A Phylogenetic Study of Oxypolis and Ptilimnium (Apiaceae) Based on Nuclear rDNA ITS Sequences

Oxypolis and Ptilimnium are two endemic genera of North America within tribe Oenantheae (Apiaceae subfamily Apioideae). Both genera are small, with 12 species currently recognized. Some members of each genus share an unusual leaf morphology. Rather than having the pinnately compound leaves that generally characterize apioid umbellifers, they have highly reduced, linear, terete, hollow, septate appendages known as rachis leaves. It has long been questioned whether the species with rachis leaves should be placed in separate genera. In this study, we use data from the internal transcribed spacer region of nuclear ribosomal DNA to explore relationships within and between these genera. A total of 147 internal transcribed spacer sequences were obtained from multiple accessions of all species of Oxypolis and Ptilimnium and for several other genera from tribe Oenantheae. These included Lilaeopsis, which also has rachis leaves, and Cynosciadium and Limnosciadium, which have rachis-like leaves. These sequence data were analyzed using maximum parsimony, maximum likelihood, and Bayesian methods. The results from each of these analyses were congruent and suggest that neither Oxypolis nor Ptilimnium as presently defined is monophyletic. Oxypolis and Ptilimnium are each separated into two clades according to leaf morphology. The rachis-leaved Oxypolis species are provisionally recognized as the genus Tiedemannia and the rachis-leaved Ptilimnium species are provisionally recognized as the genus Harperella, pending further investigation. The relationships among these four clades and the genera Cynosciadium, Daucosma, and Limnosciadium are not clear. Nevertheless, it appears that the rachis-leaf habit has evolved multiple times in the tribe. Geographic structure is apparent in the phylogenetic trees and, pending further study, may suggest new taxa; in addition, the presence of O. occidentalis on the Queen Charlotte Islands, well-separated from other populations of this species, suggests that it may have survived the last glaciation in a refugium in this area. Keywords—Oenantheae, Queen Charlotte Islands, rachis leaves, refugium. Oxypolis Raf. and Ptilimnium Raf. are two closely related genera of tribe Oenantheae (Apiaceae subfamily Apioideae) endemic to North America. Both genera are small, with a total of 12 species currently recognized (Table 1). Oxypolis canbyi and Ptilimnium nodosum are federally endangered. These genera share several traits with other members of tribe Oenantheae, including glabrous leaves and stems, fascicled roots, and a preference for moist or wet habitats. The plants are of particular interest because some members of each genus exhibit a unique morphological trait known as rachis leaves, while others have compound leaves typical of subfamily Apioideae. Rachis leaves are linear, terete, hollow, and septate and are equivalent to the rachis of a pinnately compound leaf in which the pinnae are not fully expressed. Instead, these pinnae are highly reduced and are transformed into nodal appendages that function as hydathodes (Kaplan 1970). Rachis leaves are an adaptation to a semiaquatic habitat, and species of Oxypolis and Ptilimnium with rachis leaves (i.e. O. canbyi, O. filiformis, O. greenmanii, and P. nodosum) spend much of the growing season at least partially submerged. Compound-leaved species of Oxypolis have pinnately or ternately compound leaves with pinnae that are generally somewhat broad (except for O. ternata in which they are long and narrow). Compound-leaved Ptilimnium species have finely dissected, pinnately decompound leaves with linear or filiform pinnae. The first Oxypolis species with compound leaves to be described was originally placed in the genus Sium L. (as Sium rigidius L.; Linnaeus 1753). The first described Oxypolis species with rachis leaves (now called Oxypolis filiformis) was originally placed in the genus Oenanthe L. (Walter 1788). Subsequently, the latter was grouped with Sium rigidius under the name Sium teretifolium Elliott. Although Elliott (1817) stated that “this plant is remarkable for its terete, fistulose leaves,” he grouped it with the compound-leaved species based on features of the fruit. Similarly, Rafinesque (1825) included taxa with both leaf types in his new genus Oxypolis. In contrast, de Candolle (1829) advocated the separation of the rachis-leaved species (Tiedemannia DC.) from the compound-leaved taxa (Archemora DC.) based on leaf morphology and anther attachment; this classification, however, was never widely accepted. Coulter and Rose (1887) argued that Tiedemannia and Archemora should not be considered separate genera because they could find no significant differences in fruit characters between them. They believed that the leaves of Oxypolis ternata (which are long, narrow, and ternate) were intermediate between compound and rachis leaves. The fruits of all species of Oxypolis are remarkably similar in appearance, being dorsally flattened and winged. However, dorsal flattening and wing formation have evolved in several independent lineages of Apiaceae, most likely as a dispersal mechanism (Theobald 1971; Plunkett et al. 1996; Downie et al. 1998; Lee et al. 2001). In addition, dorsal flattening and wing formation can develop in different ways (Theobald 1971), therefore such similarities in fruit morphology cannot be taken as evidence of close relationship without