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Leo1 is a component of the Polymerase-Associated Factor 1 (PAF1) complex, an evolutionarily conserved protein complex involved in gene transcription regulation and chromatin remodeling. The role of leo1 in vertebrate embryogenesis has not previously been examined. Here, we report that zebrafish leo1 encodes a nuclear protein that has a similar molecular structure to Leo1 proteins from other species. From a genetic screen, we identified a zebrafish mutant defective in the leo1 gene. The truncated Leo1(LA1186) protein lacks a nuclear localization signal and is distributed mostly in the cytoplasm. Phenotypic analysis showed that while the initial patterning of the primitive heart tube is not affected in leo1(LA1186) mutant embryos, the differentiation of cardiomyocytes at the atrioventricular boundary is aberrant, suggesting a requirement for Leo1 in cardiac differentiation. In addition, the expression levels of markers for neural crest-derived cells such as crestin, gch2, dct and mitfa are greatly reduced in leo1(LA1186) mutants, indicating a requirement for Leo1 in maintaining the neural crest population. Consistent with this finding, melanocyte and xanthophore populations are severely reduced, craniofacial cartilage is barely detectable, and mbp-positive glial cells are absent in leo1(LA1186) mutants after three days of development. Taken together, these results provide the first genetic evidence of the requirement for Leo1 in the development of the heart and neural crest cell populations. (C) 2010 Elsevier Inc. All rights reserved.

Ninety-eight satellite breakup events have resulted in 7458 trackable objects being cataloged by the Space Surveillance Network (SSN), of which 2940 are still in orbit. The vast majority (96%) of this debris resides in altitudes with orbital periods below 127 min. The remnants from these fragmentations now account for 44% of the total cataloged population (6567) and 57% of the low-Earth-orbit (LEO) population. The accurate assessment of the effects of breakups can only be performed by looking at all aspects of on-orbit population growth. The total population is divided into lower LEO (LEO1), upper LEO (LEO2), high-Earth-orbit (HEO), and geosynchronous orbit (GEO) regimes. Most of these subsets of the total population have individually exhibited linear growth rates combining to result in a catalog population increase of 240/yr. The debris generated by satellite breakups is the most variable portion of the population due to the randomly spaced large additions by these events and cyclic cleansing by solar activity. Cessation of fragmentations can significantly improve our LEO environment as it already has in the 1000-2000 km (LEO2) region. An analysis of objects in geosynchronous transfer orbits (GTO) shows that there is a great dependence on the inclination and argument of perigee of a GTO satellite to the hazard it poses to GEO satellites. (All data are current as of December 8, 1989.)