- Top of page
- 1 Material and methods
- 2 Results
- 3 Discussion
Abstract Toxicodendron is a genus in the Rhus complex of Anacardiaceae with a disjunct distribution between eastern Asia and North America, extending to southeastern Asia and the neotropics. Nuclear (internal transcribed spacer, external transcribed spacer, and NIA-i3) and chloroplast (ndhF and trnL-F) sequences were used to construct phylogenetic relationships of Toxicodendron. Phylogenetic analysis of these data strongly support Toxicodendron as a monophyletic group distinct from other genera of the Rhus complex, and the phylogeny does not fully corroborate classification at the sectional level. Two temperate disjunct lineages were detected, one from section Toxicodendron and the other between the eastern North American Toxicodendron vernix and the eastern Asian Toxicodendron vernicifluum. Their divergence times were estimated to be 13.46 (7.95–19.42) and 7.53 (2.76–12.86) mya, respectively. The disjunction between section Griffithii (taxa from warm temperate to tropical Asia) and Toxicodendron striatum (from the neotropics) was supported and their divergence time was estimated to be 20.84 (11.16–30.52) mya in the early Miocene. Our biogeographic results and the paleontological data support the Bering land bridge as the most likely route to explain the temperate disjunctions, yet the tropical disjunction in Toxicodendron seems to be best explained by the North Atlantic land bridge hypothesis.
The cashew family (Anacardiaceae) consists of more than 80 genera and 600–750 species mostly distributed in tropical Africa, Asia, and the Americas, with a small number of species in subtropical and temperate areas (McNair, 1925; Cronquist, 1981; Pell, 2004; Wannan, 2006; Pell et al., 2008). Toxicodendron Mill. is one of a few genera with a primary distribution in temperate to subtropical regions (Gillis, 1971; Zomlefer, 1994). The genus comprises approximately 24 species showing a disjunct distribution in temperate North America and eastern Asia (Li, 1952; Gray, 1846; Graham, 1972; Wen, 1999, 2001). A few taxa of Toxicodendron also occur in the tropics ranging from Central America to northernmost South America and southeastern Asia.
Toxicodendron is well-known for possessing skin-irritating oil (urushiol) that can cause severe allergic reactions to humans. Species of the genus also have lacquer in the phloem, and the lacquer is important for making anticorrosives or decorative paint. Taxa of this genus were often included in Rhus L. s.l. (Barkley, 1937a, 1963; Young, 1979; Cronquist, 1981). Miller et al. (2001) showed that Rhus should be delimited more narrowly, and that Toxicodendron and several other genera including Actinocheita F. A. Barkley, Cotinus Mill., Malosma Nutt. ex Abrams, Melanococca Blume, Metopium P. Browne, and Searsia F. A. Barkley are best segregated from Rhus.
In comparison with other genera in the Rhus complex, Toxicodendron generally has axillary inflorescences, whitish to dune-colored fruits without glandular pubescence, and toxic catechol in its resin (Brizicky, 1963; Gillis, 1971). However, the delimitation of Toxicodendron remains controversial. For example, Brizicky (1963) suggested treating Toxicodendron as a subgenus of Rhus because of their overall similarities in the inflorescence and the structure of flowers and fruits. Gillis (1971), however, preferred to treat Toxicodendron as a separate genus because of its suite of distinctive characters (e.g., presence of poisonous resin, absence of red glandular hairs on pedicels and fruits, and significantly smaller pollen grains).
Toxicodendron is commonly delineated by four sections (Venenata, Toxicodendron, Griffithii, and Simplicifolia). Section Toxicodendron (five species) comprises usually woody climbers with trifoliate leaves, short and subulate filaments, elongate anthers, and pendulous infructescences. The most well-known member of this section is Toxicodendron radicans, which is widely distributed throughout eastern North America and eastern Asia and is commonly known as poison ivy due to its superficial resemblance to English ivy (Hedera helix L. of Araliaceae) and Boston ivy [Parthenocissus tricuspidata (Siebold & Zuccarini) Planch. of Vitaceae]. Section Simplicifolia is represented by only one narrowly distributed species of northern Borneo in southeastern Asia. Its habit is very similar to that of section Toxicodendron as a deciduous woody climber, but its leaves are simple and subcoriaceous (Gillis, 1971). Section Venenata includes trees or woody shrubs with pinnately compound leaves, long filaments, and short globose anthers, mainly from eastern Asia. It is the largest section in the genus with approximately 18 species distributed throughout eastern Asia from Japan to Indonesia (Fig. 1). One species has a broad distribution within North America, ranging from southern Quebec to Florida, while another occurs in Central and South America from Veracruz, Mexico to the highlands of Colombia (Gillis, 1971; Min, 1980). Section Griffithii contains five species and has been segregated from section Venenata by Min (1980) based on their thick coriaceous leaves, erect infructescences, and mature fruits with irregular dehiscence of exocarps.
Two disjunct lineages between eastern Asia and North America have been reported in the genus, one from section Toxicodendron and the other from section Venenata. Toxicodendron radicans is perhaps one of the earliest examples cited by Gray (Gray, 1846, 1859) as showing a floristic connection between eastern Asia and eastern North America (also see Gillis, 1971, 1975; Wen, 1999). This is one of two examples that show such a disjunction at the species level, with the other being Phryma of Phrymaceae (Li, 1952; Wen, 1999; Nie et al., 2006). The distributional pattern in section Venenata is also intriguing with most species distributed throughout eastern Asia from Japan to Indonesia and only two species broadly distributed from North, to Central, to northern South America.
Most disjunct taxa between eastern Asia and North America are north temperate elements with only a few occurring in the subtropical and tropical regions (Li, 1952; Wen, 1999, 2001; Ying, 1983). Biogeographic studies so far have largely focused on temperate taxa in the Northern Hemisphere (Wen, 1999; Donoghue et al., 2001). However, it is important to include the close relatives from tropical and subtropical regions to further understand the evolution of the temperate disjunctions in the Northern Hemisphere (Lavin & Luckow, 1993; Wen, 1999, 2001; Ickert-Bond et al., 2007). A few examples of biogeographic disjunctions between temperate eastern Asia and North America with tropical extensions have been surveyed. One example can be found in Rhus s. str. Phylogenetic and biogeographic studies have suggested a migration route from North America into Asia by way of the Bering land bridge (BLB) with an ancient split in the late Eocene (Yi et al., 2004, 2007).
Although morphological data seem to support the separation of Toxicodendron from Rhus s.l., rigorous phylogenetic analyses of molecular sequence data and biogeographic analyses have never been used to test the status and evolution of Toxicodendron in the complex. The use of multiple genes from different genomes, especially low-copy nuclear genes, is needed to fully address these questions in a biogeographic context (Wang et al., 2000; Peng & Wang, 2008). In this study, we use nuclear [internal transcribed spacer (ITS), external transcribed spacer (ETS), and NIA-i3] and chloroplast data (trnL-F and ndhF) to: (i) test the monophyly of Toxicodendron in the Rhus complex; (ii) construct phylogenetic relationships within the genus; and (iii) reconstruct the biogeographic history of Toxicodendron, especially concerning the temperate and tropical intercontinental disjunctions between the Old and the New World.