Three sets of substitution lines of the spring wheat variety Chinese with chromosomes from the donor varieties Thatcher, Hope and Timstein were used to study the genetics of awning, earliness, lodging, plant height, spike density, 1000-kernel weight and yield. The various substitution lines, each representing a genotype that differs from that of the recipient variety only with respect to the genes carried by the substituted chromosome, were studied in replicated field trials so that environmental effects on the character in question could be easily removed by appropriate analysis. This permitted a comparison of the genetic effects of individual chromosomes against the standard based on the performance of a population of like genotypes.Genes conditioning awning were associated with seven chromosomes. Studies of earliness indicated that time of heading is conditioned by (a) major genes that differentiate spring and winter growth habit, and (b) genes that modify the expression of growth habit genes to a greater or lesser extent. Differences in spike density among the lines were due to minor genes only; the same was true for plant height. Lodging, protein content, 1000-kernel weight and yield were found to be conditioned by polymeric or multiple genes on many chromosomes; the effects of these individual genes though small were not usually equal.Where a substituted chromosome brings about a significant departure in character expression from that of the recipient variety, a method is outlined whereby the number of genes on a particular chromosome can be determined. The merits of the substitution method are discussed, and it is concluded that it is valuable, and gives a high degree of precision in genetic studies of polyploid organisms and that under certain conditions its effectiveness is similar to that of the backcross method for incorporating characters controlled by one or two genes into a given line or variety.

The phylogeny of the polyploid wheats has been the subject of intense research and speculation during the past 70 years. Various experimental approaches have been employed to ascertain the diploid progenitors of these wheats. The species having donated the D genome to Triticum aestivum has been unequivocally identified as Aegilops squarrosa. On the basis of evidence from many studies, Triticum monococcum has been implicated as the source of the A genome in both Triticum turgidum and Triticum aestivum. However, numerous studies since 1968 have shown that Triticum urartu is very closely related to Triticum monococcum and that it also carries the A genome. These studies have prompted the speculation that Triticum urartu may be the donor of this chromosome set to the polyploid wheats. The donor of the B genome to Triticum turgidum and Triticum aestivum remains equivocal and controversial. Six different diploid species have been implicated as putative B genome donors: Aegilops bicornis, Aegilops longissima, Aegilops searsii, Aegilops sharonensis, Aegilops speltoides, and Triticum urartu. Until recently, evidence presented by different researchers had not permitted an unequivocal identification of the progenitor of the B genome in polyploid wheats. Recent studies, involving all diploid and polyploid wheats and putative B genome donors, lead to the conclusion that Aegilops speltoides and Triticum urartu can be excluded as B genome donors and that Aegilops searsii is the most likely source of this chromosome set. The possibility of the B genome having arisen from an AAAA autotetraploid or having a polyphyletic origin is discussed. Key words: phylogeny; Triticum aestivum; Triticum turgidum; A, B, and D genomes.

Our investigation of 460 true-breeding lines confirms a long-standing observation that natural phenotypic and genetic variability in the diploid wheat Triticum monococcum L. is limited. The modes of inheritance of 12 morphological characters are discussed in light of the extensive information available on the genetics and cytogenetics of many of these characters in the related wheat Triticum aestivum. Analysis of data from appropriate crosses, complementation studies, and observations of phenotypes of F1s and F2s from crosses between lines expressing dominant traits indicate that each of these characters is determined by one major gene. A multiple allelic series exists at each of the Hg (glume pubescence) and Hn (node pubescence) loci. The genes for six of these characters fall into two closely linked groups. Genes Bg (glume colour) and Hg are the same distance apart as in Triticum aestivum, indicating that at least this segment of chromosome 1A has been highly or completely conserved since the origin of the polyploid wheats. The genes Sg (glume hardness), La (lemma awn length), Fg (false glume), and Lh (head type) are also very closely linked, with the outside markers being only 4 map units apart. The dominant and recessive alleles of genes determining these characters should serve as excellent markers for linkage and chromosomal mapping because of their complete penetrance and constant expressivity. Tentative assignments of genes and linkage groups identified in this investigation to specific chromosomes of T. monococcum have been made on the basis of known chromosomal locations of A genome genes in T. aestivum. The tentative assignments could be verified using a variety of genetic and cytogenetic approaches. It is suggested that a thorough study of the genetic heritage of einkorn wheat will require the use of induced mutants since natural genetic variability is low in this species.Key words: Triticum, characters, inheritance, linkage, mapping, A genome.

To help elucidate the origin of the B genome in polyploid wheats, karyotypes of Triticum turgidum, Triticum monoccum, and all six purported B genome donors were compared. The analysis utilized a common cytological procedure that employed the most advanced equipment for the measurement of chromosome lengths at metaphase in root tip cells. A comparison of the karyotypes of T. turgidum and T. monococcum permitted the identification of B genome chromosomes of T. turgidum. These consist of two SAT pairs, one ST pair, three SM pairs, and one M pair of homologues. Comparisons of the chromosomes of the B genome of T. turgidum with the karyotypes of the six putative B genome donors showed that only the karyotype of Aegilops searsii was similar to the one deduced for the donor of the B genome in T. turgidum, suggesting that Ae. searsii is, therefore, the most likely donor of the B genome to the polyploid wheats. Support for this conclusion has been derived from geographic, DNA-hybridization, karyotype, morphological, and protein data reported since 1977. Reasons why the B genome donor has not been unequivocally identified are discussed.Key words: phylogeny, karyotypes, Triticum turgidum, Triticum monococcum, B genome, B genome donors.

Electron microscopy of synaptonemal complex spreads from autotetraploid Triticum monococcum (2n = 4x = 28) revealed a minimum mean of 3.59 multivalents per zygotene–pachytene nucleus. The range of values was from 1 to 6 multivalents per nucleus. Most of the multivalents were quadrivalents with single, medially located pairing partner switch points. Lateral element pairing switches, particularly the few multiple switches, were often accompanied by extensive asynapsis around the switch point. The synaptonemal complex multivalent frequency is considerably higher than the metaphase I quadrivalent frequency previously reported for the same material. Calculations of expected pachytene quadrivalent frequency from metaphase I data, using several published theoretical models, gave values that did not agree with the results obtained here. The difference between the multivalent frequencies at pachytene and metaphase I does not appear to be the result of a correction process. Instead, it could be caused by a combination of preferential pairing or crossing-over and the effects of the position of partner switches and asynapsis associated with switches. Key words: autotetraploid, multivalents, synaptonemal complex, pairing effects.