Originally published as Genetics Published Articles Ahead of Print on February 19, 2006.

Genetics, Vol. 172, 2529-2540, April 2006, Copyright © 2006
doi:10.1534/genetics.106.055772

Euchromatin and Pericentromeric Heterochromatin: Comparative Composition in the Tomato Genome

Ying Wang^*,,1, Xiaomin Tang, Zhukuan Cheng, Lukas Mueller^*,, Jim Giovannoni^,** and Steve D. Tanksley^*,,2

^* Department of Plant Breeding and Genetics, Department of Plant Biology, Boyce Thompson Institute for Plant Research and ^** U.S. Department of Agriculture–Agricultural Research Service, Plant, Soil, and Nutrition Lab, Cornell University, Ithaca, New York 14853 and Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, People's Republic of China

² Corresponding author: Department of Plant Breeding and Genetics, 248 Emerson Hall, Cornell University, Ithaca, NY 14853.
E-mail: sdt4{at}cornell.edu

Eleven sequenced BACs were annotated and localized via FISH to tomato pachytene chromosomes providing the first global insights into the compositional differences of euchromatin and pericentromeric heterochromatin in this model dicot species. The results indicate that tomato euchromatin has a gene density (6.7 kb/gene) similar to that of Arabidopsis and rice. Thus, while the euchromatin comprises only 25% of the tomato nuclear DNA, it is sufficient to account for 90% of the estimated 38,000 nontransposon genes that compose the tomato genome. Moreover, euchromatic BACs were largely devoid of transposons or other repetitive elements. In contrast, BACs assigned to the pericentromeric heterochromatin had a gene density 10–100 times lower than that of the euchromatin and are heavily populated by retrotransposons preferential to the heterochromatin—the most abundant transposons belonging to the Jinling Ty3/gypsy-like retrotransposon family. Jinling elements are highly methylated and rarely transcribed. Nonetheless, they have spread throughout the pericentromeric heterochromatin in tomato and wild tomato species fairly recently—well after tomato diverged from potato and other related solanaceous species. The implications of these findings on evolution and on sequencing the genomes of tomato and other solanaceous species are discussed.

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