Recent evolutionary radiation and host plant specialization in the Xylella fastidiosa subspecies native to the United States.

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 (ρ/θ = 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.

Selenium biotransformations in an insect ecosystem: effects of insects on phytoremediation.

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.

Biotransfer of selenium: effects on an insect predator, Podisus maculiventris.

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.