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Partial 16s rRNA gene sequences of 16 strains of the genera Photorhabdus and Xenorhubdus were determined by direct sequencing of PCR products. Aligned sequences were subjected to phylogenetic analysis by maximumlikelihood and maximum-parsimony methods. Distance matrix and phylogenetic analysis did not separate the genera unambiguously. Taxonomic grouping of the bacteria closely paralleled taxonomic grouping of their nematode associates and their geographic origins. We found at least two well-supported taxonomic groups in PhotorhQbdus species, which suggests that the genus Photorhabdus is coevolving with the nematodes and may be polyspecific.The genera Photorhabdus and Xenorhabdus consist almost entirely of bacterial symbionts of entomopathogenic nematodes belonging to the families Heterorhabditidae and Steinernematidae (6). The bacterial symbionts are carried monoxenically in a special vesicle in the intestine in infective juveniles of species of the genus Steinernema in the Steinernematidae (4) and throughout the entire intestine and pharynx of infective juveniles of species of the genus Heterorhabditis in the Heterorhabditidae (12,23). The nematodes provide protection and transport for their bacterial symbionts. The bacterial symbionts establish and maintain suitable conditions for nematode reproduction (21,22) and provide nutrients and antimicrobial substances that inhibit the growth of a wide range of microorganisms (2, 6).Since the original description of the genus Xenorhabdus by Thomas and Poinar (30), strains of this taxon have been isolated easily from entomopathogenic nematodes around the world (3, 5). Xenorhabdus luminescens (30) has been transferred to a new genus, the genus Photorhabdus (6). The genus Xenorhabdus contains four species: Xenorhabdus nematophilus, Xenorhabdus bovienii, Xenorhabdus poinarii, and Xenorhabdus beddingii (3). The taxonomy of the genera Photorhabdus and Xenorhabdus has been examined by several methods. Major studies have been based on both phenotypic (4,5,14,30) and genotypic traits (6, 11, 14, 24). Few studies have been conducted to determine intergeneric relatedness; however, the branching of genus level clusters remains unclear (24). A number of isolates have been studied in some detail but have not been named, and none of the DNA relatedness groups have been described as separate species because too few strains have been studied (6). Phylogenetic studies are generally lacking at intergeneric and generic levels, and the taxonomy of the two genera is still incomplete.Comparison of 16s rRNA gene sequences has proved to be extremely useful for determining phylogenetic relationship among eukaryotic and prokaryotic organisms (32) and has been used in determining the relatedness of the genera Photorhabdus and Xenorhabdus (24). Because only a few strains have been studied, the variation among strains may not be fully evaluated. In this study, the partial 16s rRNA gene sequences of 16 Photorhabdus and Xenorhabdus strains were determined by direct sequencing of PCR products with an...

Two experiments were conducted to examine the influence of foliar nitrogen, terpenes, and phenolics of Douglas-fir on the development of gypsy moth larvae. In the first experiment, foliar concentrations of nitrogen and allelochemicals were manipulated by fertilizing 3-year-old potted seedlings with 0 or 200 ppm nitrogen. Concentrations of foliar nitrogen (0.33-2.38%) were negatively correlated with the phenolics (15.8-24.4 mg/g). Sixth-instar larvae previously reared on current-year Douglas-fir needles were allowed to feed on these seedlings. Pupal weights (312.8-995.6 mg) were positively correlated with levels of foliar nitrogen, negatively correlated with amounts of foliar phenolics, and uncorrelated with terpene concentrations. In the second experiment, terpene and phenolic extracts from Douglas-fir foliage were incorporated at natural levels into artificial diets with high and low levels of protein nitrogen. Neonate larvae grew faster and were larger on the high nitrogen control diet (4.1-4.5%), however, fourth instars performed better on the control diet with low nitrogen levels (2.5-2.7%). Foliar terpenes incorporated into diet had little effect on neonate fitness, but may induce subtle physiological changes in later instar larvae. Phenolics, alone or in combination with terpenes, excessively suppressed growth and survival, with no individuals living through the fourth instar, regardless of the nitrogen level. Incorporating foliar phenolic extracts into artificial diet caused unnatural levels of toxicity and failed to clarify the effects of Douglas-fir phenolics on gypsy moth fitness. Foliar nitrogen is a key factor influencing gypsy moth development on Douglas fir, but may be mitigated to some degree by phenolics.

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