1,720,964 research outputs found
Divergenza evolutiva nelle tartarughe delle Galápagos
No full textLe tartarughe delle Galápagos rappresentano uno dei molteplici endemismi che hanno avuto origine nell'arcipelago delle Galapagos, il cui nome si origina dalla forma a sella del carapace di questi animali. Nonostante la loro notorietà, le origini evolutive di queste tartarughe, la storia della loro colonizzazione dell'arcipelago, ed i livelli di divergenza tra le varie popolazioni sono ancora incerti. Reperti fossili testimoniano che tartarughe giganti erano diffuse in passato in tutti i continenti (tranne Australia ed Antartide). In tempi storici questi giganti sono sopravvissuti solo in remote isole oceaniche (Mascarene, Seychelles, e Galápagos). La popolazione nelle Mascarene si è estinta nel 1804. Nelle Seychelles una popolazione sopravvive nell'atollo di Aldabra. Solo nelle Galápagos queste tartarughe persistono con popolazioni distinte in diverse località. Purtroppo anche nelle Galápagos la maggioranza delle sottospecie è a rischio di estinzione (tre delle quindici sottospecie descritte sono estinte nello scorso secolo). In collaborazione con Jeff Powell (Yale University), James Gibbs (SUNY University) e Michel Milinkovitch (University of Bruxelles) abbiamo intrapreso un progetto di studio a lungo termine sui livelli e la dinamica di variazione genetica esistenti in questa specie. In questa sede
presentiamo i dati di sequenza a livello del DNA mitocondriale e nucleare. Abbiamo sequenziato regioni multiple di DNA mitocondriale (mtDNA) e nucleare per produrre una filogenesi molecolare del gruppo, individuare la specie continentale a loro più vicina, analizzare il livello di differenziamento genetico tra le varie sottospecie e confrontarlo con il corrispondente differenziamento morfologico. Dai quattro ai venti individui di ciascuna sottospecie sono stati sequenziati per complessivamente 8Kb di frammenti di DNA sia nucleare che mitocondriale.
Come outgroups abbiamo utilizzato le tre specie sudamericane di Geochelone (G. carbonaria, G. denticulata, e G. chilensis) ed una specie congenerica africana, G. pardalis. I dati molecolari indicano chiaramente che la specie vivente più vicina alle tartarughe delle Galapagos è Geochelone chilensis. All'interno dell'arcipelago i dati sul DNA mitocondriale permettono di identificare gli individui di ciascuna isola e permettono di interpretare la dinamica di colonizzazione delle isole. Studi basati su analisi morfologiche e di allozimi non consentivano la discriminazione delle diverse sottospecie. Oltre allo studio della variazione genetica a livello mitocondriale sono state sequenziate sei regioni nucleari teoricamente ad alto tasso evolutivo (5 introni e la regione ITS del DNA ribosomale) per lo stesso gruppo di individui. Livelli e dinamiche di variabilità genetica in entrambe le regioni sono state confrontate e discusse, con particolare riferimento alla loro utilità per la discriminazione di taxa di origine recente. I siti diagnostici per ciascuna sottospecie sono stati inoltre utilizzati per identificare la linea paterna e materna di individui in cattività di origine incerta. Questo risultato è di notevole importanza pratica per la gestione sia di popolazioni naturali che in cattività di questa specie
Nuclear DNA variation in Galápagos tortoises: a study on introns and ITS based polymorphisms
No full textThe Galápagos giant tortoises, Geochelone nigra, are the largest living tortoises and one of the two remaining species of giant tortoises in the world. Within the archipelago, only 1l of the subspecies survive to the present. Most of the subspecies are endangered.
The decline of the populations is primarily due to human impact. Buccaneers and whalers began in the 17th century to remove tortoises from the islands and use them as a source of fresh meat. Introduced ani maIs such as feral goat, pigs, dogs, rats and continued poaching represent more threats to the surviving populations. Although these tortoises have become a symbol of the conservation efforts to preserve the unique fauna of the Galápagos Islands, little is known about the levels of genetic divergence between the different subspecies. Previous work on several mtDNA has produced the first DNA based phylogeny of the group and shown the presence of fixed nucleotide differences between most of the 11 surviving subspecies (Caccone et al., 1999; Caccone et al., in prep.). Here we present data on fast evolving nuclear DNA regions. We assayed variation in five nuclear introns, located in the creatine kinase, actin, calmodulin, and aldolase genes, and in the rDNA ITS region. We compare the levels and pattems of genetic variation in nuclear regions with the ones obtained from the mtDNA ones. Moreover, we address thc finding of diagnostic DNA markers, which coupled with the mtDNA ones help diagnose each subspecies
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Extreme difference in rate of mitochondrial and nuclear DNA evolution in a large ectotherm, Galapagos tortoises
No full textWe sequenced approximately 4.5 kb of mtDNA from 161 individuals representing 11 named taxa of giant Galapagos tortoises (Geochelone nigra) and about 4 kb of non-coding nuclear DNA from fewer individuals of these same I I taxa. In comparing mtDNA and nucDNA divergences, only silent substitutions (introns, ITS, mtDNA control region, and synonymous substitutions in coding sequences) were considered. mtDNA divergence was about 30 times greater than that for nucDNA. This rate discrepancy for mtDNA and nucDNA is the greatest yet documented and is particularly Surprising for large ectothermic animals that are thought to have relatively low rates of mtDNA evolution. This observation may be due to the somewhat unusual reproductive biology and biogeographic history of these organisms. The implication is that the ratio of effective population size of nucDNA/mtDNA is much greater than the usually assumed four. The nearly neutral theory of molecular evolution predicts this would lead to a greater difference between rates of evolution. (C) 2004 Elsevier Inc. All rights reserved
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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