Cell and molecular mechanisms involved in the migration of cortical interneurons

Since the discovery that the vast majority of the GABA-containing interneurons of the cerebral cortex arise in the subpallium, considerable effort has been put into the description of the precise origin of these neurons in subdivisions of the ganglionic eminence and in the migratory routes they follow on their way to the developing cortex. More recently, studies have focused on the molecular and cellular mechanisms that guide their migration. Investigations of the molecular mechanisms involved have demonstrated important roles for numerous transcription factors, motogenic factors and guidance molecules. Here, we review results of very recent analyses of the underlying cellular mechanisms and specifically of the movement of the nucleus, cytoplasmic components and neuritic processes during interneuron migration.

Branching and nucleokinesis defects in migrating interneurons derived from doublecortin knockout mice

Type I lissencephaly results from mutations in the doublecortin (DCX) and LIS1 genes. We generated Dcx knockout mice to further understand the pathophysiological mechanisms associated with this cortical malformation. Dcx is expressed in migrating interneurons in developing human and mouse brains. Video microscopy analyses of such tangentially migrating neuron populations derived from the medial ganglionic eminence show defects in migratory dynamics. Specifically, the formation and division of growth cones, leading to the production of new branches, are more frequent in knockout cells, although branches are less stable. Dcx-deficient cells thus migrate in a disorganized manner, extending and retracting short branches and making less long-distant movements of the nucleus. Despite these differences, migratory speeds and distances remain similar to wild-type cells. These novel data thus highlight a role for Dcx, a microtubule-associated protein enriched at the leading edge in the branching and nucleokinesis of migrating interneurons.

Nucleokinesis in tangentially migrating neurons comprises two alternating phases: forward migration of the Golgi/centrosome associated with centrosome splitting and myosin contraction at the rear

During rodent cortex development, cells born in the medial ganglionic eminence (MGE) of the basal telencephalon reach the embryonic cortex by tangential migration and differentiate as interneurons. Migrating MGE cells exhibit a saltatory progression of the nucleus and continuously extend and retract branches in their neuritic arbor. We have analyzed the migration cycle of these neurons using in vitro models. We show that the nucleokinesis in MGE cells comprises two phases. First, cytoplasmic organelles migrate forward, and second, the nucleus translocates toward these organelles. During the first phase, a large swelling that contains the centrosome and the Golgi apparatus separates from the perinuclear compartment and moves rostrally into the leading neurite, up to 30 mum from the waiting nucleus. This long-distance migration is associated with a splitting of the centrioles that line up along a linear Golgi apparatus. It is followed by the second, dynamic phase of nuclear translocation toward the displaced centrosome and Golgi apparatus. The forward movement of the nucleus is blocked by blebbistatin, a specific inhibitor of nonmuscle myosin II. Because myosin II accumulates at the rear of migrating MGE cells, actomyosin contraction likely plays a prominent role to drive forward translocations of the nucleus toward the centrosome. During this phase of nuclear translocation, the leading growth cone either stops migrating or divides, showing a tight correlation between leading edge movements and nuclear movements.

Population genetic structure of wild Phaseolus lunatus (Fabaceae), with special reference to population sizes

To set up an in situ conservation strategy for Phaseolus lunatus, we analyzed the genetic structure of 29 populations in the Central Valley of Costa Rica. Using 22 enzyme loci, we quantified the proportion of polymorphic loci (P(p)), the mean number of alleles per locus (A), and the mean effective number of alleles per locus (A(e)), which equaled to 10.32%, 1.10, and 1.05, respectively. The total heterozygosity (H(T)), the intrapopulation genetic diversity (H(S)), and the interpopulation genetic diversity (D(ST)) were 0.193, 0.082, and 0.111, respectively. The genotypic composition of the analyzed populations showed a deviation from the Hardy-Weinberg proportions (F(IT) = 0.932). This disequilibrium was due to either genetic differentiation between populations (F(ST) = 0.497) or nonrandom mating within populations (F(IS) = 0.866). From the level of genetic differentiation between populations and the private alleles frequencies estimates, gene flow was calculated: Nm(W) = 0.398 and Nm(S) = 0.023, respectively. The results suggested that wild Lima bean maintains most of its isozyme variation among populations. Significant positive correlation was observed between population size and P(p), A, and H(o) (observed heterozygosity), whereas no correlation was observed with the average fixation index of population (F). The loss of genetic variability in populations was attributed to inbreeding and the bottleneck effects that characterized the target populations. In situ conservation and management procedures for wild Lima bean are discussed.

Soil seed bank and seed dormancy in wild populations of lima bean (Fabaceae): considerations for in situ and ex situ conservation

Seed dormancy and its impact on the soil seed bank for wild Lima bean (Phaseolus lunatus) populations were studied in the Central Valley of Costa Rica. Five populations were selected in contrasted environments. In all cases, distribution of seeds in the soil was limited to 3 cm depth. No innate dormancy was observed but combination of hard seed coat and hilum opening controlled by environmental conditions were responsible for an induced dormancy and the constitution of a persistent seed bank. Breaking of this dormancy was obtained by a brief elevation of temperature from 25° to 45°C. Impacts of this phenomenon concern both genetic and demographic aspects of in situ conservation of the species. Consequences on ex situ conservation are mainly related with the regeneration of the seed collection.

Use of chloroplast DNA polymorphisms for the phylogenetic study of seven Phaseolus taxa including P. vulgaris and P. coccineus

The genetic variability of seven Phaseolus taxa has been evaluated on the basis of molecular data and the results have used to clarify the phyletic relationships between several taxa of the P. coccineus L. complex. Chloroplast DNA (cpDNA) from 33 populations was digested with six restriction endonucleases, revealing some polymorphisms that made it possible to divide most of the taxa into two main groups: the subspecies of P. coccineus on the one hand, and P. vulgaris L., P. polyanthus Greenman and P. costaricensis (Freytag and Debouck) on the other hand. P. polyanthus is closer to P. vulgaris than the other taxa of the second group and should be considered as a separate species. The position of the wild species P. costaricensis is intermediate between P. coccineus and P. polyanthus. P. glabellus shows sufficient polymorphisms at the cpDNA level to be recognized as a separate species, as previously suggested from total seed-protein electrophoretic studies. These results favour the hypothesis of a common phylogeny for P. vulgaris, P. polyanthus, P. costaricensis and P. coccineus from a single wild ancestor. Although cpDNA is generally known to be uniform at the intraspecific level, some additional polymorphisms were also detected within P. vulgaris, P. polyanthus and P. coccineus. Further studies are required to understand the significance of the latter.

Improving efficiency of breeding for higher crop yield

Exclusive selection for yield raises, the harvest index of self-pollinated crops with little or no gain in total bipmass. In addition to selection for yield, it is suggested that efficient breeding for higher yield requires simultaneous selection for yield's three major, genetically controlled physiological components. The following are needed: (1) a superior rate of biomass accumulation. (2) a superior rate of actual yield accumulation in order to acquire a high harvest index, and (3) a time to harvest maturity that is neither shorter nor longer than the duration of the growing season. That duration is provided by the environment, which is the fourth major determinant of yield. Simultaneous selection is required because genetically established interconnections among the three major physiological components cause: (a) a correlation between the harvest index and days to maturity that is usually negative; (b) a correlation between the harvest index and total biomass that is often negative, and (c) a correlation between biomass and days to maturity that is usually positive. All three physiological components and the correlations among them can be quantified by yield system analysis (YSA) of yield trials. An additive main effects and multiplicative interaction (AMMI) statistical analysis can separate and quantify the genotype × environment interaction (G × E) effect on yield and on each physiological component that is caused by each genotype and by the different environment of each yield trial. The use of yield trials to select parents which have the highest rates of accumulation of both biomass and yield, in addition to selecting for the G × E that is specifically adapted to the site can accelerate advance toward the highest potential yield at each geographical site. Higher yield for many sites will raise average regional yield. Higher yield for multiple regions and continents will raise average yield on a world-wide basis. Genetic and physiological bases for lack of indirect selection for biomass from exclusive selection for yield are explained.