On the mod resc model and the evolution of Wolbachia compatibility types.

Cytoplasmic incompatibility (CI) is induced by the endocellular bacterium Wolbachia. It results in an embryonic mortality occurring when infected males mate with uninfected females. The mechanism involved is currently unknown, but the mod resc model allows interpretation of all observations made so far. It postulates the existence of two bacterial functions: modification (mod) and rescue (resc). The mod function acts in the males' germline, before Wolbachia are shed from maturing sperm. If sperm is affected by mod, zygote development will fail unless resc is expressed in the egg. Interestingly, CI is also observed in crosses between infected males and infected females when the two partners bear different Wolbachia strains, demonstrating that mod and resc interact in a specific manner: Two Wolbachia strains are compatible with each other only if they harbor the same compatibility type. Here we focus on the evolutionary process involved in the emergence of new compatibility types from ancestral ones. We argue that new compatibility types are likely to evolve under a wider range of conditions than previously thought, through a two-step process. First, new mod variants can arise by mutation and spread by drift. This is possible because mod is expressed in males and Wolbachia is transmitted by females. Second, once such a mod variant achieves a certain frequency, it can create the conditions for the deterministic invasion of a new resc variant, allowing the invasion of a new mod resc pair. Furthermore, we show that a stable polymorphism might be maintained in natural populations, allowing the long-term existence of "suicidal" Wolbachia strains.

MRI dosimetry: a fast quantitative MRI method to determine 3D absorbed dose distributions.

RATIONALE AND OBJECTIVES: Magnetic resonance imaging (MRI) techniques seem to be very promising for 3D dosimetry studies, but long imaging acquisition time limits their use. A new fast T1 mapping protocol, easy to implement on a conventional MR imager, has been used to determine dose distributions on Fricke gels. METHODS: The method has been tested on manganese chloride (MnCl2) doped ferrous gelatin gels. The T1 measuring times range from 1 minute 40 seconds to 3 minutes 30 seconds for a 256x256 matrix image. RESULTS: The two- and three-dimensional profiles agree with those obtained with conventional dosimetry techniques (ion chambers). The precision and the spatial resolution principally depend on the signal-to-noise ratio of the used imaging RF coil. For example, for a surface coil, the accuracy is about 2.5% with a 1.56 mm spatial resolution. CONCLUSION: These preliminary results support the feasibility of the proposed technique for accurate MRI dosimetry studies and also have potential for various clinical quantitative MRI applications.