Lrk (Lr10 receptor-like kinase) and Tak ( Triticum aestivum kinase) belong to the receptor-like kinase (RLK) supergene family in higher plants. Three Lrk / Tak gene regions spanning greater than 600 kb were identified via a genome-wide survey of barley gene-rich BAC clones. Two Lrk / Tak gene clusters are positioned on barley chromosome 3 (3H) and another is localized on chromosome 5 (1H), with each Lrk and Tak open reading frame physically positioned in a back-to-back orientation. Thirteen new Lrk / Tak -like fragments were cloned from the two clusters on 3H and the single cluster on 1H, respectively, and compared phylogenetically with other grass Lrk / Tak -like genes, including a 280-kb Lrk / Tak cluster on rice chromosome 1S. Physically clustered Lrk / Tak -like genes always form monophyletic groups; this suggests that the primary mechanism of expansion of the Lrk / Tak RLK super family was by tandem duplication, of which most members were duplicated after speciation of the Poaceae. Cultivar-dependent transcript accumulation of some Lrk / Tak family members on 3H, as revealed via Barley1 GeneChip microarray analysis, is consistent with the hypothesis of subfunctionalization of Lrk/Tak members following tandem duplication.
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