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Published online 26 August 2005
Published in Crop Sci 45:2113-2119 (2005)
© 2005 Crop Science Society of America
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CROP BREEDING, GENETICS & CYTOLOGY

Analysis of Genetic Factors Influencing the Developmental Rate of Globally Important CIMMYT Wheat Cultivars

Janny van Beema,*, Volker Mohlerb, Rudy Lukmanc, Maarten van Ginkeld, Manilal Williamd, José Crossad and Anthony J. Worlande

a 5007 Riverside Oaks Dr., Kingwood, TX 77345
b Chair of Agronomy and Plant Breeding, Centre for Life and Food Sciences Weinhenstephan, Technical Univ. Munich, Am Hochanger 2, 85350 Freising, Germany
c Seameo Biotrop, Southeast Asian Regional Centre for Tropical Biology, Jalan Raya Tajur km. 6, P.O. Box 116, Bogor, Indonesia
d CIMMYT, Lisboa 27, Apto. Postal 6-641, 06600 Mexico D.F
e Deceased), Dep. of Crop Genetics, John Innes Centre, Norwich Research Park, Colney, Norwich, NR4 7UH, UK

* Corresponding author (jvanbeem{at}kingwoodcable.net)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The rate at which wheat (Triticum aestivum L.) reaches anthesis and other developmental stages depends largely on the variation of three major factors: vernalization (Vrn), photoperiod (Ppd), and earliness per se (Eps). The objectives of this study were to characterize a set of genotypes for the presence of developmental genes and their potential association to adaptability and to identify diagnostic molecular markers for the vernalization gene Vrn-D1. Fifty-one genotypes were crossed with testers containing known Vrn and Ppd genes. To determine earliness per se and its interaction with temperature, genotypes were vernalized for 8 wk and placed in a 24-h photoperiod regime at two temperatures. To validate molecular markers for Vrn-D1, the cultivars were genotyped with closely linked SSR markers Xgwm292-5D and Xgwm212-5D. Segregation analysis on F2 populations of field-grown plants showed that Vrn-D1 was present in 66% of the cultivars, while Vrn-A1, Vrn-B1, and Vrn4 were found in 41, 39, and 8% of the lines, respectively, either solely or in combination with other Vrn genes. A low percentage of cultivars appeared to have a constitutive genetic character for early flowering (Eps), while most of the cultivars may have temperature-sensitive genes that respond to local environmental changes. Vrn-D1 was reliably detected in 76.5% of the genotypes using the 215-bp allele of microsatellite marker Xgwm292-5D, whereas Xgwm212-5D did not reveal sufficient allelic variation within the tested wheats.

Abbreviations: CS, Chinese Spring • Eps, earliness per se • MAS, marker-assisted selection • PCR, polymerase chain reaction • Ppd, photoperiod • SSR, simple sequence repeats • TD, Triple Dirk • Vrn, vernalization


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
THE DEVELOPMENT of broadly adapted, high yielding wheat varieties has been one of CIMMYT's primary goals. To achieve this, CIMMYT's breeding strategy is based initially on the selection of diverse germplasm, thereby minimizing the vulnerability in growing related cultivars over wide areas. Segregating materials are then shuttled between alternate sites to combine high yield potential, disease resistance, and quality traits. Lastly, superior lines are tested in international multilocation trials through the distribution of advanced lines to agricultural research institutions. The outcome has been a pyramiding of genes that gives CIMMYT-derived cultivars a broad adaptability spanning more than one country and often, more than one continent (Rajaram et al., 2002). It is estimated that in developing countries, CIMMYT-derived spring wheats are sown on approximately 95 million hectares and facultative–winter wheats are sown on almost 25 million hectares (Braun et al., 1996). Furthermore, 36 million hectares, or 58% of the world's wheat-producing area, is sown to varieties derived directly or indirectly from CIMMYT germplasm (Rajaram and Hettel, 1995). It is, therefore, imperative that CIMMYT germplasm carry a high degree of yield stability in international environments while providing a wide spectrum of disease resistance and end-use quality characteristics within each wheat-growing region.

To achieve geographical and environmental adaptability, it is important that wheat varieties flower at times that are appropriate to a particular environment. Growth of the flower structure must occur during an optimal period with ample time for flower development and grain fill. The flowering habits of wheat are determined primarily by a complex group of genes for Vrn, Ppd, and Eps that interact with the environment to regulate the rate and development of floral primordia. Because of the use of diverse genetic material and multiple selection sites, most CIMMYT wheat germplasm contains different combinations of Vrn, Ppd, and Eps genes that lengthen or shorten wheat's life cycle, avoiding abiotic stresses that are present in global wheat-growing areas. The adaptability of CIMMYT semidwarf wheats to diverse environments depends to a large extent on the variation and effect that these three factors have on flower development.

Vernalization insensitivity or low vernalization response is known to be under control of an orthologous series of dominant alleles at the Vrn-A1, Vrn-B1, and Vrn-D1 loci that are located in the long arms of group 5 chromosomes (Worland 1996). Other orthologous series of Vrn genes are also predicted to exist in chromosomes of homeologous groups 1 (Vrn-3) and 4 (Vrn-2) as evidenced from orthology with barley (Laurie et al., 1995) and T. monococcum L. (Dubcovsky et al., 1998). Vrn4, present in Triple Dirk F, is located on the long arm of chromosome 5D, while the chromosomal origin of Vrn-B4 has yet to be determined (McIntosh et al., 2003). In addition to these major vernalization genes, genes with minor effects have also been reported on chromosome 7B (Vrn-B4 or Vrn5; Law 1966).

The response to photoperiod is mediated primarily by the orthologous gene series Ppd-A1, Ppd-B1, and Ppd-D1 on the short arms of group 2 chromosomes (Worland et al., 1998). Genes on other chromosomes, particularly 3D, 4B, 6B, and all chromosomes on group 1, have also been implicated in determining photoperiod response (Worland et al., 1998).

Vrn and Ppd gene systems have been investigated in various studies related to adaptation, and results have shed some light on the benefits that could be obtained through their conscious manipulation within spring or winter types. Stelmakh (1993) evaluated 27 genetic effects of three Vrn genes and concluded that the highest yield was predicted for varieties containing Vrn-D1. He recommended greater involvement of Vrn-D1 donors, particularly when breeding spring cultivars with improved adaptation to late drought and heat stress. Similarly, Worland (1996) reported a yield advantage of 35% in southern European environments with the introduction of a single photoperiod-insensitive gene, Ppd-D1. The benefits were reflected in accelerated maturity, which allows the crop to escape the hot, dry conditions of late summer, while sensitive genotypes in northern Europe showed a longer grain fill period and higher grain yields.

Numerous studies have reported the effects of Eps on the rate of development of wheat. Eps may be expressed as the minimum number of days to reproductive growth, once vernalization and photoperiod requirements are satisfied. This factor was initially reported by Syme (1968), who found that wheat's basic development period was influenced by mean daily temperature. Ford et al. (1981) coined the term "earliness genes" and proposed that they were different from genes controlling photoperiod sensitivity. More recently, Miura and Worland (1994) found that Eps genes have striking effects on ear-emergence time by reducing the number of days to heading independently of environmental stimuli. Slafer (1996) re-examined the assumptions that earliness genes are independent of photoperiod and vernalization and that differences in earliness genes apply only from the vegetative period to floral initiation. Numerous publications have concluded that genetic factors related to earliness are polygenic in nature and may be located on chromosomes 2B (Scarth and Law, 1983), 3A, 4A, 4B, and 6B (Hoogendoorn, 1985b), and 3A (Miura and Worland, 1994). Kato and Wada (1999) determined that broad-sense heritability of earliness ranged from 0.90 to 0.99 and could be efficiently optimized through artificial selection.

Although the influence that earliness per se genes exert on developmental rate is considered minimal when compared with photoperiod and vernalization, they may cause developmental variations that ensure grain fill by avoiding harsh environmental conditions. This intrinsic ability to accelerate or delay development by a few days may be primarily a result of a dynamic interaction between genotype and temperature fluctuations (Slafer and Rawson, 1995). Despite indications that specific combinations of Vrn, Ppd, and Eps genes may result in ideal wheat developmental patterns, there is little information on requirements for broad or specific adaptation.

The purpose of this research was to characterize CIMMYT cultivars for the presence of development genes and study the potential association of specific gene combinations with adaptability. Thus, the objectives of the present study were to (i) characterize CIMMYT-derived cultivars for Vrn, Ppd, and Eps; (ii) test whether earliness per se is a constitutive genotypic characteristic that delays or accelerates flowering depending on environmental conditions; and (iii) identify diagnostic molecular markers for the vernalization response gene Vrn-D1 to significantly increase the efficiency of germplasm characterization and plant selection.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Forty-one CMMYT-derived and 10 non-CIMMYT wheat cultivars were chosen on the basis of their origin and target environment. While some cultivars were selected on the basis of their narrow margin of adaptation, others were selected because of their wide adaptation and extensive use in various international breeding programs. Table 1 shows the origin of each cultivar and the environment to which it is best suited (irrigated, high rainfall, or drought). Details of the names, parentages, pedigrees, and common ancestors of all varieties may be viewed in Skovmand et al. (1997). All genotypes were crossed with a series of near-isogenic Triple Dirk (TD) testers bred by Pugsley (1971)(1972), each of which contains a single vernalization gene Vrn-A1, Vrn-B1, Vrn-D1, or Vrn4. Parents and F2 populations were grown at CIMMYT's station in Toluca, Mexico (19°17' N, 99°40' W). The F2 seed was space-planted with 8 to 10 cm between plants in May 2000 with a 13-h photoperiod and an average temperature of 17°C. It was known from previous experiments that late-spring sowing at this location prevented vernalization requirements from being satisfied and resulted in a vegetative growth habit for vernalization-sensitive genotypes. Days to anthesis were recorded for 500 to 800 plants in each F2 population. Anthesis was defined as the number of days from sowing to when 50% of the spikes extruded at least one anther (Bell and Fischer, 1994). Late-flowering segregants characterized by a vegetative growth habit were assumed to represent the vernalization genotype vrn-A1vrn-B1vrn-D1vrn4. Segregation analysis of flowering vs. vegetative growth habit was done in September 2000. To confirm results, a second set of parents and F2s was planted at the same location in May 2001. Chi-square analysis was conducted to determine whether segregation ratios for winter vs. spring growth habits agreed with expected ratios following procedures described by Snedecor (1946)(p. 188–192).


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  		Table 1. Pedigree, origin, target environments, Vrn genotype, SSR allele, and earliness per se for 51 genotypes.

 
To determine the earliness per se effect, 30 seeds per genotype were imbibed for 48 h at room temperature and vernalized for 8 wk at 4°C in Petri dishes lined with filter paper to maintain moisture. After vernalization, seedlings were standardized by selecting those 4 to 8 mm in length. Twenty-four seedlings per cultivar were sown in pots containing equal quantities of peat moss, soil, and sand, at a rate of four seedlings per pot. Twelve seedlings per cultivar were placed in an environment-controlled chamber under a "warm" temperature regime, with day/night temperatures of 23/12°C. The remaining 12 seedlings were placed in a "cool" temperature regime, with day/night temperatures of 16/6°C. Both chambers were maintained at a constant photoperiod regime of 10 h of natural light supplemented by 14 h of low-intensity tungsten bulb illumination. It was assumed that a 24-h light regime and complete vernalization treatment would satisfy all vernalization and photoperiodic requirements, thus removing the effects of Vrn and Ppd. The experiment was conducted in a randomized complete block design with three replicates per cultivar (i.e., each pot was considered a replicate). Pots containing the same cultivar were placed together to ease data collection and were re-arranged randomly on a weekly basis to minimize positional effects within each chamber. Earliness per se in this study was defined as the number of days from transplanting to flowering and was recorded for each plant. Linear regression analysis was used to determine how much of the earliness in flowering under field conditions was due to earliness per se (vernalization and photoperiod effects removed). An analysis of variance (ANOVA) was conducted for earliness per se with SAS PROC GLM software (SAS Institute Inc., 1990) to examine the difference between cultivars and the interaction of cultivars with temperature.

To determine the Ppd gene composition, all genotypes were crossed with Mara containing Ppd-D1, Chinese Spring containing Ppd-B1, and C591 containing Ppd-A1. To completely satisfy the vernalization requirement, F2 seeds were imbibed for 48 h at room temperature and vernalized for 8 wk at 4°C. The vernalized F2 seed was space-planted with 8 to 10 cm between plants at CIMMYT's station in Ciudad Obregon, Mexico (27°29' N, 109°56' W). The sowing date in January 2002 coincided with a 10-h photoperiod and an average temperature of 16°C. It was assumed that a short-day season during the early growth stages would not satisfy Ppd requirements, thus making the segregation for Ppd genes evident. Flowering dates were recorded for 100 to 200 plants in each F2 population, and segregation analysis of early vs. late-flowering plants was done in May 2002.

Nuclear DNA extraction from primary leaves of 51 genotypes, the TD series, and 94 individuals of Vrn-D1 mapping population Chinese Spring (CS) x CS(Hope5D) essentially followed the procedure of Saghai-Maroof et al. (1984), with modifications described by Huang et al. (2000). Primer sequences and PCR protocols for wheat SSR markers Xgwm212-5D and Xgwm292-5D were as described in Röder et al. (1998). Forward primers were labeled with 6-FAM for fragment detection on an ABI PRISM 377 sequencer (Applied Biosystems, Foster City, CA). Two-point analysis of Vrn-D1 and Xgwm292-5D segregation data from 94 single chromosome recombinant lines (SCRLs) of cross Chinese Spring (CS) x CS(Hope5D) was conducted with the computer program MAPMAKER version 3.0b (Lander et al., 1987).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Segregation analysis of the Vrn genotype performed on F2 populations in the first experiment (May 2000) demonstrated an uneven distribution of the four Vrn genes, with Vrn-D1 being the most frequent and Vrn4 being the rarest. Results from the confirmation experiment planted at the same location in May 2001 were virtually identical to those in the first experiment. Table 1 summarizes the Vrn characterization of all genotypes. Vrn-D1 was present as the sole vernalization gene in 25% of the cultivars and was also found in combination with Vrn-A1, Vrn-B1, or Vrn4 in 41% of the cultivars. Vrn-A1 and Vrn-B1 were found in similar proportions (41 and 39%, respectively), either singly or in combination with other Vrn genes. Vrn4 was found in 8% of the cultivars tested, always in combination with other Vrn genes. The cultivar HD2329 contained all four Vrn genes, while the spring wheat varieties Embrapa 16 and Hubei showed segregation of vegetative growth habits when crossed with all testers, indicating the presence of unknown Vrn genes.

Differences in genotypic frequencies of vernalization genes might aid in explaining the selection and geographical adaptation of CIMMYT-derived cultivars. This study found that success in terms of wide adaptation may be associated with the presence of the Vrn-D1 gene, as demonstrated by its high frequency. It is interesting to note that the three most widely adapted varieties (Pastor, Attila, and Kauz) contained Vrn-D1 as the sole vernalization gene. Every year an estimated 8 to 9 million hectares worldwide are sown to these varieties, which are still being used to introduce high-yield genes. The Vrn-D1 gene was initially introduced to CIMMYT cultivars through the Japanese cultivar Akakomugi, which was used to create Mentana, an Italian cultivar (Stelmakh, 1990). Mentana was used by N.E. Borlaug to develop the semidwarf varieties Lerma Rojo 64 and Sonora 64, which were, in turn, responsible for the wide distribution of Vrn-D1 in South and Southeast Asian cultivars. It would appear that conscious selection of Vrn-D1 in CIMMYT's breeding program may have broadened the adaptation of wheat. Furthermore, national programs aiming to breed more widely adapted cultivars may benefit by including Vrn-D1 to produce developmental patterns suited to multiple target areas in their breeding programs. These results are in agreement with those found by Stelmakh (1998), who reported an increased presence of Vrn-D1 in cultivars adapted to growing areas near the equator. Early-flowering photoperiod-insensitive wheats exposed to stress at grain-fill were found to have a marked yield advantage if they carried Vrn-D1.

From previous studies on earliness factors (Flood and Holloran, 1984), it is known that 8 wk of cold treatment and 24 h of light will satisfy wheat's vernalization and photoperiod requirements. Any residual differences in days to flowering could be attributed to earliness per se genes. The ANOVA conducted on Eps indicated that the differences between genotypes and the interaction between genotype and temperature were highly significant (P < 0.0001) (Table 2). The LSD0.05 of genotypes for earliness per se under warm and cool temperature regimes was 5.9 and 2.1 d, respectively. The effect of temperature on earliness per se varied widely in the genotypes tested (Table 1). Except for one genotype with the same flowering date under both regimes, all cultivars flowered later under the cold regime, with delays ranging from 4 to 26 d.


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  		Table 2. Mean squares from analysis of variance for earliness per se. Fifty-one vernalized genotypes were planted in pots and grown in a controlled environment with 10 h of natural light supplemented by 14 h of low-intensity tungsten bulb illumination. The experiment was grown under warm and cool temperature regimes with day/night temperatures of 23/12°C and 16/6°C, respectively.

 
The earliness per se response to temperature was a continuum ranging from insensitivity to extreme sensitivity. Cultivars with relative insensitivity (about 10% of cultivars tested) exhibited delays in flowering of 0 to 7 d under the cold temperature regime. Among these was the cultivar CNO79/PRL, with 63 d to flowering under both temperature regimes, and four cultivars with differences in flowering of less than 8 d between temperature treatments (Temuco 1024.95, Temuco 1032.94, Irena, and Roque F 73). For this group, the data suggest that earliness genes may not be temperature genes but constitutive genetic characters for early flowering. Most genotypes (69%) fell into a second group, with delays of 12 to 18 d because of cooler temperatures. They did not exhibit changes in ranking with respect to one another, indicating the presence of similar earliness per se genes when exposed to the same temperature changes. A third group of genotypes exhibited a strong sensitivity to cold temperature (21% of the cultivars studied). Among the most sensitive were the cultivars Nesser, Gen3*/PVN, and Chilero, with flowering delays of 25 to 26 d. This response suggests that the earliness genes, in this group, are possibly temperature genes that are greatly affected by environmental conditions and used as a buffer to accelerate or delay maturity, allowing the crop to escape frost, heat stress, and drought.

In this study, the rate of development in most CIMMYT genotypes was responsive to temperature. Our results are in agreement with numerous studies (Angus et al., 1981; Slafer and Rawson, 1994) that found all genotypes to be responsive to temperature but with genotypic variation in sensitivity. Results reported by Slafer and Rawson (1995) found that in four genotypes the intrinsic earliness factor is a complex interaction between temperature and development. The present study examined 51 genotypes and found that earliness per se factors appear to be related to temperature, with most cultivars flowering earlier in warm temperatures, but the degree of temperature sensitivity varied widely and resulted in temperature x genotype interactions. Earliness factors also appeared to be constitutive in some cultivars, with flowering being independent of temperature.

The current study also set out to determine the relationship between earliness per se (vernalization and photoperiod effects removed) and earliness in flowering under field conditions. It is generally assumed that Vrn and Ppd genes have the strongest influence over early developmental stages and that these genes mask the effect of the earliness per se genes. The number of days to flowering for 51 cultivars grown under field conditions was averaged for 2000 and 2001 planting dates. Earliness per se for cultivars grown under the cool temperature regime was chosen over the warm regime since field conditions in Toluca at planting time closely resemble the 16/6°C day/night temperature in the cool regime. Figure 1 shows the regression analysis between days to flowering under field conditions and earliness per se, with a significant coefficient of 0.66. These results indicate that although the earliness per se genes have thus far been considered minor when compared with other developmental genes, they are nonetheless strong enough to cause earlier flowering dates in the presence of Ppd and Vrn.



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  		Fig. 1. Relationship between the average number of days to flowering for 51 cultivars grown under field conditions in Toluca, Mexico, and earliness per se for cultivars grown under the cool temperature regime (vernalization and photoperiod effects removed). ***Indicates significance at P = 0.001.

 
To identify the genotypes' Ppd composition, we performed a segregation analysis on F2 populations following protocols similar to those used in identifying Vrn composition. Two notable exceptions in the protocols were seedling vernalization (to eliminate the Vrn effect) and sowing date under a short-day regime (to make photoperiodic effects more evident). We expected to see early- vs. late-flowering plants but failed to detect true segregation among F2 populations. All populations had early and late segregants, and it was unclear from our analysis whether the cultivars contained the Ppd gene found in the testers.

There may be two reasons for not achieving our goal: first, the populations may have required an earlier sowing date. A January sowing date meant that the F2 populations experienced a light regime of more than 10 h of sunlight at 2 to 3 mo of development. It would have been more appropriate to plant the populations in November to ensure that the first 3 to 4 mo of development were under a short-day regime. Second and more importantly, there are earliness per se genes influencing the flowering dates and confounding the effect of the photoperiod genes. As discussed above, in some varieties earliness per se genes may delay the flowering date by up to 4 wk in response to temperature fluctuations. Theoretically, it may be possible to remove the earliness per se effect if the delay in flowering is known for the location and sowing time of the F2 populations. However, this may pose new problems, since previous studies (Scarth and Law, 1983; Hoogendoorn, 1985b; Miura and Worland, 1994) have found that numerous earliness genes are located on different chromosomes and presumably segregate concurrently with Ppd genes. A more rigorous study is necessary to develop protocols that would separate the effects of photoperiod and earliness per se genes.

Another important aim of the present study was to validate molecular markers for Vrn-D1, the most frequent vernalization response gene. Fifty-one genotypes and the TD series were analyzed with microsatellite markers Xgwm292-5D and Xgwm212-5D, which were found to be closely linked to Vrn-D1 in SCRL population Chinese Spring (CS) x CS(Cheyenne 5D) (Snape et al., 2001).

The close genetic linkage between Vrn-D1 and Xgwm292-5D (2.8 cM) was confirmed with segregation data from 94 SCRLs of cross CS x CS(Hope5D). The size of the Xgwm292-5D marker allele inherited from CS was 221 bp, whereas the size of the allele inherited from Hope was 217 bp. Five different marker alleles at the Xgwm292-5D locus were found within the 51 wheat genotypes tested (Table 1). The 215-bp allele was the most frequent (60.8%), followed by 223 and 213 bp (13.7% each), and 211 and 217 bp (5.9% each). The 215-bp microsatellite allele was also present in TD-E, the Vrn-D1 tester line and, therefore, is a potential indicator of the presence of Vrn-D1. The remaining TD lines displayed the 211-bp allele. Experiments comparing different Vrn gene compositions in 51 genotypes showed that the presence or absence of Vrn-D1 was reliably detected in 39 (76.5%) genotypes when using the 215-bp allele of microsatellite Xgwm292-5D (Table 1). Eight genotypes shown to carry Vrn-D1 displayed a microsatellite allele other than 215 bp. The cultivars Hubei, Pitta, Sids 4, and Inqalab 91 were not shown to contain the Vrn-D1 allele in segregation analyses, but did show the 215-bp allele at the Xgwm292-5D locus.

Microsatellite marker locus Xgwm212-5D, which had previously mapped 3.3 cM distal to Vrn-D1 (Snape et al., 2001), showed a linkage distance of 3.9 cM to Vrn-D1 in CS x CS(Hope5D). Xgwm212-5D exhibited marker alleles of 104 and 106 bp in CS and TD-E, respectively, whereas Hope and TD exhibited marker alleles of 102 bp. The marker Xgwm212-5D did not exhibit variation among the CIMMYT wheats and, with the exception of Milan/Sha7 (104 bp), all cultivars showed a marker allele of 102 bp. Therefore, the marker Xgwm212-5D does not qualify as a selection tool for Vrn-D1.

The complexity of genes involved in regulating photoperiod and vernalization response in wheat makes them ideal candidates for using marker assisted selection (MAS) strategies for their selection. In MAS procedures, the location of a frequently used marker is expected to be as close as possible to the target gene to have a low recombination frequency between the target gene and the marker. However, in the case of Vrn-D1 and Xgwm292-5D, a linkage distance of about 3 cM is apparently not effective enough, given that (i) the 215-bp allele was detected in vrn-D1 genotypes, and (ii) Vrn-D1 was found to be associated, albeit at low frequencies, with the other four alleles at the Xgwm292-5D locus. The assumption that different microsatellite alleles of Xgwm292-5D are linked with the Vrn-D1 allele is supported by the occurrence of a 221-bp marker allele in the cultivar Chinese Spring. These results suggest that it would be necessary to test for polymorphism before starting MAS in crosses of Vrn-D1 donors with wheat cultivars that were not genotyped in this study. In light of these results, the use of marker Xgwm292-5D in MAS is limited and closer flanking markers to the Vrn-D1 locus must first be identified. As suggested by Peng et al. (2000) and Stepien et al. (2004), it is possible that the accuracy of MAS for Vrn-D1 will be improved if marker haplotypes, rather than a single marker, are used. Furthermore, the recent cloning of Vrn-Am1 from diploid wheat T. monococcum (Yan et al., 2003) should permit the development of perfect markers for Vrn-B1 and Vrn-D1, as has been done for Vrn-A1 (Sherman et al., 2004). Vrn phenotyping results from this study will provide a basis for marker validation once these markers are developed.


   ACKNOWLEDGMENTS
 
The authors wish to thank the researchers of the CIMMYT Wheat Program (Mexico), Technical University München, and John Innes Centre who contributed their time and patience to this study. Funding for this research was provided by Bundesministerium für Wirtschaftliche Zusammenarbeit und Entwicklung, project # 97.7860.6-001.00.

Received for publication November 17, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome