Phylogenetic Relationships and Infraspecific Variation in Canadian Arctic Poa Based on Chloroplast DNA Restriction Site Data
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Color profile: Disabled Composite Default screen 679 Phylogenetic relationships and infraspecific variation in Canadian Arctic Poa based on chloroplast DNA restriction site data Lynn J. Gillespie and Ruben Boles Abstract: Infraspecific variation and phylogenetic relationships of Canadian Arctic species of the genus Poa were stud- ied based on chloroplast DNA (cpDNA) variation. Restriction site analysis of polymerase chain reaction amplified cpDNA was used to reexamine the status of infraspecific taxa, reconstruct phylogenetic relationships, and reexamine previous classification systems and hypotheses of relationships. Infraspecific variation was detected in three species, but only in Poa hartzii Gand. did it correspond to infraspecific taxa where recognition of subspecies ammophila at the spe- cies level is supported. Additional variation in P. hartzii ssp. hartzii is hypothesized to be the result of hybridization with Poa glauca in the High Arctic and subsequent introgression resulting in repeated transfer of P. glauca DNA. The variation in Poa pratensis L. had a geographical rather than taxonomic basis, and is hypothesized to correspond to in- digenous arctic versus introduced extra-arctic populations. In P. glauca Vahl cpDNA variation was detected only in western Low Arctic and boreal populations and may represent greater variation where the species survived the Pleisto- cene glaciations. Cladistic parsimony analysis of cpDNA restriction site data mostly confirms recent infrageneric classi- fication systems. Poa alpina L., along with the non-arctic Poa annua L. and Poa sect. Sylvestres, formed the basalmost clades. The remaining taxa group into two main clades: one consisting of Poa sects. Poa, Homalopoa, Madropoa and Diocopoa; the second, of Poa sects. Secundae, Pandemos, Abbreviatae and Stenopoa. Poa sect. Poa, comprising Poa arctica R. Br. and P. pratensis, is a strongly supported monophyletic group, not closely related to P. alpina. Poa hartzii is confirmed as a member of a paraphyletic or weakly supported P. sect. Secundae. Poa glauca and Poa abbreviata R. Br. are distinct members within a generally unresolved Poa. sect. Stenopoa–Abbreviatae complex Key words: Poa, Canadian arctic, chloroplast DNA, restriction site analysis, infraspecific variation, phylogeny. Résumé : En se basant sur la variation de l’ADN chloroplastique, les auteurs ont étudié la variation infraspécifique et les relations phylogénétiques d’espèces du genre Poa de l’Arctique canadien. Il ont utilisé l’analyse des sites de restric- tion de l’ADN, amplifié par PCR, afin de réexaminer les systèmes de classifications antécédents et les hypothèses de relations. On décèle de la variation infraspécifique chez trois espèces, mais seulement chez le Poa hartzii correspond- elle à des taxons infraspécifiques où la reconnaissance de la sous-espèce ammophila est bien supportée. On formule l’hypothèse que la variation additionnelle repérée chez le P. hartzii subsp. hartzii résulterait de l’ hybridation avec le Poa glauca dans le haut-arctique et d’une introgression subséquente conduisant à un transfert répété de l’ADN de le P. glauca. La variation décelée chez le Poa pratensis a une base géographique plutôt que taxonomique et on propose l’hypothèse qu’elle correspond à des populations indigènes vs des populations extra-arctiques introduites. Chez le P. glauca, on ne décèle une variation du cpADN que chez les populations boréales et du bas arctique occidental, ce qui pourrait représenter une variation plus importante là où les espèces ont survécu à la glaciation du Pléistocène. L’analyse cladistique en parcimonie, des données sur les sites de restriction du cpADN, confirment largement les systè- mes de classification infragénériques récents. Le Poa alpina,aveclePoa annua et les Poa sect. Sylvestres non- arctiques, forment les clades les plus fondamentaux. Les autres taxons se regroupent en deux clades principaux, un comprenant les Poa sects. Poa, Homalopoa, Madropoa et Diocopoa, et le second les Poa sects. Secundae, Pendemos, Abbreviatae et Stenopa. Les Poa sect. Poa, comprenant le P. arctica et le P. pratensis, constituent un groupe monophy- létique fortement supporté, pas étroitement relié au P. alpina.LeP. hartzii se voit confirmé comme membre du groupe, paraphylétique ou faiblement supporté, Poa sect. Secundae.LeP. glauca et le Poa abbreviatae sont des membres dis- tincts à l’intérieur du complexe généralement irrésolu Poa sect. Stenopoa-Abbreviatae. Mots clés : Poa, Arctique canadien, ADN chloroplastique, analyse des sites de restriction, variation infraspécifique, phylogénie. [Traduit par la Rédaction] Gillespie and Boles 701 Received March 08, 2001. Published on the NRC Research Press Web site on June 6, 2001. L.J. Gillespie1 and R. Boles. Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, ON K1P 6P4, Canada. 1Corresponding author (e-mail: [email protected]). Can. J. Bot. 79: 679–701 (2001) DOI: 10.1139/cjb-79-5-679 © 2001 NRC Canada J:\cjb\cjb79\cjb-06\B01-036.vp Thursday, May 31, 2001 8:19:32 AM Color profile: Disabled Composite Default screen 680 Can. J. Bot. Vol. 79, 2001 Introduction The first floristic study of the entire Canadian Arctic Ar- chipelago treated eight species of Poa and an additional The bluegrass genus Poa L. in the Canadian Arctic has been three infraspecific taxa (Porsild 1957, 1964). Poa abbreviata studied recently as part of floristic (McLachlan et al. 1989; R. Br., Poa arctica R. Br., Poa glauca Vahl, Poa hartzii Aiken et al. 1996a, 1996b) and taxonomic studies (Soreng Gand., and Poa alpigena var. colpodea (Th. Fr.) Schol. were 1991b). This research, based on morphology, has highlighted considered widespread in the Flora area; Poa arctica ssp. areas of persisting taxonomic confusion that warrant further caespitans (Simm.) Nannf., and Poa arctica var. vivipara study. The present investigation uses restriction site analysis of Hook., as primarily eastern Arctic; Poa alpigena Lindm. and chloroplast DNA (cpDNA) amplified by polymerase chain re- Poa alpina L., as Low Arctic, reaching only the southernmost action (PCR) to address these taxonomic problems and to study Arctic islands; and Poa flexuosa Sm. and Poa nascopieana phylogenetic relationships and biogeographic hypotheses Polunin, with restricted distributions in the eastern Low Arc- among Canadian Arctic species of Poa. The genus is very tic. This work was partly based on previous regional floristic widespread in the Canadian Arctic, from sea level to close to studies of the Canadian Arctic Islands (Polunin 1940, 1955). the upper altitudinal limit of vegetation, and as far north as Five taxa of Poa (P. abbreviata, P. alpigena var. colpodea, northern Ellesmere Island. Species occupy a diversity of habi- P. arctica, P. glauca, and P.“×hartzii”) were included by tats, including densely vegetated mesic tundra, dry barren McLachlan et al. (1989) in their treatment of grasses of the slopes and ridges, sand plains and hills, and mesic and wet Queen Elizabeth Islands, the northernmost island group. The mossy meadows. Ecologically important as a component of two infraspecific taxa of P. arctica treated by Porsild (1957, successional vegetation, several species grow in naturally dis- 1964), subspecies caespitans and variety vivipara, were both turbed and often enriched sites such as riverbanks, eroding synonymized under P. arctica, whereas P. hartzii was treated slopes, areas around animal burrows and remains, ancient as a hybrid species. However, the status of each of these dwelling sites, and recent human-disturbed sites. three taxa plus that of P. alpigena var. colpodea was consid- With over 500 species, Poa is renowned to be a taxonomi- ered to be uncertain. This study was expanded by Aiken et cally difficult genus. In North America alone the genus com- al. (1996a, 1996b) to cover all the Canadian Arctic Islands. prises 71 species, 37 infraspecific taxa, and 3 named hybrids In this study, P. hartzii was not formally treated as a hybrid (Soreng and Kellogg 2001). Species are often difficult to de- taxon but was discussed as a possible hybrid, and P. fine and distinguish from closely related congeners. High alpigena var. colpodea was not considered distinct from the levels of polyploidy, high incidence of asexual reproduction, Low Arctic P. alpigena, which was treated here as Poa and frequent occurrence of hybridization and introgression pratensis ssp. alpigena (Lindm.) Hiitonen. In addition, the are some of the factors believed to be responsible for this Low Arctic species P. alpina was included, and P. arctica taxonomic challenge (Clayton and Renvoize 1986; Kellogg ssp. caespitans was treated as a distinct taxon following R.J. 1987). The paucity of clearly distinguishing, discrete mor- Soreng (herbarium annotations, 1990). The two additional phological characters has led to a reliance on suites of over- species mapped by Porsild (1957, 1964) as occurring on lapping quantitative characters for identification, and, as a Baffin Island, P. flexuosa and P. nascopieana, were ex- consequence, unsatisfactory keys. Furthermore, many spe- cluded, as unconfirmed or of uncertain status, respectively. cies of Poa appear to vary morphologically depending on the The authors remained uncertain of the status of many of microenvironment; arctic species, in particular, have been these taxa and recommended that more detailed studies be described as phenotypically plastic with broad ecological carried out to resolve the persisting taxonomic uncertainties. tolerances