The Hemiparasitic Plant Phtheirospermum (Orobanchaceae) Is Polyphyletic and Contains Cryptic Species in the Hengduan Mountains of Southwest China

Abstract

Phtheirospermum (Orobanchaceae), a hemiparasitic genus of Eastern Asia, is characterized by having long and viscous glandular hairs on stems and leaves. Despite this unifying character, previous phylogenetic analyses indicate that Phtheirospermum is polyphyletic, with Phtheirospermum japonicum allied with tribe Pedicularideae and members of the Ph. tenuisectum complex allied with members of tribe Rhinantheae. However, no analyses to date have included broad phylogenetic sampling necessary to test the monophyly of Phtheirospermum species, and to place these species into the existing subfamiliar taxonomic organization of Orobanchaceae. Two other genera of uncertain phylogenetic placement are Brandisia and Pterygiella, also both of Eastern Asia. In this study, broadly sampled phylogenetic analyses of nrITS and plastid DNA revealed hard incongruence between these datasets in the placement of Brandisia. However, both nrITS and the plastid datasets supported the placement of Ph. japonicum within tribe Pedicularideae, and a separate clade consisting of the Ph. tenuisectum complex and a monophyletic Pterygiella. Analyses were largely in agreement that Pterygiella, the Ptheirospermum complex, and Xizangia form a clade not nested within any of the monophyletic tribes of Orobanchaceae recognized to date. Ph. japonicum, a model species for parasitic plant research, is widely distributed in Eastern Asia. Despite this broad distribution, both nrITS and plastid DNA regions from a wide sampling of this species showed high genetic identity, suggesting that the wide species range is likely due to a recent population expansion. The Ph. tenuisectum complex is mainly distributed in the Hengduan Mountains region. Two cryptic species were identified by both phylogenetic analyses and morphological characters. Relationships among species of the Ph. tenuisectum complex and Pterygiella remain uncertain. Estimated divergence ages of the Ph. tenuisectum complex corresponding to the last two uplifts of the Qinghai-Tibet Plateau at around 8.0-7.0 Mya and 3.6-1.5 Mya indicated that the development of a hot-dry valley climate during these uplifts may have driven species diversification in the Ph. tenuisectum complex.

Keywords: Hengduan Mountains; Orobanchaceae; Phtheirospermum; Pterygiella; cryptic species; phylogenetic incongruence.

FIGURE 1

Plants of the Phtheirospermum tenuisectum complex. (A–D) Morphological variation in Ph. tenuisectum: (A) dissected leaves with wide lobes, and (B) flowers with spreading lower-lip and entire calyx lobes from Kangding (population T6, see Figure 2 and Supplementary Table S1, hereafter); (C) yellow flower with drooping lower-lip and branched calyx lobes from Lijiang (T12); and (D) yellow flower with spreading lower-lip and branched calyx lobes (T2). (E,F) Phtheirospermum sp. 1 from Lijiang (D1 and D2): (E) a whole plant and habitat; (F) flowering branch. (G–I) Morphological variation in Phtheirospermum sp. 2: (G) a flowering plant, and (H) leaves/bracts and flower with long and entire calyx lobes from Batang (H7); and (I) leaves with linear lobes from Zhongdian (H2). (J,K) Ph. muliense: (J) plants and habitat; (K) vegetative branch.

FIGURE 2

Geographical sampling of the Ph. tenuisectum complex in Southwest China. The river systems are highlighted in blue. The color of circles corresponds to taxa. Numbers and letters indicate populations; these abbreviations are also used in the Figures 3–6. More information regarding collection vouchers can be found in Supplementary Table S1.

FIGURE 3

Bayesian inference (BI) tree inferred from the nrITS dataset. ML Bootstrap values are presented under branches, and BI posterior probabilities are shown above branches. The topology of some clades with short branch lengths appear on the right. The bottom scale bar represents the number of substitutions per site.

FIGURE 4

Bayesian inference tree inferred from the combined plastid dataset. The seven plastid markers include matK, rbcL, rps2, rps16, trnK-matK, trnH-psbA and trnL-F. ML Bootstrap values are presented under branches, and BI posterior probabilities are shown above branches. The topology of some clades with short branch lengths appear on the right. The bottom scale bar represents the number of substitutions per site.

FIGURE 5

Phylogenetic network of the Ph. tenuisectum complex and Pterygiella spp. using all loci. The scale bar on the top left represents the number of substitutions per site.

FIGURE 6

A simplified maximum clade credibility tree of Lamiales from BEAST divergence time analysis. The estimated age of nodes is presented above the branch. Node bars represent the 95% highest posterior density (HPD) interval. Six calibrated (red) and 13 key stem/crown nodes (black) were annotated by letters and/or numbers.