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Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105
* Corresponding author (s.kianian{at}ndsu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: CMS, cytoplasmic male sterility • G x E, genotype by environment • LDN (Dic 1A), Langdon (T. diccocoides 1A) • (lo), Aegilops longissima cytoplasm • NC, nuclear–cytoplasmic • Rf, restoration of fertility gene • scs, species cytoplasm specific gene • (vent), Aegilops ventricosa cytoplasm • Vi, vitality gene
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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In general, durum wheat is a more sensitive indicator of alloplasmic compatibilities than hexaploid wheat. In certain alloplasmic situations where Triticum aestivum L. is self-fertile, T. turgidum L. may only be partially fertile or completely sterile (Maan, 1992). The potential use of cytoplasmic male sterility (CMS) in the production of hybrid wheat incited a search for cytoplasm that would maintain male sterility, but not cause other deleterious effects. The interaction of T. timopheevii cytoplasm with the T. aestivum nucleus was discovered to have no apparent adverse effects on plant development, and alloplasmic plants were consistently male-sterile (Wilson and Ross, 1962). The interaction of T. timopheevii cytoplasm with the T. turgidum nucleus, however, produced weak, sterile plants (Maan, 1992). The enhanced NC compatibility found in hexaploid wheat was thought to be associated with the D-genome (Maan, 1992).
Most species of Triticum and Aegilops have unique cytoplasms. The cytoplasm D- and M- genome Aegilops species, including Ae. longissima Schweinf. & Muschl. (lo), are compatible with common wheat, but partially or completely incompatible with the nuclear genome of durum wheat (Sasakuma and Maan, 1978). Compatibility between the nuclear genome of durum wheat and the (lo) cytoplasm was produced by the introduction of several sources of scs genes (Maan, 1992). The scsti gene from T. timopheevii is located on the long-arm of chromosome 1A (Anderson and Maan, 1995), scsae is located on the long arm of chromosome 1D (1DL) from common wheat, and scsun is located on a telocentric chromosome that is homeologous to group 4 from Ae. longissima (Maan, 2004, unpublished data). The (lo) durum lines with scs genes were male-sterile and, when crossed to normal durum, produced segregating seed types: plump, viable seeds containing the alien scs gene and shriveled, inviable seeds without the scs gene. A Vi gene of spontaneous origin (Maan, 1992) produced fertility in the (lo) durum lines with a scsti, scsae, or scsun gene. Similarly, the scsspt gene on the short arm of chromosome 2 derived from Ae. speltoides Tausch (Maan and Kianian, 2001b) produced compatibility between durum wheat and Ae. ventricosa Tausch cytoplasm (vent). The resulting (vent) durum line was male-sterile and partially female fertile, and when crossed to normal durum, produced plump and viable seeds. Female gametes without scsspt did not function. The Vi gene that did produce fertility in the (lo) durum lines did not produce fertility in the (vent) durum line (Maan and Kianian, 2001b).
The CMS trait is of particular interest in alloplasmic wheat (wheat with alien cytoplasm) for the production of commercial hybrids (Lucken, 1987; Anderson and Maan, 1995). The commercial production of hybrid wheat creates an opportunity for wheat improvement. Advantageous characteristics include, but are not limited to, hybrid uniformity, gene pyramiding within a hybrid, and hybrid vigor. Controlled pollination of CMS lines in wheat has been approached in several ways. One proposal is a system based on alloplasmic T. aestivum with T. tauschii Coss. cytoplasm (Franckowiak et al., 1976). Instead of introducing a nuclear gene from a third species, a mutagen is used to induce the male sterility that is specific for T. tauschii cytoplasm. Euplasmic (true cytoplasm) T. aestivum could then be used as the male parent to produce hybrids (Franckowiak et al., 1976). A second approach for pollination control involves using a nuclear gene for male sterility and an alien chromosome (Wilson and Driscoll, 1983).
Traditionally, restoration of fertility (Rf) genes transferred from the alien species that donated the cytoplasm have been used to restore fertility in the hybrids (Anderson and Maan, 1995). Introduction of Rf genes into agronomically adapted wheat failed to produce restorer lines that would impart male fertility to hybrids (Lucken, 1987). However, screening has shown that there is a high degree of instability and environmental sensitivity associated with fertility restoration (Jo
t and Lucken, 1983). Hybrid breeding programs must therefore be designed for selecting complementary Rf gene combinations, allowing accumulation of beneficial genes and the elimination of deleterious genes (Lucken, 1987). Fertility genes customarily act as single, dominant loci to suppress the CMS phenotype (Hanson, 1991). However, in the situation of an alloplasmic durum nucleus in (lo) durum, Rf genes have no effect on compatibility (Maan et al., 1999; Maan and Kianian, 2001a). On the other hand, a two-gene system consisting of scs and Vi genes restores fertility to (lo) durum. Thus, these genes have great potential for use in a hybrid wheat production scheme (Maan, 1991).
The initial steps of understanding of NC interactions in relation to scs genes in wheat have been pursued (Anderson and Maan, 1995; Maan, 1991, 1992, 1995; Maan et al., 1999; Maan and Kianian, 2001a, 2001b; Simons et al., 2003). As with any new genetic combination, effects of the scs gene on agronomic performance must be investigated before the hybrid system can be implemented into a breeding program. Field comparisons were conducted on durum lines homozygous for the scsti gene and sib-lines lacking this gene to determine if the scsti gene plays any role on specific agronomic characteristics: days to heading, plant height, lodging resistance, grain yield, and kernel test weight. The lack of detrimental effects associated with these characteristics would justify further evaluation of the scs/Vi compatibility system in development of a hybrid durum wheat production system.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Field Evaluation
The F2:4 homozygous durum lines described earlier (lines 1–10) were evaluated at three locations: Langdon, Prosper, and Casselton, ND, in the 2001 growing season. The 10 progeny lines were accompanied by the parental lines, LDN (Dic 1A) and (d) scsti scsti, and four durum genotypes released by the North Dakota State University: ‘Ben’ (Elias and Miller, 1998), ‘Lebsock’ (Elias et al., 2001), ‘Maier’ (Elias and Miller, 2000a), and ‘Mountrail’ (Elias and Miller, 2000b), as checks (lines 11–16, respectively). The sixteen genotypes were sown in a randomized complete block design with four replicates at each location. Each experimental unit contained four 2.5-m rows 30-cm apart at a seeding rate of 25 seeds per 0.30 m. The center two rows of the four-row plots, an area of 1.75 m2, were harvested at Prosper and Langdon. At Casselton, all four rows were harvested (3.5 m2).
Phenotypic Evaluation
Agronomic data were collected on height, heading date, lodging, yield, and test weight. Height was measured from ground level to the tip of the spike, excluding awns, and reported in centimeters. Heading date was determined when at least 50% of spikes in the plot had emerged from the flag leaf sheath and expressed as the number of days from the seeding date. Lodging scores were measured on a scale of 0 to 9, with 0 being no lodging observed and 9 indicating maximum lodging. Lodging data were collected before harvest of the plots. Grain yield and test weight were measured on cleaned seeds and converted to kg ha–1 and kg m–3, respectively.
Statistical Analysis
Phenotypic data were compared for all genotypes, and then parents and progeny were partitioned into four comparison classes between lines containing two doses of the scsti gene and lines containing no scsti gene. Five genotypic comparisions were evaluated: (i) comparing the parental genotypes, (ii) the progeny lines containing two copies of the scsti gene as compared with the parental line containing the scsti gene, (iii) the progeny line deficient of scsti genes compared with the null parent, (iv) comparison of the two progeny classes (with and without the scsti gene), and (v) comparison of all experimental lines to the four check lines. Appropriate LSDs were calculated to compare the various groups. Homogeneity between locations was assessed using the Bartlett's test to determine if location data could be combined and compared (Steel and Torrie, 1980). Data for heading date (HD), height (HT), lodging (LDG), and grain yield (YD) were combined across locations. An ANOVA was performed on the combined locations data using SAS software (SAS Institute, 2002). Test weight data were heterogeneous and therefore data was not combined across locations. Genotype and environment were considered random effects, as they were representative of a larger group of genotypes or area, respectively. F tests were considered significant at P 
0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Analysis of the first four genotypic comparisons listed in the methods revealed no significant differences in HD, HT, LDG, and YD (Table 1). A lack of significance in comparing the parental classes suggests differences in HD were not likely an effect of the scsti gene. The overall lack of significant difference indicates the interactions between the scs gene and the environment are not triggering any detrimental effects in the durum lines.
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Plant heights were distinctly different between the experimental and check lines. The experimental lines, on average, were 15 cm taller than the check cultivars (Table 1). This height difference could have resulted in the higher lodging score of the experimental lines when compared with the checks. Both parents were taller and had higher lodging than the checks. Progeny from a cross between tall parents that lodge are expected to be tall and to lodge.
Unique weather conditions at each location influenced the degree of lodging observed; however, the lines responded in a relatively ordered fashion. Lodging scores were only evaluated at the Prosper and Casselton locations. The cultivar Langdon (LDN) is known to have a weak straw and is very susceptible to lodging. This was apparent from the lodging score means of lines which had LDN (Dic 1A) as a parent. Therefore, the high degree of lodging is most likely not an effect of the scsti gene, but instead an effect of the LDN (Dic 1A) parent.
It was thought that one of the effects of the scsti gene may be enhanced grain yield potential. However, field yield data did not support our hypothesis. Check lines yielded almost 27% more (
9 g plot–1) than the experimental lines (Table 1), but this was not statistically significant. Reduced yield may have been caused by lodging and the exposure of the unadapted materials to environmental stresses.
Test weight was the only trait that had heterogeneous error terms between locations. Therefore, ANOVA was performed for each location separately. There were no significant differences within the experimental lines or between the experimental lines and checks at Langdon (Table 2). However, significant differences were observed at Casselton between the parental lines and between the experimental lines and checks. At Prosper, significant differences were found between the scs parent and the progeny lines containing two copies of the scs gene (Table 2). These differences could be due to G x E interaction or the T. dicoccoides chromosome 1A substitution rather than the scsti, since the scsti line had similar test weight to the (d)-- lines (Table 2).
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Determination that the scsti gene does not induce any detrimental effects in durum wheat is a step forward when considering the scs/Vi NC compatibility system as a viable source for producing hybrid durum wheat. These two genes could provide efficient means of producing the hybrids with the benefits of natural male-sterility associated with the scs genes and the use of the Vi gene to restore fertility. Used together, it would be possible to produce NC compatibility between an alien cytoplasm and a wheat nucleus through restoration of seed plumpness, plant vigor, and male fertility.
At this point it would be valuable to conduct a similar study with the Vi gene to determine its influence, if any, on agronomic characteristics in durum wheat. The Vi gene is believed to be a spontaneous mutation of an Rf gene on chromosome 1BS. Traditionally, wheat Rf genes have been used to produce highly productive field-adapted lines for use in research and for production of commercial hybrid wheat. Many conventional wheat cultivars grown around the world also carry Rf genes that are able to restore fertility and plant vigor in wheat lines having alien cytoplasms (Busch and Maan, 1978). However, these Rf genes do not restore fertility in all intraspecific combinations (Maan, 1992). Studies on lines with different cytoplasms indicate that the scs/Vi combination is a more universal restorer of compatibility/fertility. For that reason, the use of a mutated Rf gene such as Vi, in conjunction with the scs gene, could prove invaluable in germplasm introgression, prebreeding, and the development of a hybrid durum wheat system.
The scsti gene and its alleles could have many positive effects on durum wheat breeding and crop improvement, especially when alien genes are needed in cultivar improvement. The abundance of scs genes and alleles provides ample germplasm to develop valuable compatibility restoration systems. Although efforts to develop hybrid wheat systems have failed in the past, crops such as maize (Zea mays L.) and rice (Oryza sativa L.) provide successful and encouraging examples of the potential benefits associated with the implementation of a hybrid system. Effective use of scs and Vi genes as a compatibility restoration system may facilitate a future for hybrid wheat. A better understanding of the scs gene's effects and attributes will allow for further exploration and exploitation of the genes in wheat and its wild relatives.
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ACKNOWLEDGMENTS |
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Received for publication June 14, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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t, M., and K.A. Lucken. 1983. Stability of fertility restoration in some F1 spring and winter wheat hybrids grown in the International Wheat Restorer Germplasm Screening Nursery. p. 675–682. In S. Sakamoto (ed.) Proc. 6th Int. Wheat Genetics Symp., Kyoto, Japan. 28 Nov.–3 Dec. 1983. Plant Germplasm Inst., Faculty of Agric., Kyoto Univ.
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