Genomic Instability in Wheat Induced by Chromosome 6 B " of Triticum speltoides Rama

A massive restructuring of chromosomes was observed during the production of a substitution of chromosome 68" from Triticum speltoides (Tausch) Gren. ex Richter for chromosome 6B of Chinese Spring wheat (Triticum aestivum L.). Deletions, translocations, ring chromosomes, dicentric chromosomes and a paracentric inversion were observed. Chromosome rearrangements occurred in both euchromatic and heterochromatic regions. Chromosome rearrangements were not observed either in the amphiploid between Chinese Spring and T. speltoides or in Chinese Spring. No chromosome rearrangements were observed in the backcross derivatives; however, after self-pollination of a monosomic substitution ( 2 n = 41) of chromosome 68" for wheat chromosome 6B, 49 of the 138 plants carried chromosome aberrations. Chromosome rearrangements were observed in both wheat and T. speltoides chromosomes. The frequency of chromosome rearrangements was high among the Bgenome chromosomes, moderate among the A-genome chromosomes, and low among the D-genome chromosomes. In the B genome, the rearrangements were nonrandom, occurring most frequently in chromosomes 1B and 5B. Chromosome rearrangements were also frequent for the 68" chromosome of T. speltoides. An intriguing aspect of these observations is that they indicate that wheat genomes can be subject to uneven rates of structural chromosome differentiation in spite of being in the same nucleus. B READ wheat owes its origin to interspecific hybridization of Triticum urartu Thum. (AA genomes) (KONAREV et al. 1978; NISHIKAWA 1983; DVORAK, MCGUIRE and CASSIDY 1988), Triticum speltoides (Tausch) Gren. ex Richter (B'B' genomes, usually designated S S ) or some other very closely related species of section Sitopsis (SARKAR and STEBBINS 1956) and Triticum tauschii Coss. (KIHARA 1944; MCFADDEN and SEARS 1946). Bread wheat, therefore, contains three differentiated genomes. CHEN and DVORAK (1984) selected an inbred line of T. speltoides that lacked the ability to suppress the wheat Phl gene which prevents heterogenetic chromosome pairing in polyploid wheat. A hybrid with Triticum aestivum cv. Chinese Spring that had essentially the same level of homoeologous pairing as wheat haploids was treated with colchicine and a 56-chromosome amphiploid was obtained. This amphiploid was used in substituting T. speltoides chromosomes for homoeologous chromosomes of Chinese Spring wheat according to the modified technique of KOTA and DVORAK (1 985). In advanced backcross populations, a massive restructuring of both wheat and T. speltoides chromosomes was observed. This paper describes this phenomenon and its chromosomal control. MATERIALS AND METHODS Materials: The original hybrid was produced by crossing the selected T. speltoides inbred line of CHEN and DVORAK Genetics 120: 1085-1094 (December, 1988) (1984) as a male parent with Chinese Spring wheat. Hybrid embryos were cultured on a modified B-5 medium (DVORAK 198 1). The amphiploid was obtained by doubling the chromosome number of the hybrid by immersing the crown in a 0.25% colchicine solution for 5 h (JENSEN 1974). To substitute T. speltoides chromosomes for wheat homoeologous chromosomes the amphiploid was recurrently backcrossed with selected Chinese Spring monotelosomics. Each monotelosomic was developed by crossing a ditelosomic with the monosomic for the same chromosome and selecting a monotelosomic progeny. Seeds of Chinese Spring and ditelosomic and monosomic stocks were provided by E. R. SEARS, University of Missouri, Columbia. T o determine the chromosome constitution of plants during backcrossing, root tips were cut from 2-day-old seedlings germinated in Petri dishes in the dark. One root tip per plant was used for determining its chromosome number and the two or three remaining roots were used for Cbanding analysis. For determination of chromosome number, root tips were pretreated at 2" for 24 hr in distilled water and fixed in 3: 1 ethanokacetic acid (v/v). These root tips were then stained in Schiff s reagent. The root tips for C-banding analysis were pretreated at 2" for 17 hr and fixed in 45% acetic acid. The C-bands were revealed according to a procedure described by DVORAK and APPELS (1 982). Nomenclature: The T. urartu genome is designated A and T. tauschii genome D. The designation of T. speltoides genome E differs from the customary designation S. This is done to emphasize its relationship to the wheat B-genome. Chromosomes for which homoeology is known are designated by arabic numerals specifying the homoeologous group followed by capital letter, specifying the genome of origin. The genomic designation of chromosome 4A and 4B are switched as proposed by DVORAK (1983). Chromosome 1086 R. S. Kota andJ. Dvorak