Rapid Invasion Despite Lack of Genetic Variation in the Erythrina Gall Wasp (Quadrastichus

The erythrina gall wasp, Quadrastichus erythrinae Kim, has recently and rapidly invaded a broad swath of the tropical and subtropical Pacific Basin, causing severe damage to most species of coral trees (Erythrina spp.). This small (length @1.5 mm) wasp attacks the photosynthetic tissue (leaves, buds, stems, flowers) of ornamental and native Erythrina, often killing the trees. This invasion poses an immediate extinction threat to native Erythrina spp. throughout Asia, Australia, and a number of Pacific archipelagos, including Hawai‘i, where populations of the endemic E. sandwicensis have been devastated. Although this pest is known to occur naturally in East Africa, the precise geographic origin of the invasions remains unknown. In this study, 1,623 base pairs of mitochondrial (cytochrome c oxidase subunit I) and nuclear DNA (elongation factor alpha) were used to confirm systematic identity and to examine genetic divergence among invasive populations from Hawai‘i, Guam, American Samoa, Japan, Singapore, Taiwan, and China. Samples from all invasive populations included in our study showed a complete lack of genetic diversity. Molecular findings confirm that a single species, Q. erythrinae, is involved in this dramatic, recent range expansion and that introductions may have been associated with population bottlenecks that have reduced genetic diversity in populations sampled. Although reductions in genetic diversity are generally considered detrimental to fitness, this study provides an example of invasion success despite a lack of detectable genetic variation. The monomorphic genetic pattern observed also suggests that Q. erythrinae initially may have been introduced to one location, and this invasive population may have subsequently served as a source for additional secondary invasions by unknown introduction vectors. Coral trees (Erythrina spp., Leguminosae/ Fabaceae) are a widespread and diverse genus of trees with approximately 110 species (Mabberly 1987), with a global tropical distribution. Their range includes not only continental endemic species across Africa, Australia, and South America but also endemic species in the islands of Hawai‘i, the South Pacific, Micronesia, and the Caribbean, where they are important components of native ecosystems. Some Erythrina species have been widely planted as ornamental trees, prized for their dramatic, seasonal flowers and variegated leaves, and other species are important agricultural resources serving as windbreaks, shade trees for crops (macadamia, coffee, cocoa), living fences, and fodder. In the Hawaiian Islands, the endemic coral tree, or wiliwili, Erythina sandwicensis, historiPacific Science (2010), vol. 64, no. 1:23–31 doi: 10.2984/64.1.023 : 2010 by University of Hawai‘i Press All rights reserved 1 This research was supported in part by a grant from the State of Hawai‘i Invasive Species Council, the USDA-CSREES-TSTAR program, and the Tri-Isle Research and Development. B.S.H. was supported by a grant from the U.S. Army Natural Resources Division during preparation of the manuscript. Manuscript accepted 27 April 2009. 2 Department of Plant and Environmental Protection Sciences, 310 Gilmore Hall, 3050 Maile Way, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. 3 Center for Conservation Research and Training, 3050 Maile Way, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. 4 Kaua‘i Agricultural Research Center, 7370 Kuamo‘o Road, Kapa‘a, Hawai‘i 96746. 5 Corresponding author: (e-mail: rubinoff@hawaii