Whole genome resequencing and complementation tests reveal candidate loci contributing to bacterial wilt (Ralstonia sp.) resistance in tomato.

Tomato (Solanum lycopersicum) is one of the most economically important vegetable crops worldwide. Bacterial wilt (BW), caused by the Ralstonia solanacearum species complex, has been reported as the second most important plant pathogenic bacteria worldwide, and likely the most destructive. Extensive research has identified two major loci, Bwr-6 and Bwr-12, that contribute to resistance to BW in tomato; however, these loci do not completely explain resistance. Segregation of resistance in two populations that were homozygous dominant or heterozygous for all Bwr-6 and Bwr-12 associated molecular markers suggested the action of one or two resistance loci in addition to these two major QTLs. We utilized whole genome sequence data analysis and pairwise comparison of six BW resistant and nine BW susceptible tomato lines to identify candidate genes that, in addition to Bwr-6 and Bwr-12, contributed to resistance. Through this approach we found 27,046 SNPs and 5975 indels specific to the six resistant lines, affecting 385 genes. One sequence variant on chromosome 3 captured by marker Bwr3.2dCAPS located in the Asc (Solyc03g114600.4.1) gene had significant association with resistance, but it did not completely explain the resistance phenotype. The SNP associated with Bwr3.2dCAPS was located within the resistance gene Asc which was inside the previously identified Bwr-3 locus. This study provides a foundation for further investigations into new loci distributed throughout the tomato genome that could contribute to BW resistance and into the role of resistance genes that may act against multiple pathogens.

The Aggregation Pheromone of Phyllotreta striolata (Coleoptera: Chrysomelidae) Revisited.

Aggregations of the striped flea beetle Phyllotreta striolata on their crucifer host plants are mediated by volatiles emitted from feeding males. The male-specific sesquiterpene, (6R,7S)-himachala-9,11-diene (compound A), was shown previously to be physiologically and behaviorally active, but compound A was attractive only when combined with unnaturally high doses of the host plant volatile allyl isothiocyanate (AITC) in field trapping experiments. This indicated that our understanding of the chemical communication in this species is incomplete. Another male-specific sesquiterpenoid, (3S,9R,9aS)-3-hydroxy-3,5,5,9-tetramethyl-5,6,7,8,9,9a-hexahydro-1H-benzo[7]annulen-2(3H)-one (compound G), has been reported from an American P. striolata population. We confirmed the presence of compound G, and investigated its interaction with compound A and AITC in a P. striolata population in Taiwan. Compound G was attractive to Taiwanese P. striolata in laboratory bioassays, but significantly more beetles were attracted to a blend of compounds A and G. Under the same conditions, P. striolata showed no preference for the blend of A and G combined with a range of doses of AITC over the sesquiterpenoid blend alone. The sesquiterpenoid blend was tested further in field trapping experiments and attracted significantly more beetles than traps baited with compound A and ecologically relevant amounts of AITC. We conclude that A and G are components of the male-specific aggregation pheromone of P. striolata in Taiwan, and that the attractiveness of the pheromone is not reliant on the presence of AITC. Our results further indicate that the male-specific sesquiterpenoid blends differ qualitatively between the Taiwanese and American populations of P. striolata.

Application of cytochrome oxidase I sequences for phylogenetic analysis and identification of thrips species occurring on vegetable crops.

Thrips are direct pests as well as vectors of important viruses infecting crop plants. One of the major constraints in studying the relationship between thrips vectors and tospoviruses is the difficulty of identifying the vector species because of high intraspecific variation among thrips populations. Molecular approaches have been used to identify species differences. In this study, partial cytochrome oxidase I (COI) sequences were used to understand the phylogenetic relationship among thrips populations, and assess their usefulness to identify and classify unknown thrips species collected from different crops. In total, 29 COI variants were obtained while examining the sequence polymorphisms in COI of 182 insects analyzed in this study, which were collected from six countries on tomato, chilli, onion, cabbage, cucumber, watermelon, Ethiopian mustard, French bean, and peanut. The phylogenetic analysis showed that the insects used in this study clustered with five distinct species-groups designated as Thrips palmi group, T. tabaci group, Frankliniella occidentalis group, Scirtothrips dorsalis group and an unclassified group. Higher intraspecific genetic variation was observed in S. dorsalis and T. palmi followed by T. tabaci and F. occidentalis. Thus, it was confirmed that the COI gene could be useful in grouping different thrips species and genera that coexist in a particular cropping system.

Effect of rearing conditions on production of sternal gland secretion, and identification of minor components in the sternal gland secretion of the predatory stink bug Eocanthecona furcellata.

Production of the male specific compound, 6,10,13-trimethyltetradecyl isovalerate by the predatory stink bug Eocanthecona furcellata (Wolff) was dramatically affected by rearing conditions. Male bugs kept isolated after eclosion produced an average of 1,948 ng of 6,10,13-trimethyltetradecyl isovalerate per bug, whereas male bugs reared in groups of 5-8 bugs produced an average of only 4 ng of 6,10,13-trimethyltetradecyl isovalerate per bug. Same-sex or mixed-sex pairs of bugs produced less than 50 ng per bug. Male bugs kept isolated for 1 wk and then grouped for 1 wk produced 3 ng of 6,10,13-trimethyltetradecyl isovalerate per bug, whereas male bugs grouped first and then isolated produced 135 ng of 6,10,13-trimethyltetradecyl isovalerate. A total of 11 minor components in relative amounts of less than 1% of the major 6,10,13-trimethyltetradecyl isovalerate were found in the sternal gland secretion. These included 6,10,13-trimethyltetradecanol, acetate, propionate, and butyrate esters of 6,10,13-trimethyltetradecanol, and isovalerate or valerate esters of homologs of 6,10,13-trimethyltetradecanol.