Explaining the ‘anomalous’ distribution of Echinodium (Bryopsida: Echinodiaceae): Independent evolution in Macaronesia and Australasia

The peculiar disjunction between Macaronesia and Australasia of the morphologically isolated pleurocarpous moss genus Echinodium is one of the most prominent questions in bryology. Echinodium as traditionally circumscribed comprises six extant species, four restricted to the Macaronesian archipelagos and two confined to the Australasian/Pacific regions. Molecular phylogenetic analyses based on plastid trnLUAA intron and nuclear ribosomal ITS sequences indicate that Echinodium is polyphyletic and split into three groups. Three of the four Macaronesian species (E. spinosum and the single island endemics E. renauldii and E. setigerum) are closely related to each other and treated as Echinodium s.str. (Echinodiaceae). Further clarification of the relationships of Echinodium s.str. with Orthostichella, a segregate of Lembophyllaceae, is needed. The remaining Macaronesian species, E. prolixum, is transferred to Isothecium (Lembophyllaceae); this systematic position is also strongly supported by leaf characters. The two Australasian species, E. hispidum and E. umbrosum, are molecularly unrelated to the Macaronesian species and are transferred to Thamnobryum in the Neckeraceae. While the molecular data suggest that the peculiar distribution pattern of ‘Echinodium’ is an artefact, the striking morphological similarity observed in Macaronesian and Australasian species cannot be dismissed. Possible explanations are: (i) parallel morphological evolution of the ‘Echinodium habit’ in Macaronesia and Australasia, or (ii) retention of a set of plesiomorphic characters in non-related groups in relict habitats, the Macaronesian laurel forest and the austral temperate rain forests, respectively. Of these hypotheses, the evolutionary parallelism hypothesis seems more plausible for several reasons, which are discussed.