Roselyne Lumaret

Phylogeographical variation of chloroplast DNA in holm oak (Quercus ilex L.)

Variation in the lengths of restriction fragments (RFLPs) of the whole chloroplast DNA molecule was studied in 174 populations of Quercus ilex L. sampled over the entire distribution of this evergreen and mainly Mediterranean oak species. By using five endonucleases, 323 distinct fragments were obtained. From the 29 and 17 cpDNA changes identified as site and length mutations, respectively, 25 distinct chlorotypes were distinguished, mapped and treated cladistically with a parsimony analysis, using as an outgroup Q. alnifolia Poech, a closely related evergreen oak species endemic to Cyprus where Q. ilex does not grow. The predominant role of Q. ilex as maternal parent in hybridization with other species was reflected by the occurrence of a single very specific lineage of related chlorotypes, the most ancestral and recent ones being located in the southeastern and in the northwestern parts of the species’ geographical distribution, respectively. The lineage was constituted of two clusters of chlorotypes observed in the ‘ilex’ morphotyped populations of the Balkan and Italian Peninsulas (including the contiguous French Riviera), respectively. A third cluster was divided into two subclusters identified in the ‘rotundifolia’ morphotyped populations of North Africa, and of Iberia and the adjacent French regions, respectively. Postglacial colonization probably started from three distinct southerly refugia located in each of the three European peninsulas, and a contact area between the Italian and the Iberian migration routes was identified in the Rhône valley (France). Chlorotypes identical or related to those of the Iberian cluster were identified in the populations from Catalonia and the French Languedoc region, which showed intermediate morphotypes, and in the French Atlantic populations which possessed the ‘ilex’ morphotype, suggesting the occurrence of adaptive morphological changes in the northern part of the species’ distribution.

Detection of hybrids in nature: application to oaks (Quercus suber and Q. ilex)

Powerful and accurate detection of first-generation (F1) hybrids and backcrosses in nature is needed to achieve a better understanding of the function and dynamics of introgression. To document the frequency of ongoing interspecific gene exchange between two Mediterranean evergreen oaks, the cork oak (Quercus suber) and the holm oak (Q. ilex), we analyzed 1487 individuals originating from across the range of the two species using eight microsatellite loci and two Bayesian clustering approaches (implemented in the programs STRUCTURE and NEWHYBRIDS). Simulated data were used to assess the differences between the two clustering methods and to back up the choice of the threshold value for the posterior probability to discriminate admixed from pure individuals. We found that the use of STRUCTURE resulted in the highest power to detect hybrids, whereas NEWHYBRIDS provided the highest accuracy. Irrespective of the approach, the two species were clearly distinguished as independent genetic entities without any prior information. In contrast with previous reports, we found no evidence for unidirectional introgression. The overall hybridization rate was very low (<2% of introgressed individuals). Only two individuals were identified as F1 hybrids and five as early backcrosses. This work shows that the combined application of the two complementary Bayesian approaches and their systematic validation with simulations, fit for the case at hand, helps gain resolution in the identification of admixed individuals.

Phylogenetic relationships and gene flow between sympatric diploid and tetraploid plants ofDactylis glomerata (Gramineae)

Phylogenetic relationships between sympatric, morphologically indistinguishable diploid and tetraploid plants ofDactylis glomerata L. (Gramineae) in Galicia (Spain) were assessed using allozyme markers for 6 distinct systems. The study exploited recent introduction in Galicia and subsequent hybridization of an alien 4xDactylis subspecies possessing distinct allozymes from those of all the native plants. Opportunities for gene exchanges between the ploidies were estimated from in situ observations of flowering, examination of progenies in 2x/4x natural and experimental crosses, and enzyme analyses. Results show a high genetic similarity between the Galician diploids and tetraploids, which possess peculiar alleles in common. Although the ploidy levels usually have distinct flowering periods, interploidal crosses do occasionally occur. Gene flow is likely much more important from the diploid to the tetraploid level. A good genetic intermixing occurs between the Galician and the alien tetraploid entities which have simultaneous flowering. Autopolyploidization of the diploids followed by various rates of hybridization is proposed as one very probable origin of natural tetraploids inDactylis.

Plant genetics: Ancient wild olives in Mediterranean forests

Early domestication and extensive cultivation have meant that staple Mediterranean fruit crops such as olives, grapes and dates exist in wild-looking forms that are secondary derivatives produced by sexual reproduction among cultivated plants (cultivars), which were initially propagated vegetatively. By using genetic markers associated with characters that render plants unsuitable for domestication, we show here that genuinely wild olive trees, which cannot be distinguished morphologically from feral forms, still survive in a few Mediterranean forests. These wild stocks are genetically distinct and more variable than either the crop strains or their derived feral forms, a finding that has important implications for the conservation of these ancient lineages.

Chloroplast DNA phylogeography suggests a West African centre of origin for the baobab, Adansonia digitata L. (Bombacoideae, Malvaceae)

The African baobab (Adansonia digitata L.) is an emblematic, culturally important, and physically huge tropical tree species whose natural geographical distribution comprises most of tropical Africa, but also small patches of southern Arabia, and several Atlantic and Indian Ocean islands surrounding the African continent, notably including Madagascar. We analysed the polymerase chain reaction–restriction fragment length polymorphism of five chloroplast DNA fragments obtained from 344 individuals of A. digitata collected from 74 populations covering the entire extant distribution range of the species. Our goal was to reconstruct the phylogeographical history of the species and, if possible, to identify its centre of origin, which has been a subject of controversy for many decades. We identified five haplotypes whose distribution is clearly geographically structured. Using several species of Adansonia and of closely related genera as outgroups, the haplotypes showed a clear phylogeographical pattern of three groups. Two are phylogenetically related to the outgroup taxa, and are distributed in West Africa. The third group is substantially more differentiated genetically from outgroup species, and it corresponds to southern and eastern Africa, Arabia and the Indian Ocean islands, including Madagascar. According to our results, the tetraploid A. digitata, or its diploid progenitor, probably originated in West Africa and migrated subsequently throughout the tropical parts of that continent, and beyond, by natural and human‐mediated terrestrial and overseas dispersal.

Genetic variation and gene flow in Alpine diploid and tetraploid populations of Lotus (L. alpinus (D.C.) Schleicher/L. corniculatus L.). I. Insights from morphological and allozyme markers

The altitudinal distribution, morphology, phenology and allozyme polymorphism at 12 loci were studied in diploid and tetraploid populations of Lotus growing at over 1800 m in the French Alps to clarify relationships between these cytotypes. In general, diploids occurred at higher elevation than tetraploids, although some sites at intermediate elevation contained both cytotypes, diploids predominating in the upper part and tetraploids in the lower part of the contact area. Evidence for an autopolyploid origin of the tetraploids was provided by tetrasomic inheritance at two enzyme loci, although no tetravalents were observed at meiosis. Diploid and tetraploid plants shared morphological traits distinct from those of other Lotus species and showed differences in size, which may be attributable to chromosome doubling. The diploid cytotype, L. alpinus, may thus be the ancestor of the Alpine tetraploids. Both cytotypes showed nearly identical suites of alleles at all loci and very similar genetic parameters, except for heterozygosity, which was higher in the tetraploid plants. However, the occurrence of few alleles specific to each ploidy level indicated limited gene flow between cytotypes, probably as a result of spatial segregation and variation in flowering time. Of the individuals in a tetraploid population, 25% showed morphological traits similar to those observed in L. corniculatus, suggesting genetic introgression between the two tetraploid species.

Chloroplast-DNA variation in cultivated and wild olive (Olea europaea L.)

Abstract Polymorphism in the lengths of restriction fragments of the whole cpDNA molecule was studied in cultivated olive and in oleaster (wild olive) over the whole Mediterranean Basin. Seventy two olive cultivars, 89 very old trees cultivated locally, and 101 oleasters were scored for ten endonucleases. Moreover, maternal inheritance of cpDNA in olive was shown by analysing the progeny of a controlled cross between two parents which differed in their cpDNA haplotypes. In the whole species, three site- and three length-mutations were observed, corresponding to five distinct chlorotypes. The same chlorotype (I) was predominant in both oleasters and cultivated olive trees, confirming that these are closely related maternally. Three other chlorotypes (II, III and IV) were observed exclusively in oleaster material and were restricted either to isolated forest populations or to a few individuals growing in mixture with olive trees possessing the majority chlorotype. An additional chlorotype (V) was characterised by three mutations located in distinct parts the cpDNA molecule but which were never observed to occur separately. This chlorotype, more widely distributed than the other three, in both cultivated and wild olive, and occurring even in distant populations, was observed exclusively in male-sterile trees showing the same specific pollen anomaly. However, in the present study, no evidence was provided for a direct relationship between the occurrence of the cpDNA mutations and male sterility. It is suggested that the large geographic distribution of chlorotype V may be related to the high fruit production usually observed on male-sterile trees. These may be very attractive for birds which are fond of olive fruit and spread the stones efficiently. Probably for the same reason, people preserved male-sterile oleasters for long periods and, in several places, used male-sterile cultivars over large areas.

Chloroplast DNA variation in the cultivated and wild olive taxa of the genus Olea L.

Abstract Polymorphism in the lengths of restriction fragments of the whole cpDNA molecule were studied in 15 taxa (species or subspecies) of the genus Olea. From restriction analysis using nine endonucleases, 28 site mutations and five length polymorphisms were identified, corresponding to 12 distinct chlorotypes. From a phenetic analysis based on a Nei’s dissimilarity matrix and a Dollo parsimony cladistic analysis using, as an outgroup, a species of the genus Phillyrea close to Olea, the ten taxa of section Olea were distinguished clearly from the five taxa of section Ligustroides which appear to posses more ancestral cpDNA variants. Within the section Ligustroides, the tropical species from central-western Africa, Olea hochtetteri, showed a chlorotype which differed substantially from those of the other four Olea taxa growing in southern Africa, supporting a previous assessment according to which O. hochtetteri may have been subjected to a long period of geographical isolation from the other Olea taxa. Within the Olea section, three phyla were identified corresponding to South and East Africa taxa, Asiatic taxa, and a group including Saharan, Macaronesian and Mediteranean taxa, respectively. On the basis of cpDNA variation, the closest Olea taxa to the single Mediterranean species, Olea europaea, represented by its very predominant chlorotype, observed in both wild and cultivated olive, were found to be Olea laperrinei (from the Sahara), Olea maroccana (from Maroccan High Atlas) and Olea cerasiformis (from Macaronesia). These three taxa, which all share the same chlorotype, may have a common maternal origin.

Chloroplast-DNA variation in the genus Lotus (Fabaceae) and further evidence regarding the maternal parentage of Lotus corniculatus L.

Abstract To resolve the maternal parentage of the tetraploid Lotus corniculatus, restriction-site variation of chloroplast DNA (cpDNA) was studied in several accessions of that species, in the four putative parental diploid species, L. tenuis, L. alpinus, L. japonicus and L. uliginosus, and in four phylogenetically more distant diploid species, L. hispidus, L. edulis, L. ornithopodoides and Tetragonolobus maritimus var. siliquosus. Evidence of cpDNA maternal inheritance was obtained by using reciprocal controlled crosses between plants of L. corniculatus and natural tetraploid individuals of L. alpinus showing very distinct restriction patterns. Interspecific cpDNA variation in the eight Lotus species and T. siliquosus was analysed by comparing cpDNA fragment patterns produced by five restriction endonucleases and totalling 304 distinct fragments. Genetic differentiation in cpDNA was very high between the L. corniculatus group and L. hispidus on the one hand, and the three other species on the other hand. Sixteen restriction-site mutations and eight length polymorphisms were identified among the five species of the L. corniculatus group and L. hispidus, Lotus uliginosus, L. alpinus and L. japonicus showed at least six DNA changes with regard to the molecule of L. corniculatus. Accordingly, these species should be excluded as maternal progenitors of L. corniculatus. Conversely, the cpDNA of L. tenuis differed from that of L. corniculatus by only two small-length mutations. As also suggested previously from an analysis of several nuclear markers, the results reported here show decisively that L. tenuis may be considered as the most probable maternal ancestor of L. corniculatus.

Plant species diversity and polyploidy in islands of natural vegetation isolated in extensive cultivated lands

Natural polyploidy is often related to a longer life span, vegetative reproduction and higher competitive ability. In this paper, we test the possibility that these characteristics may favour the survival of polyploid taxa under conditions of long-term habitat fragmentation. In islands of natural vegetation isolated in extensive vineyards located in the South of France and in a large neighbouring area of natural vegetation, plant species richness and the relative abundance of polyploid taxa were assessed according to island size, isolation and vegetation structure. High species richness was observed, with numerous species restricted to the islands, suggesting that these may constitute refugia. However, species richness was not related to island size or to degree of isolation except for the flora of the woody areas. A very positive effect of area fragmentation on plant richness was observed, which is probably attributable to relatively low species overlap among the islands. Particularly high species richness was observed in open areas, provided that these were not extensively colonized by shrubs which seem to be responsible for local extinction of many annual taxa. Polyploids, which comprised mostly perennial herbs and woody species, were predominant in all the islands and in the large reference area. In open habitats invaded by shrubby species, a higher relative frequency of polyploids was observed in islands than in the reference area. Moreover, polyploid taxa were present in a larger number of islands than the diploid taxa, which were often restricted to a single island, suggesting that, after a long period of isolation, the polyploids may still have a lower probability of extinction. Evidence was obtained from vegetation structure analysis that diploid and polyploid annual herbs were restricted to open habitats and were both eliminated by shrubby species. Conversely, the diploid perennial herbs were also significantly affected by shrub colonization whereas the polyploids were mostly present in shrubby areas. This suggests that the higher competitive ability of polyploid perennial herbs may constitute a critical factor responsible for their wider distribution over the islands. We report the implications of our findings on conservation strategies, more particularly for a Mediterranean flora.