Isolement reproductif | EPFL Graph Search

The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species. The mechanisms of reproductive isolation have been classified in a number of ways. Zoologist Ernst Mayr classified the mechanisms of reproductive isolation in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it. The mechanisms are genetically controlled and can appear in species whose geographic distributions overlap (sympatric speciation) or are separate (allopatric speciation). Pre-zygotic isolation mechanisms are the most economic in terms of the natural selection of a population, as resources are not wasted on the production of a descendant that is weak, non-viable or sterile. These mechanisms include physiological or systemic barriers to fertilization. Allochronic speciation Any of the factors that prevent potentially fertile individuals from meeting will reproductively isolate the members of distinct species. The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering. An example of the ecological or habitat differences that impede the meeting of potential pairs occurs in two fish species of the family Gasterosteidae (sticklebacks). One species lives all year round in fresh water, mainly in small streams. The other species lives in the sea during winter, but in spring and summer individuals migrate to river estuaries to reproduce. The members of the two populations are reproductively isolated due to their adaptations to distinct salt concentrations. An example of reproductive isolation due to differences in the mating season are found in the toad species Bufo americanus and Bufo fowleri.

Molecular heterogeneity of the ganglionic eminence and its relationship to the birth of neocortical interneurons

Susan Willi Monnerat

Neocortical function and malfunction depends critically on a spectrum of inhibitory interneurons. To gain novel insight in normal brain function and mechanisms leading to diseases, such as epilepsy or schizophrenia, the knowledge about the specific functions of each interneuron type appears crucial. To date, the mystery about the diversity of neocortical interneurons remains still unresolved, even though much effort was devoted to the characterization of mature interneurons in the neocortex. Ten years ago, the discovery of the ganglionic eminence (GE) as origin of neocortical interneurons allowed a novel approach to assess their various identities, and thus prompted researchers to follow the fate of cells accommodated in this region. Despite the knowledge of the source location and its subdivision into three GEs, their molecular identity remained unknown apart from few genes, and consequently unabled the specific isolation of precursor cells. To this purpose, we performed extensive microarray analysis of the three GEs at various developmental timepoints to reveal their molecular heterogeneity and identify novel precursor cell surface markers that would enable their prospective isolation by flow cytometry and subsequent fate assessment in vitro and in vivo. Surprisingly, molecular heterogeneity of the three GEs was restricted to few differentially expressed genes. However, the differences in gene expression became more apparent at late developmental stage that may reflect the increased specification during development. Among the three GEs, the LGE and CGE resembled molecularly each other most, while the MGE and CGE were most distantly related to each other. Furthermore, our genome-wide expression study suggested that the CGE, a structure of so far controversial nature, was rather a unique structure than a fusion of the MGE and LGE. Analysis across developmental timepoints revealed that the MGE underwent tremendous changes in gene expression when compared to the differential expression existing between the homochronic MGE and cerebral cortex. In our microarray screen, we could identify Boc as a novel marker of LGE/CGE-resident precursor cells. Taking advantage of Boc that encodes a cell surface molecule, we could isolate the Boc+ cells from the GE by flow cytometry, and demonstrate that in contrast to Boc– cells, they self-renewed and differentiated into oligodendrocytes, astrocytes, and neurons, and thus identify them as progenitor cells. Furthermore, immunohistochemical studies suggested that Boc+ GE cells may correspond to radial glia, the embryonic neural stem cells. The specific expression of Boc in the LGE and CGE, but not in the MGE during their entire lifetime suggested that Boc is a reliable marker for LGE/CGE-resident precursor cells. To assess the fate of Boc+ GE cells, we performed homotypic transplantations of Boc+ and Boc– cells onto organotypic slice cultures, and observed that in contrast to Boc– cells, Boc+ cells did not migrate from the GE to the neocortex suggesting that Boc+ precursor cells of the GE do not generate neocortical interneurons. In future, additional fate-mapping studies of Boc+ GE cells, such as in vivo transplantation, should shed more light on the ability of Boc+ LGE/CGE precursor cells to generate neocortical interneurons, and dependent on the outcome serve to construct a cell-lineage tree of neocortical interneurons. Thus, Boc will remain a useful tool to investigate developmental processes underlying the generation of neocortical and/or other neurons, such as striatal neurons.

EPFL2007

A Qualitative and Quantitative Structural Landscape Analysis Case Study in Monteverde, Costa Rica

Forest fragmentation translates a geographical isolation process and has been shown to influence the abundance, the movements and persistence of many species. The structure of the highly fragmented forests of Monteverde, Costa Rica, may exercise a relevant influence on the species richness and individual abundance of many forest-dwelling understory bird species.

2003

Molecular heterogeneity of the ganglionic eminence and its relationship to the birth of neocortical interneurons

Susan Willi Monnerat

EPFL2007