Ant-plant symbioses in Africa and the neotropics : history, biogeography and diversity

Book ChapterSymbiotic ant-plant relationships afford an excellent opportunity to analyze the effects of both historical and ecological factors on the evolution of mutualisms. Occurring in tropical forests throughout the world, all myrmecophytic plants provide food and permanent housing to ants; the ants, in turn, are known or presumed to protect their hosts from herbivores or competition or to provision them with nutrients. Despite this underlying similarity, ant-plant symbioses differ in diversity and nature on different landmasses

Paysages bioculturels et biomimétisme à un niveau écosystémique

In a wide range of environments —but which all have in common the scarcity of some key resource that is crucial for life— there have emerged ecosystems that show remarkably regular spatial patterns in the distribution of vegetation, in topography, or both. These patterns have been shown to result from the activities of organisms that can be classed as “ecosystem engineers.” In these constraining environments, “engineer” organisms, acting in concert with physical mechanisms, convert an initially homogeneous landscape into a landscape composed of two kinds of patches, one kind in which the crucial resource is concentrated, the other kind in which the crucial resource is depleted. Thus, in environments where a crucial resource is generally scarce, engineer organisms create patches of habitat in which the resource is relatively abundant. This allows organisms to persist in environments that would otherwise be marginal (or worse) for their survival. The process by which this occurs is termed “spatial self-organization” and the phenomenon is called “self-organized patchiness.” In some of the environments where strikingly patterned, self-organized ecosystems occur, farmers construct cultural landscapes that are remarkably similar to the constructions of the “engineer” organisms. This resemblance between artefacts made by humans and constructions made by non-human living beings raises numerous questions. Does the resemblance reflect independent convergence on similar adaptive solutions to constraining environments, or do human and non-human engineers interact in one or more ways? The objective of this presentation is to explore these questions

Phylogeographic support for horizontal gene transfer involving sympatric bruchid species

BACKGROUND: We report on the probable horizontal transfer of a mitochondrial gene, cytb, between species of Neotropical bruchid beetles, in a zone where these species are sympatric. The bruchid beetles Acanthoscelides obtectus, A. obvelatus, A. argillaceus and Zabrotes subfasciatus develop on various bean species in Mexico. Whereas A. obtectus and A. obvelatus develop on Phaseolus vulgaris in the Mexican Altiplano, A. argillaceus feeds on P. lunatus in the Pacific coast. The generalist Z. subfasciatus feeds on both bean species, and is sympatric with A. obtectus and A. obvelatus in the Mexican Altiplano, and with A. argillaceus in the Pacific coast. In order to assess the phylogenetic position of these four species, we amplified and sequenced one nuclear (28S rRNA) and two mitochondrial (cytb, COI) genes. RESULTS: Whereas species were well segregated in topologies obtained for COI and 28S rRNA, an unexpected pattern was obtained in the cytb phylogenetic tree. In this tree, individuals from A. obtectus and A. obvelatus, as well as Z. subfasciatus individuals from the Mexican Altiplano, clustered together in a unique little variable monophyletic unit. In contrast, A. argillaceus and Z. subfasciatus individuals from the Pacific coast clustered in two separated clades, identically to the pattern obtained for COI and 28S rRNA. An additional analysis showed that Z. subfasciatus individuals from the Mexican Altiplano also possessed the cytb gene present in individuals of this species from the Pacific coast. Zabrotes subfasciatus individuals from the Mexican Altiplano thus demonstrated two cytb genes, an "original" one and an "infectious" one, showing 25% of nucleotide divergence. The "infectious" cytb gene seems to be under purifying selection and to be expressed in mitochondria. CONCLUSION: The high degree of incongruence of the cytb tree with patterns for other genes is discussed in the light of three hypotheses: experimental contamination, hybridization, and pseudogenisation. However, none of these seem able to explain the patterns observed. A fourth hypothesis, involving recent horizontal gene transfer (HGT) between A. obtectus and A. obvelatus, and from one of these species to Z. subfasciatus in the Mexican Altiplano, seems the only plausible explanation. The HGT between our study species seems to have occurred recently, and only in a zone where the three beetles are sympatric and share common host plants. This suggests that transfer could have been effected by some external vector such as a eukaryotic or viral parasite, which might still host the transferred fragment. REVIEWERS: This article was reviewed by Eric Bapteste, Adam Eyre-Walker and Alexey Kondrashov

Natural hybridization between a clonally propagated crop, cassava (Manihot esculenta Crantz) and a wild relative in French Guiana

International audienceBecause domestication rarely leads to speciation, domesticated populations often hybridise with wild relatives when they occur in close proximity. Little work has focused on this question in clonally propagated crops. If selection on the capacity for sexual reproduction has been relaxed, these crops would not be expected to hybridise with their wild relatives as frequently as seed-propagated crops. Cassava is one of the most important clonally-propagated plants in tropical agriculture. Gene flow between cassava and wild relatives has often been postulated, but never demonstrated in nature. We studied a population of a wild Manihot sp. in French Guiana, which was recently in contact with domesticated cassava, and characterized phenotypes (ten morphological traits) and genotypes (six microsatellite loci) of individuals in a transect parallel to the direction of hypothesized gene flow. Wild and domesticated populations were strongly differentiated at microsatellite loci. We identified many hybrids forming a continuum between these two populations, and phenotypic variation was strongly correlated with the degree of hybridisation as determined by molecular markers. Analysis of linkage disequilibrium and of the diversity of hybrid pedigrees showed that hybridisation has gone on for at least three generations and that no strong barrier prevents admixture of the populations. Hybrids were more heterozygous than either wild or domesticated individuals, and phenotypic comparisons suggested heterosis in vegetative traits. Our results also suggest that this situation is not uncommon, at least in French Guiana, and demonstrate the need for integrated management of wild and domesticated populations even in clonally propagated crops

Evolutionary ecology of symbiotic ant-plant relationships

Journal ArticleAbstract.--A tabular survey of ant-plant symbioses worldwide summarizes aspects of the evolutionary ecology of these associations. Remarkable similarities between ant-plant symbioses in disjunct tropical regions result from convergent and parallel evolution of similarly preadapted ants and plants. Competition among ants has driven evolutionary specialization in plant-ants and is the principal factor accounting for parallelism and convergence. As habitat specialization accompanied the evolutionary radiation of many myrmecophytes, frequent host shifts and de novo colonizations by habitat-specific ants both inhibited species-specific coevolution and co-cladogenesis, and magnified the diversity of mutualistic partners