Multiscale assessment of patterns of avian species richness

The search for a common cause of species richness gradients has spawned more than 100 explanatory hypotheses in just the past two decades. Despite recent conceptual advances, further refinement of the most plausible models has been stifled by the difficulty of compiling high-resolution databases at continental scales. We used a database of the geographic ranges of 2,869 species of birds breeding in South America (nearly a third of the world's living avian species) to explore the influence of climate, quadrat area, ecosystem diversity, and topography on species richness gradients at 10 spatial scales (quadrat area, ≈12,300 to ≈1,225,000 km2). Topography, precipitation, topography × latitude, ecosystem diversity, and cloud cover emerged as the most important predictors of regional variability of species richness in regression models incorporating 16 independent variables, although ranking of variables depended on spatial scale. Direct measures of ambient energy such as mean and maximum temperature were of ancillary importance. Species richness values for 1° × 1° latitude-longitude quadrats in the Andes (peaking at 845 species) were ≈30–250% greater than those recorded at equivalent latitudes in the central Amazon basin. These findings reflect the extraordinary abundance of species associated with humid montane regions at equatorial latitudes and the importance of orography in avian speciation. In a broader context, our data reinforce the hypothesis that terrestrial species richness from the equator to the poles is ultimately governed by a synergism between climate and coarse-scale topographic heterogeneity.

Cell cycle-regulated nuclear import and export of Cdc47, a protein essential for initiation of DNA replication in budding yeast.

The CDC47 gene was isolated by complementation of a cdc47 temperature-sensitive mutant in Saccharomyces cerevisiae and was shown to encode a predicted polypeptide, Cdc47, of 845 aa. Cdc47 belongs to the Cdc46/Mcm family of proteins, previously shown to be essential for initiation of DNA replication. Using indirect immunofluorescence microscopy and subcellular fractionation techniques, we show that Cdc47 undergoes cell cycle-regulated changes in its subcellular localization. At mitosis, Cdc47 enters the nucleus, where it remains until soon after the initiation of DNA replication, when it is rapidly exported back into the cytoplasm. Cdc47 protein levels do not vary with the cell cycle, but expression of CDC47 and nascent synthesis of Cdc47 occur late in the cell cycle, coinciding with mitosis. Together, these results show that Cdc47 is not only imported into the nucleus at the end of mitosis but is also exported back into the cytoplasm at the beginning of S phase. The observation that Cdc47 is exported from the nucleus at the beginning of S phase has important implications for how initiation of DNA replication is controlled.