Cytological study of Polystichum (Dryopteridaceae) species from southern South America

Abstract

Polystichum is one of the most diverse genera of ferns, with 360–400 species distributed worldwide. South America harbors ~40 species, clustered in three centres of diversity, namely, the Northern and Central Andes Center (NCC), the Brazilian Center (BC) and the Southern South America Center (SSC). To increase our understanding of the systematic relationships within Polystichum, mitotic chromosomes and spore features were studied in nine species from Argentina and Chile. All species presented the basic number x = 41, with different ploidy levels (2x, 4x and 8x). In general, chromosomes were homogeneous in size (average length 2.50–5.75 µm) and mostly subtelocentric; centromeres were inconspicuous and secondary constrictions were frequently observed. All species presented 64 spores per sporangium, suggesting normal sexual reproduction. Significant differences in spore size were found among species and it was positively correlated with ploidy level. A relationship between sum total chromosome length and ploidy level was observed. However, there was also a reduction in single-chromosome length in the polyploids, pointing to genome downsizing. Our results agree with previous records, with diploids being frequent among NCC species and absent among SSC species. In addition to sharing very specific morphological characters, SSC species are cytologically characterised by being polyploids (4x and 8x). A literature survey covering 116 species of Polystichum revealed that Australian and New Zealand Polystichum species exhibit similarly high frequencies of polyploidy. In the case of P. tetragonum (2n = 164), endemic to the Juan Fernandez archipelago, our data suggested that it was originated by transoceanic migration from a South American ancestor, probably also tetraploid.

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“…The taxonomic composition in PN Contulmo showed a predominance of species with CN=22 (e.g., one species for each of the Ugni, Luma, Galium, Podanthus, and Chaetanthera genera; Jara Seguel and Urrutia Estrada, 2020), unlike other PWA. Biogeographically, it is worth noting that most of the genera shared among the PWA have a Gondwanean distribution, which is present in southern South America and Oceania (Jara Seguel et al, 2006;2014;Chacon et al, 2012a;Morero et al, 2015). Some of these genera are part of the paleo-endemic flora, mainly ferns and conifers, displaying a recurrent presence in the region that includes the PWA studied here (Southern region according to Scherson et al, 2017).…”

“…However, some PWA do not fit into this trend: for example, PN Nahuelbuta has high species diversity (311 species) but low ICNH (value of 26.8), whereas all three, MN Cerro Ñielol, Nevertheless, all 13 PWA studied here showed higher ICNH than was previously estimated for 243 Chilean species of vascular plants with an ICNH of 22.4 (Peruzzi et al, 2014), but until that date, the databases did not include several polyploid species of pteridophytes which were added in our study. The ICNH for the six PWA mentioned above could be explained by the presence of polyploid species -25 in total-representing four genera of ferns and 12 genera of angiosperms with variable CN, ranging from 28 to 328, many of them being tetraploid, hexaploid or octoploid (Jara Seguel et al, 2006;Jara Seguel and Urrutia, 2012;Jara Seguel and Urrutia Estrada, 2018;Morero et al, 2015) (Figure 1A, B, and C). It is worth noting that diploid angiosperm species are the predominant plant group within the dataset (49 species in total), although they present a lower CN ranging from 8 to 32 as compared to polyploid taxa.…”

“…However, the geographic coordinates of various sites that they describe coincide with the PWA located in the Andean range, Central valley and Nahuelbuta range (see appendix A with supplementary material in Scherson et al, 2017). An additional organisational level to analyse genetic diversity is the chromosomal (Stebbins, 1971;Levin, 2002;Windham and Yatzkievych, 2003;Severns and Liston, 2008;Peruzzi et al, 2012;Morero et al, 2015).…”

Typifications and synonymy in Polystichum (Dryopteridaceae) from Chile and Argentina

“… Polystichum Roth ( Dryopteridaceae ) is a worldwide, taxonomically complex genus of ± 360–400 species ( Zhang and Barrington 2013 ), characterized by highly variable species and convergent morphology ( Kessler et al 2005 ). Frequent hybridization, polyploidy (especially allopolyploidy), and apomixis hinder taxonomic delimitation of species ( Little and Barrington 2003 , Morero et al 2015 ).…”

“…Recent work has revealed that the polystichums of Chile and Argentina pertain to two different monophyletic lineages, an exindusiate tropical Andean clade ( Condack 2012 , Condack et al 2013 , McHenry and Barrington 2014 ) and an indusiate austral South American clade ( Morero et al 2015 , Barrington, unpublished data). A significant insight is that the tropical Andean taxa extend down the east face of the Andes and can reach the sea in southernmost Brazil and Uruguay ( Condack 2012 ), whereas the austral Andean taxa are mainly confined to the subantartic region from 37° S to Cabo de Hornos ( Morero et al 2015 ).…”