Comparative genomics of plant chromosomes.

Comparative 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.