Evolution and conservation genetics of an insular hemiparasitic plant lineage at the limit of survival: the case of Thesium sect. Kunkeliella in the Canary Islands

Abstract

Premise: The diversification of island flora has been widely studied. However, the role of environmental niches in insular radiation processes has been less investigated. We combined population genetic analyses with species distribution modelling to clarify the genetic relationships, diversification patterns, species niche requirements, and conservation of Thesium sect. Kunkeliella, a clade of rare hemiparasitic plants endemic to the Canaries.

Methods: We studied the three extant Thesium species and a new taxon from La Palma Island. We developed 12 microsatellites and performed population genetic analysis and studied the demographic history of the group. To evaluate the role of niche conservatism in the diversification of the group, we performed species distribution modelling (ESM) with four algorithms.

Results: All species presented moderate genetic diversity values for rare endemics. Thesium canariense (Gran Canaria) showed high differentiation, whereas T. subsucculentum, T. retamoides (Tenerife), and La Palma populations are closely related. The lineage may have undergone a recent diversification with colonization proceeding east to west, with T. canariense as sister to the others. We detected a climatic niche shift, as taxa showed different distributions across the temperature gradient. There is enough evidence to describe La Palma populations as a new species.

Conclusions: We characterized the evolutionary history of Thesium sect. Kunkeliella by integrating genetic and ecological assessments. Our results indicate that this clade has undergone a recent radiation process with niche differentiation among species. The results increase our knowledge about insular radiations and will inform the conservation management of the study species.

Keywords: Santalaceae; endangered species; endemic; insular radiation; microsatellites; niche conservatism; species distribution modeling.

Figure 1

(A) Photographs of Thesium species included in the study. From left to right, Thesium palmense, T. subsucculentum, T. retamoides, T. canariense. Image credits: Ángel Palomares (T. palmense) and Pedro Luis Pérez Paz (T. subsucculentum, T. retamoides, T. canariense). (B) Geographic context of the Canary Islands (in red box). (C) Localities sampled for every studied taxon, and the former distribution of the extinct T. psilotocladum. (D) Bar plot of co‐ancestry inferred from Bayesian cluster analysis implemented on structure and clumpp (K = 4)

Figure 2

(A) Neighbor joining dendrogram based on Nei's (1983) genetic distance among the 10 localitites of Thesium. (B) Representation of the most probable demographic scenario (Scenario 3) with the ABC method implemented in DIYABC (Cornuet et al., 2014). t1, t2, t2a: time scale of divergence times measured in generations since the present (t = 0). N1, N2, N3, and Na refer to effective population sizes, respectively, of T. palmense (LP)T. canariense, T. retamoides, T. subsucculentum, and from a nonsampled ancestral population. (C) Principal coordinate analysis based on the genetic distance among individuals

Figure 3

(A) Contours of topoclimatic suitability defined by Ensemble of Small Models (ESM), defined as the threshold value of maximum True Skill Statistics (TSS) evaluation score. (B) Principal component analysis of topoclimatic niche hypervolume

Figure 4

Photographs of Thesium palmense sp. nov. (Location: Andén de Los Calzones Rotos). (A) Shrub, (B) flower, (C) fruit. Image credit: Ángel Palomares

Figure 5

Holotype of Thesium palmense sp. nov., deposited in the TFC Herbarium, University of La Laguna