Phylogeny of Styrax Based on Morphological Characters, with Implications for Biogeography and Infrageneric Classification

Abstract

It has been suggested that the characters on which the current infrageneric classification of the genus Styrax is based are not reliable. However, to date no alternative classification for the genus has been proposed. To address this problem, a phylogenetic analysis of Styrax was conducted using 34 morphological characters. Parsimony analysis suggested that none of the currently recognized subdivisions of Styrax are monophyletic except section Pamphilia. This section has traditionally been considered a distinct genus but the analysis placed it in a highly nested position, consistent with its recent transfer into Styrax. The biogeographical history of Styrax was inferred with Fitch parsimony analysis and dispersal-vicariance analysis. Results suggest that 1) the presence of Styrax in South America is the result of migration from southern North America; 2) back-migration from South America to North America has occurred in both montane and lowland tropical lineages; and 3) a migration from Eurasia to North America has occurred in four lineages. The phylogeny provides evidence for a single origin of morphological gynodioecy within the genus, a floral reduction series among the gynodioecious species, and a host shift by gall-forming aphids (tribe Cerataphidini) that parasitize Styrax. Included is a revised infrageneric classification based on the phylogeny with a key and descriptions, and the establishment of two new combinations (Styrax series Cyrta and Styrax series Benzoin). The genus Styrax L. consists of approximately 130 species of trees and shrubs distributed in eastern and southeastern Asia, the New World (mainly tropical), and the Mediterranean region (Fig. 1). Styrax is delimited from the other ten genera of Styracaceae by a stamen tube that is attached high on the petals, ovules with two integuments, and a thick, indurate seed coat. Traditionally the Styracaceae have been placed in the Ebenales (Cronquist 1988; Thorne 1992; Takhtajan 1997) but recent molecular data suggest affinities of at least part of the family (including Styrax) to the Ericales (Morton et al. 1996; Soltis et al. 1997). The most recent comprehensive taxonomic treatment of Styrax is that of Perkins (1907). In this treatment, Styrax was divided into section Foveolaria (Ruiz & Pav.) Perkins, comprising taxa with 3-5 ovules per ovary (two species, Greater Antilles and Peru) and section Styrax, with about 16-24 ovules per ovary (remaining species). Section Styrax was in turn divided into series Styrax (= Imbricatae [Giirke] Perkins; about 35 species) and series Valvatae (Giirke) Perkins (about 95 species). According to Perkins' (1907) infrageneric key, the species of series Styrax possess imbricate corolla aestivation, whereas the species of series Valvatae and section Foveolaria possess valvate corolla aestivation. Pamphilia Mart. ex A.DC., a strictly South American genus of six species, was maintained as a separate genus in Perkins (1907). However, it has recently been transferred to Styrax by Wallnofer (1997) as a section on the basis of its overall similarity to other species of Styrax from South America, particularly S. foveolaria Perkins and S. nui B.Walln. Section Pamphilia differs from the other members of Styrax by the possession of five (vs. ten or more) stamens and near-basal (vs. axile) placentation, and by the lack of placental obturators. Perkins' (1907) classification of Styrax has been used as a basis foI most regional treatments of Styracaceae (e.g., Hwang 1987; Svengsuksa and Vidal 1992; Hwang and Grimes 1996) despite suggestions in the literature that some of the infrageneric taxa may not be monophyletic (van Steenis 1932; Fritsch 1997). Perkins (1907) acknowledged a number of anomalous species within ser. Valvatae that were often polymorphic for aestivation type. This was affirmed by van Steenis (1932, 1949), who consequently did not recognize the two series in studies of the Malesian species of the genus. Wallnofer (1997) has documented infraspecific variation in the number of ovules per carpel in sections Pamphilia and Foveolaria. Although it is clear that corolla aestivation and ovule number are not likely to serve as a definitive basis for the infrageneric classification of Styrax, a comprehensive revision based on an alternative set of criteria has not been attempted. The objective of