Taxonomy of Echium (Boraginaceae) species from Cape Verde Islands

The morphological variation of endemic species from the Macaronesian Islands has long attracted attention of many taxonomists. The taxonomy of the endemic Echium L. species (E. hypertropicum Webb, E. stenosiphon Webb and E. vulcanorum A.Chev.) from Cape Verde Islands was revised, on the basis of in situ collected material and on herbarium specimens. Our results revealed that the patterns of morphological variation correspond closely to the geographic localisation of the islands groups, and that habit and floral morphology are important for distinguishing the two southern taxa, E. hypertropicum and E. vulcanorum, from the northern species, E. stenosiphon. This species has the same ecological preferences (e.g. altitude; exposition; soil type) in São Nicolau, São Vicente and Santo Antão Islands. However, the differences in indumentum observed among E. stenosiphon specimens account for the occurrence of different taxonomic groups distributed in these Northern Islands. The two southern species, E. hypertropicum and E. vulcanorum, are morphologically similar, but have different ecological preferences, a pattern of variation similar to other Macaronesian Echium species. Furthermore, the morphological variation observed among the three Cape Verde species, discussed within the general context of Echium taxonomy and closely related genera (e.g. Lobostemon, Echiostachys and Pontechium), suggests that the systematic relationships are beyond the actual circumscription of these genera.