Page 1 : y ,, \,, \ 369:, , vy, | wybrid varieties :, i ., ; . ; = ", ail nybrid D * T @) yields up to 200 nuts per plant per year. The coconut examplevslso provice 4, 4 i) evidence for cytoplasmic effect on hybrid vigour (compare the performances of thet xD, a, ale p x T hybrids). In addition to being higher yielding, the T x D and D x T hybrids are’ earlier, “ty in first flowering (earlier by about 10-11 months) than both their parents. In addition to the above, ‘ty | girst two hybrids, viz, WCT x CDO and CDO x WCT, eight more coconut hybrids have been, \ released for commercial cultivation., , N 99.13. SYNTHETIC. VARIETIES, , ‘he The possibility of commercial utilization of synthetic varieties in maize was first suggested by, Hayes and Garber in 1922. Synt ieties have been of great value in the breeding of those, ¢ross-pollinated crops Where pollinati troLis difficult)e.g., forage crop species, many clonal, th @ops like cacao, alfalfa, clovers, etc. Even in maize improvement, synthetic varieties have been, , dl. important, andi India there Had been a considerable emphasis on synthetic development. The, ‘kl maize programme of CIMMYT, Mexico is based on population improvement; the end-product, , | of such a programme is usually a syhthetic variety. The same applies, albeit to a lesser extent,, mul to the pearlmillet improvement programme of ICRISAT, Hyderabad, India., , , , , , , , , aah, 1) 92.14. DEFINITIONS, A synthetic variety is produced by crossing in all combinations a number of lines that combine, , well with each other>and is maintained by open-pollination in isolation. Some breéders use this, farm in the following restricted sense: a synthetic variety is the F', generation regularly reconstructed ., , SJ, , , , Uh, é| from the parental lines, and is not maintained by open-pollination. Jt would be seen that such a, , synthetic variety is essentially a mixture of several single cross hybrids. In this chapter, however,, 4 the term is used to describe varieties maintained by open-pollination after their production from, j_ the parental lines., , | Another term, composite, is sometimes used as a synonym for synthetic, which is not, entirely correct. A composite variety is produced by mixing the seeds of several phenotypically, , a, f, ‘outstanding lines and encouraging open-pollination to produce crosses in all combinations among, , , , v\, , a, , x, , re, , hem, The Ines used a produce a composite variety are rarely tested for combinitig ability with ~~, , | Sach other. Consequently, the yields of composite varieties cannot be predicted in advance, which, | {SiContrast to the synthetics; this is because the yields of all the Fis among the component, lines are not available. Like synthetics, composites are commercial varieties and are maintained, by open-pollination in isolation. Another type of population, called germplasm complexes are, | produced by mixing seeds from several lines or populations of diverse genetic origin. The, , objective of germplasm complexes is to serve as reservoirs of genes and gene combination; they, , | re experimental populations, and not commercial varieties., , 22.15, OPERATIONS IN PRODUCTION OF SYNTHETIC VARIETIES, , By definition, a synthetic variety consists of all possible crosses among a number of lines that, combine well with each other. The lines that make up a synthetic variety may be inbred lines,, Short-term inbred lines clones, open-pollinated varieties or other populations tested for GCA-or, , Jor combining ability With each other. The various operations involved in the production of, y_with each ole, , synthetic varieties are depicted in Fig. 22.6 and are briefly described below., , |W, , 22.15.1. Evaluation of Lines for GCA :, GCA of the parental lines of synthetic varieties is generally estimated byftopcross or polycross) WYY—, , lest because these varieties exploit that portion of heterosis, which is produced by GCA. Polycross, , Scanned with CamsScanner

Page 2 : Plant Breeding : Principles and Methog,, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , rp, , , , , , , , , , , , , , , , , , , , , , , 370 ii Topcross or polyereas test for Qo,, ing lines select ., VALUATION x outstanding ed as paren, PARENTAL, OF 5 FOR GCA TESTER, LINES INBREDS, SHORT- | (eg., AN OPENTERM INBREDS, POLLINATED, SYNTHETICS, VARIETY), OPEN-POLLINATED . :, POPULATIONS, CLONE Method 1. Equal seed from ali the, STEP 2. parental lines is mixed and planted in, PRODUCTION isolation. Open-pollinated seq, OF THE harvested as the synthetic varigyy, SYNTHETIC aa (Sym), VARIETY METI I., AR COMPOSITED SEED OF, ALL PARENTAL LINES, Method 2. The parental lines are, planted in a crossing block, and aj, OPEN-POLLINATED' ible intercrosses are made ai, possi Mong, SEE RRENTAL them. Equal seed from all the crosses, LINES HARVESTED is mixed to produce the synthetic, METHOD I. :, CROSSING BLOCK variety (Syn,), ALL POSSIBLE INTERCROSSES, MADE, AND EQUAL SEED FROM, ALL THE CROSSES COMPOSITED, ¥ Seed of the synthetic variety may be, sees multiplied for one or two generations, MULTIFLICATION before distribution by open-pollination, , , , , , , , , , SEED MULTIPLICATION inv Isolation, , (OPEN-POLLINATION IN, ISOLATION), , Fig. 22.6. Operations involved in the production of synthetic varieties., , , , , , , , , , refers to the progeny of a line produced by pollination with a random sample of pollen froma, number of selected Tines; this is the most commonly used test in forage crops. Polycross progely, , are generally produced by open-pollination in isolation among the selected lines. The limes thi!, , have high GCA are selected as parents of a synthetic variety. —, , 22.15.2. Production of a Synthetic Variety, , A synthetic variety may be produced in one of the followi iment, i owing two ways; thi i erimenl, evidence suggests that both the methods yield comparable scale. a, , 1 =, vy a amounts of seed from all the parental lines, i.e., Syng generation, are mixed and, Planted in isolation. Open-pollination is allowed-and-i oduce crosses 9, , all possible combinations among the parental lines. The seed from this pop! ie, , eyniheae variony opulation raised from this seed is the Sym; generation, All possible Crosses, , among the s i : tat ‘ ui, mounts of seed from : elected parental lines ‘are made in isolation. Ba e, , all ict, Pailin eee the crosses 1s composited to produce the synthetic variety? ", 4 m this composited seed is the Sym, generation, , 29. Hcg ', 2.15.3. Multiplication of Synthetic Varieties, , b yo aif, ations bene st itis generally multiplied by open-pollinaliO™y, istribution for cultivation. This is done 10, , V7, , , , , , Scanned with CamsScanner

Page 3 : 4, \ pybrid varieties 871, , 4 commercial quantities of seed, and is, ag. sugarbeets, the synthetic varieti, in the Sym Beneration itself., , The open-pollinated progeny from the Syn; generation is termed as Sy, that from Synz as, Syns, etc. The performance of Syny is expected to be lower than that of Syn; due to the production, , 2) of new genotypes and(decrease in the level o heterozygosilyys a consequence of random mating. | “<7”, %| flowever, there would not be noticeable decline in the subsequent generations (Syn3, Syng, Sys,, , equilibrium for any genéis reached after one) wo, , © common practice in most of the crops. Bul in some crops,, es are distributed for cultivation without seed increase, ie.,, , , , , , , , etc.) produced by open-pollination since th, , generation oF cory at mating. The synthetic Varieties are usually maintained by open-pollinated, nanomnaen pe further improved through population improvement schemes, particularly through, recurrent selection., , , , , , 22.16. MERITS OF SYNTHETIC VARIETIES, , Synthetic varieties offer several unique advantages in comparison to hybrid varieties, which are, listed below., , 1. Synthetic varieties offer a feasible means of utilizing heterosis in crop species where —~, pollination control is difficult, and production of hybrid varieties would not be, commercially viable., , 2. Farmers can use the grain produced from a synthetic varicty as seed to raise the next ~, crop. If care is taken to avoid contamination by forcign pollen, and to select a sufficiently, large number of plants to avoid inbreeding, the synthetics can be maintained for several, years from open-pollinated seed. As a result, farmers do not have to purchase new seed, every year., , 3. In variable environments, synthetics are likely to perform better than hybrid \ varieties Y, Because of their wider genetic base as compared to that of hybrid varieties. ", , 4. Inthe case_of synthetic Varieties, cost of seed is relatively lower than that of hybridy”, , varieties. In a country like India, where most of the farmers have limited financial, Tesources, synthetic varieties are likely to be more attractive than hybrid varieties., , 5. Seed production of hybrid varieties is a more skilled operation than that of synthetic, varieties. — —, , 6-Synthetic varieties are good reservoirs of genetic variability, ~, , 7. There is good evidence that the performance of synthetic varieties can be considerably, improved through population improvement without appreciably reducing variability., This offers a possibility for a continuous improvement in synthetic varieties., , , , 22.17. DEMERITS OF SYNTHETIC VARIETIES, 1. The performance of synthetic varieties is usually lower than that of the single or double, cross hybrids. This 1s because synthetics (and composites) exploit only GCA, while the, hybrid varieties exploit both GCA and SCA., 2. The performance of synthetics is adversely affected by lines with relatively poorer, - The performance of synthetics 18 00n~-=- n, , GCA. Such lines often have to be included to increase the number of parental lines, “inaking up the synthetics because lines with outstanding GCA are limited in number., , [2 Synthetics can be produced and maintained only in cross-pollinated crop species, while, - hybrid varieties can be produced both in self- and cross-pollinated crops., , Scanned with CamsScanner

Page 4 : ~ suggested by’Sewall Wrig] in 1922 and may be represented as follows, , , , , , , , , , , , , , , , , , , plant Breeding : Principles and Meth, RMANCE OF SYNTHRy, f, q), , 372, , 22.18. FACTORS DETERMINING THE PERFO:, VARIETIES, , The yield of Syn, as’ noted earlier, is |, , of new gene combinations and, to some extent,, , consequences of random mating in Syn., , due chiefly to CH the Produc 4, , ower than that of Sy :, heterozygosity, both being, , a loss in, , 22.18.1. Decline in Heterozygosity ., Random mating in Syn, leads to a marginal to appreciable loss in heterozygosity in Sym |, compared to that in Syn), The magnitude of decline in heterozygosity 10 Synz depends ony,, , number of inbreds involved in a synthetic, and on the proportion of inbreds having differey, alleles at a given locus. In case of a synthetic constitu breds, the number of parti |, , fed from six inl, , wabrods having the same genotype at a given locus could be 6, 5, 4 or 3, and the per oxy, , heterozygosity at this locus in the Syn, generation produced by method 1 (Section 2.15.2) wit), , be 0, 33.3, 53.3 and 60 per cent, respectively. In the Synz generation, produced by random mating, , in Synj, there will be a decline of 5.5 to 10 per cent in heterozygosity, depending on the, , proportion of inbreds having different alleles at the given locus. This decline ranges from 8-124,, 4.1-8.2% and 3.1-7.1% in the synthetics constituted from five, seven and eight inbreds., , When short-term inbreds are used for production of s nthetic varieties, some of the inbreds, , he gene in question. Tt can be readily shown that this situation, , would be heterozygous for tl, reduces the maximum heterozygosity attainable in Syn), and the magnitude of decline in, thetic constituted from six short-term inbreds, |, , heterozygosity in Synz. In the case of a syn, hetrozygosity in Syn, may tange from 16.7 (one inbred heterozygous, the remaining, , homozygous for the same allele) to 53.3% (four inbreds heterozygous, one inbred homozygous, for one and another homozygous for the other allele). In this case, the magnitude of reduction, , in heterozygosity in Synz may range from 1.4-5.8%, the values being appreciably lower than, |, , those given above., ‘ a, , , , 22.18.2. Decline in Syn, Performance, ‘The yield of Synz, as noted earlier, is lower than that of Syn, mainly due to the loss in heterozygos', , as a result of random mating. This decrease in Syng generation would depend upon (1) the number, of parental lines (Syrp populations) constituting tt the synthetic, and (2) on the difference in yielding, me it |, , abilities of Syn, and Syno generations. Syny i: i nthetic |}, , : Oy ANE 270 Bene nes the first generation synthetic produced by m4, , ifvall possible combinations the n parental lines (designated as Syng). This ladon was fits! |, A |, , , , , , , Syn, = Syn, —[(Syn, - Syng)/n], , where, Syn, is the pe, 2 performance ynthetic variety after one generation of random, , Syn, is the perform: ic i re!, P le ota ae : eae n the first generation synthesised from the paren, oe ‘ ee, , wena aur oer possible single crosses among n parental lines), SY"o is, . «og the F, , arental lines; arid thi be T pa Ental tines ¢ (Wl, “is the number 0} re 1, dom |, , synthetic variety. The, _ + performance of Syri, mating is expected to be contparable ben i a sanaevent generations obtained by ran I 1, yn2, provided co inati ign por, ntamination by foreig' Od |, , and inbreedin :, i are avoided. Avail ;, of Synz populations quite flay, evidence shows that the above formula estimates the ”, , matings |, tal lies, , Scanned with CamScanner

Page 5 : Hybrid Varletles, , 373, , 22.18.38. Enhancing the Performance of Syn, atl, It is apparent from the above relationship that the performance of Syn, can be improved as follows: (1) by increasing the number o, , f lines entering into the synthetic, (2) by increasing, the,, performance ‘OF Spay and (3) BY WAPOVINE TRE performance of song of he parental lines., ~ 22.18.31. Increasing the Number of Parental Lines. It is clear from the above formula, that, theoretically, an increase in the number of lines entering a synthetic would improve the, performance of its Syn. generation. TI his improvement would'be more noticeable with the smaller, values of 7, ¢.g., 1/2, 1/3, 1/4, 1/5, etc., but would become less noticeable as the n becomes, larger, €.g., 1/10, 1/11,-1/12, ete. Practically, lines with outstanding GCA in any crop are limited, in number. Therefore, as n is increased, lines with poorer GCA would have to be included in the, , synthetic. This would reduce the performance of Syn; and, therefore, that of Syn). Thus n cannot, , be increase Parcertain level without adversely affecting the performance of Syn, unless, several lines with Outstanding GCA are available. Generally,, , Lee J Ll a compromise has to be made, between thes two opposing forces. Obviously, the appropriate n would depend upon the GCA, of the available lines. If several lines with high GCA are av:, , a ailable, a larger n would be more, appropriate than when fewer lines with high GCA are available. ;, , In practice, the number of lines entering a synthetic variety varies form 3-15, but 4-10 is, the most common number. Some of the maize composites recommended for commercial cultivation, in India have as many as 18 (Composite C,) or even 22 (Composite Agaiti 76) parental ‘lines., In contrast, some other composites are crosses between two populations of maize, ec. g., composite, Tarun is derived from SynP200 x Kisan, or are derived from a single population, e.g., Chandan, Safed composite is an improved version of Zapolote Chico, an exotic variety, and Makki Safed, 1 composite is derived from Bhodipur White open-pollinated variety of maize., , 22.18,3.2. Increasing the Performance of Syn;. The performance of Syn, is the average, performance of all single crosses among the parental lines. ‘Clearly, Syn, performance, fepends, upon GCA_of the parental lines. As noted previously, the Syn, performance is limited b the, nonavailability of a sufficient number of lines with outstanding s GCA. At the same tir f the |, number of lines-is_ kept too low, say, 3 and 4, Syn, performance would be high, but that of Spy oa, would be adversely affected due to the Tow number of parental lines., , 22.18.3.3. Improving the Performance of Parental Lines (Sym). Synz performance can, , be improved _by increasing the performance of parental lines themselves. This can be achieved, in the following three ways. First, second and third cycle inbreds may be developed as their, performance would be better than that of the first cycle ones. Second, inbreds may be isolated, after the population has been subjected to recurrent selection for GCA. Finally, short-term inbred, , lines or even noninbred lines or populations may be used as parents., , , , , , , , , , , , The use of short-term inbreds is an attractive idea. Plants differ in GCA in the early stages, Of inbreeding process, and even So plants differ for GCA. Thus synthetic varieties may be, Constituted from plants or lines that have undergone only limited inbreeding, e.g., for one or two, 8enerations, or no inbreeding at all. Theoretically, the performance of a Synz from noninbred lines, Were expected to be higher than that of a Syn derived from inbred lines if the performance of, Syn Populations were comparable or equal. Further, the peak performance of Synz would be, attained with a relatively smaller number of noninbred lines than with inbred lines. It may be, Pointed out that synthetics constituted by crossing of populations subjected to RSGCA and RRS, , Would serve the same purpose as developing synthetic varieties from short-term inbreds or, noninbreds., , Scanned with CamScanner