Present Status And Future Prospects For Heterosis Breeding Biology Essay

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The sunflower (Helianthus annuus L., 2n = 34) belongs to the family compositae. It was given its name, "helianthus" (Gr-helios = sun, anthos = flower), in Europe in the 18th century. Sunflower is one of the main crops around the world, cultivated on a surface of approximately 21 million hectares (Škorić et al., 2007). It is one of the three crop species along with soybean and rapeseed which account for approximately 78% of the world vegetable oil. Heterosis of these crops has been exploited only over the past few decades. Hybrid sunflower became a reality with the discovery of cytoplasmic male sterility and effective male fertility restoration system during 1970. Sunflower hybrid breeding was started economically in discovering CMS by Leclercq in 1960 and restorer genes by Kinman in 1970 (Miller and Fick, 1997). First sunflower hybrids were produced in US in 1972 and reached 80% of production in five years (Fick and Miller, 1997). Single-cross hybrids quickly became dominantly in sunflower cultivars in the world. Hybrids were preferred by farmers due to high yield and quality potential, homogeneity, same time maturing and easy possibility of cultural applications worldwide. Hybrid vigor has been the main driving force for acceptance of this oilseed crop. Utilization of heterosis has allowed sunflower to become one of the major oilseed in many countries of Eastern and Western Europe, Russia and South America and is an important crop in the USA, Australia, South Africa, China, India and Turkey. Of the approximately 16.5 million hectares of sunflower grown in the major producing countries, 11.5 million hectares are planted to hybrids. Utilization of heterosis has allowed sunflower to become one of the major oilseed in many countries of Eastern and Western Europe, Russia and South America and is an important crop in the USA, Australia, South Africa, China, India and Turkey. Of the approximately 16.5 million hectares of sunflower grown in the major producing countries, 11.5 million hectares are planted to hybrids (Miller, 1998). Present day sunflower cultivars contain more than 40% oil and 18-20% protein. Sunflower oil is of good quality as it contains high proportion of linoleic acid which is a polyunsaturated fatty acid. It is also a good source of calcium, phosphorus, nicotinic acid and vitamin E. There are a number of advantages of growing sunflower for oil compared with other cultivated species. No special machinery is needed to produce this crop. It can be grown as catch crop in many situations. Being drought resistant it is well suited for rainfed as well as irrigated areas. Sunflower seeds contain about 42 percent high quality edible oil. The crop duration is such that it can be sandwiched between two cotton, rice or potato crops. So it has a great potential to make the country self sufficient in edible oil. It is needed to improve the yield potential of sunflower varieties/hybrids to increase sunflower production of the country.

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Hybrids due to their higher yield potential are being used throughout the world for increasing the productivity of almost all crops including sunflower. With an area of 2.1 million hectares and production of 1.25 million tons is one of the most important oilseed crops of India. India's share in total world production of sunflower is about 6%, accounting for 10.0% of world acreage. Sunflower is grown year round as a sole crop and it also fits well as an intercrop with legumes, in double cropping and in three-crop rotations. Despite the premier position the crop holds in the vegetable oil economy of the country, the average productivity level is low (629 kg/ha) as compared with the world's productivity (1240 kg/ ha). After the introduction of the crop in India in the early 1970s, a need for hybrids was recognized. Experimental hybrids were developed in 1974-75 using the cms and restorer lines introduced from USA.

The first hybrid from public sector was released for commercial cultivation in 1980. Since then the hybrid breeding program has been quite successful and 29 productive hybrids were developed by both public (18) and private sectors (11). These hybrids are intended for different agroproduction situations, which are occupying 95% of the crop-grown area. According to the website of the directorate of oilseeds research Hyderabad, India, eleven open pollinated and sixteen hybrids have been released under AICORPO. Despite these successes with the crop, the major problems threatening sunflower productivity in India are the stagnating and unstable yields and vulnerability to various biotic stresses. The major diseases attacking sunflower crop in India are Alternaria leaf spot (Alternaria helianthi), rust (Puccinia helianthi Schw.), downy mildew (Plasmopara halstedii (Farl.), Rhizopus head rot and a sunflower necrosis disease (a disease of recent origin). The crop is affected at all growth stages. Disease incidence is highly unpredictable and it tends to fluctuate from year to year, season to season and location to location, thus necessitating region-specific management strategies. Among the various approaches to manage these diseases, host plant resistance is the most reliable and economical to the end users. Plant breeding efforts to develop varieties/hybrids with inbuilt tolerance to the major diseases are constrained by the narrow genetic base of the cultivated sunflower. Development of hybrids from Inbred Lines

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Hybrids are also developed from inbreds derived from the same narrow gene pool. A gene bank with about 1000 accessions is available in the country but it is characterized by unacceptable levels of intra accessional heterogeneity in many lines thus restricting the scope for utilization of the material in breeding programs. Most studies on breeding for disease resistance were confined to screening of the available cultivar germplasm against the diseases under natural conditions and hence, these sources could not be converted into usable forms. With the exception of downy mildew disease, the released cultivars rated as tolerant/resistant are based on their field reaction to the pathogen. However, none of these were bred from parents that were classified as genetically resistant to a particular disease.

Wild Helianthus species serve as potential sources of novel genetic variability and several desirable characteristics such as resistance to biotic and abiotic stresses, cytoplasmic male sterility, fertility restorer genes and oil quality have been successfully introgressed into cultivated sunflower (Seiler, 1992). The growing needs for additional genetic variability to improve the cultivated sunflower makes it necessary to collect, maintain, characterize, evaluate and utilize the wild sunflower germplasm. Concerted efforts are required to incorporate additional genetic variability from reliable sources by integrating modern biotechnological tools and conventional breeding methods. (Sujatha, 2006). Recently, a study of genetic diversity among 177 public sunflower inbred lines was carried out. These inbred lines were developed and released by USDA -ARS from 1970-2005. Target region amplified polymorphism (TRAP) marker technique was foiund suitable for fingerprinting of sunflower inbred lines. These inbred lines will be useful for further hybrid breeding programme in sunflower (Yue et al., 2009).

Hybrid Selection

Selection of adaptable hybrids combined with use of recommended production practices are important factors for profitable sunflower production. Hybrids now are planted on over 99 percent of the USA acreage. Hybrids replaced open-pollinated varieties because of their increased yield, pest resistance, uniformity, stalk quality and self compatibility. Growers should use several criteria in hybrid selection. First, they should take an inventory of available hybrids being marketed in their area. Seed yield potential is an important trait to consider when looking at an available hybrid list. Yield trial results from university experiment stations and from commercial companies should identify a dozen or so consistently high yielding hybrids for a particular area. Results from strip tests or demonstration plots on or near growers' farms should be evaluated. Yield results from previous years on an individual's farm and information from neighbors is also valuable. The best producing hybrids in a region may produce approximately 2,000 pounds per acre with good soil fertility and favorable soil moisture, or up to 3,000 pounds per acre in the most favorable growing conditions. North Dakota's average yield ranges from 1,200 to 1,400 pounds per acre. An important trait to consider is pest resistance or tolerance. Hybrids are available with tolerance to rust, Verticillium wilt, certain races of downy mildew, and early ripening. Growers should check with their local seed dealer or sunflower seed company representative to obtain this information. Stalk quality, another trait to consider, is a stalk reaction to damage from several pests. Hybrids with good stalk quality are easier to harvest and field losses generally are reduced. Good stalk quality in sunflower also allows the crop to withstand damages and field losses due to high winds. Uniform stalk height at maturity is another important trait to consider.

Oil percentage should be a trait to consider in hybrid selection. Several environmental factors influence oil percentage, but the hybrid's genetic potential for oil percentage also is important. If the market price is influenced by oil percentage, then high oil hybrids should be considered. Current hybrids have oil percentages ranging from 38 to over 50 percent. Domestic sun oil crushers have been paying a premium for higher oilseed in recent years. Hybrids with oil percentages in the 40-45 range, on a 10 percent moisture basis, should be selected. Growers should examine physiological maturity ratings to take advantage of sunflower hybrids, especially when considering late plantings. Growers need a hybrid that will mature well within the average frost-free period. Compared to the mid-late 1970s, the maturity ratings of sunflower hybrids available today are much broader in range of maturity (earliest to full season). Test weight also is an important trait for consideration, especially for hybrids selected for late planting or replanting. A test weight of 25 pounds per bushel is required to make official USDA grade, but recently developed hybrids have test weights ranging from 28 to 32 pounds per bushel. Self compatibility, the ability of the sunflower plant to pollinate itself in the absence of insect pollinators, is another desirable trait in hybrids. Finally, the last item to consider is the purchase of the seed from a reputable seed company and dealer with a good technical service record if production problems do occur and assistance or consultation is required.

Cytoplasmic male sterility for developing Sunflower hybrids

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The discovery of cytoplasmic male sterility (CMS) in sunflower by Leclereq (1969) and subsequent identification of genes for fertility restoration have resulted in the development of commercial hybrids since 1972. However, all the sunflower hybrids that are commercially grown have a single source of CMS discovered by Leclereq leading to homogeneity and potential risk that was evident in case of maize. Diversification of CMS sources is inevitable in any hybrid-breeding program. Fortunately in sunflower more than 62 new CMS sources of different origin have been reported (Serieys, 1999). The diversity of the new sources was assessed mainly based on cytoplasmic male sterility and fertility restoration systems. Only a few investigators have described interactions between cytoplasms and nuclear genes in the expression of several qualitative and quantitative characters and beneficial cytoplasmic nuclear interactions have been reported in various crops. In sunflower, a unique cytoplasmic nuclear interaction caused reduction in chlorophyll, photosynthetic rate and overall reduction in vigour and positive effect and oil content At present, the main aim of sunflower breeding is the obtaining of high yield commercial hybrids. Sunflower hybrids are object of breeding attention because of their agronomic and economic advantages over varieties (high productivity, oil content, disease resistance, etc.). The central component of sunflower hybrid development is cytoplasmic male sterility (cms). The obtaining of hybrids with high heterosis effect became possible after the discovery of the first cms source by P. Leclercq (Leclercq, 1969) and detection of fertility restoration genes by M. Kinman (Kinman, 1970).

Development of sterile cms analogues of lines used in sunflower breeding programs for commercial hybrid development is one of the practical applications of cms investigations. CMS PET-1 is a cms source which is widely used of in sunflower hybrid development. Such cytoplasmic uniformity presents a potential risk for hybrid sunflower production. The utilization of different cytoplasmic backgrounds in hybrid development will improve general variability of the sunflower and lessen the threat of epiphytotics. The different cms lines and genetic stocks registered are presented in table 1.Thirteen new sterile cms analogues on the base of different sunflower cms sources have been obtained. The obtained backcrosses have been evaluated for main agronomic traits, including the resistance to the important sunflower pathogens. As a result of this study, the most prospective cms analogues were designated for future use in breeding programs. (Tavoljanskiy et al., 2004). Cytoplasmic male sterility (CMS) in commercial sunflower hybrids is thought to be derived from a related wild species, Helianthus petiolaris, yet CMS lines are known to carry the chloroplast DNA genotype of H. annuus. Commercial sunflower hybrids released from Argentina are listed in table 3.To clarify the origin of sunflower CMS, polymerase chain reaction-based strategy was developed for detecting CMS in sunflower and surveyed more than 1,200 plants representing 55 accessions of H. annuus and 26 accessions of H. petiolaris. 160 progeny from three crosses were tested for strict maternal inheritance of organelle DNAs to determine if the apparent discrepancy in the species donor of the mitochondrial DNA and chloroplast DNA genotypes in CMS lines might result from low-frequency maternal or biparental inheritance of either organelle. No CMS cytotypes were observed in natural populations of either H. annuus or H. petiolaris, and strict maternal inheritance of organelle DNA was observed. Pollen scarcity has been studied in an Indian hybrid KBSH-1. In case of scarcity of pollen during pollination for hybrid seed production of KBSH-1 or when there is excess availability of pollen from restorer line-6D-1 due to lack of synchrony and to have better utility of available pollen, pollen grains may be stored either in refrigerator or in earthen pots filled with water and covered with wet cloth and utilized properly when male sterile line-CMS234A is receptive, to get increased yield (Sumathi et al., 2007). The sunflower hybrids for downey mildew resistance released by ICAR India are listed in table 2. Private sector seed companies have also released many important hybrids for cultivation, some recent ones are listed in table 4.

Future prospects

Contrary to conventional breeding methods, the F1 hybrid method facilitates the genetic diversification of sunflower genotypes through a more efficient exploration of the Helianthus genus with its potential gene pools for superior quantitative and qualitative traits. The genetic progress already achieved will be further increased by modern methods of biotechnology and genetic engineering which will allow development of a new super productive type of sunflower. The yield advantage of FI hybrids compared with best open-pollinated varieties stands demonstrated. Unfortunately all presently available F1 hybrids are based on the petiolaris CMS source. More efforts must be directed towards diversification of exploitable CMS sources. With the rapid pace that characterizes the development of the modern molecular genetics, the sunflower hybrid breeding can not progress correspondingly without the joint efforts of scientists and research institutes involved. Establishment of an FAO research network on sunflower was a step in the right direction The main current problems of sunflower hybrid breeding are the further improvement of productivity by increasing seed and oil yields and the reduction of genetic vulnerability to diseases and heat or water stress encountered in various ecological zones.( Vranceanu 1998). New sources of CMS and fertility restoration genes will help reduce the genetic vulnerability of commercial sunflower hybrids because of the current use of single cms cytoplasm ,PET1 (French) derived from H petiolaris Nutt and a few fertility restoration genes These new cms and corresponding fertility restoration lines will provide cytoplasmic diversity for hybrid sunflower production (Jan and Vick 2006)