Origins of cheating and loss of symbiosis in wild Bradyrhizobium.

Rhizobial bacteria nodulate legume roots and fix nitrogen in exchange for photosynthates. These symbionts are infectiously acquired from the environment and in such cases selection models predict evolutionary spread of uncooperative mutants. Uncooperative rhizobia - including nonfixing and non-nodulating strains - appear common in agriculture, yet their population biology and origins remain unknown in natural soils. Here, a phylogenetically broad sample of 62 wild-collected rhizobial isolates was experimentally inoculated onto Lotus strigosus to assess their nodulation ability and effects on host growth. A cheater strain was discovered that proliferated in host tissue while offering no benefit; its fitness was superior to that of beneficial strains. Phylogenetic reconstruction of Bradyrhizobium rDNA and transmissible symbiosis-island loci suggest that the cheater evolved via symbiotic gene transfer. Many strains were also identified that failed to nodulate L. strigosus, and it appears that nodulation ability on this host has been recurrently lost in the symbiont population. This is the first study to reveal the adaptive nature of rhizobial cheating and to trace the evolutionary origins of uncooperative rhizobial mutants.

In situ phylogenetic structure and diversity of wild Bradyrhizobium communities.

Bacteria often infect their hosts from environmental sources, but little is known about how environmental and host-infecting populations are related. Here, phylogenetic clustering and diversity were investigated in a natural community of rhizobial bacteria from the genus Bradyrhizobium. These bacteria live in the soil and also form beneficial root nodule symbioses with legumes, including those in the genus Lotus. Two hundred eighty pure cultures of Bradyrhizobium bacteria were isolated and genotyped from wild hosts, including Lotus angustissimus, Lotus heermannii, Lotus micranthus, and Lotus strigosus. Bacteria were cultured directly from symbiotic nodules and from two microenvironments on the soil-root interface: root tips and mature (old) root surfaces. Bayesian phylogenies of Bradyrhizobium isolates were reconstructed using the internal transcribed spacer (ITS), and the structure of phylogenetic relatedness among bacteria was examined by host species and microenvironment. Inoculation assays were performed to confirm the nodulation status of a subset of isolates. Most recovered rhizobial genotypes were unique and found only in root surface communities, where little bacterial population genetic structure was detected among hosts. Conversely, most nodule isolates could be classified into several related, hyper-abundant genotypes that were phylogenetically clustered within host species. This pattern suggests that host infection provides ample rewards to symbiotic bacteria but that host specificity can strongly structure only a small subset of the rhizobial community.

Highly elevated PSA and dietary PhIP intake in a prospective clinic-based study among African Americans.

African-American men die from prostate cancer (PC) nearly twice as often as white US men and consume about twice as much of the predominant US dietary heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a genotoxic rat-prostate carcinogen found primarily in well-cooked chicken and beef. To investigate the hypothesis that PhIP exposure increases PC risk, an ongoing prospective clinic-based study compared PC screening outcomes with survey-based estimates of dietary PhIP intake among 40-70-year-old African-American men with no prior PC in Oakland, CA. They completed food-frequency and meat-cooking/consumption questionnaires and had a prostate-specific antigen (PSA) test and digital-rectal exam. Results for 392 men indicated a 17 (+/-17) ng/kg day mean (+/-1 s.d.) daily intake of PhIP, about twice that of white US men of similar age. PhIP intake was attributable mostly to chicken (61%) and positively associated (R(2)=0.32, P<0.0001) with saturated fat intake. An odds ratio (95% confidence interval) of 31 (3.1-690) for highly elevated PSA > or =20 ng/ml was observed in the highest 15% vs lowest 50% of estimated daily PhIP intake (> or =30 vs < or =10 ng/kg day) among men 50+ years old (P=0.0002 for trend) and remained significant after adjustment for self-reported family history of (brother or father) PC, saturated fat intake and total energy intake. PSA measures were higher in African-American men with positive family history (P=0.007 all men, P<0.0001 highest PSA quartile). These preliminary results are consistent with a positive association between PhIP intake and highly elevated PSA, supporting the hypothesis that dietary intervention may help reduce PC risk.

Evolution of nitrogen fixation in spatially structured populations of Rhizobium.

Symbiosis between legumes and nitrogen-fixing bacteria is thought to bring mutual benefit to each participant. However, it is not known how rhizobia benefit from nodulation of legume hosts because they fix nitrogen only after differentiating into bacteroids, terminally differentiated cells that cannot reproduce. Because free-living rhizobia can reproduce, and may benefit from the increase of plant root exudates stimulated by nodulation, evolution of symbiotic nitrogen fixation may depend upon kin selection. However, unrelated nonmutualists may also benefit from increased plant exudates and nitrogen-fixing populations are therefore vulnerable to invasion by nonfixing, saprophytic Rhizobium. The access of nonfixing Rhizobium to the plant exudates associated with nodules depends upon the spatial structure of the Rhizobium populations within the soil. We investigate the influence of spatial structure on the evolution of N-fixation within a Rhizobium population using a mathematical model. Our model demonstrates that spatial structure is necessary for the evolution of N-fixation and that N-fixation is more likely to evolve with increasing degrees of spatial structure. In fact, we identify three dynamic outcomes that depend upon the relative strength of the costs of N-fixation relative to the degree of spatial structure and benefits resulting from nodulations. If the costs are relatively high, N-fixation will not evolve; if the costs are relatively low, N-fixing genes will fix in the population, but at intermediate conditions, a stable mixture of N-fixing bacteria and nonfixing bacteria will be maintained. The conditions for coexistence of N-fixing bacteria and nonfixing bacteria expand under a saturating relationship between nodule numbers and N-fixing genotype frequency.

Two books in one: natural history and evolutionary theory.

Evolutionary Ecology across Three Trophic Levels: Goldenrods, Gallmakers and Natural Enemies by W.G. Abrahamson and A.E. Weis Princeton University Press, Monographs in Population Biology, 1997. $29.95/£24.95 hbk (xiii+456 pages) ISBN 0 691 01208 3.

Laparoscopy for the impalpable testes. Initial experience of one center.

Twenty three patients with impalpable tests underwent diagnostic laparoscopy in our hospital between 1988 and 1991, in an attempt to evaluate this modality of investigation. Four of them had bilateral undescended, i.e. total of 27 tests, 12 on the right side and 15 on the left. Eight patients were adults, age between 23 and 38, average 30 years, 15 were children, age between 1 and 9, average 5 years. Twenty testes were intraperitoneal (74%), 6 intracanalicular (22.2%) and one absent (3.8%). Thirteen testes were removed and 2 were diagnostic only, the rest had successful orchiopexy. No operative or postoperative morbidity. Laparoscopy was diagnostic in all cases, assisted in planning the surgical procedure in 21 testes and saved one patient the surgical incision. Laparoscopy is a safe procedure, has a definite role in the management of impalpable testes, for diagnosis and planning of surgery. Future prospects of operative laparoscopy needs to be developed and evaluated.

The ecology and evolution of host-plant resistance to insects.

Genetic techniques have yielded new insights into plant-herbivore coevolution. Quantitative genetic tests of herbivory theory reveal that in some cases insect herbivores impose selection on resistance traits. Also, some resistance traits are costly while others appear not to be, and genetic models can explain these results. Genetic variation in plant resistance influences insect community structure by modifying interactions of herbivores with competitors and natural enemies. Therefore, models of multispecies coevolution are more realistic than pairwise coevolutionary models. Ecological genetics will facilitate further theoretical and empirical exploration of multispecies coevolution of plants and herbivores.

The effect of nitrogen and phosphorus addition on the growth, reproduction, and nutrient dynamics of two ericaceous shrubs.

Aboveground growth, reproduction, and foliar nitrogen and phosphorus contents of two ericaceous shrub species were compared over two seasons in (a) an undisturbed shrub bog (pocosin), and (b) a factorial fertilization design in which three levels each of nitrogen and phosphorus were added in all possible combinations. One species, Zenobia pulverulenta, is deciduous whereas the other species, Lyonia lucida, is evergreen. In the nutrient-poor undisturbed pocosin the two species exhibited similar foliar nitrogen and phosphorus concentrations and aboveground growth rates. Neither species flowered. In response to nutrient-addition Zenobia increased growth rates more than Lyonia. Foliar phosphorus concentrations of both species increased in response to enhanced phosphorus availability. in the first season neither species flowered in any treatment. In the second season Zenobia flowered only in the fertilized plots, with the most flowering in the high phosphorus treatments. I conclude that, by virtue of high growth rates and efficient use of nutrients and despite differences in leaf phenology and morphology, both Lyonia and Zenobia are successful in a competitive community under conditions of extremely low phosphorus availability. However, unlike Lyonia, Zenobia can take advantage of temporarily increased nutrient availability, which occurs following fire in the pocosin, to increase growth and reproduction.