatin on the short arms9 (Supplementary Figure SF1). In these cases, RH map will be extremely useful for chromosome level assembly of the genome.

It has been previously shown that the use of a combination of RH panels constructed with different irradiation doses provides superior mapping results14,15,31. Here we used two contrasting doses of irradiation to make resourceful combination of RH panels: the standard for mammals at 5000RAD and a high-resolution at 15000RAD. The 15000RAD panel can be used to obtain a high-resolution map, to order markers in a genomic region of interest at the fine scale, and to resolve gene order in complex regions such as MHC. Furthermore, since the high resolution panel is derived from a male donor, it can be used for mapping the Y chromosome. However the 15000RAD panel alone may not be sufficient to map the whole genome because of a likely fragmentation of linkage groups. On the other hand, a lower-resolution backbone map (5000RAD) is likely to produce whole-chromosome linkage groups suitable for chromosome assembly.

An important feature to note in camel-hamster hybrids was the high mean retention frequency (about one-third of hybrids had RF > 65%) observed per hybrid clone. The overall mean RF of the final set of 5000RAD 93 hybrids (47.7%) was significantly higher than that of hybrids from most other domestic animal species (21–34.2%) produced under a similar radiation dose (5000RAD). The overall mean RF was 28% for the cattle RH panel (BovR543), 21% for dog (RHDF500044), 26% for horse (Equine RH500045), 30.6% for pig (SSRH500046), 25% for sheep (USUoRH500047), 27.3% for river buffalo (BBURH500048) and 34.2% for goat (CHRH500049). Similarly, much lower overall mean RF of 21.9% and 23.6% was observed in the 6000RAD chicken50 and duck42 panels respectively. Surprisingly, the dromedary camel panel followed the pattern of high retention observed in alpaca RH panel (Perelman et al. Unpublished). The reason for such high uptake and retention of camelid chromosomes by recipient cells is currently unknown.

Retention of donor genome in hybrids can be influenced by several factors, including but not limited to (i) fusion efficiency of donor and recipient cells, (ii) location of markers used for screening radiation hybrids, (iii) radiation dose, and (iv) integration and replication of donor chromosomes in recipient cells. The fusion efficiency (number of hybrid colonies per million irradiated donor cells fused) depends on the compatibility of structure and composition of membranes from donor and recipient cells51, compatibility of culture conditions (e.g. optimal temperatures for the growth of donor and recipient cells42), growth inhibiting effects of donor DNA integration/recombination with the recipient genome52, choice of selective marker (thymidine kinase or hypoxanthine phosphoribosyl transferase), ability of selective marker gene to be transcribed and translated in a rodent environment, and the ability of selective marker protein to function normally in hybrid cells so as to enable their survival51. In general, the shorter the evolutionary distance between donor and recipient species, the better the fusion efficiency. In mammals, the fusion efficiency ranged from 10 × 10−6 cells in dogs44, 22.4 × 10−6 cells in cattle43, 28.9 × 10−6 cells in goat49 and 37.3 × 10−6 cells in horse45, while the fusion efficiency in non-mammalian vertebrates was much lower ranging from 0.5–1 × 10−6 cells in zebrafish53 to 1.4 × 10−6 cells in chicken and 3.5 × 10−6 cells in duck42. In the present study, the fusion efficiency was estimated to be 588.9 × 10−6 cells for 5000RAD panel, which is about 15–20 times higher than reported for other mammalian species. For the 15000RAD panel, the fusion efficiency was 76.3 × 10−6 cells. Although the fusion efficiency dropped with higher dose of irradiation, it was still 2–3 times higher than reported for other mammalian species. Higher retention of camel genome observed in the hybrids might be partly explained by the higher fusion efficiency observed between irradiated camel fibroblasts and hamster cells.

The location of markers used for screening the hybrids can also play an important role in the estimation of the retention of donor genome. Markers in the centromeric regions of chromosomes often show higher retention frequencies as it has been reported in several species including the pig54, mouse55, rhesus macaque56, chicken and duck42. Probably, this could be due to the ability of centromere containing fragments to achieve replication in hybrid cells without being integrated into recipient chromosomes56. Although, the number of such “independent” non-integrated chromosomal fragments seems rather low based on few FISH experiments on hybrid clones45,57,58. In case of the chicken and duck, the retention frequency of markers located in micro chromosomes (that are often closer to centromeres) have been reported to be higher than those located in macro chromosomes42,51. However, in the present study, the markers were designed from sequences distributed throughout the dromedary camel genome (including genic regions) and most were not located near centromeres. Out of 44 genotyped markers, 28 had a known position on the chromosomes based on the alpaca genome map9 and on unpublished alpaca RH data: only 1 marker had a location close to a centromere and 6 markers were close to telomeres. We also hypothesize that the high retention frequency might be related to the dromedary genome composition, particularly the heterochromatin (some particular repeats) that make irradiated fragments of camel chromosomes “stick” together and efficiently get into the hamster cell. Both alpaca and dromedary have quite prominent heterochromatin blocks, unlike many other RH-mapped animals59 (Supplementary Figure SF1).

Interestingly, one third of the hybrids retained about 50% of camel genome, even at the high dose of irradiation. This clearly indicated the higher level of camel genome retention in radiation hybrids as compared to other mammalian species. The overall mean RF of the highly selected subset of hybrids derived from donor cells irradiated at a dose of 12000RAD was reported to be 30.6% in cattle (88/204 hybrids60), 35% in pig (90/243 hybrids61) and 31.8% in sheep (90/208 hybrids62). Another possible explanation for higher donor retention could be the relatively better rate of integration between camel and hamster chromosomes in hybrids. Although donor DNA fragments can be maintained as additional independent chromosomes without being necessarily integrated into recipient genome, efficient integration can lead to higher retention rate63. Potentially, this could have occurred as a result of certain camel sequences that preferentially recombined with hamster chromosomes56.

Large quantities of DNA from radiation hybrid panels are generally required to type several thousand markers for building genome-wide maps using conventional PCR techniques33,64,65,66,67. This requires large-scale cultures leading to the spike in culturing cost, increased time and labor expenses. However, many other genotyping strategies like SNP microarrays14,36, survey sequencing based on NGS technologies, qPCR based Integrated Fluidic Circuits Dynamic Array42 have evolved recently that do not require large quantities of DNA, thus eliminating the need for clone expansion and intensive culturing. In the case of the dromedary radiation hybrids, high yield of DNA ranging from 164.2 µg to 1621.6 µg was produced within three passages of culture. Analysis of radiation hybrid panels derived from early passages not only results in higher retention frequencies, but also minimizes the occurrence of ambiguous genotyping results52. A highly stringent threshold was applied on discordance and difference rates for hybrid selection and hence the camel 5000RAD RH panel is expected to have good stability with reduced variation in signal intensities during genotyping.

In conclusion, we herein report the construction of two radiation hybrid panels for dromedary. The camel 5000RAD RH panel has a retention rate of 47.7%, which is close to the ideal frequency of 50% with good stability. A high yield of RH panel DNA can be used for more than four thousand PCR based genotyping reactions. The 15000RAD camel panel has a mean retention frequency of 39.9% and is suitable for high-resolution mapping. These important genetic resources are available upon request for camel genome researchers and are expected to be highly helpful for constructing high resolution maps as well as building a camel genome assembly at the chromosome level.

Availability of Data/Resource

The DNA for the two dromedary camel RH panels (5000RAD and 15000RAD) is available upon email request to the corresponding author K.Periasamy@iaea.org or Official.mail@iaea.org.