Circumscription and biogeographic patterns in the eastern North American-east Asian genus Stewartia (Theaceae: Stewartieae): Insight from choloroplast and nuclear DNA sequence data

Castanea, Sep 2002 by Prince, Linda M

ABSTRACT

Stewartia is a genus of approximately 26 species which are frequently divided into two subgenera or genera based on the leaf duration. All evergreen species (= Hartia) are found in the Old World. The deciduous species (= Stewartia sensu stricto) are dominant in the Old World but there are two representatives in the southeastern United States, Stewartia ovata and S. malacodendron. Maximum parsimony and likelihood data analyses of molecular DNA sequence data from both the nuclear and chloroplast genomes produce similar estimates of phylogeny for the group. All evergreen species sampled are more closely related to each other than to any of the deciduous species. The two New World deciduous species are more closely related to the evergreen species than to the Old World deciduous species. These findings complicate earlier vicariance biogeography hypotheses for the genus and challenge the recognition of Hartia, the latter of which disagrees with many published classification systems.

INTRODUCTION

Stewartia sensu lato (s.l.) is a genus of 23-26 species (Ye 1982, Li 1996) and the sole member of tribe Stewartieae in Theaceae. Prince and Parks (2001) confirmed the monophyly of the family and tribe using rbcL and matK gene sequences of the chloroplast genome. The majority of species are distributed in warm temperate and subtropical forests of the Old World (Figure 1), with only two species (S. malacodendron and S. ovata) in the southeastern United States. Diagnostic characters by which Stewartia can be distinguished from other members of the family Theaceae include: narrowly to broadly winged petioles; sclereids restricted to the petiole and petiole wing (versus throughout the leaf in other members of the family); nearly basal, axile placentation; ascending ovules; a capsular fruit that splits to reveal 2-4 narrowly winged or wingless seeds per locule; fruit lacking a persistent central columella; seeds flattened and containing a small, straight embryo and copious endosperm (Keng 1962).

Stewartia are often divided into two genera, Hartia, the evergreen species which are restricted to the Old World, and Stewartia sensu stricto (s.s.), the deciduous species which are found in both the Old and New World. The only other deciduous member of the family is the monotypic genus Franklinia, an endemic plant of Georgia, United States, which is now extinct in the wild (Bozeman and Rogers 1986). Hartia was erected by Dunn in 1902 to recognize a plant collected in Yunnan, China. According to Dunn, characteristics distinctive to the genus were the greater connation of the anther filaments into a staminal tube and the more numerous seeds per locule than is found in Stewartia. Wu (1940) expanded the list of characters to include an evergreen habit and the presence of a conspicuously winged or inflated petiole that enclosed the terminal bud or lateral shoot. Keng (1962) recommended merging the two genera based on anatomical data. Spongberg (1974) agreed with the broad circumscription of Stewartia as proposed by Keng.

There is no general consensus on whether one or two genera should be recognized in Stewartieae, nor on subgeneric classification. Ye (1982) recognized two genera, Hartia with 15 species and Stewartia with 12 species. Each genus was further subdivided into three sections based on number of flowers per inflorescence, bract shape, size, and texture, degree of style fusion, and capsule shape. Li (1996) distributed the evergreen species (under the generic name Stewartia) into several subgenera based on the degree of style connation, inflorescence type, and bracteole and sepal shape and size. Both classifications are provided in Table 1 along with appropriate author citations which will be omitted from the remainder of the text, figures, and tables. There are as many differences as similarities in the grouping of taxa into sections indicating considerable disagreement on the significance of particular morphological characters. Other classifications of the 20th century include Cheng (1934) and Yan (1981), both of which maintain Hartia; and Wu (1940) and Keng (1962), both of which recognize the evergreen species only as a distinct subgenus within Stewartia.

The distribution of Stewartia s.l. fits the classical eastern Asia-eastern North America vicariance disjunction (see recent reviews by Boufford and Spongberg 1983, Boufford 1998, and Wen 1998). Vicariance biogeography patterns have been attributed to migration and extinction events associated with major climatic and geological changes in evolutionary history, leading to isolation of a once widely distributed species into localized populations which ultimately speciate. Recent wide-spread extinction has been documented in North Temperate latitudes due to glaciation events of the Pleistocene. China appears to have the greatest biodiversity of woody plant species found in all of the North Temperate zone. This higher diversity is probably due to more extensive refugial zones during glaciation due to mountain ranges which formed barriers to limit the advancement of glaciers into southern China, thereby reducing the rate of extinction.