Salmonella: Now you see it,now you don't

Diseases caused by Salmonella species are characterized by bacterial invasion of host cells. Salmonella invasion requires a genetic locus (inv) with homology to bacterial systems involved in specific protein export and organelle assembly. Until recently, the actual Salmonella invasion factors exported or assembled by the inv system remained unidentified. It now appears that Salmonella produces novel appendages upon contact with host cells. These appendages are transient, appearing and disappearing rapidly from the bacterial surface. Appendages are altered in strains unable to invade due to mutations within the inv/spa locus. Therefore, a role for the invasion locus has been identified, providing another example of bacterial pathogens responding to signals provided by the host cell surface.     

Human parthenogenetic embryo stem cells: appreciating what you have when you have it

In this issue of Cell Stem Cell, Kim et al. (2007b) show, by genome-wide single-nucleotide polymorphism analysis, that human ES cells claimed to originate from a somatic cell nuclear transfer (SCNT) embryo are actually parthenogenetic. This now permits an appreciation of the future therapeutic utility of this type of cell.     

Host‐associated genomic differentiation in congeneric butterflies: now you see it,now you do not

Ecotypic variation among populations may become associated with widespread genomic differentiation, but theory predicts that this should happen only under particular conditions of gene flow, selection and population size. In closely related species, we might expect the strength of host‐associated genomic differentiation (HAD) to be correlated with the degree of phenotypic differentiation in host‐adaptive traits. Using microsatellite and Amplified Fragment Length Polymorphism (AFLP) markers, and controlling for isolation by distance between populations, we sought HAD in two congeneric species of butterflies with different degrees of host plant specialization. Prior work on Euphydryas editha had shown strong interpopulation differentiation in host‐adapted traits, resulting in incipient reproductive isolation among host‐associated ecotypes. We show here that Euphydryas aurinia had much weaker host‐associated phenotypic differentiation. Contrary to our expectations, we detected HAD in Euphydryas aurinia, but not in E. editha. Even within an E. aurinia population that fed on both hosts, we found weak but significant sympatric HAD that persisted in samples taken 9 years apart. The finding of significantly stronger HAD in the system with less phenotypic differentiation may seem paradoxical. Our findings can be explained by multiple factors, ranging from differences in dispersal or effective population size, to spatial variation in genomic or phenotypic traits and to structure induced by past histories of host‐adapted populations. Other infrequently measured factors, such as differences in recombination rates, may also play a role. Our result adds to recent work as a further caution against assumptions of simple relationships between genomic and adaptive phenotypic differentiation.     

When to believe what you see

The recent X-ray crystal structure of a hammerhead ribozyme derived from Schistosoma mansoni containing the rate-enhancing peripheral domain has a catalytic core that is very different from the catalytic core present in the structure of the "minimal" hammerhead, which lacks a peripheral domain (Martick and Scott, 2006). The new structure reconciles many of the disagreements between the minimal hammerhead structure and the biochemical data on the cleavage properties of chemically modified hammerheads. The new structure also emphasizes the dynamic nature of small RNA domains and provides a cautionary tale for everyone who tries to use structure to understand function.