AFLP analysis of relationships among cassava and other Manihot species

 Despite the worldwide importance of cultivated cassava (M. esculenta Crantz) its origin and taxonomic relationships with other species in the genus have not been clearly established. We evaluated a representative sample of the crop’s diversity and six wild taxa with AFLPs to estimate genetic relationships within the genus. Groupings of accessions of each species by data analysis corresponded largely with their previous taxonomic classifications. A mixed group, consisting of Manihot esculenta subsp. flabellifolia and M. esculenta subsp. peruviana, was most similar to cassava, while M. aesculifolia, M. brachyloba, and M. carthaginensis were more distant. Species-specific markers, which may be useful in germ-plasm classification or introgression studies, were suggested by the unique presence of AFLP products in samples of each of the three wild species. Heterogeneity of similarities among individuals of certain species suggested the existence of intraspecific gene pools, a hypothesis that was supported by morphological or ecogeographic evidence with varying degrees of success. Quantitative assessment of genetic diversity revealed greater homogeneity among cassava accessions than among itsclosest wild relatives. The demonstration of unique genetic diversity in the two M. esculenta subspecies and their genetic similarity to the crop supports the hypothesis that these materials may be the ancestors of cassava. Received: 4 November 1996 / Accepted: 20 December 1996     

Mechanism of nitrite oxidation and oxidoreductase systems inNitrobacter agilis

Chemiosmotic coupling mechanisms operate in the electron transfer reactions from: nitrite to O₂, NO₂ ^– to NAD⁺, ascorbate to O₂, NADH to O₂, and NADH to NO₃ ^–. The enzyme systems catalyzing these reactions are named NO₂ ^–:O₂ oxidoreductase, ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, ascorbate:O₂ oxidoreductase, NADH:O₂ oxidoreductase, and NADH:NO₃ ^– oxidoreductase, respectively. All of the oxidoreduction reactions are exergonic with the exception of the ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase system, which involves reversed electron flow against the thermodynamic gradients. The mechanism for nitrite oxidation was found to be quite different from that of ascorbate oxidation; both systems were insensitive, however, to rotenone, amytal, antimycin A, and 2-n-heptyl 4-hydroxyquinolineN-oxide. These compounds, on the other hand, severely inhibited the electron transfer reactions catalyzed by NADH:O₂ oxidoreductase, NADH:NO₃ ^– oxidoreductase, and the ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, indicating a common pathway of electron transport in these oxidoreductase systems. Cyanide inhibited all systems except the NADH:NO₃ ^– oxidoredctase. The uncoupler carbonyl cyanide-m-chlorophenyl hydrazone strongly inhibited NO₂ ^–:O₂ oxidoreductase and ATP-dependent NO₂ ^–:NAD⁺ oxidoreductase, which indicates the involvement of energy-linked reactions in both systems; the uncoupler caused a marked stimulation of the NADH:O₂ oxidoreductase and NADH:NO₃ ^– oxidoreductase without affecting the ascorbate:O₂ oxidoreductase activities.     

Optimization theory in plant evolution: An overview of long-term evolutionary prospects in the angiosperms

Examination of literature shows that a number of authors regard outbreeding and heterozygosity as the prevalent factors associated with long-term successful evolution in the angiosperms. A number of plant evolutionists, however, have doubted the truth of such an assumption. Everincreasing reports of the existence of arboreal angiospermous apomixis in tropical forests of the Neotropics and the Far East undermined a thinking which, recently, has rested on optimality. Finding apomixis in trees surprised authors, who held biased opinions about the determinism of outbreeding as the major guiding factor in the evolutionary history of the angiosperms. The thinking that apomixis may turn out to be a regular mating system of the flowering plants met with the approval of some authors, who wondered about the true penetration of the phenomenon among the higher plants. The fact that one-third of all known flowering plants are autogamous has cast further doubt on the deterministic infallibility of outbreeding and successful long-term evolution. Despite claims that the breeding system is directly involved with fitness, while determining the course of optimized evolution, there is comparatively little hard evidence to substantiate a hypothesis which, in the last analysis, has rested principally on common sense. Rather, if continuing field research happens to unveil new cases of woody angiospermous apomixis, a prediction is advanced that the next two biomes to show regular incidence of the phenomenon are Africa’s paleotropical savannas and humid forests. If evolution is partly or wholly dependent on the breeding system to proceed, current knowledge supports views that further enhancement of organic diversification vis-à-vis selection and adaptednesss rests on three major tested mating systems: outbreeding, inbreeding and apomixis.     

Diffusion of Messages from an Electronic Cigarette Brand to Potential Users through Twitter

Objective

This study explores the presence and actions of an electronic cigarette (e-cigarette) brand, Blu, on Twitter to observe how marketing messages are sent and diffused through the retweet (i.e., message forwarding) functionality. Retweet networks enable messages to reach additional Twitter users beyond the sender’s local network. We follow messages from their origin through multiple retweets to identify which messages have more reach, and the different users who are exposed.

Methods

We collected three months of publicly available data from Twitter. A combination of techniques in social network analysis and content analysis were applied to determine the various networks of users who are exposed to e-cigarette messages and how the retweet network can affect which messages spread.

Results

The Blu retweet network expanded during the study period. Analysis of user profiles combined with network cluster analysis showed that messages of certain topics were only circulated within a community of e-cigarette supporters, while other topics spread further, reaching more general Twitter users who may not support or use e-cigarettes.

Conclusions

Retweet networks can serve as proxy filters for marketing messages, as Twitter users decide which messages they will continue to diffuse among their followers. As certain e-cigarette messages extend beyond their point of origin, the audience being exposed expands beyond the e-cigarette community. Potential implications for health education campaigns include utilizing Twitter and targeting important gatekeepers or hubs that would maximize message diffusion.