Evolution of Rhaphithamnus venustus (Verbenaceae), A Gynodioecious Hummingbird-Pollinated Endemic of the Juan Fernandez Islands, Chile

Abstract

Rhaphithamnus Miers. (Verbenaceae) consists of two species restricted to southern South America. Rhaphithamnus spinosus (A. L. Juss.) Mold. occurs in mainland Chile and adjacent Argentina; R. venustus (philippi) Robinson is endemic to the two islands (Masatierra and Masafuera) of the Juan Fernandez Archipelago. Both species are related to Citharexylum Miller, a genus distributed from Peru northward along the Andes. Youthful geological ages of the Juan Fernandez Islands (1-4 myr) and occurrences of the sister group also on the con.tinent suggest that R. venustus evolved from R. spinosus rather than the reverse. Morphologically the two species differ primarily in corolla length and color and stem armament, with R. spinosus with shorter (ca. 12 mm) and bluish flowers and usually with axillary thorns in contrast to longer (ca. 25 mm) and purple flowers and lack of thorns for R. venustus. Studies of pollinators reveal bees, flies, beetles, and infrequently hummingbirds for R. spinosus and exclusively hummingbirds for R. venustus. Rhaphithamnus spinosus is hermaphroditic and partially self-compatible, whereas R. venustus is gynodioecious and with an unknown compatibility system. In the latter species female flowers appear to be in an early stage of evolution because anthers are still fully formed, but usually without pollen grains. Embryological studies reveal breakdown of pollen mother cells (and tapetal cells) during meiosis. We hypothesize that evolution of floral features in R. venustus is a result of a change from primarily insect to hummingbird pollination; loss of thorns may result from absence of herbivores in the Islands. Gynodioecy in Rhaphithamnus may have as its selective basis reduction of inbreeding depression otherwise brought on by geitonogamy in scattered individuals of small populations.