Extremely low levels of chloroplast genome sequence variability in Astelia pumila (Asteliaceae, Asparagales)

Introduction

Astelia pumila (G.Forst.) Gaudich. is a dioecious, cushion-forming perennial herb. It is one of the main constituents of so-called Magellanic moorland (Godley, 1960), which prevails in the hyperoceanic fjord and channel landscape of West and Fuegian Patagonia of southwestern South America (Schmithüsen, 1956). The species occurs from 40°S to Cape Horn at 56°S, and on the Falkland Islands (Islas Malvinas). In the northern part of its range, it is found on the highest summits of the Chilean Cordillera de la Costa, which harbour isolated cushion peat bog outposts. Similar moorland enclaves occur also in the Northwest Patagonian Andes (Heusser, Heusser & Hauser, 1992; Villagrán et al., 1998; Pfanzelt, García & Marticorena, 2013). South of 47°S, the zonal vegetation is composed of cool-temperate Nothofagus rainforest and cushion peat bogs, where A. pumila is very abundant (Pisano, 1983; Gajardo, 1994). East of the Andes, in the arid Patagonian steppe, it is too dry for cushion peat bog development. Astelia pumila is probably insect-pollinated, however, flower visitors have never been observed during our own fieldwork. Its yellow berries were assumed to be bird-dispersed (Skottsberg, 1905). The species is probably tetraploid (2n = 64; Moore, 1983), with flow cytometric evidence that some individuals are hexaploid (S Pfanzelt, 2013, unpublished data).

Astelia pumila belongs to Asteliaceae, a small-sized family with three genera and 36 species from the circum-Pacific region, with most species occurring in the Southern Hemisphere (Birch, 2015). They grow in a variety of habitats, i.e., in forests, alpine fellfields and wetlands (Bayer, Appel & Rudall, 1998). The infrafamilial phylogenetic relationships were established by Birch, Keeley & Morden (2012), based on DNA sequence data from chloroplast (trnL, psbA-trnH, rps16 and petL-psbE) and nuclear (NIA-i3) regions. Birch (2015) revised the infrageneric classification of Astelia, but the placement of sect. Micrastelia, containing only A. pumila, remained unresolved (incertae sedis).

Together with other dominant cushion peat bog plant species, Astelia pumila is being used as a study system for the reconstruction of the ice-age history of Magellanic moorland with phylogeographic methods (Pfanzelt, Albach & von Hagen, 2017; Šarhanová et al., 2018). Previous genomic resources of A. pumila did not exist and our preliminary search for variable chloroplast markers did not produce satisfactory results. Consequently, the chloroplast genomes of five A. pumila individuals, sampled from almost the entire distribution range of the species, were assembled and compared, with the aim to identify phylogeographically informative chloroplast regions.

Here, the complete chloroplast genome sequence of A. pumila is reported and its intraspecific sequence variability assessed. Until now, there was no complete chloroplast genome sequence available of lower Asparagales, neither of Asteliaceae, nor of further astelioid genera (Boryaceae, Blanfordiaceae, Lanariaceae and Hypoxidaceae). Research on chloroplast genome evolution in Asparagales has been primarily focused on orchids (e.g., Kim & Chase, 2017; Lin et al., 2017; Roma et al., 2018) and Asparagaceae (e.g., Steele et al., 2012; McKain et al., 2016; Floden & Schilling, 2018). Major structural rearrangements have been documented in the chloroplast genome of parasitic and mycoheterotrophic species (e.g., Barrett et al., 2014), but in photoautotrophic members of the order, deviations from the typical land plant chloroplast genome structure are restricted to the loss of single genes (Meerow, 2010; McKain et al., 2016) and slightly shifting single copy-inverted repeat boundaries (Dong et al., 2018). Therefore, we did not expect the chloroplast genome of A. pumila to show large structural changes. However, the sequence data presented here may prove helpful to enhance our understanding of the evolutionary dynamics of the monocot plastome, through narrowing the sampling gap between orchids on the one hand and higher Asparagales on the other hand.

Materials & Methods

Results

The total lengths of the individual A. pumila chloroplast genome sequences vary from 158,215 bp to 158,221 bp due to indel variation (Fig. 1). The large and small single copy regions have lengths of 85,981–85,983 bp and 18,182–18,186 bp, respectively. The inverted repeat regions have a length of 27,026 bp. GC content is 37.8%. Genome annotation predicted a total of 113 genes, including 30 protein-coding genes and four rRNA genes.

Intraspecific chloroplast sequence variation was very low in A. pumila. Over a length of 158 kb, 37 variable sites were found, of which 18 were indels, 18 single nucleotide polymorphisms (SNPs), and one a 3-bp mutation (Table 2). The latter occurred in an imperfect repetitive region and was treated as a single mutation event. All the SNPs and the 3-bp mutation were autapomorphies of which 10 occurred in non-coding regions, i.e., introns or spacers. Of the eight SNPs occurring in coding regions, four represented non-synonymous mutations. Read coverage at the SNP sites ranged from 139−2,484 (mean 468, s.d. 541). The 18 observed indels had lengths of 1−2 bp. Indel variation was always associated with homopolymer runs of maximally 15 bp length. No differences in homopolymer lengths were observed when cross-checking the Geneious mappings with the contigs of the VelvetOptimizer de novo assemblies, so indel variation was not a software-related artefact. The NeighborNet showed a star-like topology (not shown).

The phylogenetic reconstruction of Asparagales, based on whole chloroplast genome sequences, recovers Orchidaceae as basally branching within the order (Fig. 2). The astelioid clade, represented in this study by Asteliaceae, is then sister to the remaining Asparagales.

Discussion

The chloroplast genome of A. pumila showed the typical quadripartite structure, i.e., a large and a short single copy region and two inverted repeats (Fig. 1). No general differences in gene order or inversions were detected when comparing the A. pumila chloroplast genome to those of related Asparagales species. In general, major structural rearrangements like, for example, the IR enlargement and inversions documented for geranium (Palmer, Nugent & Herbon, 1987) or the 22-kbp inversion that marks an early evolutionary split in Asteraceae (Jansen & Palmer, 1987), have not been detected yet in the chloroplast genome of any Asparagales species. However, gene loss has been documented for some taxa throughout higher Asparagales (Meerow, 2010; Steele et al., 2012; McKain et al., 2016). These missing genes—clpP, ndhF, rpl32, rps16, and rps19—are all present in the chloroplast genome of A. pumila. In orchids, basally branching within Asparagales, degradation of the ndh gene complex has been frequently observed, especially among heterotrophic species (Neyland & Urbatsch, 1996; Chang et al., 2006; Lin et al., 2017). By contrast, all eleven ndh genes are maintained in A. pumila. McKain et al. (2016) identified the rps19 gene to be the most dynamic in Agavoideae (Asparagaceae). There, it was either missing, pseudogenized, or present at different positions, either within the LSC or the IR. In A. pumila, rps19 is found within the IR, close to the LSC-IR boundaries. Located between the rps19 and the psbA genes, there is a partial rpl22 gene, truncated at the LSC-IR_A junction. This kind of gene order was classified as Type IIIg by Wang et al. (2008), a configuration typically found in Asparagales and Commelinales. In other Asparagales, e.g., Asparagus densiflorus and Crinum asiaticum, the LSC-IR_A junction lies downstream of the rps19 gene and the IR_A does not include a partial rpl22 gene. The structural dynamics of the LSC-IR junctions carry a phylogenetic signal, since there is an IR expansion trend in monocots: basally branching groups have generally shorter IRs than derived ones (Wang et al., 2008).

Eighteen indels of 1−2 bp length were observed among the five A. pumila accessions compared, all of which were associated with A or T homopolymer stretches of 8 to 15 bp length. It has been shown that the indel error rate of the Illumina sequencing platforms increases after long homopolymer runs by up to two orders of magnitude (Ross et al., 2013). Therefore, indel variation associated with homopolymer stretches should be treated with caution, although the main sequencing errors of Illumina platforms are substitution type miscalls (Kircher, Stenzel & Kelso, 2009) with the general indel error rate being about an order of magnitude lower (Laehnemann, Borkhardt & McHardy, 2016).

Intraspecific chloroplast sequence variability was very low, although the geographical sampling covered almost the entire distribution range and included an accession from the distant Falkland Islands. The five compared chloroplast genome sequences differed only in 37 variable sites, of which 18 were indels associated with homopolymer stretches and thus of unclear reliability (see preceding paragraph). The remaining variable sites were all autapomorphies, without any phylogenetically informative content. This contrasts with previous studies on intraspecific chloroplast sequence variability in Jacobaea vulgaris (32 SNPs observed within 17 individuals, of which 11 were parsimony-informative sites; (Doorduin et al., 2011) and Theobroma cacao (78 SNPs segregating within 10 individuals; (Kane et al., 2012), in which genetic structuring could be observed.

Given the non-existence of genetic structuring in A. pumila, it may be speculated that West and Fuegian Patagonia, and the Falkland Islands, have been colonized only recently, probably after the last glacial. Clearly, the sampling in the present study is not adequate to allow for firm conclusions on the Pleistocene history of A. pumila, but such a scenario would fit to the classical biogeographic hypothesis brought forward by Villagrán (1988; 2001), based on palynological data: Magellanic moorland species migrated northwards during the last glacial and survived in the lowlands of south-central Chile. From there, they recolonized the large Patagonian Channel region after the disintegration of the Patagonian Ice Sheet, which had reached the continental shelf south of c. 42°S at the height of the last glacial (Denton & Hughes, 1981; Porter, 1981; Moreno et al., 2015). In order to properly and adequately quantify population genetic diversity and identify phylogeographic patterns in A. pumila, >350 individuals from almost 40 populations were genotyped at seven nuclear microsatellite loci. These data are currently being analysed and will, together with palaeodistribution modelling, shed light on the open question where A. pumila survived the glacials (Pfanzelt et al., 2019, unpublished data).

The phylogenetic reconstruction, based on whole chloroplast genome sequences, recovered Orchidaceae as basally branching within Asparagales. Asteliaceae was then retrieved as sister to the remaining clades of the order (Fig. 2). This topology is in accordance with previous multi-gene-based phylogenetic analyses of Asparagales (Seberg et al., 2012). The chloroplast genome of A. pumila is the first to be reported for a member of the astelioid clade of basal Asparagales. This is a major improvement in terms of published chloroplast genomes from that order, as especially orchids and subfamily Agavoideae (Asparagaceae) are very well sampled (McKain et al., 2016). Furthermore, the generated information—whole genomic DNA shotgun sequences of five A. pumila individuals and RNA-Seq data of one of them—represents a valuable genomic resource, e.g., for the identification of nuclear single copy genes. Such markers may prove useful to ascertain the still unresolved infrageneric placement of sect. Micrastelia, which contains only A. pumila as its single member.

Conclusions

The comparison of whole chloroplast genome sequences of five A. pumila accessions, sampled from almost the entire distribution range of the species, revealed extremely low levels of sequence variability. The genomic resources generated in the course of the present study may prove useful for future work on Astelia, e.g., for the development of single-copy nuclear markers. These could be employed to ascertain the yet unresolved phylogenetic placement of A. pumila within the genus.

Supplemental Information