Department of Genetics and Plant Breeding, G.B. Pant Univ. of Agric. & Tech., Pantnagar, 263 145, India
Fax : 91-05944-33473, Ph. : 91- 05944-33864, E-Mail : firstname.lastname@example.org
The Brassica oil crops are the world’s third most important source of edible oil. In India, rapeseed-mustard is the second most important group of oilseed crop. Exploitation of yield heterosis is now being viewed as an important avenue to break the yield barrier. Heterosis in brassica has been known since 1954, however, its effectiveness at commercial level has been demonstrated only during the past one decade. On an average, 25% standard heterosis for yield has been observed in hybrids.
Effective genetic mechanisms for commercial hybrids seed production include cytoplasmic male sterility, genetic male sterility, sporophytic incompatibility and fertility restoration. A number of alloplasmic lines have been developed following sexual or somatic hybridization. However, lack of stable fertility restores have hampered the exploitation of these CMS systems for producing commercial hybrid seed. The first B. napus hybrid (PGSH-51) has been commercialized while in B. campestris it is near commercialization. Hybrids in B. juncea have yet to make a headway due to non-availability of stable fertility restorer system and lower realizable heterosis due to biological penalty associated with current systems.
Private seed companies like Mahyco, Pro-agro, Hindustan Lever Ltd. etc. are also engaged in the development of commercial Brassica hybrids. Three B. juncea hybrids developed by Mahyco are already in the market since 1997.
Brassica hybrids with the required level of disease/insect resistance and acceptable oil quality have very good prospects in future. Thus, hybrids breeding in Brassica seems to be the best short term strategy to increase edible oil production in India.
KEYWORDS: Brassica species, fertility restoration, heterosis, male sterility, self-incompatibility.
Rape-seed mustard is the India second most important source of edible oil, being exceeded only by groundnut. It is cultivated in about 6.81 million ha which produce about 6.96 million tons of grains with productivity level of 1022 kg/ha (FAO 1997). Development of successful hybrids in maize in 1930s provide an important impetus for breeders of other crops to exploit the available heterosis by means of commercial hybrids in other crops. Brassica hybrid development programme was initiated during 1989 under ICAR aided project, “Promotion of research and development efforts on hybrids in selected crops-subcrop brassica”. Recent investigations leading to the development of new CMS lines and identification of maintainers and fertility restoration lines have brightened the prospects of development of commercial hybrids in brassica. This article reviews the current status and future prospects for breeding brassica hybrids.
Heterosis was first reported in brown sarson by Singh and Mehta (1954). Subsequently many studies have estimated the extent of heterosis for seed yield. The results indicate significant level of heterosis 13 to 91% in B. juncea (Banga and Labana 1984, Kumar et al 1990, Rai 1995, Thakur and Bhateria 1993, Baishakh et al .1994, Verma et al. 1998) 25 to 110% in B. campestris (Patnaik and Murty 1978, Verma et al. 1989, Dhillon et al. 1990, Varshney and Rao 1997, Yadav et al. 1998) and 10 to 72% in B. napus (Rai 1995, Dhillon et al. 1996, Thakur and Sagwal 1997). It was also observed that hybrids between genetically distant group showed greater heterosis than within the group combinations. Significant levels of heterosis for yield contributing parameters have been reported. Attention must however, be paid to the fact that the branch number, siliqua number and seed number and weight constitutes the prime component of yield heterosis are the character with poor heritability, and are severely affected by population density (Labana et al. 1978, Pradhan et al. 1993, Thakur and Sagwal, 1997). Unfortunately the majority of these studies report heterosis over mean parental value or better parental value and based on small plot size, data based on superiority to commercial variety and in CMS combination are rare.
Critical studies on gene action on yield and yield components in brassica are very few and most suffer from small population sizes and limited evaluation at one location or for one year. Studies on combining ability in relation to heterosis have been done by Singh 1973, Amrithadevarathinam et al. 1976, Sachan and Singh (1986), Sachan and Singh (1988), Wani and Srivastava (1989), Diwakar and Singh (1993), Thakur and Bhateria (1993), Kumar et al. (1997) and Pandey (unpublished data) with a few exceptions all the studies showed significant GCA and SCA effects for yield and component characters indicating that both additive and non-additive gene action were important in the inheritance of these traits. As anticipated yield components usually showed a preponderance of GCA variances compared to SCA variances (Labana et al. 1978, Dhillon et al. 1990).
Among crop species B. juncea, B. napus and B. carinata are predominantly self pollinating while B. campestris ecotypes with the exception of yellow sarson are cross-pollinated (Labana et al. 1992). In Indian mustard the out crossing varied from 7.6 to 22% (Labana and Banga 1984, Chauhan et al. 1987, Dhillon and Labana 1988, Ram Bhajan et al. 1991, Abraham 1994). In B. napus, outcrossing upto 16% was observed by Banga (unpublished data). Insect pollination has observed as an important component in B. juncea (Labana and Banga 1984).
Genetic male sterility
A number of sources of genetic male sterility are available in B. juncea (Badwal and Labana 1983, Banga and Labana 1983a, Banga and Labana 1985), B. campestris v. brown sarson (Das and Pandey 1961, Chowdhury and Das 1966, 1967a, b, 1968, Katiyar 1983), B. campestris v. yellow sarson (Chowdhury and Das 1966, Singh et al. 1987, Ram Bhajan et al. 1993, Gupta et al. 1997), and B. campestris v. toria (Singh et al. 1984, Anonymous 1997). Most of them are spontaneous in origin and exhibit monogenic inheritance. The exploitation of genetic male sterility is not economically viable as extra labour is required to rouge out the fertile plants from the ms line before anthesis. No linked seedling markers or pleiotropic effect of the male sterility gene has been found which could make it possible to identify the male fertile plants before the initiation of flowering
Cytoplasmic male sterility (CMS)
In B. juncea cytoplasmic male sterility was first time reported by Rawat and Anand (1979). The male sterility was stable over a wide range of environment conditions. Subsequently, the male sterility inducing cytoplasm was transferred to B. carinata (Anand 1987) and B. campestris (Banga, unpublished). A limitation to this system is much reduced nectaries and the frequent occurrence of malformed flowers.
The back cross substitution of the B. juncea genome into the cytoplasmic background of B. carinata resulted in a stable male sterile line (Banga et al. 1983). Banga et al. (1988) reported that breakdown of sterility at higher temperatures is a major limitation of nap system. Raphanus based CMS was transferred to B. juncea (Labana and Banga 1989). Resulted male sterility is highly thermostable but three problems arose in the utilization of this system are chlorophyll deficiency at low temperature (< 13 0C), low nectar production and lack of fertility restoration. Banga and Gurjeet (1994) observed thermosensitivity of sterility expression and yield penalty associated with the polima CMS system. ISN 126 and GLS 8909 acted as perfect maintainers for this system.
Male sterility conferred by ogu cytoplasm of Raphanus stativus has been transferred to B. juncea cv RLM 198 from male sterile B. napus through repeated back crossing and selection. The resulted male sterile B. juncea is however, highly chlorotic and late (Kirti et al. 1995). Gulati et al. (1994) and Raut et al. (1996) observed that B. oxyrrhina CMS in B. juncea background would be ideal for their exploitation in the hybrid programme. Enarthrocaprus lyratus cytoplasm causes male sterility in oilseed rape has been reported by Gundimeda et al. (1992). Banga and Banga (1997) has developed lyratus based CMS line in B. napus.
Alloplasmic B. juncea and B. napus have been obtained based on B. oxyrrhina, Trachystoma balli, Moricondia arvensis, Diplotaxis siifolia and Sinapis alba cytoplasm. Male sterility was found stable and coupled with high seed fertility (Prakash and Chopra 1988, Batra et al. 1990, Prakash and Chopra 1990, Rao et al. 1994, Kirti et al. 1995, Prakash et al. 1995, Prakash et al. 1996, Rao and Shivanna 1996, Prakash and Kirti 1997). Five cytoplasm : Erucastrum canariense, E. abyssinicum, Brassica cossoneana, Sinapis tubescence and Diplotaxis erucoides are in the process of introgression in B. juncea. In B. napus CMS development programme has been in progress with four CMS systems namely polima, tournefortii, lyratus and oxyrrhina (Anonymous, 1997).
In order to deploy CMS systems to develop commercial brassica hybrids, it is essential to have effective restorer lines. Complete fertility restorers could be developed for polima, tournefortii and lyratus CMS sources in B. napus (Banga and Gurjeet 1994, Banga et al. 1994, Banga and Banga 1997). Fertility restorers have been identified for trachystoma and moricondia based CMS lines of B. juncea (Kirti et al. 1997, Prakash and Kirti 1997). Angadi and Anand (1988) also reported restorer in B. juncea but that was not found suitable for commercial exploitation . As a results of these investigations functional 3 line system have become available in B. juncea and B. napus for the first time.
Lack of restorers has hampered the exploitation of several other CMS systems for producing commercial hybrid seed (Amandeep and Banga 1996, Banga and Amandeep 1996, Singh and Verma 1997). The major difficulty in finding restorer in natural accessions is the multilocus control of incompatibility between the mitochondrial and nuclear genome (Anand et al. 1985, Downey and Chopra 1996).
Utilization of self-incompatibility mechanism present in toria and lotni brown sarson types has been suggested to produce hybrids in these crops (Rai 1980). No major achievement reported so far in India.
Use of male gametocides
Very few researchers have evaluated the use of male gametocides in brassica (Chopra et al. 1960, Banga and Labana 1983b, 1984b, Banga et al. 1986). It was not found promising. Thus the male gametocides do not seem to have immediate practical utility.
Brassica oilseed crop has been proven to be one of the few commercially important crop plants to respond to all the biotechnologies. The incompatibility between mitochondrial and nuclear genomes have been overcome by generating mitochondrial recombination through protoplast fusion (Kirti et al .1991, Kirti et al. 1993, Kirti et al. 1995). Diversification of cytoplasmic male sterility systems has been accomplished through somatic hybridization (Kirti et al . 1992, Kirti et al. 1995, Armugam et al. 1996, Gaikwad et al. 1996). The molecular basis of CMS has been studied by analysing mitochondrial DNA modification (Pradhan et al. 1991, Kirti et al. 1993, Kirti et al. 1995, Mohapatra et al. 1998, Rao et al. 1998). The genetic diversity of parental lines and hybrids has been characterized by using isozyme markers (Banga et al. 1984, Sekhon and Gupta 1995, Prabhu et al. 1996, Pandey and Singh 1999). Embryo culture and protoplast fusion for developing alloplasmic lines is being used (Rao and Shivanna 1997, Sarmah and Sarla 1997, 1998). Transgenic based on barnase and barstar system is being evaluated (Ghosh 1998).
A hybrid of B. napus based on tour CMS (PGSH-51) has been released in India and given 18% more yield than check (Downey and Chopra 1996). Technology to develop GMS based hybrids in yellow sarson was perfected (Anonymous 1997). Seed production technique has been standardized for B. napus hybrid seed production (Rai 1995, Anonymous 1997). Three Mahyco Brassica juncea hybrids are under marketing sine 1997 (Zehr et al. 1997). Recently a canola type B. napus hybrid Hyolla 401, has been notified for commercial cultivation.
Hybrid brassica breeding should provide farmers with an opportunity to improve productivity, particularly in potential high-yield areas and where conventional breeding has apparently reached a yield plateau (Dhillon et al. 1996). The CMS system has been found to be the most effective and practical for developing brassica hybrids, several CMS sources have been identified but only a few of them have been deployed to develop commercial brassica hybrids (Banga 1992). Three new CMS-fertility restoration systems which are now at final stages of development, appear to be very promising for B. juncea. As hybrids technology has been perfected in B. napus, current emphasis is on developing canola quality hybrids in this crop. Technology to develop GMS/SI based hybrids in yellow sarson and toria has good prospects in year to come.
Per-se performance, genetic diversity and the combining ability of parental lines are generally used to predict heterosis. However, with availability of molecular marker technologies in brassica, it is easier to determine genetic diversity among perspective parental lines and tag heterotic gene blocks with molecular markers and subsequently transfer these in selected parental lines to improve their combining ability.
Identification of new CMS system and transfer of an available CMS system into genetic background of maintainer lines involves five or six back crossing to develop stable CMS lines. This is long and cumbersome process. The success in protoplast culture in brassica makes it possible to produce cybrids which enable immediate transfer of cytoplasmic male sterility into elite brassica cultivars (Mukhopadhyay et al. 1994). This approach should be explored widely to develop genetically diverse CMS lines more expeditiously. Improved techniques of hybrids seed production should be developed to economize seed production cost (Anand et al. 1985, Banga et al. 1995).
The use of hybrids brassica is a strategy to lift the yield ceiling of brassica to help the India’s meet the future projected demand, which will increase due to increasing populations and rising incomes. Though, we have very much succeeded in developing and releasing the first commercial hybrid cultivar in B. napus, which has very large acreage and could make much of dent on oilseeds production in the country is not yet in sight. So, the work on this area of research has to be accelerated. More basic work is needed on the male sterility, fertility restoration, heterosis, combining ability and hybrid seed production techniques. The adoption of F1 hybrids in brassica will ultimately depend on (1) the magnitude of the yield advantage obtained, (2) the cost/benefit ratio of using hybrid versus pure line seed, and (3) the efficiency of seed production, certification and distribution agencies available in the country. Progress during the next decade will determine how much hybrids in this major oilseed crop can help to increase India oilseed production.
This work was supported by a grant from the Council of Scientific and Industrial Research, Government of India.
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