Recurrent origin of peripheral, coastal (sub)species in Mediterranean <i>Senecio</i> (Asteraceae)

Abstract

<p><b><i>Background</i></b>: It is argued that coastal endemic taxa may evolve in parallel at the periphery of the distributional range of a widespread species.</p> <p><b><i>Aims</i></b>: We tested this hypothesis for the origins of three peripheral, coastal isolates of <i>Senecio, S. glaucus</i> ssp. <i>glaucus</i> (Israel), <i>S. g</i>. ssp. <i>coronopifolius</i> p.p. (Sicily), and <i>S. hesperidium</i> (Morocco), from widespread <i>S. glaucus</i> ssp. <i>coronopifolius</i>. We also determined the relative roles of selection vs. genetic drift in shaping phenotypic divergence in ssp. <i>glaucus</i> and <i>S. hesperidium</i>, using Lande’s test of neutral morphological change.</p> <p><b><i>Methods</i></b>: We surveyed morphological and/or allozyme variation in the three peripheral isolates and mainly inland populations of <i>S. g</i>. ssp. <i>coronopifolius.</i></p> <p><b><i>Results</i></b>: Genetic data supported independent origins of the coastal taxa from nearby populations of ssp. <i>coronopifolius</i>. These descendant and ancestral populations showed pronounced morphological but weak genetic differentiation. Phenotypic similarities between ssp. <i>glaucus</i> (Israel) and <i>S. hesperidium</i> (Morocco) in plant height and floral traits may have resulted from parallel divergent selection from ssp. <i>coronopifolius</i>, though drift remains an alternative cause in <i>S. hesperidium</i>.</p> <p><b><i>Conclusions</i></b>: Our results indicate parallel ecotype formation and (sub)speciation in <i>Senecio</i> in which primarily selective vs. neutral determinants promoted the recurrent origin of coastal types in, respectively, Israel and Morocco.</p