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First published online November 7, 2008; doi:10.3732/ajb.0800214 American Journal of Botany 95: 1621-1631 (2008) © 2008 Botanical Society of America, Inc.

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The age and biogeography of Citrus and the orange subfamily (Rutaceae: Aurantioideae) in Australasia and New Caledonia¹

² Centre for Plant Biodiversity Research CSIRO Plant Industry, GPO Box 1600, Canberra, ACT, 2601 Australia ³ School of Botany and Zoology, The Australian National University Canberra, ACT, 0200 Australia

ABSTRACT

The geological history of Australasia, New Caledonia, and Southeast Asia, has been complex, resulting in competing biogeographic hypotheses for taxa found here. Alternative hypotheses—Gondwanan vicariance, rafting terranes, long-distance dispersal—may be distinguished by different predicted divergence times between disjunct sister taxa. Taxa within Rutaceae subfamily Aurantioideae are ideal for testing these hypotheses because of their distributions. Therefore, the ages of Rutaceae and Aurantioideae were estimated using molecular dating. One data set comprised 51 sequences of rbcL and atpB with sampling across rosids and three fossil calibrations: crown Fabales+Fagales+Rosales (>94 Ma), Fabaceae (>51 Ma) and stem Ailanthus, Simaroubaceae (>52 Ma). Another data set comprised 81 Aurantioideae using >8 kb of chloroplast sequence and secondary calibration. Confidence in estimated divergence times was explored by varying the root age, dating method (strict, local, and relaxed clocks), and inclusion of internal calibrations. We conclude that the Rutaceae crown diverged in the Eocene (36.4–56.8 Ma, mean 47.6), whereas the Aurantioideae crown originated in the early Miocene (12.1–28.2 Ma, mean 19.8). This young age suggests that Gondwanan vicariance does not explain the distributions of extant Aurantioideae. Taxa found in New Caledonia may have arrived by separate transoceanic dispersal events.

Key Words: Aurantioideae • biogeography • Citrus • fossil calibration • Gondwana • molecular dating • New Caledonia • Rutaceae • trans-oceanic dispersal

Received for publication 27 June 2008. Accepted for publication 16 September 2008.

FOOTNOTES

¹ The authors thank R. J. Bayer for access to unpublished data.

⁴ Author for correspondence (Bernard.Pfeil{at}csiro.au)

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