Background: Pbx genes encode TALE class homeodomain transcription factors that pattern the developing neural tube, pancreas, and blood. Within the hindbrain, Pbx cooperates with Hox proteins to regulate rhombomere segment identity. Pbx cooperates with Eng to regulate midbrainhindbrain boundary maintenance, and with MyoD to control fast muscle cell differentiation. Although previous results have demonstrated that Pbx is required for proper eye size, functions in regulating retinal cell identity and patterning have not yet been examined.

Retinal ganglion cells (RGCs) extend axons that exit the eye, cross the midline at the optic chiasm, and synapse on target cells in the optic tectum. Class three semaphorins (Sema3s) are a family of molecules known to direct axon growth. We undertook an expression screen to identify sema3s expressed in the retina and/or brain close to in-growing RGC axons, which might therefore influence retinal-tectal pathfinding. We find that sema3Aa, 3Fa, 3Ga, and 3Gb are expressed in the retina, although only sema3Fa is present during the time window when the axons extend. Also, we show that sema3Aa and sema3E are present near or at the optic chiasm. Furthermore, sema3C, 3Fa, 3Ga, and 3Gb are expressed in regions of the diencephalon near the path taken by RGC axons. Finally, the optic tectum expresses sema3Aa, 3Fa, 3Fb, and 3Gb. Thus, sema3s are spatiotemporally placed to influence RGC axon growth. Developmental Dynamics 236:2918 -2924, 2007.

Although the arrangement of internal organs in most metazoans is profoundly left–right (L/R) asymmetric with a predominant handedness, rare individuals show full (mirror-symmetric) or partial (heterotaxy) reversals. While the nematode Caenorhabditis elegans is known for its highly determinate development, including stereotyped L/R organ handedness, we found that L/R asymmetry of the major organs, the gut and gonad, varies among natural isolates of the species in both males and hermaphrodites. In hermaphrodites, heterotaxy can involve one or both bilaterally asymmetric gonad arms. Male heterotaxy is probably not attributable to relaxed selection in this hermaphroditic species, as it is also seen in gonochoristic Caenorhabditis species. Heterotaxy increases in many isolates at elevated temperature, with one showing a pregastrulation temperature-sensitive period, suggesting a very early embryonic or germline effect on this much later developmental outcome. A genome-wide association study of 100 isolates showed that male heterotaxy is associated with three genomic regions. Analysis of recombinant inbred lines suggests that a small number of loci are responsible for the observed variation. These findings reveal that heterotaxy is a widely varying quantitative trait in an animal with an otherwise highly stereotyped anatomy, demonstrating unexpected plasticity in an L/R arrangement of the major organs even in a simple animal.This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.

Anatomical left-right (L/R) handedness asymmetry in C. elegans is established in the four-cell embryo as a result of anteroposterior skewing of transverse mitotic spindles with a defined handedness. This event creates a chiral embryo and ultimately an adult body plan with fixed L/R positioning of internal organs and components of the nervous system. While this “dextral” configuration is invariant in hermaphrodites, it can be reversed by physical manipulation of the early embryo or by mutations that interfere with mitotic spindle orientation, which leads to viable, mirror-reversed (sinistral) animals. During normal development of the C. elegans male, the gonad develops on the right of the midline, with the gut bilaterally apposed on the left. However, we found that in males of the laboratory N2 strain and Hawaiian (“Hw”) wild isolate, the gut/gonad asymmetry is frequently reversed in a temperature-dependent manner, independent of normal embryonic chirality. We also observed sporadic errors in gonad migration occurring naturally during early larval stages of these and other wild strains; however, the incidence of such errors does not correlate with the frequency of L/R gut/gonad reversals in these strains. Analysis of N2/Hw hybrids and recombinant inbred advanced intercross lines (RIAILs) indicate that the L/R organ reversals are likely to result from recessively acting variations in multiple genes. Thus, unlike the highly reproducible L/R asymmetries of most structures in hermaphrodites, the L/R asymmetry of the male C. elegans body plan is less rigidly determined and subject to natural variation that is influenced by a multiplicity of genes.

Commissure formation requires adaptive responses of axonal growth cones to midline cues. We describe the zebrafish habenular commissure, the sole axonal crossing point in the dorsal forebrain of teleosts, as a model for studying commissure development. We have used lipophilic dyes and electroporationmediated transfection as in vivo labelling techniques to document habenular commissure formation. These analyses have revealed novel growth cone dynamics at the midline. In embryos with mutations in esrom, commissural axons navigate to the roof plate of the diencephalon, but stall and ultimately fail to cross. While stalled, growth cones are dynamic and exhibit extensive protrusive activity. esrom encodes the zebrafish ortholog of PAM/PHR/Highwire/RPM-1, an immense molecule with E3 ubiquitin ligase activity that interacts with multiple signal transduction pathways.We are using genetics, immunohistochemistry, receptor affinity probe assays, transient unilateral transfection of the brain, and live imaging to elucidate the axon guidance signals and growth cone responses required for proper habenular commissure formation, and to understand how this process is disrupted in esrom. Axon segments within the commissure, but not elsewhere, have EphB receptors localized to the plasma membrane. Esrom is also required for proper retinal axon sorting and topographic mapping, a process dependent on Eph-Ephrin signaling. We are exploring the hypothesis that Esrom acts in a pathway to regulate Eph signaling in the growth cone during habenular commissure formation.Midline axon guidance and Eph signaling offer a new context for understanding the cellular functions of Esrom, an enigmatic and complex molecule. EphBs are required for proper corpus callosum and habenular commissure formation in mammals. We speculate that mechanisms of habenular commissure formation in teleosts may share common mechanisms with dorsal forebrain decussation in higher vertebrates.UNC-6/Netrin signaling is involved in guiding axons towards their target cells (by attraction, using its receptor UNC-40/DCC) and away from their targets (by repulsion, using UNC-40 and its other known receptor UNC-5/RCM). UNC-6 also mediates the migration of other cell types including the distal tip cell (DTC), the leading cell of the gonad. In mutants of these three genes, the dorsoventral phase of DTC migration frequently fails to occur. In order to identify additional components of the UNC-6/netrin guidance system, our laboratory conducted a genetic screen in the background of a weak UNC-40 mutant to identify enhancers of DTC migration. From this sensitized screen, we isolated 13 mutants in DTC migration including enh-2. On its own enh-2, has no DTC defects but like other components of the UNC-6 guidance system, enh-2 appears to affect dorsoventral migration in multiple cell types, including neurons. We found that enh-2 has increased axon guidance defects and mispositioning of the touch neurons. Interestingly, we found that enh-2 does not appear to affect axon guidance of the motor...