SEVEN Host Shifts, the Evolution of Communication, and Speciation in the Enchenopa binotata Species Complex of Treehoppers REGINALD B. COCROFT, RAFAEL L. RODRÍGUEZ, AND RANDY E. HUNT Speciation in animals is promoted by the evolution of of insect adaptation to their host plants can lead to assorta- behavioral differences that reduce attraction, mating, and tive mating as a pleiotropic effect (Wood and Keese 1990; fertilization between individuals in diverging populations Craig et al. 1993; Wood 1993; Feder 1998; Berlocher and (Mayr 1963; West-Eberhard 1983; Eberhard 1985, 1994, Feder 2002; see also Jiggins et al. 2005). Colonization of a 1996; Coyne and Orr 2004). Behavioral traits involved in new host environment may also have a profound influence communication between the sexes often provide the most on the evolution of communication. immediate contributions to reproductive isolation (Blair In this chapter we examine the role of communication 1955; Claridge 1990; Ryan and Rand 1993; Bridle and systems in the diversification of the Enchenopa binotata Ritchie 2001; Gerhardt and Huber 2002; Kirkpatrick and species complex of treehoppers (Hemiptera: Membracidae), Ravigné 2002). Consequently, identifying the evolutionary a clade of 11 sap-feeding species distributed across eastern forces that lead to changes in sexual communication is nec- North America. Based on the career-long series of studies by essary to understand the evolution of behavioral isolation T. K. Wood and colleagues, this group provides one of the and its contribution to divergence and speciation. most widely cited examples of sympatric speciation in The relationship between sexual communication and spe- plant-feeding insects (Tauber and Tauber 1989; Wood 1993; ciation depends on the extent of interactions between indi- Berlocher and Feder 2002; Coyne and Orr 2004). The viduals from the diverging populations. When geography E. binotata complex is especially promising for studies of the and/or ecology prevent such interactions during the specia- relationship between host shifts and behavioral isolation tion process, differences in mating signals and preferences (Landolt and Phillips 1997; Etges 2002), because of the rich may become important upon secondary contact, at which understanding of its natural history, comparative biology, point the differences may be enhanced by selection against and communication behavior. hybridization (Howard 1993; Kelly and Noor 1996; Coyne We first discuss the role of ecological factors in promot- and Orr 1997; Jiggins and Mallet 2000; Servedio and Noor ing assortative mating among populations of E. binotata 2003). In contrast, when the geography and ecology of spe- on ancestral and novel species of host plant. We then ciation do lead to interactions between individuals from the examine the role of sexual communication in behavioral diverging populations (i.e., when speciation occurs in sym- isolation among E. binotata species in the present. We patry), behavioral causes of reproductive isolation are explore sources of divergent sexual and natural selection important from the outset (Kondrashov et al. 1998; that could alter the evolutionary trajectory of mating Kondrashov and Kondrashov 1999; Kirkpatrick and Ravigné signals and preferences after a host shift. We also consider 2002). ways in which developmental influences on sexual com- Sympatric speciation is implicated in the diversification munication may affect gene flow between populations on of host-specific, plant-feeding insects (Mallet 2001; ancestral and novel hosts (before any evolutionary change Berlocher and Feder 2002; Bush and Butlin 2004). In these in communication systems has occurred), and how devel- insects, which comprise a large fraction of animal diversity opmental plasticity may generate changes in sexual selec- (Price 2002; Bush and Butlin 2004), speciation is often asso- tion regimes. We will argue that for plant-feeding insects, ciated with changes in host-plant use. Host shifts have sexual communication systems provide an important link widespread consequences for life-history traits, and aspects between host use, assortative mating, and divergent 88 selection and may be a key component of sympatric speci- hypothesis that local adaptation to a novel host can occur ation through host shifts. in sympatry with the original host is supported by the com- bination of host fidelity during oviposition and mating, Ecological Isolation in the E. binotata Complex assortative mating among allochronically shifted age classes, and host-related ecological divergent selection. Thomas K. Wood and colleagues developed the E. binotata In Enchenopa treehoppers, allochrony and host fidelity complex as a model for evaluating the role of host shifts in reduce, but do not eliminate, interactions among individu- promoting genetic divergence in sympatry (Wood and als on ancestral and novel hosts. The potential for interac- Guttman 1983; Wood 1993). (A complete list of Wood’s tions depends on the degree of allochrony among host- publications is provided by Deitz and Bartlett [2004].) The shifted populations, the extent of host fidelity, and the central message of this research is that successful coloniza- dispersal distance between plants (Wood 1980; Wood et al. tion of a novel host can lead to immediate assortative mat- 1999). There can be substantial overlap in mating periods ing between populations on the original and novel hosts, between species on different hosts. In a study in which pop- facilitating a response to divergent selection imposed by dif- ulations of six E. binotata species were established on natal ferences between the host species. host plants in a common environment, the average date of The most immediate factor contributing to assortative mating differed by only one to two days between species on mating after a host shift in Enchenopa is a change in life-his- certain hosts (Wood and Guttman 1982); as a consequence, tory timing caused by differences in host-plant phenology the three- to four-week mating periods overlapped almost (Wood and Guttman 1982; Wood and Keese 1990; Wood completely. et al. 1990). Species in the E. binotata complex have one Overlap of mating periods leads to interactions between generation per year, with eggs deposited into host-plant tis- individuals from different hosts, at least in experimental sue in late summer and fall, and developing in the spring populations in close proximity. In a study by Wood (1980), (Wood 1993). Egg development is triggered by hydration over 40% of courtships were between males and females of from the flow of sap through plant stems, which happens different species. Strikingly, however, only 6% of the mat- on different schedules in different host species (Wood et al. ings were between heterospecifics. The common occurrence 1990). As a consequence, egg hatch occurs at different times of mixed-species courtship, coupled with the rarity of mis- on different hosts. Depending on the rate of nymphal mating, suggested that behavioral isolation resulting from development on different hosts, the timing of adult eclo- courtship interactions may be even stronger than ecological sion and mating will also differ (Wood 1993). In an elegant isolation. Dispersal between host plants by mate-searching experiment that isolated the effects of allochrony from males occurs not only in cage experiments, but also in the other potential sources of reproductive isolation, Wood and field. For example, in one sample of individuals collected Keese (1990) manipulated the phenology of Celastrus scan- during the breeding season on Cercis canadensis in Missouri, dens plants containing egg masses of E. binotata from a sin- 88% of the males were E. binotata ‘Cercis,’ while 12% of the gle population to create a series of age classes. This experi- males were from another host-associated species in the ment resulted in significant assortative mating by age, even complex (R.B.C., unpublished data). (Note: The species in though age classes were only five days apart (Wood and this complex have not yet been formally named, and here Keese 1990). Assortative mating apparently was a conse- we refer to them using the name of their host plants.) quence of differences in the timing of female receptivity, as We suggest that behavioral isolation arising from differ- well as in male mortality schedules. ences in sexual communication systems may act synergisti- Assortative mating after a host shift is also promoted by cally with allochrony and host fidelity to promote isolation host fidelity. In an experiment in which males and females and facilitate divergence among host-shifted populations. of six species were placed in a common cage and allowed to That is, while the isolation that arises as a consequence of choose mates and host plants, most matings occurred on differences in host phenology is clearly important in initiat- the female’s natal host (Wood 1980). A later study showed ing divergence, the evolution of behavioral isolation may be assortative mating by host among experimentally host- important for completing the process of speciation (Rundle shifted populations, as a consequence of female and male and Nosil 2005). host fidelity (Wood et al. 1999). Females also show high In the following sections, we examine the relationships host fidelity in oviposition (Wood 1980). between host shifts, sexual communication, and assortative Host shifts lead not only to assortative mating, but also to mating in the E. binotata complex. We suggest that sexual divergent natural selection.