Evolutionary radiation of Asteromyia carbonifera (Diptera: Cecidomyiidae) gall morphotypes on the goldenrod Solidago altissima (Asteraceae) JOHN O. STIREMAN III 1 *, ERIC M. JANSON 2 , TIMOTHY G. CARR 3 , HILARY DEVLIN 1 and PATRICK ABBOT 2 1 Department of Biological Sciences, Wright State University, 3640 Colonel Glenn Highway, Dayton, OH 45435, USA 2 Department of Biological Sciences, Vanderbilt University, Box 351634 Station B, Nashville, TN 37235, USA 3 Department of Ecology and Evolutionary Biology, Cornell University, E145 Corson Hall, Ithaca, NY 14853, USA Received 21 April 2008; accepted for publication 3 June 2008 Population divergence of phytophagous insects is often coupled to host-plant shifts and is frequently attributed to the divergent selective environments associated with alternative host-plants. In some cases, however, divergence is associated with the use of alternative host-plant organs of a single host species. The basis of within-host radiations such as these remains poorly understood. In the present stusy, we analysed the radiation of Asteromyia gall midges occurring both within one host plant species and within a single organ on that host. In this system, four morphologically distinct Asteromyia gall forms (morphs) coexist on the leaves of goldenrod Solidago altissima. Our analyses of amplified fragment length polymorphism and DNA sequence data confirm the genetic differen- tiation among midges from three gall morphs and reveal evidence of a genetically distinct fourth gall morph. The absence of clear gall morph related clades in the mitochondrial DNA derived phylogenies is indicative of incomplete lineage sorting or recent gene flow, suggesting that population divergence among gall forms is recent. We assess the likely history of this radiation and use the results of phylogenetic analyses along with ecological data on phenology and parasitism rates to evaluate potential hypotheses for the mode of differentiation. These preliminary analyses suggest that diversification of the Asteromyia gall morphs is likely shaped by interactions between the midge, a symbiotic fungus, and parasitoid enemies. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 840–858. ADDITIONAL KEYWORDS: adaptive radiation – AFLP – cryptic species – ecological speciation – fungal mutualism – gall midge – multi-trophic interactions – within-host divergence. INTRODUCTION Insect diversity is strongly associated with phytoph- agy (Mitter, Farrell & Wiegmann, 1988; Farrell, 1998). At the root of this association are the special- ized relationships that most insects have with their host plants. Although some are extreme polyphages, most phytophagous insects are restricted to either one or a small set of related host plant species (Strong, Lawton & Southwood, 1984; Dyer et al., 2007). When successful, shifts between host plants can result in divergent selection, assortative mating, and/or spatial isolation between populations on different hosts (Futuyma & Moreno, 1988; Jaenike, 1990; Berlocher & Feder, 2002). Differential adaptation to ecological niches and reproductive barriers associated with the use of alternative host plants is thought to encourage ecological specialization and eventually lead to the emergence of reproductively isolated host races or species (Feder et al., 1997; Rundle & Nosil, 2005; *Corresponding author. E-mail: john.stireman@wright.edu Biological Journal of the Linnean Society, 2008, 95, 840–858. With 6 figures © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 840–858 840