Physical mapping methods that do not rely on meiotic recombination are necessary for complex polyploid genomes such as wheat (Triticum aestivum L.). This need is due to the uneven distribution of recombination and significant variation in genetic to physical distance ratios. One method that has proven valuable in a number of nonplant and plant systems is radiation hybrid (RH) mapping. This work presents, for the first time, a high-resolution radiation hybrid map of wheat chromosome 1D (D genome) in a tetraploid durum wheat (T. turgidum L., AB genomes) background. An RH panel of 87 lines was used to map 378 molecular markers, which detected 2312 chromosome breaks. The total map distance ranged from $3,341 cR 35,000 for five major linkage groups to 11,773 cR 35,000 for a comprehensive map. The mapping resolution was estimated to be $199 kb/break and provided the starting point for BAC contig alignment. To date, this is the highest resolution that has been obtained by plant RH mapping and serves as a first step for the development of RH resources in wheat.
Aegilops longissima S. & M. and Ae. sharonenis Eig were crossed with Triticum dicoccum Schrank ‘Khapli’ pollen, and F1 hybrids were crossed several times with T. aestivum L. em. Thell. pollen to substitute the T. aestivum nucleus into the cytoplasm of these Aegilops species. Partially male and female‐sterile plants having a monosomic substitution (20″+2′) or a monosomic addition (21″+1′) were obtained from these nuclear‐substitution backcrosses. These plants had normal anther extrusion, abundant pollen, and incomplete scattered seed set on bagged or crossed heads, and the reciprocal cross with euploid T. aestivum as a female gave complete seed set. Euploid, 21” plants were not obtained from crosses in either direction, most offspring having again 20″+2′ or 20″+1′, and having abundant pollen but partial seed set on bagged heads. All the selfed progeny of plants with a monosomic substitution or monosomic addition had a disomic substitution or disomic addition, respectively, and were fertile. Evidently, male and female gametes with one Aegilops chromosome were functional, and those without the Aegilops chromosome did not function.There was thus an apparent gametocidal action of the sporophyte having an Aegilops chromosome, on the gametes lacking this chromosome. Occasionally malesterile euploid T. aestivum plants with Ae. sharonensis cytoplasm were obtained as a result of rare functional female gametes without the critical Ae. sharonensis chromosome.
Radiation hybrid (RH) mapping is based on radiation-induced chromosome breakage and analysis of chromosome segment retention or loss using molecular markers. In durum wheat (Triticum turgidum L., AABB), an alloplasmic durum line [(lo) durum] has been identified with chromosome 1D of T. aestivum L. (AABBDD) carrying the species cytoplasm-specific (scs ae ) gene. The chromosome 1D of this line segregates as a whole without recombination, precluding the use of conventional genome mapping. A radiation hybrid mapping population was developed from a hemizygous (lo) scs ae Ϫ line using 35 krad gamma rays. The analysis of 87 individuals of this population with 39 molecular markers mapped on chromosome 1D revealed 88 radiation-induced breaks in this chromosome. This number of chromosome 1D breaks is eight times higher than the number of previously identified breaks and should result in a 10-fold increase in mapping resolution compared to what was previously possible. The analysis of molecular marker retention in our radiation hybrid mapping panel allowed the localization of scs ae and 8 linked markers on the long arm of chromosome 1D. This constitutes the first report of using RH mapping to localize a gene in wheat and illustrates that this approach is feasible in a species with a large complex genome.
A monosomic set of male‐sterile ‘Chris’ (Triticum aestivum L.) with T. timopheevi Zhuk. cytoplasm was used to determine the chromosomal location of Rf‐genes in six male fertility restoring lines (FR‐lines) of T. aestivum L. The six FR‐lines were: R1‐Lee (Nebraska 542437/T. aestivum ‘Lee’), R2‐Sonora 64 (T. timopheevi/2*T. aestivum ‘Marquis’//‘Sonora 64’), R3 (T. timopheevi/3*T. aestivum ‘Marquis’), R4 (amphidiploid T. timopheevi‐Ae. squarrosa/3*T. aestivum ‘Dirk’), R5 (T. Zhukovsky Men. and Er.,/3*T. aestivum ‘Justin’), and T. aestivum ‘Primepi’. The relative proportions of male‐sterile and male‐fertile plants in the F2 and F3 from monosomic F1 indicated that chromosomes 1A and 7D of R1‐Lee, R2‐Sonora 64, R3, R4, and R5 had Rf‐genes. R2‐Sonora 64 and R5 each had a third Rf‐gene on chromosomes 6B and 7B, respectively. Primepi had Rf‐genes on chromosomes 1B and 5D. The inheritance of fertility restoration in control crosses and in some of the noncritical crosses indicated that R1‐Lee, R2‐Sonora 64, and R5 each had three Rf‐genes, and R3, R4, and Primepi each had two. The deficiency of certain chromosomes influenced the penetrance and expressivity of the Rf‐genes in certain crosses.
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