Genetic and physical mapping of chromosome 4E of tall wheatgrass, Thinopyrum elongatum, and, Molecular detection of rDNA sequences indicates a novel genome donor in the polyploid genus Thinopyrum

When present as a disomic addition in wheat, chromosome 4E of the wheatgrass Thinopyrum elongatum has the unique ability to promote meristem survival and new tiller outgrowth following whole plant senescence, a trait termed post-sexual cycle regrowth or PSCR. Comprehensive genetic and physical mapping of 4E was undertaken to better characterize this chromosome and to locate the region occupied by the PSCR gene(s). Fifty-six CAPS and AFLP markers were placed on the resulting maps. Twenty-five markers were placed in a linkage map based on a cross between amphiploids of both Th. elongatum and its relative Th. scirpeum. Thirty-one markers were assigned to a physical map of 4E, composed of seven chromosomal bins based on chromosomal deletions and translocations induced by Aegliops cylindrica chromosome 2C. Three unique translocations of 4E chromatin onto wheat chromosomes were produced in this study. Deletion line phenotypes implicate the centromeric region of the 4E short arm as critical for the PSCR response. In an effort to elucidate relationships in this complex genus, additional studies on the nature and inheritance of rDNA sequences in Thinopyrum spp. were performed, through sequence analysis and real-time PCR. Results revealed a unique sequence present in Th. intermedium and Th. ponticum that has homology to the annual wild grass Crithopsis delileana, which could be a previously unidentified ancestor of these Thinopyrum polyploids. The implication of this annual grass in the evolution of a uniformly perennial genus may provide tools to better study and map genes involved in determining the differences between annual and perennial life strategies.