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Biological Control 111 (2017) 45–52 Contents lists available at ScienceDirect Biological Control journal homepage: www.elsevier.com/locate/ybcon Hybridization affects life-history traits and host specificity in Diorhabda spp. MARK ⁎ E.V. Bitumea,d, , D. Beanb, A.R. Stahlkec, R.A. Hufbauerd a Exotic and Invasive Weeds Research Unit, U.S. Department of Agriculture – Agricultural Research Service, Albany, CA, United States b Colorado Department of Agriculture, Palisade, CO, United States c Department of Biological Sciences, Institute for Bioinformatics and Evolutionary Studies, University of Idaho, Moscow, ID, United States d Bioagricultural Science and Pest Management, Plant Sciences C129, Colorado State University, Fort Collins, CO, United States ARTICLE INFO ABSTRACT Keywords: Hybridization is an influential evolutionary process that has been viewed alternatively as an evolutionary dead- Biological control end or as an important creative evolutionary force. In colonizing species, such as introduced biological control Life-history traits agents, hybridization can offset losses in genetic variation due to population bottlenecks and genetic drift. fi Host speci city Increased genetic variation associated with hybridization could benefit biological control programs, by Hybrid vigor increasing the chances of establishment success. However, hybridization also can lead to the emergence of Inbreeding transgressive phenotypes, potentially including changes in host use; an important consideration when assessing Invasive species ff Host use potential non-target impacts of planned agents. In a series of laboratory experiments, we investigated the e ects Non-target impacts of hybridization between three species of Diorhabda released to control invasive Tamarix (saltcedar), evaluating effects on development time, adult mass, and fecundity over two generations for all three cross types, and over a third generation for one cross. Depending on the cross, hybridization had either a positive or neutral impact on the measured traits. We evaluated preference for the target (saltcedar) relative to a non-target host Tamarix aphylla (athel), and found hybridization influenced preference in two of the three cross types, demonstrating the possibility for hybridization to shape host use. The overall effects of hybridization varied by cross, suggesting that the outcome of hybridization will be difficult to predict a priori. 1. Introduction particularly when the genetic distance between parents is small (Mallet, 2005). Hybridization is an influential evolutionary process that has been Hybridization between recently diverged taxa may be particularly viewed alternatively as an evolutionary dead-end, because hybrids are beneficial in colonizing populations, which typically pass through often less fit than the parental species (Mayr, 1963; Dobzhansky, 1970) strong bottlenecks in population size, losing genetic variation, and or as an important creative evolutionary force (Anderson and Stebbins, potentially becoming inbred (Ellstrand and Schierenbeck, 2000; 1954; Ellstrand and Schierenbeck, 2000). On the detrimental side, Dlugosch and Parker, 2008; Rius and Darling, 2014; Laugier et al., hybrid breakdown, or outbreeding depression, can decrease perfor- 2016). In the planned release of specialized biological control agents, mance of hybrid individuals across a suite of traits linked to fitness, the goal is for the intentionally released population to establish and such as development time, mortality, and fecundity (Burton et al., propagate (Seastedt, 2015), to feed on the target host (typically an 1999; Edmands, 2002). On the positive side, hybridization can increase invasive weed or insect), and not shift to use other, non-target hosts. fitness relative to parents directly through heterozygote advantage Biological control programs have a fairly low success rate (< 50%), (overdominance of beneficial traits) (Edmands, 2002; Hedrick and mostly due to lack of establishment of agents in their new environment Garcia-Dorado, 2016; Lee et al., 2016) or by masking deleterious alleles (Van Driesche et al., 2010). As an evolutionary mechanism, hybridiza- (heterosis) and reducing inbreeding depression, and indirectly through tion might allow these establishing populations to better face adaptive increasing genetic variation and thus facilitating adaptive evolution challenges posed by a novel environment. There is some evidence that (Fisher, 1930). Additionally, hybridization can facilitate the formation releasing different “strains” or ecotypes of biological control agents in of novel genotypes, potentially producing ‘transgressive’ phenotypes an effort to increase genetic variation might improve establishment that fall outside the range of either parent (Rieseberg et al., 1999). success (Hopper et al., 1993; Henry et al., 2010). New evidence Alternatively, in some cases hybridization can have minimal effects, suggests that increased genetic variation can be even more important ⁎ Corresponding author at: Exotic and Invasive Weeds Research Unit, USDA-ARS, 800 Buchanan Street, Albany, CA 94701, United States. E-mail address: [email protected] (E.V. Bitume). http://dx.doi.org/10.1016/j.biocontrol.2017.05.009 Received 7 February 2017; Received in revised form 12 May 2017; Accepted 22 May 2017 Available online 26 May 2017 1049-9644/ Published by Elsevier Inc. E.V. Bitume et al. Biological Control 111 (2017) 45–52 than augmenting population size in promoting population growth athel for both hybrid offspring and the parental species. (Frankham, 2015; Hufbauer et al., 2015; Frankham, 2016). Colonizing populations also experience novel environments in which transgressive 2. Materials & methods phenotypes may, by chance, have higher fitness than parental pheno- types (Ellstrand and Schierenbeck, 2000). In addition to potential 2.1. Organism benefits for biological control, hybridization, by increasing variation and potentially altering phenotypes, warrants careful consideration of The beetles used in our experiments were from samples originally potential risks as well, particularly with respect to host use in the collected from saltcedar in Eurasia and North Africa. Descendants of biological control of weeds. There is robust evidence that host range of these samples were used to establish laboratory populations maintained specialized agents used in weed biological does not evolve readily, and at the Palisade Insectary, Biological Pest Control Program, Colorado that non-target effects that occasionally occur (e.g. (Pemberton, 2000) Department of Agriculture, Palisade, CO (CDA Palisade). Colonies were have largely been predicted from host-specificity testing prior to release maintained on cuttings of saltcedar, including T. ramosissima, T. (Van Klinken and Edwards, 2002). chinensis, and their hybrids (Gaskin and Schaal, 2002). In a study To continue this robust record of predictably safe biological control, investigating 110 saltcedar plants from a wide range in the USA, Gaskin examining potential risks posed by hybridization is crucial. Only a few and Kazmer (2009) found that 83–87% of plants were some level of studies have examined hybridization in weed biological control agents hybrid between the two species, with proportion T. ramosissima higher (Hoffmann et al., 2002; Mathenge et al., 2010; Szűcs et al., 2012). Szűcs in the north, and proportion T. chinensis higher in the south, in a clear et al. (2012) found that hybridization improved performance in vital latitudinal gradient (Williams et al., 2014). While we expect various life-history traits, which could improve control of the target pest. In levels of hybridization to be present, we fed all beetles with cuttings contrast, Hoffmann et al. (2002) and Mathenge et al. (2010) found that from a restricted set of 3 saltcedar sampling sites on the 39th parallel, hybridization between two biotypes of Dactylopius could alter host which would make the individual plants likely to be hybrid, and not specificity relative to parental lines. In the Dactylopius system, each parental (Williams et al., 2014, Fig. 1). Beetles were reared in 7.5-liter biotype is specific to a single cactus species, and hybrids between them capacity plastic containers with mesh siding for ventilation in incuba- generally can feed on both hosts, and thus are suggested for release in tors under a light regime of 16:8 and 27 °C/16 °C. Diorhabda carinata areas with both hosts (Mathenge et al., 2010). Additional research (“C” hereafter) used in this study were originally collected in 2002 near along these lines will improve our understanding of the consequences of Karshi (Qarshi), Uzbekistan (38.86 N, 65.72 E; elevation 350 m), and hybridization for biological control programs, including evaluating the Diorhabda sublineata (“S” hereafter) originated near the town of Sfax, degree to which it will be possible to draw general conclusions versus Tunisia (34.66 N, 10.67 E, elevation 10 m). Colonies of both of these research being needed on a case-by-case basis. species were maintained in the laboratory since their collection prior to To that end, we quantify the effects of hybridization between our experiments. Diorhabda elongata (“E” hereafter) were collected from biological control agents in the genus Diorhabda that were released to Sfakaki, Crete, Greece (35.83 N, 24.6 E, elevation 7 m) and in 2004 control Tamarix (saltcedar, or tamarisk) in North America. Saltcedar in they were first released upstream of Esparto, CA along Cache Creek in North America is comprised of a hybrid swarm