HYBRID BREEDING AND POSSIBILITY OF APOMIXIS IN EXPLOITATION OF HETEROSIS IN PIGEONPEA
K.B.Wanjari,
ICAR Emeritus Scientist
Dr Panjabrao Deshmukh Krishi Vidyapeeth, Akola, 444 104, Maharashtra.
(email: )
1. Introduction:
In broader sense pulses are those legume species whose kernels either whole or split are utilized for consumption by human. They form a major source of protein. Pulses very much complement the cereals in vegetarian diet to balance the amino-acid profile in the nutrition. Many of these legumes form an inherent symbiotic association with nitrogen fixing bacteria viz., Rhizobium which has great significance in inclusion of pulses in the cropping system. The merits associated with the pulse crops have played important role in the historical developments in agriculture.
Pigeonpea is one of the major pulse crops of dry land agriculture due to its deep tap root system and inherent drought resistance. It is an important source of proteins in the diet of majority of vegetarian population. It is also a source of protein rich feed for cattle. Ability to produce high amount of biomass per unit area makes it more useful as fodder as well fuel for the rural masses. High amount of organic matter added to the soil through foliage drop after senescence and symbiotic association with Rhizobium is an important character contributing towards soil improvement. Very strong tap root system after decomposing improves soil airation. The dried biomass of stem and branches are useful for thatching the roof and making walls of low cost huts in rural area. The multifarious uses of pigeonpea plant are responsible to value it very high in the economy of our farmers.
India has the distinction of being the world leader in exploitation of heterosis in crops like cotton, pearlmillet, castor, pigeonpea etc. Heterosis is mainly capitalized on the non additive gene action. The level of heterosis expression in the hybrid is largely dependant on allelic frequency differences among the parents and presence of certain level of dominance. Use of genetic male steriles available in pigeonpea can ensure 100% out crossing. As a result breeding methods in pigeonpea can be either the standard classical methods for self pollinated crops as well as those for cross pollinated species which may include F1 hybrid breeding and population improvement. Stable source of genetic male sterility (Reddy et al., 1978) and cytoplasmic-genetic male sterility (Tikka et al., 1997; Saxena and Kumar, 2003) and predominance of abundant natural out pollination (Kadam et al, 1945; Abrams, 1967; Saxena and Sharma, 1990) prompted to go further in hybrid breeding in this crop and developed interest in F1 hybrid breeding in recent decades. Recently, apomixis for fixation of heterosis expressed in F1 hybrid and to use the seed harvested from the hybrid repeatedly is being tried and it is a topic of interest from the point of avoiding hybridization for commercial use of hybrid seed.
2. Distribution:
Major area of pigeonpea cultivation lies in Asia. India is a major growers of pigeonpea covering about 90 per cent of the World hectarage under the crop (Nene and Sheila, 1991). Other Asian countries growing pigeonpea are Myanmar, Nepal, Srilanka, Bangla-desh, Pakistan and Thailand. In Africa, Kenya is a largest pigeonpea grower followed by Uganda, Malvi and Tanzania. Dominican Republic and Venezuela in Central America have considerable area under pigeonpea, while other American states like Puerto Rico, Jamaica, Panama, grows this crop on few thousand ha. Australia has taken interest in this crop in last few years and has consideration towards production of the crop on few thousand ha.
Major consumers of pigeonpea lie in Asian countries more particularly in India and are considered as a potential market for export by Australia and African countries. Myanmar has also considerable export of pigeonpea. The value is still improved in the open global economy when countries like Myanmar, Australia, few east African countries are successfully dealing it as an export commodity. In India, pigeonpea accounts for about 14 per cent of the area under pulses and 20 per cent of the pulses production in the country. Major area lies in Maharashtra, Madhya Pradesh, Karnataka, Andhra Pradesh and Gujarat.
3. Development of hybrid pigeonpea:
Development of a particular approach in crop improvement is based on the convenience. The convenience is decided by various considerations such as the status of the crop improvement work, available varieties and available germplasm. This makes one to enter into technically easy or complicated strategies. Other considerations are about the resources in terms of manpower, finance, field and laboratory facilities available at the disposal of a breeder.
Pigeonpea improvement in early ages started with selection from landraces. Later it was emphasized to incorporate resistance to major diseases like Fusarium wilt and sterility mosaic. Unfortunately the pigeonpea improvement was restricted to conventional breeding may be due to limited resources and could not achieve gains in productivity potential (Saxena, 2002) to cope up with increasing demand. It has now greater value than any time in past and invites research policies to have new non-conventional approaches like heterosis breeding through F1 hybrid.
3.1 Feasibility of heterosis breeding in pigeonpea:
Heterosis is a function of dominance of genes and genetic divergence among the parents(H=dy2). Pigeonpea has been considered technically suitable for heterosis breeding due to predominance of non-additive genetic variance for the trait like grain yield and other important yield components (Reddy et al. 1981, Sidhu and Sandhu, 1981; Saxena and Sharma, 1990). Germplasm present wide rage of genetic diversity. Abundant natural out pollination (Saxena et al. 1990) can be utilized to avoid tedious hand pollination. The need of male sterility to avoid mechanical emasculation had been fulfilled through discovery of genetic male sterility (Reddy et al. 1978 and Wallis et al. 1981, Wanjari et al. 1995). These points were favorable in commercial seed production of F1 hybrids in Pigeonpea. F1 hybrids breeding based on genetic male sterility has thus, been attempted successfully in late nineties (Rathnaswami et al., 1997; Wanjari et al., 1998; 1999b, Khorgade et al., 2000; Saxena, 2005).
3.3 Constraints in adoption of GMS based hybrids:
None of the hybrids based on GMS could have sufficient hybrid seed on commercial scale due to which they did not find place on farmer’s field. Efforts were made to analyze the constraints in promotion of GMS based hybrids. Niranjan et al. (1998) concluded that the cost of hybrid pigeonpea seed is within the affordable limits and the hybrid advantage is salable but the technology itself suffers with major bottlenecks, when it comes to large-scale seed production. Imperfection in hybrid seed production technology for large scale multiplication, inherent low yielding ability of seed parents from early maturity group, working limitations in rouging 50 per cent fertile plants from female parent i.e. GMS, heavy damage from pod borers, difficulties in determining genetic purity and imposition of quality control and lack of seed production knowledge, inputs and trained manpower in the initial phase of hybrid development were some of the limitations in promotion of GMS based hybrids. Hence, special efforts were needed to deal these aspects. Subsequently, CGMS system could be established (Tikka et al 1997, Saxena and Kumar, 2003).
3.4 Development of CGMS system
Many seed growers in India are trained in CMS-based hybrid seed production in maize, sorghum and millet. Hence it was thought that, the development of cytoplasmic genic male-sterile lines in pigeonpea would effectively overcome the seed production inefficiencies of genetic male-sterility based hybrids. The first attempt to develop CMS in pigeonpea using the crossable wild relatives of pigeonpea was made as early as 1981 by Reddy and Faris. They crossed Cajanus scarabaeoides, a wild species with fertile F1 plants of Cajanus cajan x C. scarabaeoides cross. The resulting BC1F1 plants were fertile but in BC1F2 generation some male-sterile segregates were identified. This male-sterility was linked with female-sterility and therefore it was not pursued further. However it attracted attention of the breeders and deliberate search of cytoplasmic male sterility was made through wide hybridization at many centers.
Ariyanayagam et al. (1995) crossed Cajanus sericeus with a short-duration advanced breeding line of pigeonpea. The F1 progeny was partially male-sterile and the backcross (BC1F1 – BC3F1) populations (2-19 plants) were found segregating for male-sterility. The maternally inherited male-sterility in the BC3F1 (15 plants) ranged between 8 - 99 per cent. Beside these initial efforts in developing CMS at ICRISAT and BARC, Trombay six Indian national centers viz. I I P R, Kanpur; I A R I, New Delhi; S D A U, S K Nagar; P D K V, Akola; T N A U, Coimbatore; and P A U, Ludhiana joined the efforts to develop CMS lines through inter-specific crosses.
The identification of male-sterile plants were reported at Dr Punjabrao Deshmukh Krishi Vidyapeeth, Akola in an interspecific cross with C. volubilis and C. lanceolatus (Srivastava et al., 1997). Wanjari (1998) used the wild species C. sericeus, C.volubilis, C. cajanifolia, C. lineatus, C. lanceolatus as sources of alien cytoplasm. Male sterility isolated from the derivatives of C.volubilis x C.cajan var. ICPL-83024 (Wanjari et al. 1999) was found to have cytoplasmic inheritance. However special efforts to establish the fertility restorer were in vain, for which it could not be utilized for hybrid breeding. Another CMS system could be stabilized using cytoplasm from C .lanceolatus (Wanjari, et al., 2003). It has been diversified in six agronomic backgrounds viz., AKT9827, AKWR-376, AKMR-875, AKMR-840, MDDRL-11, AKT-8811 (Anonymous, 2007).
Tikka et al., (1997) established stable cytoplasmic male sterility using C. scarabaoides as a source of cytoplasm. They selected 14 male-sterile plants from an F2 population of cross C. scarabaeoides x C. cajan. For maintaining this male-sterility a number of lines were crossed. The progeny of ICPL288 produced all male-sterile plants. This male-sterile line, designated as GT 288A, was found stable over environments (Saxena et al., 2004). It has been successfully transferred in various agronomically desirable genotypes at various pulses breeding centers. Later CMS system was derived from a cross involving cytoplasm from Cajanus scarabaeoides (Saxena and Kumar, 2003) and C. cajanifolius (Saxena et al., 2005)
3.5 Development of fertility restorer amenable to autogamous hybrids:
Practical utility of CMS for hybrid development in any species where seed is of economic importance, exists only after availability of the fertility restorer lines. The utilization of diverse germplasm may lead to identification of fertility restorer lines. Some of the restorers produced hybrids which did not set seeds when placed inside the net net-cages. It is likely to be due to the inability of hybrid to have effective self pollination. Although it is not pre-requisite, the auto-gammy in the hybrid would be highly desirable to make it more productive.
3.6 Fertility restorers for cms lines based on cytoplasm from C. scarabaoides :
Using several derivatives from F5 population of cross Cajanus scarabaeoides x Cajanus cajan and some pigeonpea germplasm lines as pollinators with the CMS based on Cajanus scarabaeoides, deliberate search for the fertility restorer led to 18 restorer lines at Sardar Krushi Nagar (Saxena et al., 2004). Among them eight were of medium maturity (135-150 days) while 10 lines were of medium-late (150-175 days) maturity group. Only one restorer (GTR-11) had determinate growth habit and the remaining were non-determinate types.
At Panjabrao Deshmukh Krishi Vidyapeeth, Akola, two cytoplasmic male sterility systems i.e. one based on C.scarabaoides cytoplasm (Tikka et al. 1997) and another based on C.volubilis cytoplasm (Wanjari et al. 1999) were used to produce hybrids with more than 250 germplasm lines as pollinator parent (Patel, 2001; Wanjari et al., 2003). All the hybrids produced on C. volubilis based AKCMS-1 were found to be sterile while few hybrids on C.scaraboides based CMS-GT288 gave an indication of availability of fertility restorer mechanism with one Kenyan germplasm line viz., ICP-10875. The expression of fertility restoration varied in terms of anther dehiscence, pollen size and pollen fertility in the hybrids. The germplasm line AK-200355 has been found to be a good source of restoration (Wanjari and Patel, 2003a). However it was associated with undesirable agronomic traits such as late flowering/maturity, determinate flowering, highly affected due to terminal drought resulting in shy pod setting. It was further diversified in desirable agronomic background.
Efforts were made to characterize the fertility restorer genes in terms of pollen and anther traits. Study of segregating population in F2 derived from fertile hybrid indicated that selection per se for fertility in the derivatives can be a criterion for obtaining good fertility restorer lines from the population (Wanjari et al., 2003). Lad and Wanjari (2004) studied F3 and BC1F2 progenies derived from the fertile plants in F2 and BC1 respectively and revealed that better anther dehiscence has been largely associated with higher pollen fertility in the derivatives. Such derivatives when used for hybridization on CMS-GT288 produced highly fertile hybrid with better autogamous seed setting inside the insect proof cages.
Saxena and Kumar (2003) reported eight more fertility restorers with the CMS based on C. scaraboides. At present a number of fertility restorers have been identified in different genetic backgrounds and these could also be used to develop base populations for the identification of high combining restorer lines. Saxena et al., (2004) expressed environment sensitivity in expression of fertility restoration in the hybrids of some of the restorers. Hence it would be necessary to confirm uniform expression of fertility in the hybrids across the seasons and locations.
3.7 Genetics of fertility restoration:
Efforts have been made to work out genetics of the fertility traits viz., yellow vs. white translucent anther color, dehiscent vs. non-dehiscent anthers and good vs poor dehiscence (based on quantity of pollen grains released). Lad and Wanjari (2005) presented very interesting segregating behavior in the selfed progenies of fertile F2 plants of CMS-GT288 x ICP10875 in F2, F3 and BC1F2 generations. The segregation pattern varied in different plant to row progenies for each of these traits. It is postulated that there is possibility of involvement of complex epistasis of more than three genes for each of the three fertility traits described by (Wanjari and Patel, 2003) viz., anther color (translucent white vs. yellow), dehiscent vs. non-dehiscent anther and quality of dehiscence (good vs. poor). However, Lad and Wanjari (2004) and Tagade (2004) brought out possibility of selection of highly fertile plants in the segregating generations which had better chances to be a good fertility restorer in subsequent generation for the CMS with C. scaraboides cytoplasm. For the selection for pollen fertility secondary criteria can be based on excellent dehiscence of the anthers (Wanjari and Patel, 2003b).