Australian Journal of Entomology (2011) 50, 405–417 Gall inducers take a leap: host-range differences explain speciation opportunity (Thysanoptera: Phlaeothripidae)aen_831 405..417 Michael J McLeish,1,2* Michael P Schwarz2 and Tom W Chapman2,3 1Department of Botany and Zoology, DST-NRF Centre of Excellence for Invasion Biology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. 2Flinders University, Sturt Road, Bedford Park, Adelaide, SA 5042, Australia. 3Department of Biology, Memorial University, St. John’s, Newfoundland, Canada, A1B 3X9. Abstract Phytophagous insects that specialise on broadly distributed plant groups are exposed to host-species diversity gradients. The gall-inducing thrips genus Kladothrips (Froggatt) that specialise on Australian Acacia Mill. (Mimosoideae: Leguminosae, subgenus Phyllodineae DC.) is expected to exhibit variation in host range that is dependent on host ecology. Host Acacia species distributions show structuring between the arid Eremean and non-arid biomes of the monsoonal tropics and temperate south-western and south-eastern Australia. We investigate two aspects of host use in: (1) the Kladothrips rugosus species complex that specialises on hosts whose distributions overlap among sibling lineages on different Acacia species; and (2) Kladothrips nicolsoni that specialises on a species that is relatively isolated from hosts of sibling lineages. First, several approaches that use DNA sequence data are combined to infer putative species among K. rugosus lineages collected from multiple Acacia species using: phylogenetic inference; statistical parsimony; amova; maximum likelihood genetic distance relationships; and generalised mixed Yule coalescent likelihood test of lineage delimitation. Second, haplotype network analysis is used to estimate population structuring of K. nicolsoni that specialises on a geographically isolated Acacia host species. Analyses indicated below species-level relationships among lineages that each induces galls on different Acacia host species. In contrast, haplotype network analysis for a gall-thrips species that is largely allopatric with hosts of sibling species indicates isolation by distance and range expansion effects. Greater host-species richness enhances gall-thrips opportunity to host shift resulting in founder effects that lead to disruptive selection. Restricted gene flow among gall-thrips populations specialising on a relatively isolated host species implies genetic drift via allopatric mechanisms. The results suggest gall-thrips speciation is driven by the combined influence of ecologically based selection with genetic drift that is largely determined by variation in host-species richness between arid and non-arid Australia. Key words Acacia, ecological opportunity, host shift, insect–plant interaction, thrips. INTRODUCTION (Crisp et al. 2004; Crisp & Cook 2007; Byrne et al. 2008; Murphy et al. 2010). Specialising on a comparatively small Aspects of the environment have profound consequences for proportion of these Acacia is the gall-inducing thrips genus how speciation proceeds (Schluter 2001). Reproductive iso- Kladothrips Froggatt (Crespi & Abbot 1999; Morris et al. lation should evolve between populations adapting to con- 2001; McLeish et al. 2007a). The taxonomy of this small trasting habitat structure, climate, resources, and the suite of group of Acacia lineages is controversial (Murphy et al. predators and competitors present (Schluter 2009). Heteroge- 2010). However, host-species affiliations as defined by the neous environments are believed to influence diversification Kladothrips phylogeny agree with older classifications that in widely distributed plant clades and the insect clades that separate section Juliflorae (Benth.) from section Plurinerves specialise on them (Austin et al. 2004; Östergård & Ehrlén (Benth.). These Acacia host-species distributions are geo- 2005; Janz et al. 2006). The Australian continent comprises graphically structured between arid and non-arid areas distinct biomes that include the arid Eremean interior and (Maslin & Pedley 1988; Maslin 2001b; Miller et al. 2003; non-arid temperate and monsoonal tropics that together Murphy et al. 2003; Bowman & Yeates 2006; McLeish et al. support an enormous diversity of endemic Acacia Mill. 2007b). Speciation of gall-thrips on each of these host sec- tions is expected to operate under different environmental and ecological contexts that are partitioned by arid/non-arid *[email protected] geographical disjunctions (MJ McLeish in review). © 2011 The Authors Journal compilation © 2011 Australian Entomological Society doi:10.1111/j.1440-6055.2011.00831.x 406 M J McLeish et al. Fig. 1. The species ranges of Plurinerves hosts of the Kladothrips rugosus and K. waterhousei complexes. Distributions of different Acacia species are shown by various overlapping outlined areas. The species range of Acacia papyrocarpa highlighted below shows Kladothrips nicolsoni sample populations. Distributions were simplified from Maslin (2001b). Solid vertical lines connect putative K. rugosus agg. species collected from multiple Acacia species. The dotted lines indicate the same putative K. waterhousei species but sampled from more than one Acacia species. Specialisation on a narrow host range does not necessarily The Kladothrips rugosus agg. (Froggatt), or species limit further evolution (Maslin 2001a; Nosil 2002; Nosil & complex, is associated with at least 14 Acacia species (Mound Mooers 2005; Tripp & Manos 2008). Ecological barriers to et al. 1996; Crespi et al. 1998; Crespi & Worobey 1998; phytophagous insect host use include fitness tradeoffs and Crespi & Abbot 1999; Kranz et al. 2000; McLeish et al. ancestral host preference (Janz et al. 2001; Ward et al. 2003). 2007b). Host species of the K. rugosus complex are distributed Host specificity of parasitic organisms in general does not over the Great Dividing Range, Murray River Basin and riv- necessarily have to correspond with historical associations and erine lowland region, Nullabor Plain and south-west Western is susceptible to ecological and geographical circumstances Australia (Maslin 2001b). Two lineages belonging to this (Janzen 1985; Nyman 2010). Differentiation in specialist phy- clade, Kladothrips nicolsoni (ex. K. rugosus Froggatt) and K. tophagous insects dependent on host ecology is influenced by maslini are described species. The clade comprises several selection driven by ecological opportunity (Janz et al. 2006; morphologically similar lineages that inhabit Acacia section Hoberg & Brooks 2008; Via 2009). Kladothrips predomi- Plurinerves. Each gall-thrips lineage is associated with a nantly exhibit host-species specificity (Crespi et al. 1998). variant of several highly discrete gall morphologies believed to However, observations support the possibility that some be extended phenotypes (Crespi et al. 1998; Crespi & species induce galls on multiple host species (Ananthakrish- Worobey 1998; Crespi & Abbot 1999; McLeish et al. 2006a). nan 1992; Morris et al. 2002; Crespi et al. 2004). Host spe- Unlike all the other members of the complex, K. nicolsoni is cialisation should favour the formation of new species via a distributed over the south-central deserts of the Eremean in shift from an ancestral to novel host plant followed by adap- relative isolation of host species of sibling lineages (Fig. 1). In tation (Bush 1969; Crespi et al. 1998). Clades comprising contrast, other members of the complex specialise on host races or sibling species offer an opportunity to evaluate host species that are largely overlapping. specificity at species and coalescent stages subject to different The aim of this study is to infer putative genetic species environmental contexts (Forbes et al. 2009). (Mayr 1969; Avise & Ball 1990) belonging to the K. rugosus © 2011 The Authors Journal compilation © 2011 Australian Entomological Society Host range and speciation opportunity 407 complex in order to detect departures from strict one-to-one Phylogenetic inference host-species specificity. We assess the genetic distinctiveness A comprehensive explanation of DNA extraction, PCR and of K. rugosus lineages and reconcile these with host Acacia alignment protocols is given by McLeish et al. (2006a). Phy- species distributions among different biomes of Australia logenetic inference was based on amplicates of up to 1245 bp (Crisp et al. 2004). A Bayesian consensus phylogram is of the COI mitochondrial gene, 444 bp of elongation factor inferred to identify the exclusivity of clades’ that might repre- one alpha (EF-1a), 472 bp of 16S (ribosomal RNA subunit) sent population-level divergences. Maximum likelihood (ML) and 549 bp of wingless nuclear genes. Up to 618 bp of the COI approach analyses based on four gene fragments are used to was amplified and included in the putative population-level generate distributions of pairwise genetic distances among the analyses. Sequence alignment was straightforward and K. rugosus species complex in order to detect below-species achieved manually by comparison with existing alignments host relationships. Statistical parsimony is used to infer aggre- (McLeish et al. 2007b). A Bayesian consensus phylogram was gates of K. rugosus lineages that are used as a priori putative inferred for the entire K. rugosus complex sample including K. species for analysis of molecular variance (amova) tests of nicolsoni, to assess branch length differences among lineages genetic variance. Fixation indices (F ) among the aggregates ST using the