Host-associated genetic differentiation in pecan leaf phylloxera Aaron M. Dickey § & Raul F. Medina* Department of Entomology, Texas A&M University, TAMU 2475, College Station, TX 77843, USA Accepted: 16 January 2012 Key words: Phylloxera notabilis, host discrimination, host race, amplified fragment length polymor- phisms, Hemiptera, Phylloxeridae, endophagy, parthenogenesis, gall inducer, water hickory, Carya il- linoinensis, Juglandaceae Abstract Host-associated differentiation (HAD) is the formation of genetically distinct host-associated popu- lations. One of the genotypic signatures of HAD is that populations exhibit stronger differentiation by host-plant species than by geographic isolation. HAD, as a mechanism promoting ecological spe- ciation, has been invoked to explain phytophagous insect diversity. Two traits proposed to promote HAD are endophagy and parthenogenesis. Using amplified fragment length polymorphisms (AFLPs), we tested for the presence of HAD in pecan leaf phylloxera, Phylloxera notabilis Pergande (Hemiptera: Phylloxeridae), an endophagous, gall inducing, and cyclically parthenogenetic insect on sympatric pecan and water hickory at a geographic mesoscale. This species shows strong HAD. Whereas the effect of collecting site was significant, accounting for 7.3% of molecular variation, host- plant species identity accounted for 63.5%. In addition, a choice test indicated that pecan leaf phyl- loxera originating from water hickory showed weak but significant preference for leaflets of the natal host, whereas pecan leaf phylloxera originating from pecan did not. This is the first such study of a species of arboreal Phylloxeridae, a poorly known insect group. This is also the first endophage and the second parthenogen shared by these two hickory species to show evidence of HAD. This hickory system could be a good parthenogen-rich counterpoint to the goldenrod system in the study of HAD in insect communities. Introduction Insect herbivores have long been considered important for the study of reproductive isolation and speciation (Walsh, 1864; Brues, 1924; Thorpe, 1930; Bush, 1975; Funk et al., 2002; Funk & Nosil, 2008). As parasites, they tend to be highly specialized, feeding on one or few host-plant taxa (Price, 1980; Bernays & Chapman, 1994). To insects, host plants are ephemeral sources of nutrition and rendezvous sites. They attract natural enemies (De Moraes et al., 1998; Kessler & Baldwin, 2001; Rasmann et al., 2005) and in some instances provide herbivores with chemical or physi- cal defenses against those enemies (Heinz & Parrella, 1994; Muller et al., 2001; Aliabadi et al., 2002; Karban & Agra- wal, 2002). In each of these roles, host plants can mediate disruptive natural selection and promote insect popula- tion divergence (Maynard, 1962; Rice & Hostert, 1993; Bush, 1994; Nosil & Crespi, 2006). Host-associated differ- entiation (HAD) is the formation of the genetically distinct subpopulations that result from this divergence (Dres & Mallet, 2002; Abrahamson & Blair, 2008) and that can ultimately lead to ecological speciation (Schluter, 2001; Rundle & Nosil, 2005). Several case studies document HAD in herbivorous insects (Dres & Mallet, 2002; Stireman et al., 2005; Sword et al., 2005; Althoff et al., 2006; Hendry et al., 2007; Pec- coud et al., 2009; Dickey & Medina, 2010; Hernandez- Vera et al., 2010) but only Stireman et al. (2005) and Dickey & Medina (2010) have specifically tested for the presence of HAD in more than one herbivore per host- plant pair. In addition, these two reports published nega- tive results, that is, cases where HAD was absent. Stireman *Correspondence: Raul F. Medina, Department of Entomology, TAMU 2475, College Station, TX 77843, USA. E-mail: rfmedina@ tamu.edu § Present address: USDA-ARS-USHRL, 2001 South Rock Road, Fort Pierce, FL 34945, USA Ó 2012 The Authors Entomologia Experimentalis et Applicata 143: 127–137, 2012 Entomologia Experimentalis et Applicata Ó 2012 The Netherlands Entomological Society 127 DOI: 10.1111/j.1570-7458.2012.01250.x