A. J. Worland

Genetic analysis of the dwarfing gene (Rht8) in wheat. Part I. Molecular mapping of Rht8 on the short arm of chromosome 2D of bread wheat (Triticum aestivum L.)

Abstract Two sets of single chromosome recombinant lines comparing 2D chromosomes from the wheat varieties ‘Ciano 67’ and ‘Mara’ with the common 2D chromosome of ‘Cappelle-Desprez’ in a ‘Cappelle-Desprez’ background were used to detect a diagnostic wheat microsatellite marker for the dwarfing gene Rht8. The genetic linkage maps place the wheat microsatellite marker WMS 261 0.6 cM distal to Rht8 on the short arm of chromosome 2D. By PCR analysis the WMS 261 alleles of ‘Mara’, ‘Cappelle-Desprez’ and ‘Ciano 67’ could be distinguished by different fragment sizes of 192 bp, 174 bp and 165 bp, respectively. A screen of over 100 international varieties of wheat showed that the three allelic variants were all widespread. It also demonstrated that a limited number of varieties carried novel WMS 261 variants of over 200 bp. Following classification of the individual recombinant lines for allelic variants at the WMS 261 locus it was possible to attribute a 7- to 8-cm reduction in plant height with the WMS 261-192-bp allele compared to the WMS 261-174-bp allele in the set of recombinant lines comparing 2D chromosomes of ‘Mara’ and ‘Cappelle-Desprez’. A height reduction of around 3 cm was detected between the WMS 261-174-bp allele and the WMS 261-165-bp allele in the recombinant lines comparing 2D chromosomes of ‘Cappelle-Desprez’ and ‘Ciano 67’.

Genetic analysis of the dwarfing gene Rht8 in wheat. Part II. The distribution and adaptive significance of allelic variants at the Rht8 locus of wheat as revealed by microsatellite screening

Abstract Wheat microsatellite WMS 261 whose 192-bp allele has been shown to be diagnostic for the commercially important dwarfing gene Rht8 was used to screen over 100 wheat varieties to determine the worldwide spread of Rht8. The results showed Rht8 to be widespread in southern European wheats and to be present in many central European wheats including the Russian varieties ‘Avrora’, ‘Bezostaya’ and ‘Kavkaz’. Rht8 appears to be of importance to South European wheats as alternative giberellic acid (GA)-insensitive dwarfing genes do not appear to be adapted to this environment. The very successful semi-dwarf varieties bred by CIMMYT, Mexico, for distribution worldwide have been thought to carry Rht8 combined with GA-insensitive dwarfing genes. Additional height reduction would have been obtained from pleiotropic effects of the photoperiod-response gene Ppd1 that is essential to the adaptability of varieties bred for growing under short-winter days in tropical and sub-tropical areas. The microsatellite analysis showed that CIMMYT wheats lack Rht8 and carry a WMS 261 allelic variant of 165 bp that has been associated with promoting height. This presumably has adaptive significance in partly counteracting the effects of other dwarfing genes and preventing the plants being too short. Most UK, German and French wheats carry an allelic variant at the WMS 261 locus with 174 bp. This could be selected because of linkage with the recessive photoperiod-sensitive ppd1 allele that is thought to offer adaptive significance northern European wheats.

WAITING FOR FINE TIMES: GENETICS OF FLOWERING TIME IN WHEAT

To maximise yield potential in any environment, wheat cultivars must have an appropriate flowering time and life cycle duration which ‘fine-tunes’ the life cycle to the target environment. This in turn, requires a detailed knowledge of the genetical control of the key components of the life cycle. This paper discusses our current knowledge of the genetical control of the three key groups of genes controlling life-cycle duration in wheat, namely those controlling vernalization response, photoperiod response and developmental rate (“earliness per se”, Eps genes). It also discuses how our ability to carry out comparative mapping of these genes across Triticeae species, and particularly with barley, is indicating new target genes for discovery in wheat. Major genes controlling vernalization response, the Vrn-1 series, have now been located both genetically and physically on the long arms of the homoeologous group five chromosomes. These genes are homoeologous to each other and to the vernalization genes on chromosomes 5H of barley and 5R of rye. Comparative analysis with barley also indicates that other series of vernalization response genes may exit on chromosomes of homoeologous groups 4 (4B, 4D, 5A) and 1. The major genes controlling photoperiod response in wheat, the Ppd-1 genes, are located on the homoeologous group 2 chromosomes, and are homoeologous to a gene on barley chromosome 2H. Mapping in barley also indicates a photoperiod response locus on barley 1H and 6H, indicating that a homoeologous series should exist on wheat group 1 and 6 chromosomes. In wheat, only a few “earliness per se” loci have been located, such as on chromosomes of homoeologous group 2. However, in barley, all chromosomes appear to carry such loci, indicating that several series of loci that affect developmental rate independent of environment remain to be discovered. Overall, comparative studies indicate that there are probably twenty-five loci, controlling the duration of the life-cycle, Vrn,Ppd and Eps genes, that remain to be mapped in wheat. There are major gaps in our knowledge of the detailed physiological effects of genes discovered to date on the timing of the life cycle from different sowing dates. This is being addressed by studying the phenology of isogenic and deletion lines in both field and controlled environmental conditions. This has indicated that the vernalization genes have major effects on the rate of primodia production, whilst the photoperiod genes affect the timing of terminal spikelet production and stem elongation, and these effects interact with sowing date.

The influence of photoperiod genes on the adaptability of European winter wheats

Photoperiod response genes play a major role in determining the climatic adaptability of European wheat varieties. Photoperiod insensitivity, in the vast majority of photoperiod insensitive European wheat varieties, is probably determined by a Ppd1 allele originally derived from the old Japanese variety Akakomugi. Analysis of the pleiotropic effects of a Ppd1 allele from the Italian variety Mara shows that, besides accelerating ear emergence time, Ppd1 also reduces plant height, tillering, and spikelet numbers. Increases in spikelet fertilities more than compensate for reduced spikelet numbers, producing increased numbers of grains per ear. In southern Europe, early flowering Ppd1 genotypes produce larger grain and greater yields. In England and Germany, pleiotropic effects of Ppd1 on yield vary annually, depending on prevailing weather conditions, from +9% to −16%, over a 10 year period in the United Kingdom. A possible alternative Ppd1 allele from the CIMMYT variety Ciano 67 was compared to that from Mara. Differences associated with complete substituted chromosomes were found to be due to linked genes rather than different Ppd1 alleles. Examination of an alternative weaker gene for photoperiod insensitivity, Ppd2, shows this to exert similar but less significant pleiotropic effects to Ppd1. In the UK, in each of three years of trialing, Ppd2 increased yield 6% more than Ppd1. Results of 10 years trialing show that in Central European countries, between areas where photoperiod sensitive or photoperiod insensitive varieties have a clear adaptive significance, the annual variations in climate make it extremely difficult for breeders to produce varieties with good adaptability to changing environmental conditions.

The relationships between the dwarfing genes of wheat and rye

SummaryThe improvement of lodging resistance by introducing major dwarfing genes, classified either as GA insensitive or GA sensitive, is one of the main strategies chosen by cereal breeders. In the present paper the current knowledge about the genetics, chromosomal localisation and the homoeoallelic relationships of the dwarfing genes in wheat and rye is reviewed. The confusing system of the symbolisation of the GA insensitive dwarfing genes/alleles in wheat is discussed and a nomenclature based on rules for gene symbolisation in wheat is proposed.

Genetical analysis of chromosome 5A of wheat and its influence on important agronomic characters

SummaryChromosome 5A of bread wheat, Triticum aestivum carries the major gene, Vrnl, which is one of the main determinants of the winter/spring growth habit polymorphism in this species. Genetical analysis of this chromosome has been carried out using single-chromosome recombinant lines to establish the pleiotropic effects of this locus and two other major genes, q determining ear morphology and bl determining the presence of awns, on important agronomic characters. The three major genes were located on the long arm of chromosome 5A with a gene order of: centromere -bl-q-Vrnl. Analysis of quantitative characters from a winter sowing revealed pleiotropic effects of Vrnl or the effects of closely linked loci on the characters plant height, tiller number and spikelet number. However effects on ear emergence time were not associated with Vrnl but with q as were effects on spikelet number and ear length. In addition a locus determining yield/plant was located between Vrnl and q. Independant loci determining height and ear length were apparent on the short arm of chromosome 5A. From a spring sowing, however, there was a large pleiotropic effect of Vrnl on ear emergence time, as well as the effects previously detected. In addition, associated with q were effects on plant height and grain size which were not expressed from the winter sowing.

Application of microsatellite markers to distinguish inter-varietal chromosome substitution lines of wheat (Triticum aestivum L.)

Wheat microsatellites (WMS) were used to test the authenticity of inter-varietal chromosome substitution lines developed using the varieties ‘Cappelle-Desprez’ and ‘Bezostaya 1’. The results demonstrated that the majority of the lines were correct. Microsatellites, with their abundance of polymorphic markers randomly distributed over the entire wheat genome, provided ideal tools for establishing the authenticity of cytogenetically developed genetic stocks of wheat.

Genetic mapping of QTL controlling tissue-culture response on chromosome 2B of wheat (Triticum aestivum L.) in relation to major genes and RFLP markers

Abstract Three quantitative trait loci (QTL) for tissue- culture response (Tcr) were mapped on chromosome 2B of hexaploid wheat (Triticum aestivum L.) using single-chromosome recombinant lines. Tcr-B1 and Tcr-B2, affecting both green spots initiation and shoot regeneration, were mapped in relation to RFLP markers in the centromere region and on the short arm of chromosome 2B, linked to the photoperiod-response gene Ppd2. A third QTL (Tcr-B3), influencing regeneration only, was closely related to the disease resistance locus Yr7/Sr9g on the long arm of chromosome 2B. The homoeologous relationships to the tissue-culture response loci Qsr, Qcg and Shd of barley are discussed. A possible influence of the earliness per se genes of wheat and barley is suggested.

Comparative genetic mapping of loci affecting plant height and development in cereals

Restriction fragment length polymorphism (RFLP) mapping data for genes determining dwarfness (GA insensitive and GA sensitive), vernalisation response and photoperiodic response in wheat, rye and barley were compared and their homoeologous relationships discussed. The GA insensitive Rht genes of wheat are not related to the GA insensitive dwarfing genes of rye or barley; however, homoeology is present for two members of the GA sensitive dwarfing genes of wheat (Rht12) and rye (Ddw1), located on the translocated segments of the long arms of chromosomes 5A and 5R, respectively. The comparative mapping of the Triticeae group 5 vernalisation response genes of wheat, rye and barley, and the group 2 photoperiodic response genes of wheat and barley, show that both gene families are located in homoeologous regions of the particular chromosomes.

Mapping of QTLs conferring resistance to bacterial leaf streak in rice.

Abstract A large F2 and a RI population were separately derived from a cross between two indica rice varieties, one of which was highly resistant to bacterial leaf streak (BLS) and the other highly susceptible. Following artificial inoculation of the RI population and over 2 years of testing, 11 QTLs were mapped by composite interval mapping (CIM) on six chromosomes. Six of the QTLs were detected in both seasons. Eight of the QTLs were significant following stepwise regression analysis, and of these, 5 with the largest effects were significant in both seasons. The detected QTLs explained 84.6% of the genetic variation in 1997. Bulked segregant analysis (BSA) of the extremes of the F2 population identified 3 QTLs of large effect. The 3 QTLs were dentical to 3 of the 5 largest QTLs detected by CIM. The independent detection of the same QTLs using two methods of analysis in separate mapping populations verifies the existence of the QTLs for BLS and provides markers to ease their introduction into elite varieties.