The ability of the shoot apical meristem to multiply and distribute its meristematic potential through the formation of axillary meristems is essential for the diversity of forms and growth habits of higher plants. In the lateral suppressor mutant of tomato the initiation of axillary meristems is prevented, thus offering the unique opportunity to study the molecular mechanisms underlying this important function of the shoot apical meristem. We report here the isolation of the Lateral suppressor gene by positional cloning and show that the mutant phenotype is caused by a complete loss of function of a new member of the VHIID family of plant regulatory proteins.

A 57-kb region of tomato chromosome 7 harboring five different genes was compared with the sequence of the Arabidopsis genome to search for microsynteny between the genomes of these two species. For all five genes, homologous sequences could be identified in a 30-kb region located on Arabidopsis chromosome 1. Only two inversion events distinguish the arrangement of the five genes in tomato from that in Arabidopsis. Inversions were not detected when the arrangement of the five Arabidopsis genes was compared with the arrangement in the orthologous region of Capsella, a plant closely related to Arabidopsis. These results provide evidence for microcolinearity between closely and distantly related dicotyledonous species. The degree of microcolinearity found can be exploited to localize orthologous genes in Arabidopsis and tomato in an unambiguous way. INTRODUCTIONArabidopsis, a small crucifer, has been adopted as a model in plant genome analysis. The small genome size of 125 Mbp and a low number of repetitive elements facilitated the assembly of comprehensive molecular marker and clone contig maps for the five Arabidopsis chromosomes (reviewed in Schmidt, 1998). Sequence analysis of the genome has been completed (The Arabidopsis Genome Initiative, 2000). The 430-Mbp rice genome is a model for monocotyledonous plants, and the rice genome project aims to decipher the entire genomic sequence for this species (Sasaki and Burr, 2000). Equally detailed studies are not feasible for many other plant genomes at present, especially given the large genome sizes of most crop plants. It needs to be established if and how information generated on the Arabidopsis and rice genomes can be used for the study of other plant genomes.Comparative genetic mapping experiments yielded evidence for the conservation of gene repertoire and colinear chromosome segments for related species. An extensive conservation of marker order was found for the 12 tomato and potato chromosomes, and five chromosomal inversions could explain differences in marker organization (Tanksley et al., 1992). For the Poaceae family, a remarkable degree of genome conservation could be established even between species that diverged as long as 60 million years ago and that differ considerably in genome size (reviewed in Gale and Devos, 1998). Comparing the genetic maps of Arabidopsis and different Brassica species also has revealed many colinear chromosome segments for species belonging to the Brassicaceae family (reviewed in Schmidt, 2000). The results of the first microsynteny studies using sequencelevel resolution in the Poaceae (Chen et al., 1997;Messing and Llaca, 1998;Tikhonov et al., 1999) and Brassicaceae families (Grant et al., 1998;Acarkan et al., 2000) support the view that genome colinearity can be observed at the level of genes.Few attempts to analyze genome colinearity between more distantly related species have been reported (Paterson et al., 1996;Devos et al., 1999;van Dodeweerd et al., 1999;Ku et al., 2000). The low degree of sequence homology in distantl...

SummaryGenome colinearity has been studied for two closely related diploid species of the Brassicaceae family, Arabidopsis thaliana and Capsella rubella. Markers mapping to chromosome 4 of A. thaliana were found on two linkage groups in Capsella and colinear segments spanning more than 10 cM were revealed. Detailed analysis of a 60 kbp region in A. thaliana and its counterpart in C. rubella showed virtually complete conservation of gene repertoire, order and orientation. The comparison of orthologous genes revealed very similar exon±intron structures and sequence identities of 90% or more were found for exon sequences. This extensive genome colinearity at the genetic and molecular level allows the ef®cient transfer of data from the well-studied A. thaliana genome to other species in the Brassicaceae family, substantially facilitating genome analysis studies for species of this family.

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A 57-kb region of tomato chromosome 7 harboring five different genes was compared with the sequence of the Arabidopsis genome to search for microsynteny between the genomes of these two species. For all five genes, homologous sequences could be identified in a 30-kb region located on Arabidopsis chromosome 1. Only two inversion events distinguish the arrangement of the five genes in tomato from that in Arabidopsis. Inversions were not detected when the arrangement of the five Arabidopsis genes was compared with the arrangement in the orthologous region of Capsella, a plant closely related to Arabidopsis. These results provide evidence for microcolinearity between closely and distantly related dicotyledonous species. The degree of microcolinearity found can be exploited to localize orthologous genes in Arabidopsis and tomato in an unambiguous way.