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Title: Evolution of Rhaphithamnus venustus (Verbenaceae), A Gynodioecious Hummingbird-Pollinated Endemic of the Juan Fernandez Islands, Chile 
Author: Sun, Byung Y; Stuessy, Tod F; Humana, Ana M; Riveros G, Magaly; Crawford, Daniel J
Date: 1996-01
Publisher: University of Hawaii Press
Citation: Sun BY, Stuessy TF, Humana AM, Riveros GM, Crawford Daniel J. 1996. Evolution of Rhaphithamnus venustus (Verbenaceae), a gynodioecious hummingbird-pollinated endemic of the Juan Fernandez Islands, Chile. Pac Sci 50(1): 55-65.
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.
ISSN: 0030-8870
URI: http://hdl.handle.net/10125/2586

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