Abstract
The Macaronesian Scrophularia lowei is hypothesized to have arisen from the widespread S. arguta on the basis of several phylogenetic studies of the genus, but sampling has been limited. Although these two annual species are morphologically distinct, the origin of S. lowei is unclear because genetic studies focused on this Macaronesian species are lacking. We studied 5 S. lowei and 25 S. arguta populations to determine the relationship of both species and to infer the geographical origin of S. lowei. The timing of S. lowei divergence and differentiation was inferred by dating analysis of the ITS region. A phylogenetic analysis of two nuclear (ITS and ETS) and two chloroplast (psbJ–petA and psbA–trnH) DNA regions was performed to study the relationship between the two species, and genetic differentiation was analysed by AMOVA. Haplotype network construction and Bayesian phylogeographic analysis were conducted using chloroplast DNA regions and a spatial clustering analysis was carried out on a combined dataset of all studied regions. Our results indicate that both species constitute a well-supported clade that diverged in the Miocene and differentiated in the Late Miocene-Pleistocene. Although S. lowei constitutes a well-supported clade according to nDNA, cpDNA revealed a close relationship between S. lowei and western Canarian S. arguta, a finding supported by the spatial clustering analysis. Both species have strong population structure, with most genetic variability explained by inter-population differences. Our study therefore supports a recent peripatric speciation of S. lowei—a taxon that differs morphologically and genetically at the nDNA level from its closest relative, S. arguta, but not according to cpDNA, from the closest Macaronesian populations of that species. In addition, a recent dispersal of S. arguta to Madeira from Canary Islands or Selvagens Islands and a rapid morphological differentiation after the colonization to generate S. lowei is the most likely hypothesis to explain the origin of the last taxon.
Highlights
In the chloroplast DNA (cpDNA) tree, all S. lowei individuals are included in a well-supported clade (PP = 1.00) along with individuals of S. arguta from the three most western sampled Canary Islands (Tenerife, La Gomera and La Palma), which pinpoints the differentiation of this clade to the Pleistocene (0.71 million years ago (Ma), 0.15–1.51 Ma HPD) (Fig 4B)
Scrophularia lowei is a species with high phenotypic and geographic differentiation with respect to S. arguta. Their phenotypic differentiation is supported by clear morphological differences, while their geographical differentiation is reflected in their distribution in different archipelagos: S. lowei is restricted to the Azores and Madeira, whereas S. arguta is found in the other Macaronesian archipelagos
A close relationship exists between western Canarian S. arguta populations and S. lowei, with both having recently diverged from a common ancestor
Summary
Scrophularia lowei and S. arguta (Fig 1), annual species with the same chromosome number (2n = 40), are characterized by small flowers (corolla 3–5 mm), calyx-lobes possessing a keel, an androecium with a well-developed staminode and white pollen grains, and conical and beaked capsules [37]. The main morphological differences between these species are the presence or absence of cleistogamous flowers in shoots or in inflorescences arising from the plant base (present in S. arguta but not in S. lowei) and different corolla colours (white, with sometimes purplish striae and margins in S. lowei vs brownish red in S. arguta; Fig 1). S. arguta is present in Macaronesia (Selvagens and Canary islands and Cape Verde) and ranges from north-western Africa to the Arabian Peninsula and the Horn of Africa, with some isolated populations on the Iberian Peninsula [38], in all cases with low number of individuals per population. Scrophularia arguta has a similar ecological habitat in Macaronesia, but prefers basaltic cliffs in lowland xerophytic areas, especially Euphorbia communities on coastal rocky outcrops; in the Canary Islands, it blooms from February to May [37, 40, pers. observ.]
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