The vertebrate body plan is established during gastrulation, when cells move inwards to form the mesodermal and endodermal germ layers. Signals from a region of dorsal mesoderm, which is termed the organizer, pattern the body axis by specifying the fates of neighbouring cells. The organizer is itself induced by earlier signals. Although members of the transforming growth factor-beta (TGF-beta) and Wnt families have been implicated in the formation of the organizer, no endogenous signalling molecule is known to be required for this process. Here we report that the zebrafish squint (sqt) and cyclops (cyc) genes have essential, although partly redundant, functions in organizer development and also in the formation of mesoderm and endoderm. We show that the sqt gene encodes a member of the TGF-beta superfamily that is related to mouse nodal. cyc encodes another nodal-related proteins, which is consistent with our genetic evidence that sqt and cyc have overlapping functions. The sqt gene is expressed in a dorsal region of the blastula that includes the extraembryonic yolk syncytial layer (YSL). The YSL has been implicated as a source of signals that induce organizer development and mesendoderm formation. Misexpression of sqt RNA within the embryo or specifically in the YSL induces expanded or ectopic dorsal mesoderm. These results establish an essential role for nodal-related signals in organizer development and mesendoderm formation.

In chordate phylogeny, changes in the nervous system, jaws, and appendages transformed meek filter feeders into fearsome predators. Gene duplication is thought to promote such innovation. Vertebrate ancestors probably had single copies of genes now found in multiple copies in vertebrates and gene maps suggest that this occurred by polyploidization. It has been suggested that one genome duplication event occurred before, and one after the divergence of ray-finned and lobe-finned fishes. Holland et al., however, have argued that because various vertebrates have several HOX clusters, two rounds of duplication occurred before the origin of jawed fishes. Such gene-number data, however, do not distinguish between tandem duplications and polyploidization events, nor whether independent duplications occurred in different lineages. To investigate these matters, we mapped 144 zebrafish genes and compared the resulting map with mammalian maps. Comparison revealed large conserved chromosome segments. Because duplicated chromosome segments in zebrafish often correspond with specific chromosome segments in mammals, it is likely that two polyploidization events occurred prior to the divergence of fish and mammal lineages. This zebrafish gene map will facilitate molecular identification of mutated zebrafish genes, which can suggest functions for human genes known only by sequence.

Transmission of Plasmodium falciparum malaria parasites requires formation and development of gametocytes, yet all but the most mature of these sexual parasite forms are absent from the blood circulation. We performed a systematic organ survey in pediatric cases of fatal malaria to characterize the spatial dynamics of gametocyte development in the human host. Histological studies revealed a niche in the extravascular space of the human bone marrow where gametocytes formed in erythroid precursor cells and underwent development before reentering the circulation. Accumulation of gametocytes in the hematopoietic system of human bone marrow did not rely on cytoadherence to the vasculature as does sequestration of asexual-stage parasites. This suggests a different mechanism for the sequestration of gametocytes that could potentially be exploited to block malaria transmission.

Studies of prokaryotic chromosome replication have focused almost exclusively on organisms with one chromosome. We defined and characterized the origins of replication of the two Vibrio cholerae chromosomes, oriCI(vc) and oriCII(vc). OriCII(vc) differs from the origin assigned by bioinformatic analysis and is unrelated to oriCI(vc). OriCII(vc)-based replication requires an internal 12 base pair repeat and two hypothetical genes that flank oriCII(vc). One of these genes is conserved among diverse genera of the family Vibrionaceae and encodes an origin binding protein. The other gene codes for an RNA and not a protein. OriCII(vc)- but not oriCI(vc)-based replication is negatively regulated by a DNA sequence adjacent to oriCII(vc). There is an unprecedented requirement for DNA adenine methyltransferase in both oriCI(vc)- and oriCII(vc)-based replication. Our studies of replication in V. cholerae indicate that microorganisms having multiple chromosomes may utilize unique mechanisms for the control of replication.