Haldane's rule in an avian system: using cline theory and divergence population genetics to test for differential introgression of mitochondrial, autosomal, and sex-linked loci across the Passerina bunting hybrid zone

Using cline fitting and divergence population genetics, we tested a prediction of Haldane's rule: autosomal alleles should introgress more than z-linked alleles or mitochondrial haplotypes across the Passerina amoena/Passerina cyanea (Aves: Cardinalidae) hybrid zone. We screened 222 individuals collected along a transect in the Great Plains of North America that spans the contact zone for mitochondrial (two genes), autosomal (four loci) and z-linked (two loci) markers. Maximum-likelihood cline widths estimated from the mitochondrial (223 km) and z-linked (309 km) datasets were significantly narrower on average than the autosomal cline widths (466 km). We also found that mean coalescent-based estimates of introgression were larger for the autosomal loci (0.63 genes/generation, scaled to the mutation rate mu) than for both the mitochondrial (0.27) and z-linked loci (0.59). These patterns are consistent with Haldane's rule, but the among-locus variation also suggests many independently segregating loci are required to investigate introgression patterns across the genome. These results provide the first comprehensive comparison of mitochondrial, sex-linked, and autosomal loci across an avian hybrid zone and add to the body of evidence suggesting that sex chromosomes play an important role in the formation and maintenance of reproductive isolation between closely related species.     

Non-cell-autonomous effects of Ret deletion in early enteric neurogenesis

Neural crest cells (NCCs) form at the dorsal margin of the neural tube and migrate along distinct pathways throughout the vertebrate embryo to generate multiple cell types. A subpopulation of vagal NCCs invades the foregut and colonises the entire gastrointestinal tract to form the enteric nervous system (ENS). The colonisation of embryonic gut by NCCs has been studied extensively in chick embryos, and genetic studies in mice have identified genes crucial for ENS development, including Ret. Here, we have combined mouse embryo and organotypic gut culture to monitor and experimentally manipulate the progenitors of the ENS. Using this system, we demonstrate that lineally marked intestinal ENS progenitors from E11.5 mouse embryos grafted into the early vagal NCC pathway of E8.5 embryos colonise the entire length of the gastrointestinal tract. By contrast, similar progenitors transplanted into Ret-deficient host embryos are restricted to the proximal foregut. Our findings establish an experimental system that can be used to explore the interactions of NCCs with their cellular environment and reveal a previously unrecognised non-cell-autonomous effect of Ret deletion on ENS development.     

Signalling between the hindbrain and paraxial tissues dictates neural crest migration pathways.

Cranial neural crest cells are a pluripotent population of cells derived from the neural tube that migrate into the branchial arches to generate the distinctive bone, connective tissue and peripheral nervous system components characteristic of the vertebrate head. The highly conserved segmental organisation of the vertebrate hindbrain plays an important role in patterning the pathways of neural crest cell migration and in generating the distinct or separate streams of crest cells that form unique structures in each arch. We have used focal injections of DiI into the developing mouse hindbrain in combination with in vitro whole embryo culture to map the patterns of cranial neural crest cell migration into the developing branchial arches. Our results show that mouse hindbrain-derived neural crest cells migrate in three segregated streams adjacent to the even-numbered rhombomeres into the branchial arches, and each stream contains contributions of cells from three rhombomeres in a pattern very similar to that observed in the chick embryo. There are clear neural crest-free zones adjacent to r3 and r5. Furthermore, using grafting and lineage-tracing techniques in cultured mouse embryos to investigate the differential ability of odd and even-numbered segments to generate neural crest cells, we find that odd and even segments have an intrinsic ability to produce equivalent numbers of neural crest cells. This implies that inter-rhombomeric signalling is less important than combinatorial interactions between the hindbrain and the adjacent arch environment in specific regions, in the process of restricting the generation and migration of neural crest cells. This creates crest-free territories and suggests that tissue interactions established during development and patterning of the branchial arches may set up signals that the neural plate is primed to interpret during the progressive events leading to the delamination and migration of neural crest cells. Using interspecies grafting experiments between mouse and chick embryos, we have shown that this process forms part of a conserved mechanism for generating neural crest-free zones and contributing to the separation of migrating crest populations with distinct Hox expression during vertebrate head development.     

Novel bacteriolytic activity associated with the style microflora of the mussel Mytilus edulis (L.)

Previous studies have shown that bacterioplankton are retained by the mussel Mytilus edulis (L.), and that lysozyme-like enzymes are associated with the crystalline style of the mussels. This study has demonstrated the presence of a lytic agent which is produced by bacteria associated with the crystalline style. The production of the agent is inhibited by chloramphenicol, and anaerobic growth conditions. The agent is heat-sensitive and nonfilterable. Attempts at the isolation of the agent in a cell-free extract have been unsuccessful. The presence of bacteria-like cells in the outer laminae of the crystalline style, coupled with the nonfilterable nature of the lytic agent, make it seem possible that these bacteria are responsible for lysis of gram-negative bacteria which are taken in with the food supply.     

Concerns of patients,MDs are transforming transfusion medicine.

Fear of HIV and AIDS has been the driving force in reducing physicians'' use of blood and blood products. Nancy Robb interviewed doctors across the country to determine steps they are taking to lower the number of transfusions and discovered that transfusion medicine in Canada has undergone a sea change.