Abstract
The Horned Lark (Eremophila alpestris) is a small songbird that exhibits remarkable geographic variation in appearance and habitat across an expansive distribution. While E. alpestris has been the focus of many ecological and evolutionary studies, we still lack a highly contiguous genome assembly for the Horned Lark and related taxa (Alaudidae). Here, we present CLO_EAlp_1.0, a highly contiguous assembly for E. alpestris generated from a blood sample of a wild, male bird captured in the Altiplano Cundiboyacense of Colombia. By combining short-insert and mate-pair libraries with the ALLPATHS-LG genome assembly pipeline, we generated a 1.04 Gb assembly comprised of 2713 scaffolds, with a largest scaffold size of 31.81 Mb, a scaffold N50 of 9.42 Mb, and a scaffold L50 of 30. These scaffolds were assembled from 23685 contigs, with a largest contig size of 1.69 Mb, a contig N50 of 193.81 kb, and a contig L50 of 1429. Our assembly pipeline also produced a single mitochondrial DNA contig of 14.00 kb. After polishing the genome, we identified 94.5% of single-copy gene orthologs from an Aves data set and 97.7% of single-copy gene orthologs from a vertebrata data set, which further demonstrates the high quality of our assembly. We anticipate that this genomic resource will be useful to the broader ornithological community and those interested in studying the evolutionary history and ecological interactions of larks, which comprise a widespread, yet understudied lineage of songbirds.
The Horned Lark (Eremophila alpestris) is a widespread species of songbird that occupies grasslands, tundras, deserts, and other sparsely vegetated habitats on five continents (Beason 1995). As is characteristic of most species in the family Alaudidae, E. alpestris is a terrestrial species that nests on the ground and relies on camouflage to avoid predation by avian predators (Donald et al. 2017). The Horned Lark has been studied extensively in terms of geographic variation and systematics (Behle 1942; Johnson 1972), population genetics (Drovetski et al. 2006, 2014; Mason et al. 2014; Ghorbani et al. 2019), physiological adaptations (Trost 1972), breeding biology (de Zwaan et al. 2019), and responses to human activity, such as agriculture (Mason and Unitt 2018) and wind energy (Erickson et al. 2014), among other focal areas. Despite extensive past and ongoing research involving E. alpestris and other alaudids, we lack a highly contiguous reference genome for the species and the family as a whole (but see Dierickx et al. 2019). Generating genomic resources for the Horned Lark and related taxa will enable studies linking phenotypic and genetic variation (Kratochwil and Meyer 2015; Hoban et al. 2016), chromosomal rearrangements (Wellenreuther and Bernatchez 2018), and many other avenues of future genomic research for non-model organisms (Ellegren 2014).
Here, we describe CLO_EAlp_1.0, a new genomic assembly that we built with DNA extracted from a wild, male lark captured from a demographically small and geographically isolated population near Toca, Boyacá, Colombia. We sampled this individual and population because it had high a priori likelihood of high homozygosity compared to larks elsewhere with much larger effective population sizes and variable patterns of connectivity to adjacent populations. To generate this de novo assembly, we used the ALLPATHS-LG pipeline (Butler et al. 2008; Gnerre et al. 2011). Given the lack of genomic resources currently available for Alaudidae, we hope this de novo assembly will inspire and facilitate future studies on the genomic biology of larks—a widespread, diverse lineage of songbirds.
Materials and Methods
Sample collection, DNA extraction, and sequencing
We captured a male E. alpestris (EALPPER07; NCBI BioSample SAMN12913182) approximately 170 km NE of Bogotá, Colombia near the town of Toca on the shores of the Embalse de La Copa in the Altiplano Cundiboyacense of the Boyacá department (5.623299, -73.184156). This population is small and represents a subspecies (E. a. peregrina) that is geographically isolated from other populations of larks, the nearest population of which is in Oaxaca, Mexico. The Colombian subspecies of Horned Lark likely underwent a population bottleneck upon colonizing the distant, high-elevation plateaus of the Altiplano Cundiboyacense region and therefore probably has high homozygosity compared to other populations, which is preferable for de novo genome assembly. We collected blood from the brachial vein, from which we subsequently extracted genomic DNA with a Gentra Puregene Blood Kit (Qiagen, Hilden, Germany) following the manufacturer’s protocol. We confirmed the sex of the individual using PCR amplification (Chu et al. 2015). After running the sample on a 1% agarose gel to confirm the presence of high molecular weight DNA, we sent the extraction to the Cornell Weil Medical School (New York, USA), where they generated a 180 bp fragment library, a 3 kb mate-pair library and a 8 kb mate-pair library. We sequenced the 180 bp library across two lanes and combined the 3 kb and 8 kb mate-pair libraries on another lane of Illumina HiSeq 2500 to perform 100 bp paired-end sequencing.
Genome assembly, polishing, and assessment
We assembled the genome with ALLPATHS-LG v52415 (Butler et al. 2008; Gnerre et al. 2011). We did not perform additional adapter removal or quality filtering with the short-insert 180 bp libraries because ALLPATHS-LG has built-in steps that remove low-quality and adapter-contaminated reads (Butler et al. 2008). Once the initial assembly had finished, we aligned the short-insert and mate-pair libraries back to the assembly genome using bwa 0.7.17-r1188 (Li and Durbin 2009) and samtools v1.9 (Li et al. 2009) and then performed three iterations of scaffold polishing using pilon v1.22 (Walker et al. 2014) with default parameters. Once scaffold polishing had finished, we ordered and correspondingly renamed the scaffolds with respect to decreasing scaffold size using SeqKit v0.7.2 (Shen et al. 2016). We assessed the contiguity of the de novo genome using the function stats.sh from BBMap v38.73 (Bushnell 2014) and estimated genome completeness with BUSCO v3 (Simão et al. 2015; Waterhouse et al. 2018) alongside HMMER v3.1b2 (Finn et al. 2011) and BLAST+ v2.7.1 (Camacho et al. 2009) to identify single-copy orthologous gene sets among birds and vertebrates. We subsequently submitted our genome to the NCBI genome submission portal, which performs an additional scan for contaminants, including adapter contamination, and removed any additional contaminant sequences that were detected.
Mitochondrial genome assembly
We also assembled the mitochondrial genome for the same individual (EALPPER07) with NOVOplasty v3.7 (Dierckxsens et al. 2017) using a ND2 sequence (GenBank Accession KF743558) from a previous study (Mason et al. 2014) as the initial seed to begin the assembly process.
Data availability
Raw output from sequencing runs and the final assembly, CLO_EAlp_1.0, are available from NCBI (BioProject PRJNA575884). Short-fragment and mate-pair libraries are also available from the NCBI SRA (SUB6392689). Outputs from BUSCO and BBMap analyses are available from figshare (https://doi:10.6084/m9.figshare.9956063; https://doi:10.6084/m9.figshare.9956042). Supplemental material available at figshare: https://doi.org/10.25387/g3.9992360.
Results and Discussion
Taken together, the three lanes of Illumina HiSeq 2500 sequencing generated 1.59 × 109 total reads (∼134x estimated coverage of a 1.2 Gb genome), including 5.45 × 108 paired-end reads for the 180 bp short-insert libraries, 1.24 × 108 paired-end reads for the 3 kb mate-pair library, and 1.27 × 108 paired-end reads for the 8 kb mate-pair library. Following scaffold polishing, the finalized CLO_EAlp_1.0 assembly consisted of 2713 scaffolds that totaled 1.04 Gb. The largest scaffold was 31.81 Mb while the scaffold N50 was 9.42 Mb and scaffold L50 was 30 (Table 1). The assembly consisted of 23,684 contigs, including a largest contig size of 1.69 Mb, a contig N50 of 193.81 kb, and a contig L50 of 1429. The average GC content of the assembly was 42.23%, which is similar to other birds (Jarvis et al. 2014; Botero-Castro et al. 2017), while the de novo genome assembly included 94.5% of single-copy orthologs from the Aves data set and 97.7% of the Vertebrata data set as identified by BUSCO (Table 2). The mitochondrial assembly pipeline generated a single mtDNA contig of 14 kb.
We opted not to assemble pseudochromosomes by aligning our de novo genome to an existing chromosome-level genome assembly (e.g., Zebra Finch (Taeniopygia guttata). While birds generally exhibit strong synteny (Derjusheva et al. 2004), avian sex chromosomes and microchromosomes are often comprised of extensive rearrangements (Volker et al. 2010). Our assembly could be further improved such that scaffolds match full chromosomes through strategies such as Hi-C (Burton et al. 2013) or ultra-long read sequencing technology (Ma et al. 2018). Functional annotation of our assembly could also be improved by generating RNA-Seq and protein libraries specifically for larks. Nonetheless, CLO_EAlp_1.0 represents a large step forward toward leveraging the natural history of larks and advanced sequencing technology to further understand avian biology.
Acknowledgments
We would like to thank Bronwyn Butcher for assistance with lab work. We also thank Leonardo Campagna for helpful advice on genome assembly and bioinformatics. Thanks to Diana Carolina Macana for assistance with field work. This research was supported by SELVA and a Doctoral Dissertation Improvement Grant from the National Science Foundation (DEB-1601072) to NAM.
Footnotes
Supplemental material available at figshare: https://doi.org/10.25387/g3.9992360.
Communicating editor: C. Marshall
- Received October 16, 2019.
- Accepted December 18, 2019.
- Copyright © 2020 Mason et al.
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
