HYBRID SEED PRODUCTION SYSTEMS IN WHEAT
Satish Kumar, C N Mishra, S K Singh, Raj Pal Meena, Vikas Gupta and Indu Sharma
Heterosis is the manifestation of heterozygosity expressed as increased vigour, size, fruitfulness and resistance to disease, insects or climatic extremes relative to either the high-parent or the mid-parent value. Varying estimates of heterosis have been reported in wheat for various traits of economic importance. It is generally known that hybrid advantage is a function of three factors:
- breeding-method efficiency (rate-of-progress);
- negative or positive effects of the system used to produce the hybrid; and
- the inherent level of heterosis.
In wheat, one of the major constraints in hybrid development has been the lack of an effective and efficient seed production system. Here we discuss two different methods of seed production in wheat.
1. Cytoplasmic Male Sterility based system
Wilson and Ross (1962) were the first to describe a workable cytoplasmic male sterile, nuclear-restoration system when they reported that Triticum timopheevii cytoplasm induces male sterility with largely neutral effects on agronomic and quality characteristics. This discovery led to the establishment of a number of research programmes to evaluate alternate cytoplasmic sources of male sterility with common wheat and the nuclear genes necessary to restore fertility to the F1 hybrid. Virmani and Edwards (1983), in a comprehensive review of cytoplasmic male sterility, reported that 15 different cytoplasms of the genera Triticum and Aegilops have been recognized and that several of these could form the basis of a CMS system. However, they also stated the T. timopheevii system has gained widespread use due to deleterious effects of the other cytoplasms on various traits and/or because no advantage existed over the T. timopheevii system.
Panayotov (1983) reviewed 129 cytoplasms of species and subspecies in five different genera and reported that 37 cause male sterility. He also reported that several of the cytoplasms depress growth and development of the wheat plant and that cytoplasmic heterosis was found among some of the fertile alloplasmic lines.
He et al. (1998) reported that the Chinese national hybrid wheat network first emphasized the use of T. timopheevii CMS, but that Ae. kotschyi and Ae. ventricosa were also being utilized. They report that three hybrids based on T. timopheevii and six hybrids based on Ae. kotschyi had a 15 percent advantage over commercial pure line cultivars. Pickett (1993) briefly reviewed the effects of 20 different cytoplasms from Aegilops, one each from Haynaldia and Secale, and five cytoplasms from the genus Triticum. He concluded that CMS derived from Triticum appeared to present fewer problems and pointed to the moderately successful commercialization of hybrids produced with the T. timopheevii CMS system in the United States.
Development of restorer lines (R-line) to effect restoration of normal pollen fertility to the F1 hybrid involves the discovery and incorporation of fertility restoration (Rf) genes. Restoration genes from a number of species and from common wheat have been identified and are scattered throughout the ABD genomes (Wilson, 1984). The species that is the cytoplasmic donor is often a source of fertility restoring genes, and a number of Rf genes have been derived from T. timopheevii (Virmani and Edwards, 1983). Fertility restoration is under complex genetic control, and is further complicated by differences among females for ease of restoration and by the effect of the environment on fertility restoration (Wilson, 1984). Wilson (1968) attempted to classify environments as "shallow sterile", "sterile" and "deeply sterile". All of this has caused much difficulty in developing agronomically acceptable R-lines that give complete fertility restoration of the F1 hybrid.
The CMS system requires complete and stable male sterility and complete restoration of fertility to the F1 hybrid in a number of environments. While R-lines have been developed that do restore complete fertility, the difficult task of continuing to make genetic progress in developing agronomically improved R-lines remains as the most significant obstacle in the continued development of CMS hybrid wheat. The use of marker-assisted selection for Rf genes, as with restriction fragment length polymorphism (RFLP), would seem to increase the probability of making agronomic progress while retaining Rf genes in the male population (Ma and Sorrells, 1995).
2. Chemical Hybridizing Agents based system
Chemical hybridizing agent (CHA) is the term that has gained the most widespread acceptance to describe the class of chemicals that have the property of affecting male and/or female sterility. McRae (1985) discusses the origin of the terms gametocide and CHA and suggests that the former entered the scientific literature as a misnomer. He suggests that CHA is sufficiently broad to embrace all modes of action and does not equate gamete with pollen. The earliest report in the literature on the effect of a CHA on wheat was by Hoagland et al. (1953), who investigated the effects of maleic hydrazide on spring wheat. Since the early report, there have been significant advances in the development of CHA technology (Mock, 1995). Compared with CMS systems, an effective CHA allows the production of large numbers of parental combinations and permits the evaluation of a number of inbreds for combining ability and/or breeding value. This substantially reduces the time required for hybrid development, as noted by a number of authors (Bruns and Peterson, 1998; Wilson, 1984
The first generation of chemical compounds to be tested as CHAs was developed for other purposes, most notably as anti-lodging or height-reduction agents. They generally caused a high degree of phytotoxicity at rates required for effective sterility. This often resulted in poor female receptivity or fertility, or failed to produce adequate male sterility over a range of environments. Ethephon, gibberellins and RH531 would be examples of these chemicals. The risk of commercial production or their utility in developing breeding populations is unacceptable with any of this class of compounds.
The second generation of compounds was developed and tested specifically for CHA activity. These were generally found to work within a narrowly defined set of conditions. Phytotoxicity often was observed outside of these conditions, which generally included a narrow window of chemical application and a restricted array of genotypes. RH-0007 (Hy-brex) - an example of this class of compounds - was used for commercial production in both the United States and Europe. Although McRae (1985) described the compound as "appearing close to the ideal", risk of commercial production with this compound proved to be rather high, and hybrid seed quality was generally compromised.
The current generation of CHAs was developed specifically for their male pollen-suppressing activity. They provide a much improved safety margin with significantly reduced phytotoxic effects. They also provide for improved seed quality and are able to be used on wide array of genotypes. The Shell compound WL 84811 and the Monsanto CHA GENESIS are examples of this class of chemistry. Development of WL 84811 has been discontinued, but GENESIS has received full commercial registration in the United States and a provisional registration in France. It is currently being used in commercial production of hybrid seed on both continents. Pickett (1993), Virmani and Edwards (1983) and McRae (1985) each list a number of chemicals that have been evaluated by various researchers for their potential as CHAs. The effects of these compounds on pollen control and phytotoxic activity are briefly described. He et al. (1998) state that CHAs have been extensively studied and list five that have been established as efficient hybridizing agents, including several made in China.
While there is a rather large number of chemicals that cause male sterility in wheat, there are very few that meet most of the above requirements. Once a chemical is identified as having some CHA activity, much work needs to be done to identify the optimum time and rate of application. Additionally, application rates can vary with environment and genotype. Rates also can be modified with the use of chemical adjuvants and surfactants.
The registered chemical GENESIS meets most of the attributes of an ideal CHA, although it is not seed applied and, while it does effectively sterilize all genotypes, different rates of application often are required for different inbreds. Virmani and Edwards (1983) report that two chemicals - zincmethyl arsenate and sodium methyl arsenate - have been used to develop two released CHA rice hybrids. However, they also report that the consensus in development of hybrid rice favours the CMS system. Advantages of using a CHA in the development and production of hybrid wheat:
- breeding procedures are simplified since this system does not require conversion and maintenance of the A-line, or the breeding of fertility restoration into male parents;
- genotypes with poor anther extrusion still can be used as female parents;
- evaluation of large numbers of lines for general and specific combining ability and for seed production characteristics is simplified;
- development and improvement of heterogeneously diverse breeding populations, such as Reid and Lancaster open-pollinated maize cultivars or the Kafir and Milo sor-ghums, is made possible.
There are also some discussed disadvantages of CHAs, including:
- reduced seed set of CHA females versus CMS females due to overdose effects;
- difficulty of optimum field application of the CHA due to rainfall or high winds;
- required evaluation of genotype x environment interactions, with the efficiency of the CHA system diminishing if many years or locations are required to determine optimum dosage rates;
- male fertility or selfing in hybrid production fields, which can result in seed lots that do not meet seed law standards for a hybrid;
- costs associated with identification, development and registration of the compound.
It should be pointed out, however, that some of these disadvantages are also issues in a CMS system. Genotypes x environment interactions are, for example, also a problem in developing males with adequate fertility restoration in an array of environments. Similarly, male fertility can be a problem with a CMS female that does not have stable cytoplasmic sterility or that has a B-line that cannot be converted to complete pollen sterility (Wilson, 1984).
A number of other hybridizing systems have been proposed. Examples include a recessive genetic male sterility system combined with chemical restoration (Wilson, 1984), nuclear male sterility (NMS), such as the XYZ system of Driscoll (1985) (see Pickett, 1993 for a review), or photothermal-sensitive systems requiring different day-length and temperature regimes (He et al., 1998; Murai, 1998). With the continued development of biotechnology, it seems likely that bioengineered hybridizing systems may also be developed. However, the use of CHAs to develop breeding populations and identify specific hybrids for conversion to nuclear inherited systems of hybrid production seems efficient and perhaps necessary.
Additional developments in the chronology of hybrid wheat would have to include advances in the development of the science of genomics, which could lead to an improved understanding and ability to exploit the phenomenon of heterosis in this complex allohexaploid. Transformation of wheat, now possible with particle gun and Agrobacterium ssp. technologies, also may lead to further investment in hybrid wheat due to genetic dominance of transformed genes and an ability to capture the value in a hybrid crop.
Satish Kumar, C N Mishra, S K Singh, Raj Pal Meena, Vikas Gupta and Indu Sharma
Directorate of Wheat Research, Karnal
Haryana (India) – 132 001