Identifying factors that promote population differentiation is of interest for understanding the early stages of speciation. Gene flow among populations inhabiting different environments can be reduced by geographical distance (isolation-by-distance) or by divergent selection resulting from local adaptation (isolation-by-ecology). Few studies have investigated the influence of these factors in small oceanic islands where the influence of geographic distance is expected to be null but where habitat diversity could have a strong effect on population differentiation. In this study, we tested for the spatial divergence of phenotypes (floral morphology and floral scent) and genotypes (microsatellites) among ten populations of Jumellea rossii, an epiphytic orchid endemic to Réunion growing in three different habitats. We found a significant genetic differentiation between populations that is structured by habitat heterogeneity rather than by geographic distance between populations. These results suggest that ecological factors might reduce gene flow among populations located in different habitats. This pattern of isolation-by-habitat may be the result of both isolation-by-ecology by habitat filtering and asynchrony in flowering phenology. Furthermore, data on floral morphology match these findings, with multivariate analysis grouping populations by habitat type but could be only due to phenotypic plasticity. Indeed floral scent compounds were not significantly different between populations indicating that specific plant-pollinator mutualism does not seem to play a major role in the population differentiation of J. rossii. In conclusion, the results from our study emphasize the importance of habitat diversity of small oceanic islands as a factor of population differentiation.
Even if the persistence of the J. fragrans population is not threatened in the short term, its genetic diversity has probably been reduced by fragmentation and over-collection. In situ conservation actions for J. fragrans and ex situ cultivation of both species are recommended in order to meet the demand of local people.
Jumellea is an orchid genus centered on Madagascar but also occurs on some of the neighboring oceanic islands (the Mascarenes and Comoros) and in southern and eastern Africa. Prior to our study the genus contained ca. 55 morphologically distinct species, of which six are found in the Comoros (three endemic, three shared with Madagascar), nine in the Mascarenes (four endemic, four shared with Madagascar), two in southern and eastern Africa (both endemic), and 41 species endemic to Madagascar. We perform Bayesian and parsimony phylogenetic analyses of Jumellea based on combined chloroplast (matK, trnL-F, rps16, ycf1) and nuclear (nrITS) data from 60 specimens representing 47 species, four subspecies, and two varieties of Jumellea: (1) to assess the phylogenetic value of growth form, leaf, bract and lip shape, and spur length, presently used for recognizing informal groups within the genus; and (2) to test the monophyly of some variable species (e.g., J. gracilipes, J. lignosa). We find no support for the informal groups of Jumellea, as all the characters tested are evolutionarily labile. Jumellea lignosa (comprising J. lignosa subsp. lignosa, subsp. tenuibracteata, subsp. acutissima, and subsp. latilabia) is not monophyletic unless subsp. tenuibracteata is excluded. Jumellea gracilipes s.l. (including J. ambongensis, J. imerinensis, and J. unguicularis) is polyphyletic. As a result, we resurrect these three latter species, and recognize J. lignosa subsp. tenuibracteata at species level. Furthermore, we propose new circumscriptions for the following species: the Comorian J. arachnantha (including the Malagasy J. sagittata); the Reunionese J. exilis (including the Malagasy J. flavescens); the Reunionese J. recta (including the Malagasy Jumellea sp. 1); the Reunionese J. recurva (including the Malagasy J. pandurata); and the Reunionese J. steno-phylla (including the Malagasy J. gracilipes 2 and 3). Finally, Jumellea arborescens and J. maxillarioides are recorded from Madagascar and the Comoros. Finally, the number of species of Jumellea has now increased from 55 to 57: seven species in the Comoros (four shared with Madagascar), nine species in the Mascarenes (four shared with Madagascar), two species in Africa, and 39 species, three subspecies, and two varieties restricted to Madagascar.
Morphological differences and identification of Jumellea rossii and Jumellea fragrans (Orchidaceae) in Reunion Island : implications for conservation. — Conservation strategies are not possible when there is confusion in the taxonomic status of a group of plants. Such confusion makes difficult for systematic biologists to identify functional biological entities. This is particularly true in the case of species that diversified fast due to adaptive radiation particularly plants from the oceanic islands. It is the case of certain orchids from Reunion Island. Among them, the “Faham” complex includes several taxa whose populations are situated at various elevations with morphological and reproductive variations. These taxa are actually merged into one name : Jumellea fragrans causing confusion in their taxonomic status. This confusion has caused numerous problems of identification and errors hindering the protection status of the species. In this study, we suggest to clarify the systematic position of the taxa composing complex Jumellea fragrans using a biometric approach at population scale. The objective is to identify morphological characters that allow distinguishing the taxa of the J. fragrans complex. Morphological characters (vegetative and floral) are measured on individuals from three natural populations representative of altitudinal range of the complex. Principal component analysis made on the characters show a separation of the individuals in two groups (Mare-Longue vs Basse-Vallée and Cilaos). Nine characters (on 14 measured) are significantly different between Mare-Longue population and the populations of Basse-Vallée and Cilaos and allow identifying both groups. Population of Mare-Longue belongs to the species Jumellea fragrans (Thouars) Schltr. and populations of Basse-Vallée and Cilaos belongs to the species Jumellea rossii Senghas. In terms of conservation, the first recommendation is to recognize two different species : J. rossii and J. fragrans. Moreover, putting J. fragrans into the «vulnerable» IUCN Red List category and the adoption of a protection status are recommended. Concerning J. rossii, more studies on geographical range and population dynamics are necessary to estimate the risk of extinction. In the same way, studies on genetic variability of both species would generate useful data for implementation of conservation strategies.
The fungal agents of root rots in West Africa. Characteristics of the main root rot fungi infecting forests and plantations in West Africa are described. The biological cycle of the parasites, type of decay, detection and control methods are reviewed and discussed.
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