Evolution of Erigeron (Compositae) in the Juan Fernandez Islands, Chile

Abstract

Six endemic species of Erigeron occur in the Juan Fernandez Islands: E. fernandezianus, E. ingae, E. luteoviridis, E. rupicola, E. stuessyi, and E. turricola. The last five are restricted to the younger and more distant island, Masafuera (1-2 million years old), and E. fernandezianus occurs on Masafuera and Masatierra (ca. 4 million years old). All species have n = 27. Phenetic analysis suggests that all six species have come from a single introduction from southern South America, perhaps coastal southern Peru or central Chile. Cladistic analysis reveals two major clades: one involving E. rupicola and E. stuessyi, and the other containing the other four species. In the latter line, E. ingae and E. turricola form a closely related paraphyletic species pair, and E. luteoviridis and E. fernandezianus represent a closely allied holophyletic unit. Phytogeographically, the genus apparently became established first on Masafuera, with subsequent (and more recent) colonization of Masatierra. Speciation appears to have followed a pattern of adaptive radiation with discernible geographic and ecological isolation. Flavonoid compounds from 56 populations of endemic species in the archipelago and from 10 populations of five species on the continent exhibit interpopulational variation, which compromises the value of flavonoids for systematic purposes. Erigeron rupicola and E. stuessyi have similar profiles that differ from the other species. The Juan Fernandez islands, located 600 km off the coast of Chile at latitude 33?S, consist of two major islands: Masatierra (MT, or Isla Robinson Crusoe), and Masafuera (MF, or Isla Alejandro Selkirk), separated in an east-west line by 150 km of ocean. The unusual flora of these islands comprises 147 native species of angiosperms, 65% of which are endemic (Skottsberg 1956; Stuessy et al. 1990), and has been the focus of several systematic and evolutionary studies including: chromosomes (Sanders et al. 1983; Spooner et al. 1987; Sun et al. 1990), flavonoids (Pacheco et al. 1985), genetic variation (Crawford, Stuessy, and Silva 1987, 1988; Crawford et al. 1990), evolution and biogeography (Sanders et al. 1987), conservation status (Sanders et al. 1982; Stuessy et al., in press), and patterns of phylogeny (Stuessy et al. 1990). The Juan Fernandez archipelago is an appropriate system in which to examine evolutionary trends because the approximate geological ages of the islands are known (3.7-4.4 million years for Masatierra and 1-2.4 my for Masafuera, Stuessy et al. 1984), the source for most of the flora is mainland South America, there are only two major islands of small dimensions (ca. 75 and 58 kM2, respectively), and the flora has relatively few species. Among endemic taxa of the islands, 25% are Compositae. Erigeron L. is the third largest genus (after the endemic genera Dendroseris D. Don and Robinsonia DC.) with six endemic species: E. fernandezianus (Colla) Harling, E. ingae Skottsb., E. luteoviridis Skottsb., E. rupicola Philippi, E. stuessyi Valdebenito, and E. turricola Skottsb. Erigeron fernandezianus is found on both islands, whereas the remaining five are restricted to Masafuera. These species differ in leaf shape (from lanceolate to spatulate), capitulum size and number, and growth form (semiglobose subshrubs to erect shrubs over 1 m tall). Ecological differentiation also occurs with different species found from near sea level to over 1000 m, and from dry and exposed areas near the coast to wet environments usually covered by fog. A morphological, chemical, and cytological study of Erigeron in the Juan Fernandez Islands