Resistance to Aphis glycines among wild soybean accessions in laboratory experiments
Introduction
The soybean aphid (Aphis glycines Matsumura) is a pest of soybean (Glycine max (L.) Merr.), in eastern Asia and in North America (Ragsdale et al., 2011). It is native to eastern Asia, and has historically been a sporadic pest of soybean there; outbreaks can cause up to 52% yield reduction (Wang et al., 1994). Soybean aphid has been an invasive pest in North America since 2000 (Ragsdale et al., 2011). Although it has been found in nearly all soybean-producing states west of the Rocky Mountains, the pest status of soybean aphid in North America is largely confined to north-central production regions (Ragsdale et al., 2011). Soybean aphid infestations in North America were projected to cost $3.6 to $4.9 billion due to yield loss and control expenditures (Kim et al., 2008a).
Insecticides are the primary method of controlling soybean aphid (Hodgson et al., 2012), but host-plant resistance offers a promising alternative management option (Hesler et al., 2013). In general, a wide diversity of plant resistance sources will optimize the efficacy and durability of resistant crop lines (Smith, 2005; Mundt, 2014). This may be especially important for management of soybean aphid because various soybean aphid biotypes are able to overcome some of the major resistance (Rag) genes that have been identified (Kim et al., 2008b; Hill et al., 2010; Alt and Ryan-Mahmutagic, 2013; Fox et al., 2014). Biotype 2 is virulent to Rag1 plants, biotype 3 surmounts resistance from Rag2, and biotype 4 overcomes resistance in plants with Rag1, Rag2 or both genes. Avirulent soybean aphids are designated as biotype 1.
The domestication of crops has undoubtedly led to the loss of important pest resistance genes (Berlinger, 2008; van Doorn and de Vos, 2013; Zhang et al., 2017), and such loss through de-selection bottlenecks seems especially acute in the domestication of soybean (Hyten et al., 2006; Guo et al., 2010). Thus, the probability of finding aphid-resistant sources could be increased by including the screening of wild relatives of soybean (Hill et al., 2004; Yang et al., 2004; Hesler, 2013; Zhang et al., 2017). This can include wild soybean, Glycine soja Siebold and Zucc., which readily crosses with domesticated soybean and for which a large accession pool is available and not widely screened (Hyten et al., 2006; Zhang et al., 2017).
Various high throughput techniques have been developed to rapidly screen soybean germplasm in the laboratory and field (Hill et al., 2004; Mensah et al., 2005; Michel et al., 2010; Hesler et al., 2012; Bhusal et al., 2014), and many of these methods are adaptable for screening wild soybean as well (Hill et al., 2004; Hesler, 2013; Hesler and Tilmon, 2017). Hesler and Tilmon (2017) recently screened for soybean aphid resistance among 337 early-maturing wild soybean accessions in a series of assays in which soybean aphid were free to colonize various test accessions within environmental chambers. However, tests for resistance should extend beyond free-choice assays, as differences among accessions may be unduly influenced by relatively high attractiveness of some susceptible accessions rather than high unsuitability of other accessions (Berlinger, 2008). Such an effect could inflate the false discovery rate of resistant accessions (Broadhurst and Kell, 2006; Pusztai et al., 2013; Ganna et al., 2013). Accordingly, the identification of putatively resistant sources from screening trials may be followed up with no-choice cage assays that confine soybean aphid to individual accessions and better reflect limited host choice that soybean aphid faces in the field (Berlinger, 2008; Hesler et al., 2017b). The objective of this study was to report the results of screening assays that identified putative soybean aphid resistance among wild soybean accessions, as well as results from further evaluation of these accessions in no-choice assays.
Section snippets
Overview of experiments
Experiments were conducted at the North Central Agricultural Research Laboratory, USDA-ARS, Brookings, SD, USA, and included eight screening assays and two no-choice assays of wild soybean accessions. Ninety-six unique, wild soybean accessions were screened, with 16 accessions tested in two assays, including 12 accessions that showed resistance in their first assay (Appendix). Accessions identified as aphid resistant in screening assays (see below) were advanced for further evaluation in the
Screening assays
Mean rankings of aphid infestation ratings differed among test accessions in each of the eight screening assays (Table 1). The mean rankings for eight test accessions (PI 65549, PI 101404 A, PI 135624, PI 407032 B, PI 407205, PI 407299, 99PI81762 and 99PI101404B) were both greater than that of the susceptible check, PI 522212 B, and not significantly different from that of the resistant check, PI 549046, in respective tandem screening assays (Table 2). Another two test accessions (PI 342618 A
Discussion
Putative resistance to soybean aphid was identified in 10 wild soybean accessions through screening assays and confirmed to varying degrees among eight accessions in no-choice assays in this study. The resistance was particularly strong in PI 135624 and PI 65549. Regardless, all eight accessions should be advanced for genetic testing in order to understand patterns of inheritance, map resistance loci, and determine any novel alleles (Zhang et al., 2016, 2017; Bhusal et al., 2013). Such
Acknowledgments
Mark West and Lawrence Madsen advised on statistical procedures. Eric Beckendorf, Philip Rozeboom, Marissa Layman, Mallory Thompson, Megan Fisher, Alyssa Baumberger, Jamie Beckman, and Ryan Ackman provided technical assistance in conducting the experiments. Randy Nelson and Todd Bedford supplied wild soybean seed. Eric Beckendorf, Sophia Conzemius, and Lauren Hesler graciously reviewed drafts of this paper. The study was funded by USDA-ARS Project 5447-21220-005-00D, South Dakota Soybean
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Current address: Department of Entomology, OARDC, The Ohio State University, 1680 Madison Ave., Wooster, OH 44691, USA.