Testing reticulate versus coalescent origins of Erica lusitanica using a species phylogeny of the northern heathers (Ericeae, Ericaceae)

https://doi.org/10.1016/j.ympev.2015.04.005Get rights and content

Highlights

  • Phylogeny of northern Ericeae, with multiple accessions of all northern Erica species and representatives of southern clade.

  • Conflict between plastid and nrITS gene trees; simulations cannot rule out incomplete lineage sorting.

  • Morphological character optimisations suggest chloroplast capture with single origin of ‘tree heathers’.

  • Species tree analyses sensitive to reticulate versus linear scenarios, but not to uncertainty in age and generation time.

Abstract

Whilst most of the immense species richness of heathers (Calluna, Daboecia and Erica: Ericeae; Ericaceae) is endemic to Africa, particularly the Cape Floristic Region, the oldest lineages are found in the Northern Hemisphere. We present phylogenetic hypotheses for the major clades of Ericeae represented by multiple accessions of all northern Erica species and placeholder taxa for the large nested African/Madagascan clade. We identified consistent, strongly supported conflict between gene trees inferred from ITS and chloroplast DNA sequences with regard to the position of Erica lusitanica. We used coalescent simulations to test whether this conflict could be explained by coalescent stochasticity, as opposed to reticulation (e.g. hybridisation), given estimates of clade ages, generation time and effective population sizes (Ne). A standard approach, comparing overall differences between real and simulated trees, could not clearly reject coalescence. However, additional simulations showed that at the (higher) Ne necessary to explain conflict in E. lusitanica, further topological conflict would also be expected. Ancient hybridisation between ancestors of northern species is therefore a plausible scenario to explain the origin of E. lusitanica, and its morphological similarities to E. arborea. Assuming either process influences the results of species tree and further evolutionary inference. The coalescence scenario is equivocal with regard the standing hypothesis of stepping stone dispersal of Erica from Europe into Africa; whereas reticulate evolution in E. lusitanica would imply that the colonisation of Tropical East Africa by E. arborea instead occurred independently of dispersals within the rest of the African/Madagascan clade.

Introduction

Erica L. is among the largest genera of flowering plants (Frodin, 2004) with 830–840 species (Oliver and Oliver, 2003, Oliver and Forshaw, 2012). Most of the immense richness of Erica is endemic to the Cape Floristic Region (CFR) of South Africa, but species of Erica and closely related Calluna Salisb. and Daboecia D.Don (Ericaceae; Ericeae, commonly referred to as ‘heaths’ or ‘heathers’) are archetypal elements of open landscapes of Europe and surrounding areas, in both the Temperate and Mediterranean biomes. These northern heathers (Nelson, 2012) have been the subject of various empirical studies addressing evolutionary and ecological questions including the environmental factors dictating species distributions (Gil-López et al., 2014, Ojeda et al., 1998), and patterns of dispersal and genetic diversity (investigated for individual species by e.g. Beatty and Provan, 2012, Désamoré et al., 2010, Désamoré et al., 2012). The tree heather, Erica arborea, in particular, has been investigated to infer dispersal patterns between Europe and Tropical East Africa (TEA) (Désamoré et al., 2010) and within TEA (Gizaw et al., 2013).

The geographically widespread E. arborea is very similar in gross morphology to another species found exclusively in Europe: E. lusitanica. They share a tall habit and white corollas, and have been grouped in different formal and informal classifications (e.g. Bentham, 1839; ‘Tree heathers’, Nelson, 2012). However, the interpretation of morphological variation in Erica is far from straightforward: there is evidence for extensive homoplasy of morphological characters (Oliver, 2000, Pirie et al., 2011). Floral characters may evolve rapidly, probably as adaptations to changing (pollinator) environments (Pirie et al., 2011, Rebelo et al., 1985, Van der Niet et al., 2014), and vegetative characters such as adaptations to recurrent fires (Ojeda et al., 2005) may have undergone similar shifts. As a result, the classic generic classification (Bentham, 1839, Hansen, 1950), based on such characters, has long been considered artificial (Hansen, 1950, Oliver, 2000). In E. lusitanica, micromorphological characters such as indumentum and seed coat sculpture appear more similar to some other northern species than they are to E. arborea (Fagúndez et al., 2010, Fagúndez and Izco, 2010, Nelson, 2012).

The obvious means to assess this kind of morphological complexity is the molecular phylogeny. However, E. lusitanica has not previously been included in phylogenetic analyses, nor have several other European species, and current knowledge of the relationship of species of Erica L. in general is limited. Evidence available to date suggests that the heathers (tribe Ericeae; Ericaceae, including Erica, Calluna and Daboecia) comprise a basal grade of ‘northern’, largely European, species (McGuire and Kron, 2005, Pirie et al., 2011) subtending a single, much larger, ‘southern’ clade (‘African/Malagasy Erica’; Pirie et al., 2011). Data that we collected in the course of ongoing work on the phylogeny of Ericeae confirmed this general pattern. The results that we report here showed much improved resolution between lineages of the northern grade, revealing strong conflict between phylogenetic trees based on plastid data and independent nuclear ITS with regard to the position of E. lusitanica.

This gene tree conflict raises an alternative hypothesis to explain homoplasy of morphological characters in E. lusitanica: instead of indicating parallel evolution of traits, it could be the result of hybridisation between morphologically dissimilar species (de Villiers et al., 2013). However, gene tree conflict can instead represent incomplete lineage sorting, being the result of coalescent stochasticity given a linear (rather than reticulate) species tree (Nichols, 2001).

As is the case in many empirical examples of gene tree conflict (Blanco-Pastor et al., 2012, de Villiers et al., 2013, Maureira-Butler et al., 2008, Pirie et al., 2009), both coalescent and reticulate scenarios are in principle plausible for E. lusitanica: The possibility of ancient hybridisation events cannot be ruled out since hybridisation between extant Erica species is documented: wild hybrids between European species include Erica × stuartii (MacFarl.) Mast. (E. tetralix L. × E. mackayana Bab.); Erica × veitchii Bean (E. arborea L. × E. lusitanica Rudolphi.); Erica × watsonii Benth. (E. ciliaris L. × E. tetralix); Erica × williamsii Druce. (E. vagans L. × E. tetralix); and Erica × nelsonii Fagúndez (E. tetralix × E. cinerea L.) (Fagúndez, 2006, Fagúndez, 2012, Nelson, 2012, Rose, 2007), and various further crosses have been achieved in cultivation (Nelson, 2012). Moreover, there are no obvious karyological barriers to homoploid hybridisation: polyploidy has not been reported in Erica and chromosome counts are constant at n = 12 for all studied species with the exception of the European species E. spiculifolia that is n = 18 (Nelson and Oliver, 2005). Coalescent stochasticity on the other hand can generally be assumed to result in greater or lesser differences between inferred gene trees depending in particular on effective population sizes through time (Nichols, 2001).

In this paper, we attempt to discern whether gene tree conflict in the northern Ericeae with regards to E. lusitanica is the result of reticulate evolution or coalescent processes. We present two independent gene trees from DNA sequences (1) of multiple plastid markers and (2) of nuclear ribosomal ITS; from samples representing multiple accessions of all northern species of Erica, Calluna vulgaris (L.) Hull and Daboecia cantabrica (Huds.) K.Koch and exemplar sampling of the large African/Madagascan Erica clade. We use coalescent simulations and ancestral state reconstructions of selected morphological characters, and use data concatenation and coalescence based approaches in order to infer and test reticulate and linear species trees under each assumption separately. Finally, we use these trees to reassess the hypotheses concerning colonisation of Tropical East Africa by the putatively closely related tree heather, Erica arborea.

Section snippets

Taxon sampling

We sampled multiple populations from across the geographic distributions of all 21 non-hybrid species and two subspecies of Erica recognised by Nelson (2012) within the northern area (including for the first time E. umbellata and E. maderensis, as well as E. platycodon and E. azorica not previously included in phylogenetic analyses of the genus), plus multiple accessions of Calluna vulgaris and Daboecia cantabrica (123 accessions in total). In addition, we sampled one naturally occurring

Phylogenetic inference

Trees inferred from individual chloroplast markers did not show supported topological conflict (data not shown), therefore the data were combined under the assumption of a single bifurcating chloroplast tree. For ITS, the total alignment length was 974, of which 31 positions were variable but parsimony uninformative and 271 parsimony informative (33 and 183 respectively, for Erica alone); CI = 0.85, RI = 0.89. For the concatenated plastid regions, the total alignment length was 10,222 positions,

A robust and representative phylogenetic hypothesis for northern Ericeae

The phylogenetic results presented here represent the first analysis including all northern species of Erica and a considerable improvement in resolution compared to previous work (McGuire and Kron, 2005, Pirie et al., 2011). Our results indicate the monophyly of most currently accepted species. A notable exception is the E. scoparia clade, or wind-pollinated ‘besom heaths’ (Nelson, 2012), formed by the south-western Mediterranean E. scoparia and its relatives of the Macaronesian islands (E.

Acknowledgments

The authors gratefully thank Mr. Kurt Kramer, Ms. Bettina Banse, the plant breeders and producers of the Azerca group, Mr. Michael Knaack and his team from Belvedere Garten in Vienna, the gardener’s team of the Botanical Garden in Bonn, and the Gartenbauzentrum team in Straelen for spending their valuable time and making available plant material for this study. Further material was kindly provided by Charles Nelson, various members of The Heather Society, and Fernando Ojeda. For financial

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