Neurons must cope with extreme membrane trafficking demands to produce axons with organelle compositions that differ dramatically from those of the cell soma and dendrites; however, the mechanism by which they accomplish this is not understood. Here we use electron microscopy and quantitative imaging of tagged organelles to show that Caenorhabditis elegans axons lacking UNC-16 (JIP3/Sunday Driver) accumulate Golgi, endosomes, and lysosomes at levels up to 10-fold higher than wild type, while ER membranes are largely unaffected. Time lapse microscopy of tagged lysosomes in living animals and an analysis of lysosome distributions in various regions of unc-16 mutant axons revealed that UNC-16 inhibits organelles from escaping the axon initial segment (AIS) and moving to the distal synaptic part of the axon. Immunostaining of native UNC-16 in C. elegans neurons revealed a localized concentration of UNC-16 at the initial segment, although UNC-16 is also sparsely distributed in distal regions of axons, including the synaptic region. Organelles that escape the AIS in unc-16 mutants show bidirectional active transport within the axon commissure that occasionally deposits them in the synaptic region, where their mobility decreases and they accumulate. These results argue against the long-standing, untested hypothesis that JIP3/Sunday Driver promotes anterograde organelle transport in axons and instead suggest an organelle gatekeeper model in which UNC-16 (JIP3/Sunday Driver) selectively inhibits the escape of Golgi and endosomal organelles from the AIS. This is the first evidence for an organelle gatekeeper function at the AIS, which could provide a regulatory node for controlling axon organelle composition. NEURONS have a unique cell biology that presents daunting membrane trafficking challenges. For example, they must selectively transport two classes of regulated secretory vesicles (synaptic vesicles and dense core vesicles) long distances into axons, but only after the vesicles have completed their maturation process in the cell soma, during which they arise from, and interact with, other organelles in the soma. Neurons must also restrict, or even prevent, the flow of some organelles, such as Golgi, lysosomes, and endosomes, into the distal synaptic region of axons, which are relatively devoid of these organelles compared to cell somas.However, under special conditions, such as the need for axon repair or growth, neurons may require these organelles in axons. The potential hazards of excessive organelle transport into axons may include organelle traffic jams within narrow axons, reduced synaptic vesicle production as synaptic vesicle proteins are transported away from the cell soma before they are assembled into mature vesicles, and the disruption of membrane trafficking pathways in the synaptic region of axons caused by the inappropriate presence of cell soma organelles.A crucial regulatory domain for controlling axon composition is the region at or near the junction of the cell soma and axon, designated th...

Aim Previous work on the tidepool copepod Tigriopus californicus revealed a curious case of incipient speciation at the southern end of the species' range in Baja California, Mexico. The present study expands on the geography of this pattern and tests for congruence between reproductive and phylogenetic patterns.Location The Pacific coast of North America, from central Baja California to south-eastern Alaska (27-57°N), including the full range of T. californicus.Methods Primary techniques included mating experiments (> 4000 crosses), phylogeny reconstruction (mitochondrial cytochrome c oxidase subunit I) and screening of single nucleotide polymorphisms (SNPs, 42 loci). Analyses used > 8000 copepods for the mating experiments, 86 copepods for the phylogeny and 41 copepods for the SNP assays. Phylogenies were constructed using Bayesian, maximum likelihood and maximum parsimony methods. ResultsPopulations were found to fall into three reproductive groups: northern and southern groups that were reproductively isolated from each other, and an intermediate group that could serve as a conduit for gene flow. The northern and intermediate populations fell into one clade while all southern populations fell into a second clade. These two clades are now separated by less than 12 km at latitude 29.35°N. Nuclear SNP data for a subset of locations confirmed striking divergence between populations on either side of this boundary. The second (southern) clade was further subdivided into two clades separated by the lagoon region of Guerrero Negro (latitude 28°N).Main conclusions Reproductive assays and molecular data (both mitochondrial and nuclear) reveal a sharp break at 29.35°N, a region with no obvious barriers to dispersal, with no evidence for mixing across this narrow transition zone. Results also showed a milder break at the Guerrero Negro Lagoon (28°N), a location where breaks have been reported for other taxa.

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Squamates may be an attractive group in which to study the influence of sex chromosomes on speciation rates because of the repeated evolution of heterogamety (both XY and ZW), as well as an apparently large number of taxa with environmental sex‐determination.

Sex chromosomes in vertebrates range from highly heteromorphic (as in most birds and mammals) to strictly homomorphic (as in many fishes, amphibians, and nonavian reptiles). Reasons for these contrasted evolutionary trajectories remain unclear, but species such as common frogs with polymorphism in the extent of sex chromosome differentiation may potentially deliver important clues. By investigating 92 common frog populations from a wide range of elevations throughout Switzerland, we show that sex chromosome differentiation strongly correlates with alleles at the candidate sex‐determining gene Dmrt1. Y‐specific Dmrt1 haplotypes cluster into two main haplogroups, YA and YB, with a phylogeographic signal that parallels mtDNA haplotypes: YA populations, with mostly well‐differentiated sex chromosomes, occur primarily south of the main alpine ridge that bisects Switzerland, whereas YB populations, with mostly undifferentiated (proto‐)sex chromosomes, occur north of this ridge. Elevation has only a marginal effect, opposing previous suggestions of a major role for climate on sex chromosome differentiation. The Y‐haplotype effect might result from differences in the penetrance of alleles at the sex‐determining locus (such that sex reversal and ensuing X‐Y recombination are more frequent in YB populations), and/or fixation of an inversion on YA (as supported by the empirical observation that YA haplotypes might not recombine in XYA females).