Closing the ring: historical biogeography of the salamander ring species Ensatina eschscholtzii

Aim The salamander Ensatina eschscholtzii Gray is a classic example of a ring species, or a species that has expanded around a central barrier to form a secondary contact characterized by species‐level divergence. In the original formulation of the ring species scenario, an explicit biogeographical model was proposed to account for the occurrence of intraspecific sympatry between two subspecies in southern California (the ‘southern closure’ model). Here we develop an alternative ring species model that is informed by the geomorphological development of the California Coast Ranges, and which situates the point of ring closure in the Monterey Bay region of central coastal California (the ‘Monterey closure’ model). Our study has two aims. The first is to use phylogenetic methods to evaluate the two competing biogeographical models. The second is to describe patterns of phylogeographical diversity throughout the range of the Ensatina complex, and to compare these patterns with previously published molecular systematic data. Location Western North America, with a focus on the state of California, USA. Methods We obtained mitochondrial DNA sequence data from 385 individuals from 224 populations. A phylogeny was inferred using Bayesian techniques, and the geographical distributions of haplotypes and clades were mapped. The two biogeographical ring species models were tested against our Bayesian topology, including the associated Bayesian 95% credible set of trees. Results High levels of phylogeographical diversity were revealed, especially in central coastal and northern California. Our Bayesian topology contradicts the Monterey closure model; however, 0.08% of the trees in our Bayesian 95% credible set are consistent with this model. In contrast, the classic ring species biogeographical model (the southern closure model) is consistent with our Bayesian topology, as were 99.92% of the trees in our 95% credible set. Main conclusions Our Bayesian phylogenetic analysis most strongly supports the classic ring species model, modified to accommodate an improved understanding of the complex geomorphological evolution of the California Coast Ranges. In addition, high levels of phylogeographical diversity in central and northern California were identified, which is consistent with the striking levels of allozymic differentiation reported previously from those regions.     

Presence of telomeric and subtelomeric sequences at the fusion points of ring chromosomes indicates that the ring syndrome is caused by ring instability

In situ hybridization of a telomeric (TTA-GGG) _n sequence to metaphases from three cases of ring chromosome, involving respectively chromosomes 4, 16, and 20, showed the presence of the cognate sequences in all three rings. To investigate whether these ring chromosomes originated by telomere-telomere fusion, we determined, by in situ hybridization, whether telomere-associated sequences and/or specific distal sequences were still present in the ring chromosomes. The finding that these sequences were preserved in all the ring chromosomes strongly indicates that they originated by telomere-telomere fusion. All three subjects carrying the ring chromosomes are affected by the so-called ring syndrome, with failure to thrive, minor dysmorphic signs and no major anomalies. The r(4) patient has the ring in mosaic form with a normal cell line and has normal intelligence. The r(16) and the r(20) patients have moderate mental retardation and suffer from seizures. We conclude that the ring syndrome, even in its more severe manifestation, is caused by ring chromosome instability.     

Enzymatic bromination of the thiazole ring

Bromination of the thiazole ring by the enzyme chloroperoxidase, produced by the fungus Caldariomyces fumago, was demonstrated. Both 2-acetoacetamido-4-methylthiazole and 2-acetamidothiazole were brominated on C-5 of the thiazole ring in the presence of chloroperoxidase, bromide, and hydrogen peroxide in 0.06 M Phosphate solution at pH 3.0. No reaction occurred in the absence of enzyme.     

Chromosome biology: the crux of the ring

SMC proteins are key components of large ring-shaped chromosomal protein complexes, such as cohesin and condensin. New evidence supports the idea that these rings topologically encircle DNA. Hints also emerge as to what it may take for DNA to enter the ring.     

Rotation of the proteolipid ring in the V-ATPase

V0V1-ATPase is a proton-translocating ATPase responsible for acidification of eukaryotic intracellular compartments and for ATP synthesis in archaea and some eubacteria. We demonstrated recently the rotation of the central stalk subunits in V1, a catalytic sector of V0V1-ATPase (Imamura, H., Nakano, M., Noji, H., Muneyuki, E., Ohkuma, S., Yoshida, M., and Yokoyama, K. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2312-2315), but the rotation of the proteolipid ring, a predicted counterpart rotor in the membrane V0 sector, has remained to be proven. V0V1-ATPase that retained sensitivity to N',N'-dicyclohexylcarbodiimide was isolated from Thermus thermophilus, immobilized onto a glass surface through the N termini of the A subunits of V1, and decorated with a bead attached to a proteolipid subunit of V0. Rotation of beads was observed in the presence of ATP, and direction of rotation was always counterclockwise viewed from the membrane side. The rotation proceeded at approximately 3.0 rev/s in average at 4 mm ATP and was abolished by N',N'-dicyclohexylcarbodiimide treatment. Thus, the rotation of the central stalk in V1 accompanies rotation of a proteolipid ring of V0 in the functioning V0V1-ATPase.     

Bacterial division: the fellowship of the ring

The Z ring, composed of the tubulin homolog FtsZ, is essential for bacterial cell division. Recently a new protein, ZapA, has been discovered that localizes to the Z ring and stabilizes it, probably by promoting the bundling of FtsZ protofilaments.     

A case of the ring 20 syndrome

A 4-year-old child with a ring 20 chromosome mosaicism, low grade developmental delay, and seizures is described.     

Cytokinesis: myosin spots the ring

Faithful actomyosin ring assembly is pivotal for successful cell division. The mechanisms by which the actomyosin ring is assembled at the correct time and place remain unclear. Recent studies in fission yeast have shown that a myosin II-containing spot may be a novel progenitor structure essential for actomyosin ring assembly.     

Unexpected twist to the Z ring

Development in Bacillus subtilis involves a switch in the location of the cytokinetic Z ring from midcell to the pole. Time lapse photography of an FtsZ-GFP fusion reveals that this switch involves a spiral intermediate and allows identification of the specific sporulation functions involved.     

Replicative helicase loaders: ring breakers and ring makers

Helicase loaders transfer the ring-shaped replicative helicases onto DNA. They assort into two classes: ring breakers, which place stabile hexameric rings on DNA via transient gaps at subunit interfaces; and helicase makers, which assemble hexameric rings around DNA from monomeric helicase units.