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Although bisexual reproduction has proven to be highly successful, parthenogenetic all-female populations occur frequently in certain taxa, including the whiptail lizards of the genus Aspidoscelis. Allozyme analysis revealed a high degree of fixed heterozygosity in these parthenogenetic species (1,2), supporting the view that they originated from hybridization events between related sexual species. It has remained unclear how the meiotic program is altered to produce diploid eggs while maintaining heterozygosity. Here we show that meiosis commences with twice the number of chromosomes in parthenogenetic versus sexual species, a mechanism that provides the basis for generating gametes with unreduced chromosome content without fundamental deviation from the classic meiotic program. Our observation of synaptonemal complexes and chiasmata demonstrate that a typical meiotic program occurs and that heterozygosity is not maintained by bypassing recombination. Instead, fluorescent in situ hybridization probes that distinguish between homologues reveal that bivalents form between sister chromosomes, the genetically identical products of the first of two premeiotic replication cycles. Sister chromosome pairing provides a mechanism for the maintenance of heterozygosity, which is critical for offsetting the reduced fitness associated with the lack of genetic diversity in parthenogenetic species., True parthenogenesis, characterized by the complete absence of male contributions, has been described for various species of reptiles, including whiptail lizards, geckos, blind snakes and rock lizards (3). Whiptail lizards [...]

Haematopoietic stem cell (HSC) niches, although proposed decades Ago (1), have only recently been identified as separate osteoblastic and vascular microenvironments (2-6). Their interrelationships and interactions with HSCs in vivo [...]

Nature 457, 97-101 (2009) In this Letter, affiliations 1 and 2 were in the wrong order. The correct affiliations are as shown here., Author(s): Yucai Xie [sup.1] [sup.2] , Tong Yin [sup.2] , Winfried Wiegraebe [sup.2] , Xi C. He [sup.2] , Diana Miller [sup.3] , Danny Stark [sup.2] , Katherine Perko [sup.2] [...]

In the mammalian brain, similar features of the sensory stimuli are often represented in proximity in the sensory areas. However, how chemical features are represented in the olfactory bulb has been controversial. Questions have been raised as to whether specific chemical features of the odor molecules are represented by spatially clustered olfactory glomeruli. Using a sensitive probe, we have analyzed the glomerular response to large numbers of odorants at single glomerulus resolution. Contrary to the general view, we find that the representation of chemical features is spatially distributed in the olfactory bulb with no discernible chemotopy. Moreover, odor-evoked pattern of activity does not correlate directly with odor structure in general. Despite the lack of spatial clustering or preference with respect to chemical features, some structurally related odors can be similarly represented by ensembles of spatially distributed glomeruli, providing an explanation of their perceptual similarity. Whereas there is no chemotopic organization, and the glomeruli are tuned to odors from multiple classes, we find that the glomeruli are hierarchically arranged into clusters according to their odor-tuning similarity. This tunotopic arrangement provides a framework to understand the spatial organization of the glomeruli that conforms to the organizational principle found in other sensory systems. [ABSTRACT FROM AUTHOR]

Rhythmic production of vertebral precursors, the somites, causes bilateral columns of embryonic segments to form. This process involves a molecular oscillator — the segmentation clock — whose signal is translated into a spatial, periodic pattern by a complex signalling gradient system within the presomitic mesoderm (PSM). In mouse embryos, Wnt signalling has been implicated in both the clock and gradient mechanisms, but how the Wnt pathway can perform these two functions simultaneously remains unclear. Here, we use a yellow fluorescent protein (YFP)-based, real-time imaging system in mouse embryos to demonstrate that clock oscillations are independent of β-catenin protein levels. In contrast, we show that the Wnt-signalling gradient is established through a nuclear β-catenin protein gradient in the posterior PSM. This gradient of nuclear β-catenin defines the size of the oscillatory field and controls key aspects of PSM maturation and segment formation, emphasizing the central role of Wnt signalling in this process. [ABSTRACT FROM AUTHOR]

A correction to the article "Detection of functional haematopoietic stem cell niche using real-time imaging" that was published in the previous issue is presented.