Nearly finished sequences for model organisms provide a foundation from which to explore genomic diversity among other taxonomic groups. We explore genome-wide microsynteny patterns between the rice sequence and two sorghum physical maps that integrate genetic markers, bacterial artificial chromosome (BAC) fingerprints, and BAC hybridization data. The sorghum maps largely tile a genomic component containing 41% of BACs but 80% of single-copy genes that shows conserved microsynteny with rice and partially tile a nonsyntenic component containing 46% of BACs but only 13% of single-copy genes. The remaining BACs are centromeric (4%) or unassigned (8%). The two genomic components correspond to cytologically discernible ''euchromatin'' and ''heterochromatin.'' Gene and repetitive DNA distributions support this classification. Greater microcolinearity in recombinogenic (euchromatic) than nonrecombinogenic (heterochromatic) regions is consistent with the hypothesis that genomic rearrangements are usually deleterious, thus more likely to persist in nonrecombinogenic regions by virtue of Muller's ratchet. Interchromosomal centromeric rearrangements may have fostered diploidization of a polyploid cereal progenitor. Model plant sequences better guide studies of related genomes in recombinogenic than nonrecombinogenic regions. Bridging of 35 physical gaps in the rice sequence by sorghum BAC contigs illustrates reciprocal benefits of comparative approaches that extend at least across the cereals and perhaps beyond.comparative genomics ͉ Oryza ͉ synteny T he grasses (Poaceae) provide most of mankind's caloric intake and a growing share of our fuel. The best-studied grasses, leading cereal crops, shared a common paleopolyploid ancestor Ϸ42-47 million years ago (mya) (1). Cereals show much colinearity of genetic maps and often have important traits controlled by quantitative trait loci at corresponding locations (2). Despite these similarities, the cereals have diverged remarkably in genome size from Ϸ430 million base pairs (MBP) in rice (3) to 15,966 MBP in wheat (3), largely due to differential repetitive DNA amplification and elimination.As a model for tropical grasses, sorghum [Sorghum bicolor (SB)] is a logical complement to rice (Oryza), in that it has biochemical and morphological specializations to improve carbon assimilation at high temperatures (C4 photosynthesis). By contrast, rice uses C3 photosynthesis more typical of temperate grasses. The Ϸ760-MBP (3) sorghum genome is a logical bridge to the Ϸ2,500-MBP (3) maize genome, and the Ϸ4,000-MBP (3) genome of sugarcane, the world's leading biomass͞biofuels crop. Sorghum shared common ancestry with maize (12 mya) and sugarcane (5 mya), much more recently than rice (42-47 mya). The most recent whole-genome duplication in sorghum appears to be Ϸ70 mya (1) vs. Ϸ12 mya in maize (4) and Ͻ5 mya in sugarcane (5), promising a higher success rate in relating sorghum genes to phenotypes by knockouts than either maize or sugarcane genes. Comparison of SB and closely related Sorghum...

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The plasma flow in Jupiter's magnetosphere observed during the Ulysses flyby is compared with previous Pioneer, Voyager and ground‐based observations. These data show that near‐rigid corotation is enforced in the inner magnetosphere, but that the azimuthal flow plateaus at 150–300 km s−1 in the middle magnetosphere plasma sheet beyond ∼20 RJ. Such flows extend through the prenoon plasma sheet to ∼45 RJ in the compressed magnetosphere observed by the Voyagers and to ∼70 RJ in the expanded system observed by Ulysses. Higher speeds of ∼500 km s−1 occur in the postmidnight plasma sheet at 75–125 RJ in Voyager data, while preliminary Ulysses evidence is presented for anticorotation in the dusk plasma sheet beyond ∼50 RJ. In the outer magnetosphere the dawn and dusk flank flows are antisunward at several hundred kilometres per second, while in the prenoon sector the flow appears to depend magnetospheric state, being corotational at 250–600 km s−1 when compressed and anticorotational (and radially in) at ∼250 km s−1 when expanded. These observations are compared with theories proposed by Hill and Vasyliunas, augmented to include the effects of solar wind coupling. This model accounts qualitatively for many features, but not for the anticorotation flows observed. We also compare the flows with the bending of the magnetic field out of meridian planes. Given the subcorotation‐anticorotation nature of the observed flow, a pervasive “lagging” configuration is expected. This accords with in situ field data, except for the dusk outer magnetosphere observed by Ulysses, where a “leading” configuration was observed in the presence of subcorotating (downtail) flow. We conclude that field bending due to the tail‐magnetopause current system dominates that due to ionospheric coupling on the dusk flank.

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