The tetraploid AABB genomic component of two varieties of common hexaploid wheat (AABBDD; 2n = 42) was reconstituted by a simple back-crossing technique in which known phylogenetic relationships between the hexaploid and tetraploid groups of Triticum were used. The reconstituted tetraploids do not closely resemble commonly described varieties of the present-day group of tetraploid species. The plants are dwarfed, lack vigor, and are partially or completely self-sterile, depending on the variety of the hexaploid source. Chromosome pairing is similar to that observed in a variety of durum wheat (AABB; 2n = 28). Synthetic hexaploids derived from hybrids between one of the reconstituted tetraploids and several strains of Aegilops squarrosa (D genome) are of normal growth and vigr and are highly fertile.

Telocentrics of hexaploid wheat, Triticum aestivum spp. vulgare CV. Chinese Spring, were used to establish the chromosome arm location and crossover distance from the centromere of genes controlling characters introduced into synthetic hexaploid wheat (2n = 42 = AABBDD) from Aegilops squarrosa (2n = 14 = DD). The chromosome arm location and the crossover distance from the centromere of each gene studied are as follows: synthetic hexaploid RL 5404 -brown glumes ( R g 2 ) , IDL, 13.3 2 3.3%; tenacious glumes ( T g ) , 2Da, 39.4 2 4.9%; inhibitor of waxy foliage ( W 2 ' ) , 2Da, 52.5 + 5.0%; adult-plant leaf rust resistance (Lr22), 2Da, 63.6 f 4.8%; purple coleoptile ( R c 3 ) , 7DS, 10.3 2 2.8%; synthetic hexaploid RL 5406 -Rg2, lDL, 1.7 2 1.0%; Tg, 2Da, 42.9 f 4.6%; W2', 2Da, 58.9 -C 4.6%; R d , 7DS, 9.8 f 2.8%. A gene for seedling leaf rust resistance (Lr21) found in RL 5406 is located on chromosome ID.

All 15 hexaploid wheats (2n = 42 = AABBDD) synthesized from various combinations of nine tetraploid wheats (2n = 28 = AABB) and seven forms of Aegilops squarrosa L. (2n = 14 = D D ) were non-free-threshing, regardless of the presence o r absence of the Q factor. Monosomic and telosomic analysis of synthetic hexaploids RL 5404 and RL 5406, produced from crosses of Tetra Canthatch (the AABB component extracted from the common wheat cultivar Canthatch) with two forms of Ae. squarrosa, revealed the presence of a partially dominant gene for tenacious glumes, Tg, on chromosome 2Da. This gene, derived from the squarrosa parent, inhibited the expression of Q located on chromosome 5A. The recessive allele tg as well as Q must be present for the expression of the free-threshing character in hexaploid wheat. On the assumption that Ae. squarrosa of the past ~pssessed Tg, as apparently do all extant forms, it is hypothesized that the primitive hexaploid progenitor of free-threshing hexaploid wheat also carried this gene and, therefore, was non-free-threshing. The mutation from Tg to tg is presumed to have occurred at the hexaploid level.L. or its wild form T. boeticum Soiss. (2n = 14 = AA), combined with a second diploid species having the B genome to produce the tetraploid wheats (2n = 28 = AABB). Second, tetraploid wheat hybridized with a third diploid, Aegilops squmrosa L. (2n = 14 = DD), to produce the first hexaploid wheat (2n = 42 = AABBDD). The source of the B genome remains controversial. Until recently, the evidence supported Aegilops speltoides Tausch as the contributor of this genome. Now, however, doubt has been expressed that this species is indeed the donor of the B set of chromosomes to the polyploid wheats (Johnson, 1972;Kimber and Athwal, 1972). Since their origin the tetraploid and hexaploid groups have diverged into numerous distinct morphological types.A very significant sequence of changes in transition from the primitive to the cultivated forms of Triticum were those associated with seed dissemination. These included rachis fragility, spikelet articulation, awn development, pubescence, grain size, and glume tenacity or threshability. Of these, one that has been extensively investigated, because of both its evolutionary significance and its importance in the practical utilization of wheat grain, is that of the free-threshing habit. Both freeand non-free-threshing forms occur in the tetraploid and hexaploid groups, while all =Joint contribution from the Department of Plant Science, University of Manitoba (Contribution No. 381) and Canada Agriculture, Research Station. 25 Dafoe Road, Winnipeg, Manitoba, R3T 2 v 9 (Contribution No. 587). From a thesis submitted by the junior author to the Faculty of Graduate Stud~es, University of Manitoba, in partial fulfilment of the requirements of the Doctor of Philosophy Degree. 2Present address:

T h e inheritance of seedling leaf rust resistance and several morphological characters derived from Aegilops sqzlnrrosn (211 = 14 = D D ) was investigated in a synthetic hexaploid wheat. T h c hexaploid was obtained by combining the tetraploid component (211 = 28 = A A B R ) extracted from thc colnmon whcat cultivar Canthatch with Ae. sqzlnrrosn 1 ar. nreyeri R.L. 5289.A major, partially dominant gene was identified that gives good resistance (typc 0;l rcaction) t o leaf rust races 1, 5, 9, 11, 15, 30, 58 and 126a. This gene was shown to bc diffcrent from the resistance genes L r l , Lr2, Lr5, LrlO, Lr16, Lr17 and LrlR. A minor second gene was also detected which gives resistance (typc 2 rcaction) to race 9 and slight resistance t o some of the other races. Each of the characters purple coleoptile, non-waxy foliage, brown glumes, and non-frce threshi~lg (tenacious glumes) of the synthetic wheat was monogenitally inhcritcd. T h c gcnc for threshability may be different from other genetic systcxns known t o affect this character. T h e gene for brown glumes was linked with thc n~a j o r gene for leaf rust resistance with a recombination value of 1.1 * 1.1%. Tllc genes for non-waxy foliage and non-free threshing were associated with an estimated linkage value of 15.1 * 2.6%.T h e results effcctivcly demonstrated thc relative ease with which genetic variation may bc incorporated into common hexaploid wheat from its ancestral diploid, Ae. sqmrrrosn, by means of a synthetic hexaploid intermediary. T h e n~cthod avoids thc difficultics and complications often encountered with the transfer of genes from morc distantly related species which d o not 11a\~e a genome in common with T. crestivrlt~~.

A partially dominant gene for adult-plant leaf rust resistance together with a linked, partially dominant gene for stem rust resistance were transferred to the hexaploid wheat cultivar 'Marquis' from an amphiploid of Aegilops speltoides × Triticum monococcum by direct crossing and backcrossing. Pathological evidence indicated that the alien resistance genes were derived from Ae. speltoides. Differential transmission of the resistance genes through the male gametes occurred in hexaploid hybrids involving the resistant 'Marquis' stock and resulted in distorted segregation ratios. In heterozygotes, pairing between the chromosome arm with the alien segment and the corresponding arm of the normal wheat chromosome was greatly reduced. The apparent close linkage between the two resistance genes, 3 ± 1.07 crossover units, was misleading because of this decrease in pairing in the presence of the 5B diploidizing mechanism. The newly identified gene for adult-plant leaf rust resistance, located on chromosome 2B, is different from adult-plant resistance genes Lr12, Lr13, and Lr22 and from that in the hexaploid accession PI250413; it has been designated Lr35. It is not known whether the newly transferred gene for stem rust resistance differs from Sr32, also derived from Ae. speltoides and located on chromosomes 2B.Key words: hexaploid, Triticum, Aegilops, aneuploid, Puccinia graminis, Puccinia recondita.