Genetic segregation and the detection of spontaneous wheat-alien translocations

Robertsonian translocation between homoeologous chromosomes is one means for introducing alien genes from related species into common wheat. This study was undertaken to determine if selection for normal segregation of an alien trait could be used to identify favourable translocations. Agrotana (an octoploid agrotriticum, probably derived from Thinopyrum ponticum) carries resistance to the wheat curl mite (Eriophyes tulipae Keifer) which is the vector of wheat streak mosaic virus. Agrotana was crossed with the winter durum Michurinka (Triticum turgidum L. em. Bowden), and resistant F₂ plants were backcrossed four times with Norstar winter wheat (T. aestivum L. em. Thell.). During the later stages of backcrossing and selfing, resistant progenies were tested for their segregation ratios; selection was made for high and stable rates of transmission of mite resistance in segregating male gametes and zygotes. The 49 descendant gametes were tested for Mendelian-like inheritance of resistance. Two lines were identified with segregations that approached Mendelian ratios (1:1 male transmission in a testcross, 3:1 in F₂ and 1:2:1 among F₃ families). Contrary to expectations, selection for stable F₂ ratios (3:1) proved more efficient than selection for normal male transmission (1:1) as a means of identifying these stocks. Chromosome painting and PCR (polymerase chain reaction) analysis indicated the recovery of two independent Robertsonian translocations, both of which probably involved the reunion of the short arm of a group 6 chromosome of Thinopyrum ponticum and the long arm of chromosome 6D. In conclusion, selection for Mendelian ratios from among backcross derivatives of an interspecific hybrid was successful in isolating desirable translocations; cytogenetic methods were only used to characterise desirable stocks once these were identified. This revised version was published online in August 2006 with corrections to the Cover Date.