Temporal effects on the composition of a population of Sinorhizobium meliloti associated with Medicago sativa and Melilotus alba

Abstract

An assessment was made of the impact of temporal separation on the composition of a population of Sinorhizobium meliloti associated with Medicago sativa (alfalfa) and Melilotus alba (sweet clover) grown at a single site that had no known history of alfalfa cultivation. Root nodules were sampled on six occasions over two seasons, and a total of 1620 isolates of S. meliloti were characterized on the basis of phage sensitivity using 16 typing phages. Plant infection tests indicated that symbiotic S. meliloti were deficient in the soil at the time of planting and that these bacteria were present at low density during the first season (<10(2)/g of soil); in the second season numbers increased markedly to about 10(5)/g of soil. Overall, 37 and 51 phage types, respectively, were encountered among the nodule isolates from M. sativa and M. alba. The data indicate significant temporal shifts in the frequency and diversity of types associated with the two legume species. Apparent temporal variation with respect to the frequency of types appeared largely unpredictable and was not attributable to any one sampling time. The results indicate an apparent reduction in phenotypic diversity over the course of the experiment. Differential host plant selection of specific types with respect to nodule occupancy was indicated by significant interactions between legume species and either the frequency or diversity of phage types. Isolates from M. sativa that were resistant to lysis by all typing phages (type 14) were unusual in that they were predominant on this host at all sampling times (between 53% and 82% nodule occupancy) and were relatively homogeneous on the basis of DNA hybridization with 98% of the isolates analysed sharing the same nod EFG hybridization profile. In contrast, those isolates from M. alba comprising type 14 were encountered at low total frequency (2%) and were genetically heterogeneous on the basis of Southern hybridization. The implications of the observed temporal and host plant variation for ecological studies are discussed.

Standing genetic variation in host preference for mutualist microbial symbionts

“…Bromfield et al [50] collected T173 from a field where M. albus and M. lupulina co-occur, and it thus represents a strain that M. lupulina can potentially encounter [51]. Previous phylogenetic analysis indicates that T173 nests within the Ensifer clade of mutualist rhizobia, including the mutualist rhizobia strain RB7 used in this study [31,51].…”

Ensifer, Phyllobacterium and Rhizobium species occupy nodules of Medicago sativa (alfalfa) and Melilotus alba (sweet clover) grown at a Canadian site without a history of cultivation

“…Single-colony isolates of bacteria from nodules of alfalfa and sweet clover were maintained at -80 uC in 20 % (v/v) glycerol. In the earlier work (Bromfield et al, 2001), a subsample consisting of 180 of 561 phage-resistant isolates (PT14) from alfalfa and all 19 PT14 isolates from sweet clover were placed in a number of distinct groups based on nodEFG hybridization. Bacterial isolates representative of each of these groups as well as several isolates randomly selected from those belonging to each of PTs 13 and 24 (susceptible to eight and one of 16 typing phages, respectively) were used for further characterization in this study.…”

“…Frequency estimates for bacterial genotypes were based on the phage type data of Bromfield et al (2001), where 810 bacterial isolates is the total sample size from alfalfa and sweet clover, respectively. Frequencies of bacterial genotypes among phageresistant isolates (PT14) were estimated by combining the nodEFG hybridization data of Bromfield et al (2001) with those obtained by RFLP/Southern hybridization in this study.…”

“…Frequency estimates for bacterial genotypes were based on the phage type data of Bromfield et al (2001), where 810 bacterial isolates is the total sample size from alfalfa and sweet clover, respectively. Frequencies of bacterial genotypes among phageresistant isolates (PT14) were estimated by combining the nodEFG hybridization data of Bromfield et al (2001) with those obtained by RFLP/Southern hybridization in this study. The frequencies of genotypes among PT14 isolates from alfalfa (A) were based on a subsample of 180 from 561 isolates and estimated from [(xA14/180) (561/810)]6100, where xA14 is the number of PT14 isolates in the subsample of 180 belonging to a particular nodEFG type and 561/810 represents the proportion of isolates belonging to PT14 in the total sample from alfalfa.…”

“…Bromfield et al (2001), characterized 1620 bacterial isolates from nodules of alfalfa and sweet clover) grown for two seasons at a Canadian site (latitude 45 u 239 6.280 N; longitude 75 u 429 15.650 W) without a known history of cultivation. Initially, bacteria able to nodulate alfalfa and sweet clover were not detected in soil, whereas in the second season their numbers increased markedly, presumably in response to the presence of legumes.…”