Subcortical re-entrant projection systems connecting cerebral cortical areas with the basal ganglia and cerebellum are topographically specific and therefore considered to be parallel circuits or "closed loops." The precision of projections within these circuits, however, has not been characterized sufficiently to indicate whether cortical signals are integrated within or among presumed compartments. To address this issue, we studied the first link of the rat cortico-ponto-cerebellar pathway with anterograde axonal tracing from physiologically defined, individual whisker "barrels" of the primary somatosensory cortex (SI). The labeled axons in the pontine nuclei formed several, sharply delineated clusters. Dual tracer injections into different SI whisker barrels gave rise to partly overlapping, paired clusters, indicating somatotopic specificity. Three-dimensional reconstructions revealed that the clusters were components of concentrically organized lamellar subspaces. Whisker barrels in the same row projected to different pontine lamellae (side by side), the somatotopic representation of which followed an inside-out sequence. By contrast, whisker barrels from separate rows projected to clusters located within the same lamellar subspace (end to end). In the neostriatum, this lamellar topography was the opposite, with barrels in the same row contacting different parts of the same lamellar subspace (end to end). The degree of overlap among pontine clusters varied as a function of the proximity of the cortical injections. Furthermore, corticopontine overlap was higher among projections from barrels in the same row than among projections from different whisker barrel rows. This anisotropy was the same in the corticostriatal projection. These findings have important implications for understanding convergence and local integration in somatosensory-related subcortical circuits. Key words: 3-D reconstruction; basal ganglia; cerebellum; cerebrocerebellar; double anterograde tracing; parallel circuits; pontine nuclei; somatosensory maps; somatotopyNeurons in the cerebral cortex project to a number of subcortical targets. Many of these neurons belong to two major corticosubcortical re-entrant circuits, one including the basal ganglia (for review, see Heimer et al., 1995;Parent and Hazrati, 1995) and another including the pontine nuclei and cerebellum (for review, see Bjaalie, 1992, 1997;Schmahmann and Pandya, 1997). Structural and functional specification have been studied extensively in the first links of these circuits, i.e., in the projections from the cerebral cortex to the pontine nuclei (Brodal, 1968(Brodal, , 1978Mihailoff et al., 1978Mihailoff et al., , 1985Wiesendanger and Wiesendanger, 1982;Bjaalie and Brodal, 1989;Leergaard et al., 2000) and the neostriatum (Webster, 1961;Malach and Graybiel, 1986;Gerfen, 1989;Alloway et al., 1999). During development, the global topography of corticopontine projections appears to be determined by simple temporal and spatial gradients operative within source (cerebral cortex) an...

To elucidate the detailed organization of corticostriatal projections from rodent somatosensory cortex, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into separate parts of the whisker "barrel" representation. In one group of rats, the two tracers were injected into different barrel columns residing in the same row; in the other group of rats, the tracers were deposited into barrel columns residing in different rows. Reconstructions of labeled axonal varicosities in the neostriatum and ventrobasal thalamus were analyzed quantitatively to compare the extent of overlapping projections to these subcortical structures. For both groups of animals, corticostriatal projections terminated in densely packed clusters that occupied curved lamellar-shaped regions along the dorsolateral edge of the neostriatum. When the tracers were injected into different whisker barrel rows, the distribution of BDA- and FR-labeled terminals in the neostriatum followed a crude somatotopic organization in which the amount of overlap was approximately the same as in the ventrobasal thalamus. When both tracers were injected into the same whisker barrel row, however, the amount of corticostriatal overlap was significantly higher than the amount of overlap observed in the ventrobasal thalamus. These results indicate that corticostriatal projections from whisker barrel cortex have an anisotropic organization that correlates with the pattern of vibrissal movements during whisking behavior.

Indirect genetic effects arise when genes expressed in one individual affect the expression of traits in other individuals. The importance of indirect genetic effects has been recognized for a diversity of evolutionary processes including kin selection, sexual selection, community structure and multilevel selection, but data regarding their genetic architecture and prevalence throughout the genome remain scarce, especially for interactions between unrelated individuals. Using a set of 411 Bay-0 x Shahdara Arabidopsis recombinant inbred lines grown with Landsberg neighbours, we examined quantitative trait loci (QTL) having direct and indirect effects on size, developmental, and fitness related traits. Using an interval mapping approach, we identified 15 QTL with direct effects and found that 13 of these QTL had significant indirect effects on trait expression in neighbouring plants. These results suggest widespread pleiotropy, as nearly all direct effect QTL have associated pleiotropic indirect effects. Paradoxically, most indirect effects were of the same sign as direct effects, creating a pattern of nearly universal positive pleiotropy that makes most covariances between direct and indirect effects positive. These results are consistent with a complex genetic basis for intraspecific interactions, but suggest that interactions between neighbouring plants are largely positive, rather than negative as would be expected for competition. In addition to their evolutionary and ecological importance, these pleiotropic relationships between DGE and IGE loci have implications for quantitative genetic studies of natural populations as well as experimental design considerations. Additionally, studies that ignore IGEs may over- or underestimate quantitative genetic parameters, as well as the effect of and variance contributed by QTL.