Introgression and Expression of the Aegilops speltoides Ph1 Suppressor (Su1-Ph1) in Tetraploid Wheat

Recombination between homoeologous chromosomes in allotetraploid wheat (Triticum turgidum, genomes AABB) and allohexaploid wheat (T. aestivum, genomes AABBDD) is inhibited by the Ph1 (pairing homoeologous) locus in the B genome. Aegilops speltoides (genomes SS), the closest extant relative of the wheat B-genome progenitor, is polymorphic for the suppression of Ph1 in hybrids between wheat and Ae. speltoides. This polymorphism was exploited in mapping the Ph1 suppressors in Ae. speltoides. Major loci were mapped on Ae. speltoides chromosome arms 3SL (Su1-Ph1) and 7SL (Su2-Ph1). Using molecular markers, Su1-Ph1 was introgressed from Ae. speltoides into hexaploid wheat cv ‘Chinese Spring’ and from it to tetraploid wheat cv ‘Langdon’. Sequential fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) karyotyping of Langdon heterozygous for Su1-Ph1 revealed the presence of a single recombined chromosome 3A with an Ae. speltoides segment replacing the distal portion of the long arm. In plants homozygous for Su1-Ph1, two such chromosomes were present. To test the effects of Su1-Ph1 on homoeologous pairing in tetraploid wheat, a chromosome from Ae. searsii (genomes S^seS^se), a sibling species of Ae. speltoides, was employed as a tester of homoeologous pairing. A double-monosomic substitution line 5S^se(5B)/5B was developed in the Langdon genetic background and was crossed with the Langdon Su1-Ph1 line. The 5S^se chromosome paired in 65 to 90% of pollen mother cells (PMCs) in the presence of Su1-Ph1. In the plants without Su1-Ph1, 5S^se paired in only 7% of PMCs. These findings indicate that Su1-Ph1 had been transferred from Ae. speltoides chromosome arm 3SL into Langdon chromosome arm 3AL and that Su1-Ph1 is active and suppresses the expression of Ph1 in the genetic background of tetraploid wheat.