The bacterial pathogen, Xylella fastidiosa, infects many plant species in the Americas, making it a good model for investigating the genetics of host adaptation. We used multilocus sequence typing (MLST) to identify isolates of the native U.S. subsp. multiplex that were largely unaffected by intersubspecific homologous recombination (IHR) and to investigate how their evolutionary history influences plant host specialization. We identified 110 "non-IHR" isolates, 2 minimally recombinant "intermediate" ones (including the subspecific type), and 31 with extensive IHR. The non-IHR and intermediate isolates defined 23 sequence types (STs) which we used to identify 22 plant hosts (73% trees) characteristic of the subspecies. Except for almond, subsp. multiplex showed no host overlap with the introduced subspecies (subspecies fastidiosa and sandyi). MLST sequences revealed that subsp. multiplex underwent recent radiation (<25% of subspecies age) which included only limited intrasubspecific recombination (/ ‫؍‬ 0.02); only one isolated lineage (ST50 from ash) was older. A total of 20 of the STs grouped into three loose phylogenetic clusters distinguished by nonoverlapping hosts (excepting purple leaf plum): "almond," "peach," and "oak" types. These host differences were not geographical, since all three types also occurred in California. ST designation was a good indicator of host specialization. ST09, widespread in the southeastern United States, only infected oak species, and all peach isolates were ST10 (from California, Florida, and Georgia). Only ST23 had a broad host range. Hosts of related genotypes were sometimes related, but often host groupings crossed plant family or even order, suggesting that phylogenetically plastic features of hosts affect bacterial pathogenicity.T he genetic typing of bacterial isolates is an increasingly important tool for understanding the epidemiology of pathogenic bacteria. It permits us to track biogeographical patterns, host specificity, and the evolutionary changes occurring within taxa. To achieve these goals, one of the most useful and widely used typing methodologies is multilocus sequence typing (MLST), introduced in 1998 (1, 2). The spread of this methodology has been rapid in the study of human bacterial pathogens; however, MLST schemes are equally important in the study of plant pathogens (3), and schemes for plant bacteria are beginning to become established, e.g., Pseudomonas syringae (4), Xylella fastidiosa (5, 6), fruit tree phytoplasmas (7), Ralstonia solanacearum (8), Xanthomonas spp. (9), and Acidovorax citrulli (10).The genetic typing of plant pathogens provides a critical first step in answering the important question of what genetic factors determine host specificity. Genome comparison among species and pathovars of Xanthomonas suggest specificity is determined by a complex set of genetic differences involving both gene content and gene sequence (11). Of these two kinds of difference, content and sequence, gene content differences are more easily studied, since it...

Phytoremediation of selenium-contaminated soils may be influenced by higher trophic levels including insects. We examined how selenium affects the behavior, survival, and development of the wasp parasitoid Cotesia marginiventris, parasitizing its natural host, the beet armyworm Spodoptera exigua, feeding on alfalfa, Medicago sativa, irrigated with water containing selenate. X-ray absorption spectroscopy was used to quantify the selenium chemical forms in each trophic level. Alfalfa partially transformed selenate to organoselenium. S. exigua contained only organoselenium, both directly absorbed from M. sativa and transformed from selenate. C. marginiventris cocoons collected shortly after larval emergence contained only organoselenium derived from the host. The surprising finding of trimethylselenonium-like species in adult parasitoids and the cocoons from which they emerged suggests that adults and pharates can detoxify excess selenium through methylation and volatilization. Adult parasitoids do not discriminate against selenium-containing alfalfa, even though alfalfa generates selenium volatiles. Parasitoids raised on selenium-fed larvae emerged later and pupae weighed less than their selenium-free counterparts. We conclude therefore that C. marginiventris can be used to control S. exigua damage to M. sativa being used to remove selenium from soils. Moreover, the presence of such insects may improve phytoremediation by increasing biotransformation of inorganic selenium and release of volatile selenium species.

Minimal information is available on the impact of various organic and inorganic forms of the ecologically and agriculturally important pollutant, selenium (Se), on insect herbivores. We conducted bioassays with artificial diet to examine the feeding responses of a generalist herbivore, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), to various forms and concentrations of Se. Two different-aged cohorts of larvae were examined in choice tests with control diets vs. test diets incorporating lethal concentrations (LC 10 , LC 30 , LC 50 , and LC 70 ) of sodium selenate, sodium selenite, seleno-DL-cystine, and seleno-DL-methionine. Tests initiated with neonates showed larvae significantly preferred control diet over diet with sodium selenate, sodium selenite, or selenocystine, but at most concentrations showed no preference between selenomethionine and control diet. Choice tests initiated with third instars demonstrated a preference for control diet over sodium selenate treatments, and sodium selenite treatments. In contrast, no significant responses were found in tests initiated with third instars offered the choice between selenocystine or selenomethionine and untreated controls. Additionally, comparisons of consumption demonstrated that inorganic selenium compounds were antifeedants whereas the organic selenium compounds tested have little antifeedant activity. The toxicity of all of the tested forms of selenium, in combination with the lack of antifeedant activity of some compounds, has the potential to affect both the distribution and diversity of terrestrial herbivores in both agricultural and natural systems. INTRODUCTIONSelenium (Se) is an essential trace nutrient important to humans and most other animals as an antioxidant, functioning as the metal cofactor for important enzymatic activity requiring glutathione peroxidase (Mayland, 1994). When present in high concentrations, Se is substituted for sulfur in sulfur to sulfur linkages of proteins.This results in an inability to form a helix structure, leading to non-functioning, malformed proteins (Lemly, 1998). The element Se can be acquired by plants, is readily biomagnified in the Selenium Effects on Herbivore Feeding Preferences 65 food chain, and is known to cause toxicosis in wildlife, domestic animals, and humans (Heinz et al., 1990;Frankenberger and Benson, 1994;Daniels, 1996).Soil Se accumulation associated with agricultural irrigation, geochemical processes, mining, and a variety of other industrial sources frequently results in significant effects on animal health (Haygarth, 1994). Within the western United States, high Se levels around the Salton Sea (Imperial County, CA) and Kesterson National Wildlife Refuge (Merced County, CA) have resulted in symptoms of Se intoxication of animals (Presser and Ohlendorf, 1987;Heinz et al., 1990;Mayland, 1994;Daniels, 1996;Lemly, 1997). However, these are not just localized problems; approximately 160,000 ha of agricultural land in the San Joaquin Valley of CA are affected by salinity and high water tables, a...

The effects of selenium (Se) accumulation in phytophagous insects on predators in the next trophic level were investigated. The generalist predator Podisus maculiventris Say (Hemiptera: Pentatomidae) was fed an herbivore Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) larvae from control diet and diets at two Se levels (0, 109, and 135 microg/g sodium selenate dry weight added). Predators reared on larvae grown on diets with sodium selenate took longer to complete each developmental stage and had significantly higher mortality rates. Predators achieving the adult stage on Se-containing hosts weighed 20% less than those feeding on control larvae. Reduced adult weight of insects has been associated with reduced fitness (longevity, egg production, etc.), which would have long-term negative impacts on population dynamics. These developmental and mortality effects resulted from biotransfer of Se, not biomagnification since the trophic transfer factor was less than 1.0 (approximately 0.85). Host larvae in Se-treatments contained significantly more total Se (9.76 and 13.0 microg/g Se dry weight host larvae) than their predators (8.34 and 11 microg/g Se dry weight predatory bugs, respectively). Host larvae and predators in the control groups did not differ in their Se content. These data demonstrate that Se in the food chain may have detrimental population level effects on insects even in the absence of biomagnification, given the host contains significantly elevated concentrations of selenium.

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