A reassessment of the course of evolution of wheat

The phylogenesis of protein α-amylase inhibitors from wheat seed and the speciation of polyploid wheats

Protein α-amylase inhibitors extracted with water from seeds of a number of Triticum and Aegilops species were characterized according to their molecular weights and action specificities towards human salivary and Tenebrio molitor L. α-amylases. Four inhibitor peaks, with molecular weights 60000, 44000, 22000 and 11000, active towards the two amylases have been detected. Another inhibitor peak with molecular weight 11000, only active towards the insect α-amylase, has been found in several species tested. Triticum urartu showed only the 22000 inhibitor peak, while other diploid Triticum species did not exhibit any inhibitory activity. All the diploid Aegilops species tested contained α-amylase inhibitors and the inhibitor patterns differed greatly even for closely related species. In general, tetraploid Triticum species (turgidun and timopheevi) exhibited amylase inhibitor patterns of higher complexity than diploid Triticum and Aegilops species.The relationships existing among the amylase inhibitor patterns of the Triticinae species tested are consistent with the hypothesis of the polyphyletic origin of tetraploid wheats by Sarkar and Stebbins (1956) and suggest that the amylase inhibitors from diploid species all derive from common ancestral genes.

THE RELATIONSHIP BETWEEN THE GENOME OF TRITICUM URARTU AND THE A AND B GENOMES OF TRITICUM AESTIVUM

Triticum urartu (2n = 14) was crossed with T. aestivum lines ditelosomic for chromosomes of the A and B genomes. Except for telosome 4Aα, the rest of the telosomes of the A genome paired in these hybrids while telosomes of the B genome did not pair. This indicates that T. urartu cannot be the donor of the B genome of 4x and 6x wheat, but carries an A genome. Compared to the rest of the hybrids, pairing of T. urartu chromosomes was significantly reduced in hybrids deficient for chromosome arms 5AS or 5BS. It is suggested that this reduction in chromosome pairing resulted from the absence of genes which promote homoelogous pairing and which are normally present on chromosome arms 5AS and 5BS in Chinese Spring.

The use of aneuploids in studies of genetics, breeding, and evolution in wheat

In wheat a unique series of aneuploids is available, ranging from all 21 possible monosomics to complex types which are simultaneously deficient for one chromosome and duplicate for another. Furthermore, lines with chromosomes from related, alien species either added to or substituted for wheat chromosomes are in common cytological use. This contribution considers the use of this range of material in studies designed to elucidate the evolutionary relationships of the species, investigations of the genetics of a polyploid with cytological diploidization, and in potential breeding manipulations.

IDENTIFICATION OF THE APPARENT B–GENOME DONOR OF WHEAT

More than 1450 accessions of wild wheats were collected throughout their area of endemism in the Near East. Two types of diploids were distinguished by their seed protein electrophoretic pattern. One of these, Triticum boeoticum Boiss., was prevalent in all wild wheat areas. The other, occurring over the entire range of the tetraploids, was identified as T. urartu Tum., a species previously known only from Transcaucasia. The tetraploid T. diococcoides Körn. Schweinf. (AeAeBeBe), occupying the western arc of the Fertile Crescent, and T. araraticum Jakubz. (AtAtBtBt) occurring in the eastern arc and Transcaucasia, gave dissimilar but remarkably uniform albumin patterns from ethanol extracts of the seeds. A different combination of boeoticum and urartu biotypes accounted for all of the albumin bands in each of the tetraploid patterns obtained from water extracts. The morphological characters of the diploids were combined in the tetraploids. Hybrids between the wild diploid species were sterile, and showed 7 II at meiosis. The synthetic amphiploid from such a hybrid showed 10 II + 2 IV or 12 II + 1 IV. The triploid hybrid T. dicoccum × T. urartu showed 6-7 II. These observations implicate T. urartu as the missing B-genome donor to both of the tetraploids.

INCOMPLETE CONCURRENCE OF EVIDENCE FROM PROTEIN ELECTROPHORESIS AND GENOME ANALYSIS REGARDING THE ORIGIN OF AEGILOPS OVATA

Electrophoresis of ethanol extracted, water soluble, seed proteins of many different biotypes of 3 diploid species of Aegilops and of the tetraploid A. ovata L. suggest that A. ovata may be descended from an allotetraploid of A. umbellulata Zhuk. and A. squarrosa L. This does not agree with the few published results of genome analysis, which suggest that A. umbellulata and A. comosa Sibth. &Smith are the ancestors of A. ovata. Hypotheses advanced to explain this incomplete concurrence of evidence from these two biosystematic methods include an insufficiency of samples, genome divergence, translocations followed by introgressive hybridization and genetic control of meitoic chromosome pairing.

DISOMIC AND DITELOSOMIC ADDITIONS OF DIPLOID AGROPYRON ELONGATUM CHROMOSOMES TO TRITICUM AESTIVUM

A set of disomic addition lines was produced in which each chromosome of Agropyron elongatum (2n = 14) was added to the chromosome complement of Triticum aestivum cv. Chinese Spring. In addition a complete set of ditelosomic addition lines involving Agropyron chromosome arms IS, IIα, IIIα, IVS, IVL, VS, VL, VIS and VIIα, was developed. Except for disomic addition line VII which is similar to Chinese Spring, each added alien chromosome pair was found to have a specific effect on plant morphology. Five disomic addition lines had reduced fertility. Agropyron chromosome arms carrying genes for anthocyanin production, waxless foliage, tenacious glumes, and several other traits were identified. The single Agropyron chromosomes in the monosomic addition lines usually have either minor effects or no effect on plant morphology and fertility.The disomic addition lines are similar in many characters to tetrasomics of the corresponding homoeologous wheat chromosomes and show characters that are not present in either Ag. elongatum or the amphiploid, T. aestivum × Ag. elongatum. Thus many of the effects are due to gene dosage. If individual disomic addition lines are compared with the amphiploid a significant role of interchromosomal gene interactions is apparent.In combination with knowledge of the homoeologous relationships among wheat and alien chromosomes, sets of disomic addition lines can provide valuable information on the evolutionary past of genomes in Triticinae. In monosomic addition lines only Agropyron chromosome IV paired with a wheat chromosome and it is concluded that it is improbable that Agropyron genomes played any role in the evolution of the polyploid series of Aegilops and Triticum.

ORIGIN OF THE FREE THRESHING CHARACTER IN HEXAPLOID WHEAT

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 = DD) were non-free-threshing, regardless of the presence or 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 2Dα. 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 possessed 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.

SATELLITE NUMBER AND SIZE IN TRITICUM MONOCOCCUM L. AND THE EVOLUTION OF THE POLYPLOID WHEATS

Biotypes of Triticum monococcum from Europe and Iran were surveyed for variations in the number and size of the chromosomal satellites. Biotypes with both one and two pairs of satellited chromosomes were found, and the satellite size varied both within and between biotypes. It is concluded that existing phylogenetic and evolutionary schemes within Triticum and Aegilops, which are based mainly on karotype evidence, are suspect. The hypothesis that Ae. speltoides is the donor of the B genome of the tetraploid and hexaploid wheats is questioned.

THE PAIRING OF AN ALIEN CHROMOSOME WITH HOMOEOLOGOUS CHROMOSOMES OF WHEAT

The homoeology of chromosome A of Triticum umbellulatum has been determined by studying its pairing relationships with wheat telocentrics in hybrids having the chromosome constitution of 20 wheat chromosomes + one wheat telocentric chromosome + seven Triticum speltoides chromosomes + chromosome A. It has been found to be homoeologous to the group-6 chromosomes of wheat.