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Hybridization and crossability in Caiophora (Loasaceae subfam. Loasoideae): Are interfertile species and inbred populations results of a recent radiation?¹

Institut für Biologie—Systematische Botanik und Pflanzengeographie, Freie Universität Berlin, Altensteinstraße 6, 14195 Berlin/Germany

ABSTRACT

Interspecific hybridization is considered a possible mechanism of plant diversification. The Andes are a hotspot of biodiversity, but hybridization in Andean taxa has so far not been investigated intensively. The current study investigates crossability in Caiophora (Loasaceae subfam. Loasoideae) by experimental interspecific hybridization of seven different species. Hand pollination was undertaken, developing fruits counted, thousand (seed) grain weights, and seed viability were examined. Cross pollination led to some fruit set in 36 of the 37 different combinations. Overall fruit set was virtually identical irrespective of the combination of parental plants. Mean germination rates were much higher in hybrid seeds, indicating a marked heterosis effect and the possible presence of an inbreeding depression in the source populations: In experimental hybridization the divergent taxa of Caiophora behave like isolated, inbred populations of a single species. Allopatry and different habitat preferences seem to be the key factors keeping the (interfertile) taxa of Caiophora apart in the apparent absence of both postmating isolating mechanisms and obvious isolating mechanisms in phenology and floral biology. Interspecific hybrids reported from the wild appear to be the result of secondary contact due to human impact.

Key Words: Andes • Caiophora • heterosis • hybridization • Peru • postmating isolating mechanisms • Loasaceae

Received for publication 7 December 2007. Accepted for publication 9 June 2008.

FOOTNOTES

¹ The authors express their gratitude to L. Rieseberg (Vancouver) and S. Renner (Munich) for helpful comments on the manuscript. The authors thank A. Cano and M. I. La Torre (Lima, Peru), F. Luebert (Santiago, Chile), E. Rodríguez (Trujillo, Peru), N. Salinas (Cuzco, Peru), H. Förther (München, Germany), N. Dostert, T. Henning, D. Kollehn, O. Mohr, C. Schwarzer and K. Weigend (Berlin, Germany) for help in the field and collecting; Prof. Dr. H. H. Hilger for space and funds for this study. Funds kindly provided by the following institutions are gratefully acknowledged: Studienstiftung des Deutschen Volkes, Deutscher Akademischer Austauschdienst, Lewis B. and Dorothy Cullman Laboratory for Molecular Systematics Studies at the New York Botanical Garden, Deutsche Forschungsgemeinschaft (Grant-nr.WE 2330/1), botconsult GmbH.

² Author for correspondence (e-mail: ackermal{at}zedat.fu-berlin.de)