The genome of the japonica subspecies of rice, an important cereal and model monocot, was sequenced and assembled by whole-genome shotgun sequencing. The assembled sequence covers 93% of the 420-megabase genome. Gene predictions on the assembled sequence suggest that the genome contains 32,000 to 50,000 genes. Homologs of 98% of the known maize, wheat, and barley proteins are found in rice. Synteny and gene homology between rice and the other cereal genomes are extensive, whereas synteny with Arabidopsis is limited. Assignment of candidate rice orthologs to Arabidopsis genes is possible in many cases. The rice genome sequence provides a foundation for the improvement of cereals, our most important crops.

We have produced a draft sequence of the rice genome for the most widely cultivated subspecies in China, Oryza sativa L. ssp. indica, by whole-genome shotgun sequencing. The genome was 466 megabases in size, with an estimated 46,022 to 55,615 genes. Functional coverage in the assembled sequences was 92.0%. About 42.2% of the genome was in exact 20-nucleotide oligomer repeats, and most of the transposons were in the intergenic regions between genes. Although 80.6% of predicted Arabidopsis thaliana genes had a homolog in rice, only 49.4% of predicted rice genes had a homolog in A. thaliana. The large proportion of rice genes with no recognizable homologs is due to a gradient in the GC content of rice coding sequences.

FOUNDATIONS OF COMPARATIVE GENOMICSComparative genomics, the study of the similarities and differences in structure and function of hereditary information across taxa, uses molecular tools to investigate many notions that long preceded identification of DNA as the hereditary molecule. Vavilov's (1922) law of homologous series in variation was an early suggestion of the similarities in the genetic blueprints of many (plant) species. Genetic analysis based on morphological and isoenzyme markers hinted at parallel arrangements of genes along the chromosomes of various taxa. These hints were later borne out at the DNA level, in seminal investigations of nightshades (Tanksley et al., 1988; Bonierbale et al., 1988) and grasses (Hulbert et al., 1990).Over the past two decades, multiple investigations of many additional taxa have delivered two broad messages: (1) In most plants, the evolution of the small but essential portion of the genome that actually encodes the organism's genes has proceeded relatively slowly; as a result, taxa that have been reproductively isolated for millions of years have retained recognizable intragenic DNA sequences as well as similar arrangements of genes along the chromosomes. (2) A wide range of factors, such as ancient chromosomal or segmental duplications, mobility of DNA sequences, gene deletion, and localized rearrangements, has been superimposed on the relatively slow tempo of chromosomal evolution and causes many deviations from colinearity. COMPARATIVE MAPS IN CROP TAXA The BrassicaceaeThe Brassicaceae comprise 360 genera, organized into 13 tribes (Shultz, 1936; Al-Shehbaz, 1973). The most economically important species are in the genus Brassica (tribe Brassiceae), six species of which are cultivated worldwide.

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