Characterization of the complete plastid genome of Butia eriospatha (Arecaceae)

Abstract Butia eriospatha is an endemic palm species from the Atlantic Rainforest in Brazil, a biodiversity hotspot. This species is currently listed in the IUCN red list as vulnerable and lacks specific plastid markers for population genetics studies. In addition, the evolutionary relationship within the genus Butia is not yet well resolved. Here, we sequenced and characterized the complete plastid genome (plastome) sequence of B. eriospatha. The complete plastome sequence is 154,048 bp in length, with the typical quadripartite structure. This plastome length and genes content is consistent with other six species from tribe Cocoseae. However, the Inverted Repeat (IR) borders show some variation among the analyzed species from this tribe. Species from the Bactridinae (Astrocaryum and Acrocomia) and Elaeidinae (Elaeis) subtribes present the rps19 gene completely duplicated in the IR region. In contrast, all plastomes sequenced from the subtribe Attaleinae (Butia, Cocos, Syagrus) present one complete CDS of rps19 and one partial copy of rps19. The difference in the IR/LSC junctions between Attaleinae and the sister clades Bactridinae + Elaeidinae might be considered an evolutionary signal and the plastome sequence of B. eriopatha may be used in future studies of population genetics and phylogeny.

The family Arecaceae, comprises approximately 188 genera and 2,585 species, distributed throughout tropical and subtropical climate of the world, but it is most diverse in tropical forest habitats, being present from Americas to Asia-Pacific region (Palmweb).
Butia (Becc.) Becc. is a genus of the family Arecaceae, indigenous from South America. This genus includes 18 species (WFO, 2019), among them Butia eriospatha (Mart. ex Drude) Becc.. This species is endemic to southern Brazil and occurs in the Atlantic Rainforest phytogeographic domain: mainly in high altitude grassland, grassland, palm grove (Heiden et al., 2020). Butia eriospatha is easily distinguished by its solitary stem with 4-5 meters in hight, globose fruit, spathe ferruginous-pubescent on outer surface, and pistillate flowers with 3-7 mm (Deble et al., 2011;Heiden et al., 2020). Among palms, B. eriospatha has a high economic importance, being used for ornamentals projects, and with edible fruits. Currently, the fruits are consumed fresh or in pulp, alcoholic drinks, jams, jellies or ice cream (Hoffmann et al., 2014).
Species of the genus Butia show the same chromosome number (2n = 2x = 32) (Correa et al., 2019) and the genus is well supported as monophyletic (Meerow et al., 2015). However, within Butia, few molecular markers were used in phylogenetic studies. The most recent study on Butia phylogeny used only one plastid marker (trnH-psbA) and two nuclear markers (ITS and WRKY19) and failed to resolve the evolutionary relationship between Butia species (Pereira, 2019).
Plastome sequences are frequently used to understand evolutionary events and efficiently resolve phylogenetic relationships (Lopes et al., 2018(Lopes et al., , 2019. Typically, the plastome is represented as a single circular molecule composed by two inverted regions (IR), a large single copy (LSC) and a small single copy (SSC), with 120 to 130 genes, encoding ribosomal RNA (rRNA), transfer RNA (tRNA) and peptides. The plastome size varies from 107 kb to 218 kb, depending on each species (Daniell et al., 2016;Menezes et al., 2018).
Here we report the complete plastome of B. eriospatha and describe its structure and gene composition. For this, B. eriospatha leaves were collected from a single individual at the Federal University of Santa Catarina -Brazil. DNA isolation with plastid DNA-enrichment was performed according to Vieira et al. (2014). The DNA was quantified using Qubit ® fluorometer (Invitrogen, Carlsbad, CA) and 1 ng of DNA was used for preparing sequencing libraries with Nextera XT DNA Sample Prep Kit (Illumina Inc., San Diego, CA). Libraries were sequenced on Illumina MiSeq (Illumina Inc.). The obtained paired-end reads (2 x 300 bp) were used for de novo assembly in CLC Genomics Workbench 8.0v.
Butia eriospatha plastome is a circular molecule with 154,048 bp in length and the typical quadripartite structure, including a large single copy with 83,805 bp, two inverted repeat regions (26,437 bp) and a small single copy with 17,369 bp ( Figure 1). This plastome assembly was realized with 457,423 plastid reads (all trimmed with 0.05 quality 2 Magnabosco et al. scores in CLC Genomics Workbench), which resulted in approximately 820x genome coverage depth. The minimum genome coverage was 206x, which shows the high coverage sequencing of this plastome. This plastome encodes 113 unique genes, being 79 protein-code genes, 30 tRNA genes and four rRNA genes (Table 1), the same number as the other Cocoseae species (Lopes et al., 2018(Lopes et al., , 2019. The 20 duplicated genes in the IRs are: eight protein-coding genes (among them, the ycf1 and rps19 are partially duplicated), eight tRNA genes, and all four rRNA genes. Among the 113 genes, 16 contain one intron (10 protein-codes genes, and six rRNA), and three contain two introns (clpP, ycf3 and rps12) ( Table 1).
The size of the plastome of B. eriospatha was compared with others plastomes from species in the same tribe: Cocoseae (Table 2). They have similar length and structure, varying in the LSC from 83,805 bp to 85,192 bp, the SSC from 17,369 bp to 17,639 bp and the IRs from 26,437 bp to 27,092 bp. This variation can be better visualized in the IRs borders ( Figure 2). Species from the subtribes Bactridinae Plastome of Butia eriospatha 3  (Acrocomia and Astrocaryum) and Elaeidinae (Elaeis) present the rps19 gene completely duplicated in the IR region. In contrast, all plastomes sequenced from subtribe Attaleinae (Butia, Cocos, Syagrus) present one complete CDS of rps19 and one partial copy of rps19, thus presenting only one functional copy of the gene (Figure 2). The intergenic spacers (IGS) between rpl22-rps19 genes and rps19-psbA genes, located in the LSC/IR B and IR A /LSC junctions, respectively, show variations in size within Cocoseae species (Figure 2). The SSC/IR A junction varies mainly in the length of the ycf1 gene portion contained within the SSC. The ndhF gene is identical in length among all species studied and part of it is located within the IR B , overlapping in part the ycf1 gene (56 bp). The differences in the IR/LSC junctions between Attaleinae and the sister clades Bactridinae + Elaeidinae migth be considered an evolutionary signal and the plastome sequence of B. eriospatha may be used in future studies of population genetics and for phylogenetic studies.