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Research Article
Two complete mitochondrial genomes from Praticolella mexicana Perez, 2011 (Polygyridae) and gene order evolution in Helicoidea (Mollusca, Gastropoda)
expand article infoRussell L. Minton, Marco A. Martinez Cruz§, Mark L. Farman|, Kathryn E. Perez§
‡ University of Houston Clear Lake, Houston, United States of America
§ University of Texas Rio Grande Valley, Edinburg, United States of America
| University of Kentucky, Lexington, United States of America
Open Access

Abstract

Helicoidea is a diverse group of land snails with a global distribution. While much is known regarding the relationships of helicoid taxa, comparatively little is known about the evolution of the mitochondrial genome in the superfamily. We sequenced two complete mitochondrial genomes from Praticolella mexicana Perez, 2011 representing the first such data from the helicoid family Polygyridae, and used them in an evolutionary analysis of mitogenomic gene order. We found the mitochondrial genome of P. mexicana to be 14,008 bp in size, possessing the typical 37 metazoan genes. Multiple alternate stop codons are used, as are incomplete stop codons. Mitogenome size and nucleotide content is consistent with other helicoid species. Our analysis of gene order suggested that Helicoidea has undergone four mitochondrial rearrangements in the past. Two rearrangements were limited to tRNA genes only, and two involved protein coding genes.

Keywords

Gene rearrangement, mitochondria, tRNA, homoplasy, convergence, phylogeny

Introduction

Helicoidea (Mollusca, Gastropoda) is a globally distributed and diverse superfamily of terrestrial mollusks (reviewed in Razkin et al. 2015). It is part of the larger Stylommatophora, a clade that accounts for around 80% of all terrestrial mollusks (Klussmann-Kolb et al. 2008) and encompasses over 100 families. Helicoid snails possess a typical pulmonate body plan with two pairs of tentacles, a usually dextrally coiled shell, and a vascularized pallial cavity that functions as a lung (Beesley et al. 1998). Taxonomic and systematic classifications of Helicoidea have differed based on morphological, ecological, and molecular characters including select mitochondrial markers (Steinke et al. 2004, Manganelli et al. 2005, Groenenberg et al. 2011). Phylogenies based on entire mitochondrial genomes (Wang et al. 2014a, Deng et al. 2016) are limited in size and scope due to the paucity of helicoid data relative to other mollusk groups (e.g. Caenogastropoda).

The mitogenome serves as a powerful evolutionary tool given its small size and fast mutation rate relative to the nuclear genome (Avise et al. 1987, Saccone et al. 1999, Funk and Omland 2003). Mitogenome organization has several qualities that make it a valuable phylogenetic marker. For example, mitochondrial gene order and content can be highly variable in metazoans (Black and Roehrdanz 1998, Mindell et al. 1998, Weigert et al. 2016). Transposition of tRNA genes is more common than movement of protein coding or ribosomal RNA genes (Xu et al. 2006) and may be weighted accordingly during analysis (Bader et al. 2008). Variability in gene length, arrangement, and strand assignment can be examined as sets of phylogenetically informative characters (Gissi et al. 2008). Many mitochondrial gene order rearrangements also represent rare events that serve as homoplasy-free evidence of common ancestry (Boore and Brown 1998, Rokas and Holland 2000, Gribaldo and Philippe 2002) provided they carry sufficient phylogenetic information (Yang 1998). In mollusks, these classes of genetic variation can occur within the same family or genus (Milbury and Gaffney 2005, Rawlings et al. 2010). Helicoideans possess typical metazoan mitogenomes, with 37 genes organized among ribosomal (16S and 12S) and transfer RNA (22, including two each for leucine and serine) genes and 13 protein coding genes (Boore 1999). The degree of genetic rearrangements and variability within and among helicoid mitogenomes, however, is poorly understood.

Within Helicoidea, only three of the constituent 19 families (Bouchet et al. 2005) are represented by complete mitogenomes on GenBank: Bradybaenidae, Camaenidae, and Helicidae. Currently unrepresented is Polygyridae, a helicoid family endemic to the Americas. Many of the most visible and commonly encountered land snails in North America are polygyrids. Nearly 300 species are described in the family, including five that are considered problematic invasives (Perez et al. 2014). We focused our efforts on Praticolella mexicana Perez, 2011, a small, globe-shaped snail (Figure 1) most likely native to Mexico and introduced to the Caribbean and the United States gulf coast (Perez 2011). The United States Department of Agriculture reports finding this species in shipments of fruit, furniture, and ornamental plants (USDA pers. comm.). Our study had two primary research aims: to sequence and annotate the mitochondrial genome of P. mexicana to examine gene order and arrangement in Polygyridae; and to explore gene order evolution in Helicoidea. Results from both aspects of the study will increase our knowledge of these gastropod groups and provide a better understanding of land snail mitochondrial genome evolution.

Figure 1.

Mitochondrial genome of Praticolella mexicana UTRGV and McAllen illustrated with an image of the species holotype (ANSP 426031). Gene order and sizes are shown relative to one another, not including non-coding regions. Genes are color coded by H (black) or L (red) strand. IUPAC single letter codes are used to identify tRNA genes.

Materials and methods

Specimen collection and DNA extraction

We collected one adult P. mexicana each from the UTRGV campus in Edinburg, Texas (26.30726; -98.1714), and from a residential neighborhood in McAllen, Texas (26.2085; -98.2254). We immersed foot tissue from each snail in reconstituted BupH™ phosphate buffered saline (Thermo Scientific) and homogenized it with a Dounce homogenizer. We used EDTA-free Protease Inhibitor Cocktail (Thermo Scientific) and the Mitochondrial Isolation Kit for Tissue (Thermo Scientific) to isolate intact mitochondria from the homogenates. Mitochondrial DNA was extracted using the Mitochondrial DNA Isolation Kit (BioVision) followed by application of Plasmid-Safe™ ATP-dependent DNase (Epicenter) to remove any remaining nuclear DNA.

Genome sequencing and assembly

We purified enriched mitochondrial DNA using the Zymoclean Genomic DNA Clean and Concentrator kit (Zymo Research) and quantified it using a BioAnalyzer (Agilent Technologies). Approximately 50 ng of total DNA was used for barcoded library construction using the Nextera DNA library prep kit (Illumina), precisely following the manufacturer’s instructions. We pooled the P. mexicana samples on one flowcell and sequenced them on the MiSeq (Illumina) platform using the 2x250 bp run mode. Barcodes and deconvolution of the pooled reads was performed automatically in the BaseSpace (Illumina) server and used their native format. The CLCBio 8.0.2 de novo genome assembly tool was used to assemble the reads using default parameter settings. Genomic contigs representing mitochondrial DNA segments were subsequently identified using the CLCBio assembly Fasta files to query a BLAST database comprising the Achatina fulica mitochondrial genome (GenBank: KJ744205).

Genome annotation

We loaded the assembly for each individual into Geneious 8 (http://www.geneious.com, Kearse et al. 2012) and used the built-in ORF finder function to identify putative coding regions. We compared the output to that generated in MITOS (Bernt et al. 2013) to determine the location and orientation of 13 protein-coding genes. ARWEN (Laslett and Canbäck 2008) and tRNAscan-SE 1.21 (Lowe and Eddy 1997) were used to identify the 22 tRNAs, and MITOS and BLAST (Altschul et al. 1990) were used to locate the two ribosomal genes. Nucleotide and codon composition analyses were conducted in DAMBE (Xia and Xie 2001).

Phylogenetic analysis of mitochondrial genes

To determine the position of Polygyridae within Helicoidea, we extracted the amino acid sequences for all 13 protein-coding genes from the two new genomes. These were combined with mitochondrial genome sequences from eight other helicoid taxa, and eight stylommatophoran and one non-stylommatophoran outgroup (Table 1). Genome fragments of Euhadra herklotsi (Z71693-701) were excluded because the gene order data (see below) could not be coded. Data for each gene were aligned separately in MUSCLE (Edgar 2004). We used IQTREE 1.4.2 (Nguyen et al. 2014) to determine that all alignments fit the mtZOA+F+I+G4 model (Rota-Stabelli et al. 2009) optimally. We assembled all alignments into a single data matrix and analyzed it in IQTREE under maximum likelihood, allowing each gene to be optimized separately. We assessed branch support using 10,000 ultra-fast bootstrap replicates (Minh et al. 2013).

Taxonomic list of mitochondrial genomes used in the study.

Taxonomy GenBank Reference
Clade Systellommatophora
Superfamily Onchidioidea
Family Onchidiidae
Onchidella celtica AY345048 Grande et al. 2004
Clade Stylommatophora
Superfamily Achatinoidea
Family Achatinidae
Achatina fulica KJ744205 He et al. 2016
Superfamily Clausilioidea
Family Clausiliidae
Albinaria caerulea X83390 Hatzoglou et al. 1995
Superfamily Helicoidea
Family Bradybaenidae
Aegista aubryana KT192071 Yang et al. 2016
Aegista diversifamilia KR002567 Huang et al. 2016
Dolicheulota formosensis KR338956 Huang et al. 2016
Mastigeulota kiangsinensis KM083123 Deng et al. 2016
Family Camaenidae
Camaena cicatricosa KM365408 Wang et al. 2014a
Family Helicidae
Cepaea nemoralis U23045 Terrett et al. 1996
Cylindrus obtusus JN107636 Groenenberg et al. 2012
Cornu aspersum JQ417194 Gaitán-Espitia et al. 2013
Family Polygyridae
Praticolella mexicana McAllen KX259343 this study
Praticolella mexicana UTRGV KX278421 this study
Superfamily Orthalicoidea
Family Cerionidae
Cerion incanum KM365085 unpublished
Family Orthalicidae
Naesiotus nux KT821554 Hunter et al. 2016
Superfamily Pupilloidea
Family Pupillidae
Gastrocopta cristata KC185403 Marquardt 2013
Pupilla muscorum KC185404 Marquardt 2013
Family Vertiginidae
Vertigo pusilla KC185405 Marquardt 2013
Superfamily Succineoidea
Family Succineidae
Succinea putris JN627206 White et al. 2011

Gene order analysis and phylogeny

For all included mitogenomes, we determined the gene order and strand assignment. Using Cornu aspersum (JQ417194) as reference (Gaitán-Espitia et al. 2013), we numbered the genes consecutively in a 5’ to 3’ direction on the H strand starting with cox1 as gene number one. With each of the genes now numbered, we proceeded to generate gene order for the other mitogenomes, using positive numbers for H strand and negative numbers for L strand. The resulting data matrix was analyzed under maximum likelihood using MLGO (Hu et al. 2014). MLGO uses a binary encoding method with probabilistic models (Lin et al. 2013) to infer gene duplications, genome rearrangements, and branch support through bootstrapping. We compared our amino acid phylogeny to the gene order tree using the Kishino-Hasegawa test (Kishino and Hasegawa 1989) as implemented in IQTREE. We also used our protein sequence phylogeny with MLGO to reconstruct ancestral genomes to study the evolution of gene order in Helicoidea.

Results

Approximately 12 million sequences reads derived from the UTRGV P. mexicana sample assembled into over 450,000 contigs, the largest of which was 14,275 bp in length. A BLAST search against the A. fulica mitogenome revealed that the largest contig was comprised entirely of mitochondrial sequence. The McAllen P. mexicana sample comprised 26 million reads that assembled into more than 300,000 contigs. The largest contig spanned 14,259 bp and was also composed entirely of mitochondrial sequence. After final sequence editing, both P. mexicana mitogenomes were found to be 14,008 bp in length.

The complete P. mexicana mitochondrial genomes (KX278421 UTRGV, KX259343 McAllen; Figure 1) possess the same genes in the same orders and orientations (Table 2). The smallest tRNA is 54 bp (tRNA-Ser1), while the largest is 68 bp (tRNA-Ser2). Non-coding regions make up 1.4% (194 bp) of the P. mexicana mitogenome. The genome has three large non-coding regions. The first region is 25 bp and sits between cox3 and tRNA-Ile. The second region is 56 bp long and exists between the two serine tRNAs. The third region is a GC-rich 89 bp segment between the tRNA-Trp and tRNA-Gln genes. Searches using BLAST (Altschul et al. 1990) found no significant matches in any database for these three non-coding regions.

Mitochondrial genome annotation for P. mexicana UTRGV.

Gene Start Stop Length Strand Start codon Stop codon Anticodon
cox1 1 1525 1525 H TTG T
tRNA-Val 1526 1587 62 H TAC
16S 1589 2579 991 H
tRNA-Leu1 2580 2642 63 H TAG
tRNA-Pro 2640 2706 67 H TGG
tRNA-Ala 2706 2768 63 H TGC
ND6 2769 3239 471 H GTG TAA
ND5 3223 4893 1671 H TTG TAG
ND1 4887 5768 882 H ATC TAG
ND4L 5768 6052 285 H GTG TAA
cytB 6054 7148 1095 H ATT TAG
tRNA-Asp 7149 7212 64 H GTC
tRNA-Cys 7209 7265 57 H GCA
tRNA-Phe 7270 7331 62 H GAA
cox2 7332 8003 672 H ATG TAG
tRNA-Gly 8007 8068 61 H TCC
tRNA-His 8063 8123 61 H GTG
tRNA-Tyr 8131 8192 62 H GTA
tRNA-Trp 8186 8248 63 H TCA
tRNA-Gln 8338 8396 59 L TTG
tRNA-Leu2 8397 8454 58 L TAA
atp8 8458 8608 151 L ATG T
tRNA-Asn 8612 8670 59 L GTT
atp6 8671 9322 652 L ATG T
tRNA-Arg 9323 9382 60 L TCG
tRNA-Glu 9383 9442 60 L TTC
12S 9443 10186 744 L
tRNA-Met 10187 10248 62 L CAT
ND3 10250 10597 348 L TTG TAA
tRNA-Ser2 10598 10665 68 L TGA
tRNA-Ser1 10723 10776 54 H GCT
ND4 10777 12100 1324 H ATG T
tRNA-Thr 12010 12162 62 L TGT
cox3 12163 12958 796 L ATT T
tRNA-Ile 12985 13044 60 H GAT
ND2 13048 13954 907 H ATG T
tRNA-Lys 13955 14008 61 H TTT

Genome size for P. mexicana is comparable with the other helicoid and stylommatophoran taxa available (Table 3). The majority of the genes (nine protein-coding, one rRNA, 15 tRNA) are located on the H strand (Figure 1). Gene overlap exists among protein coding and tRNA genes. The overall base composition of all taxa based on the H strand shows anti-cytosine bias along with excesses of thymine and guanine (Table 3). Protein-coding genes comprise 76.2% of the total P. mexicana genome, and five start (ATC, ATG, ATT, GTG, TTG) and two stop (TAA, TAG) codons are used. Incomplete stop codons (T) are used for atp6, atp8, cox1, cox3, ND2, and ND4 (Table 2). No downstream stop codons were found for those six genes.

Nucleotide and skew statistics for the mitochondrial genomes used.

Whole genome composition
Species Size (bp) A% C% G% T% A+T% AT skew GC skew
Achatina fulica 15057 0,280 0,171 0,195 0,355 0,634 -0,118 0,064
Aegista aubryana 14238 0,313 0,145 0,164 0,379 0,692 -0,095 0,062
Aegista diversifamilia 14039 0,325 0,133 0,157 0,386 0,711 -0,086 0,083
Albinaria caerulea 14130 0,328 0,138 0,155 0,379 0,707 -0,073 0,059
Camaena cicatricosa 13843 0,319 0,135 0,167 0,379 0,698 -0,086 0,108
Cepaea nemoralis 14100 0,262 0,189 0,213 0,336 0,598 -0,125 0,058
Cerion incanum 15177 0,298 0,158 0,185 0,360 0,657 -0,095 0,077
Cylindrus obtusus 14610 0,258 0,166 0,219 0,358 0,615 -0,162 0,137
Dolicheulota formosensis 14237 0,284 0,131 0,167 0,418 0,702 -0,191 0,120
Gastrocopta cristata 14060 0,308 0,136 0,172 0,384 0,692 -0,110 0,116
Helix aspersa 14050 0,307 0,136 0,165 0,392 0,699 -0,121 0,097
Mastigeulota kiangsinensis 14029 0,295 0,144 0,182 0,379 0,674 -0,125 0,118
Naesiotus nux 15197 0,336 0,120 0,147 0,397 0,733 -0,083 0,100
Praticolella mexicana McAllen 14008 0,289 0,126 0,188 0,398 0,686 -0,159 0,198
Praticolella mexicana UTRGV 14008 0,288 0,126 0,188 0,398 0,686 -0,160 0,198
Pupilla muscorum 14149 0,325 0,129 0,153 0,393 0,718 -0,094 0,083
Succinea putris 14092 0,339 0,109 0,122 0,430 0,769 -0,113 0,055
Vertigo pusilla 14078 0,326 0,123 0,155 0,397 0,722 -0,098 0,116

Maximum-likelihood analysis of our protein sequence dataset of 19 taxa and 4,011 aligned amino acid positions yielded a single tree (Figure 2). Helicoidea, Bradybaenidae, and Helicidae were recovered as well-supported monophyletic groups (100% bootstrap support), as were Pupilloidea and Pupillidae. Bradybaenidae and Camaena were well-supported sister taxa, and P. mexicana was positioned as sister to the remaining helicoid taxa. A maximum likelihood phylogeny of gene order again supported the monophyly of Bradybaenidae, Helicidae and Pupillidae, and suggested a sister relationship of Bradybaenidae with P. mexicana (Figure 3). Helicoidea, well-supported in the analysis of protein sequences, was not recovered as monophyletic though branch support was low (Figure 2). The gene order phylogeny represented a significantly less likely topology than the protein phylogeny (Δ likelihood = 1246.275, p << 0.01).

Figure 2.

Maximum likelihood phylogeny of Stylommatophora protein coding genes. Analysis in IQTREE yielded a single tree (log likelihood = -89104.188) under the mtZOA+F+I+G4 model. Branch support >50% is shown based on 10,000 ultra-fast bootstrap replicates. Helicoidea, Bradybaenidae, and Helicidae were recovered as monophyletic. Nodes A-E refer to rearrangements shown in Figure 4.

Figure 3.

Maximum likelihood phylogeny of gene order. Analysis in MLGO yielded a single tree. Branch support >50% is shown based on 100 bootstrap replicates. Bradybaenidae and Helicidae were recovered as monophyletic, but Helicoidea was not.

Given the protein topology represented the more likely relationships among our included taxa, we reconstructed ancestral gene orders predicted by MLGO with that topology. The results suggested a pattern of four rearrangements in the helicoid mitochondrial genome (Figure 4), assuming the fewest number of gene rearrangements. Starting with the hypothetical ancestral helicoid mitogenome (Figure 2 node A), P. mexicana had (tRNA-Tyr, tRNA-Trp) transposing with (tRNA-Gly, tRNA-His). The Helicidae+Camaena+Bradybaenidae ancestor maintained the same order as the hypothetical ancestral helicoid. Two unique rearrangements followed in Helicidae (Figure 2 node C); tRNA-Pro moved from between tRNA-Leu1 and tRNA-Ala to between ND6 and ND5, and (tRNA-Ser2, ND4) transposed with (tRNA-Thr, coxIII). The ancestor of Camaena+Bradybaenidae (Figure 2 node D) showed the same (tRNA-Tyr, tRNA-Trp) and (tRNA-Gly, tRNA-His) rearrangement as P. mexicana. Finally, a unique rearrangement is seen in Aegista (Figure 2 node E) The ND3 gene moved from between tRNA-Met and tRNA-Ser1 to between tRNA-Tyr and tRNA-Trp.

Figure 4.

Ancestral gene order reconstructions for Helicoidea. Columns (A–E) correspond to labeled nodes in Figure 2. IUPAC single letter codes are used to identify tRNA genes. Rearrangements in red and blue are unique to Helicidae. The convergent rearrangement seen in Bradybaenidae, Camaena, and Praticolella is shown in yellow. The green rearrangement is unique to Aegista.

Discussion

Gastropod mitogenomes tend to be compact (Boore 1999) even while carrying non-coding regions of varying sizes (Grande et al. 2008). Both mitogenomes sequenced from P. mexicana encode the standard 37 metazoan genes and possess intergenic non-coding regions. We believe that the 56 bp non-coding region between tRNA-Ser1 and Ser2 may represent the putative mitochondrial origin of replication (POR) and control region for P. mexicana. The POR is usually an AT-rich sequence that may contain palindromic stretches of nucleotides. The 56 bp region in P. mexicana is comparable in size to the presumed POR in Camaena and Bradybaenidae and is similarly AT-rich, though it is often adjacent to cox3 in pulmonate snails (Gaitán-Espitia et al. 2013). Both new mitogenomes have the same gene order and strand orientations, and have the highest GC skew and third highest AT skew (Perna and Kocher 1995) among the species examined. Strand-specific bias in nucleotide composition is a common feature among metazoan mitogenomes (Hassanin et al. 2005) and may be a constraint of organellar function (Asakawa et al. 1991).

The two P. mexicana mitogenomes differ by 71 bp, which was fewer differences than seen in Cornu aspersum from Chile (107-149 bp), the only other stylommatophoran with more than one mitogenome available (Gaitán-Espitia et al. 2013). Our preliminary intraspecific mitogenome divergence in P. mexicana (0.5%) is comparable to that seen in other metazoans such as butterfly (Vanlalruati et al. 2015) and catfish (Wang et al. 2014b) species. The majority of the differences represent substitutions in non-coding regions or third codon positions. Our results also show that P. mexicana uses six different start codons and incomplete stop codons for mitochondrial gene expression. The invertebrate mitochondrial genetic code uses multiple alternate start codons (Osawa et al. 1992, Jukes and Osawa 1993), and partial stop codons are found in other land snail mitogenomes (Groenenberg et al. 2012, Wang et al. 2014a, Yang et al. 2015). Post-transcriptional adenylation is predicted to complete incomplete stop codons (Ojala et al. 1981).

Mitochondrial gene rearrangements are common across Metazoa (Black and Roehrdanz 1998, Boore et al. 2004, Mwinyi et al. 2009, Wang et al. 2016) often including the movement of tRNA genes (Mueller and Boore 2005, Dowton et al. 2009). In Helicoidea, two of the four rearrangements we observed involve tRNA genes only. These tRNA rearrangements are the most common type seen in mitogenomes (Xu et al. 2006) The (tRNA-Tyr, tRNA-Trp) and (tRNA-Gly, tRNA-His) rearrangement seen in bradybaenids, camaenids, and polygyrids represents a convergent restructuring of the genome. These homoplastic events involving tRNA genes were first observed in insects (Flook et al. 1995) and have been reported across Arthropoda. The fourth rearrangement we observed is unique to Aegista, involving the movement of the ND3 protein coding gene. While less frequent than tRNA rearrangements, those involving protein coding genes without multiple gene inversions or transpositions have been shown in other mollusks (Grande et al. 2008, Osca et al. 2015).

Our protein sequence data support previous works showing the monophyly of Helicoidea, Helicidae, and Bradybaenidae. Previous work has suggested close relationships between Bradybaenidae, Camaenidae, and Polygyridae, but the monophyly of the former two families remains in question (Wade et al. 2001, Cuezzo 2003, Wade et al. 2006, Wade et al. 2007, Razkin et al. 2015). Unfortunately, with the relatively small number of mitogenomes available, our data were unable to resolve these issues. Our gene order phylogeny further supported the monophyly of Helicidae and Bradybaenidae, suggested close relationships between Bradybaenidae, Camaena, and P. mexicana, but did not suggest a monophyletic Helicoidea despite its consistent recovery elsewhere (Wade et al. 2001, Grande et al. 2004, Wade et al. 2007, Razkin et al. 2015). This finding was unexpected, since comparisons of gene trees to gene order phylogenies tend to produce similar results across the tree of life when using whole nuclear genomes (Wolf et al. 2001). Differences that do arise have been attributed to the phylogenies deriving from uncorrelated datasets, with gene sequence trees being driven by point mutations in the coding DNA sequence while the gene orders vary through rearrangement (Sankoff et al. 1992). While many studies examine mitochondrial gene order in the context of sequence phylogenies (e.g. Aguileta et al. 2014, Weigert et al. 2016), we found no studies that used MLGO or similar older programs (e.g. MGRA, Alekseyev and Pevzner 2009) to generate mitochondrial gene order phylogenies. More thorough and inclusive analyses are needed to determine to what extent the two types of phylogenies can be congruent and how that congruence speaks to mitogenomic evolution (Leigh et al. 2011).

Acknowledgements

RLM is supported in part by faculty research and development funds and a Barrios fellowship from UHCL. MAMC and KEP would like to acknowledge UTRGV Science Education Grant #52007568 funded by the Howard Hughes Medical Institute, ADVANCE Institutional Transformation Grant (NSF# 1209210), UTRGV Faculty Research Council, and College of Sciences for financial support. This work was supported in part by the National Science Foundation (under grant HRD-1463991). Any opinions, findings, and conclusions or recommendations are those of the authors and do not necessarily reflect the views of NSF. Jolanta Jaromczyk at UK Healthcare Genomics assisted with genome assembly.

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