The origin of a zebra chromosome in wheat suggests nonhomologous recombination as a novel mechanism for new chromosome evolution and step changes in chromosome number

An alloplasmic wheat line, TA5536, with the zebra chromosome z5A was isolated front an elymus trachycaulus/Triticum aestivum backcross derivative. This chromosome was named zebra because of its striped genomic m situ hybridization pattern. Its origin was traced to nonhomologous chromosome 5A of wheat and 1H(1) of Elymus; four chromatin segments were derived front chromosome 1H(1) and five chromatin segments including the centromere from 5A. In this study, our objective was to determine the mechanism of origin of chromosome z5A, whether by nonhomologours recombination or by multiple translocation events. Different crossing schemes were used to recover recombinants containing various Elymus chromatin segments of the z5A chromosome. In addition, one z5AL telocentric chromosome and three z5AL isochromosomes were recovered. The dissection of the Elymus segments into different stocks allowed us to determine the chromosomal origin of the different chromosome fragments oil the basis of tire order of the RFLP markers employed and suggested that the zebra chromosome originated Front nonhomologous recombination. We present a model of possible mechanism(s) of chromosome evolution and step changes in chromosome number applicable to a wide range of organisms.