Department of Botany

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Browsing Department of Botany by Subject "Africa"

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    Disperser Limitation and Recruitment of an Endemic African Tree in a Fragmented Landscape
    (Wiley, 2009-04) Cordeiro, Norbert J.; Ndangalasi, Henry J.; McEntee, Jay P.; Howe, Henry
    Forest fragmentation may have positive or negative effects on tropical tree populations. Our earlier study of an endemic African tree, Leptonychia usambarensis (Sterculiaceae), in the East Usambara Mountains of Tanzania, found poorer recruitment of seedlings and juveniles in small fragments compared to continuous forest, and concomitant reduction of seed-dispersal agents and seed dispersal. However, the possibility that other biotic or abiotic consequences of the fragmentation process contribute to diminished recruitment in fragments was left open. Here we test whether excessive seed predation, diminished fecundity, low seed quality, or adverse abiotic effects acted independently or in concert with reduced seed dispersal to limit seedling and juvenile recruitment in fragments. Extended observations of disperser activity, a seed placement experiment, seed predator censuses, and reciprocal seedling transplants from forest and fragment sources failed to support the alternative hypotheses for poorer seedling and juvenile recruitment in fragments, leaving reduced seed dispersal as the most plausible mechanism. Poorer recruitment of this species in forest fragments, where high edge-to-area ratios admit more light than in continuous forest, is particularly striking because the tree is an early successional species that might be expected to thrive in disturbed microhabitats.
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    A Reassessment of the Epidemiology of Rice Yellow Mottle Virus Following Recent Advances In Field And Molecular Studies
    (Elsevier, 2009-02) Traore, Oumar; Galzi, Agnès P.; Fatogoma, Sorho; Sarra, Soungalo; Rakotomalala, Mbolarinosy; Sangu, Emmanuel; Kanyeka, Zakaria L.; Sere, Y.; Konaté, Gnissa; Fargette, Denis
    The available knowledge on the epidemiology of Rice yellow mottle virus (RYMV) is reassessed in the light of major advances in field and molecular studies of the disease it causes in rice. Previously un-described means of transmission by mammals and through leaf contact have been discovered recently. Several agricultural practices, including the use of seedbed nurseries, have also contributed to a massive build-up of RYMV inoculum. Phytosanitation is now known to be critical to reduce disease incidence in rice. A new model of the ecology of RYMV in which man plays a central role has emerged. Furthermore, estimates of the evolutionary rate of change of RYMV provided a time-frame for its epidemiology, the first attempt for a plant virus. Earlier interpretations of the patterns of virus diversity which assumed a long-term evolution, and assigned a major role to adaptive events had to be discarded. In contrast, a wave-like model of dispersal of RYMV, which postulates its initial diversification in East Africa, followed by westward spread across the continent, was developed, refined and dated. The most salient -- and largely unexpected -- finding is that RYMV emerged recently and subsequently spread rapidly throughout Africa in the last two centuries. Diversification and spread of RYMV has been concomitant with an extension of rice cultivation in Africa since the 19th century. This major agro-ecological change increased the encounters between primary hosts of RYMV and cultivated rice. It also modified the landscape ecology in ways that facilitated virus spread.
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    Towards a Phylogeny for Coffea (Rubiaceae): Identifying Well-supported Lineages Based on Nuclear and Plastid DNA Sequences
    (Oxford University Press, 2008-01) Maurin, Olivier; Davis, Aaron P.; Chester, Michael; Mvungi, Esther F.; Jaufeerally-Fakim, Yasmina; Fay, Michael F.
    †Background and AimsThe phylogenetic relationships between species ofCoffeaandPsilanthusremain poorlyunderstood, owing to low levels of sequence variation recovered in previous studies, coupled with relativelylimited species sampling. In this study, the relationships betweenCoffeaandPsilanthusspecies are assessedbased on substantially increased molecular sequence data and greatly improved species sampling.†MethodsPhylogenetic relationships are assessed using parsimony, with sequence data from four plastid regions[trnL – Fintron,trnL – Fintergenic spacer (IGS),rpl16intron andaccD – psa1IGS], and the internal transcribedspacer (ITS) region of nuclear rDNA (ITS 1/5.8S/ITS 2). Supported lineages inCoffeaare discussed within thecontext of geographical correspondence, biogeography, morphology and systematics.†Key ResultsSeveral major lineages with geographical coherence, as identified in previous studies based on smallerdata sets, are supported. Other lineages with either geographical or ecological correspondence are recognized for thefirst time.CoffeasubgenusBaracoffeais shown to be monophyletic, butCoffeasubgenusCoffeais paraphyletic.Sequence data do not substantiate the monophyly of eitherCoffeaorPsilanthus. Low levels of sequence divergencedo not allow detailed resolution of relationships withinCoffea, most notably for species ofCoffeasubgenusCoffeaoccurring in Madagascar. The origin ofC. arabicaby recent hybridization betweenC. canephoraandC. eugenioidesis supported. Phylogenetic separation resulting from the presence of the Dahomey Gap is inferredbased on sequence data fromCoffea.