Showing papers on "Heterosis published in 1980"

Crossbreeding in Beef Cattle: Evaluation of Systems

Abstract: Summary The basic objective of beef cattle crossbreeding systems is to optimize simultaneously the use of both nonadditive (heterosis) and additive (breed differences) effects of genes. Experimental results evaluating rotational crossbreeding systems indicate that high levels of heterosis are sustained in successive generations and that the relationship between loss of heterosis and loss of heterozygosity approaches linearity. Major differences among breeds have been demonstrated for most characters that contribute to production efficiency. Results based on both experimentation and computer simulation indicate that differences in additive genetic merit of breeds for specific characters can be used in specific crossbreeding systems to synchronize genetic resources with other production resources and to provide for complementarity through terminal sire breeds. Rotational crossbreeding systems have the advantage of using heterosis in all females and progeny in a self-contained commercial herd; however, fluctuation between generations in additive genetic composition relll~es use of breeds that are generally compatible. This requirement restricts the use that can be made of breed differences to synchronize germ plasm resources with other production resources and eliminates the use of complementarity other than in a combined breed-rotation, terminal-sire system. A static terminal-sire crossbreeding system provides opportunity to synchronize germ plasm resources with other production resources in about 50% of the cow herd, to use maximum (FI) heterosis in about 67% of the calves marketed and to use complementarity in

Effective crossbreeding systems utilizing Zebu cattle.

Abstract: The basic objectives and procedures for successful crossbreeding of beef cattle were considered, with special reference to the levels of realized hybrid vigor that might be anticipated for different crossbreeding programs involving the American Brahman (Z1) and European (Ej) breeds. Intralocus heterozygosis levels with respect to breed origin of genes (breed heterozygosis) were computed for various crossbreeding systems. These values were weighted by approximate relative F1 heterosis levels for different breed combinations (1.0 for Z1Ej and .3 for E1E2) to yield the estimated relative hybrid vigor levels for total performance in alternative crossbreeding systems. The estimated average maternal heterosis levels for the various systems, were respectively 0, 33, 67, 66, 50, 54 and 49% for production of F1 (EZ) calves, three-breed terminal cross calves, two-breed-of-sire (E1--Z1) rotation crosses, three-breed-of-sire (Z1--E1--E2) rotation crosses, two-breed (Z X E) synthetics, three-breed (Z1 X E1E2) synthetics and four-breed (Z1E1 X E2E3) synthetics. The respective values for calf heterosis levels were 50, 57, 67, 66, 50, 54 and 49%. Successful crossbreeding requires (1) the choice of appropriate breed combinations for the environment and production system management programs to support the increased production potential or crossbred cattle.

Heterosis in rice.

Abstract: The nature and extent of heterosis was estimated for grain yield and its related characters in 10 parent diallel crosses in rice. The performance of their F2’s was also studied at two locations. The high heterotic effects for grain yield were observed in IR 8 X Sona and Jhona 349 X Basmatti 370 hybrids which gave 42–44 and 56• 35 percent heterosis, respectively over better parent and 42–44 and 31–53 percent heterosis, respectively over the best variety IR-8. The high positive heterotic effectsfor spikelets/panicle and high negative heterotic effects for grains/panicle indicated occurrence of high percentage of spikelets sterility which contributedto the lack of heterosis in grain yield in many crosses. The crosses C 12329 x BJ-1, Jhona 349 x TK. M-6 and TKM-6 x Basmati 370 showed very highlevel of negative heterosis for grains/panicle which was due to very high spikelet sterility in these crosses On the basis of yield performance of F2’s at two locations, the crosses IR 8 x Sona, IR 8 x Jhona 349, Sona X BJ-1 and IR 2153-43-2-5-4 X BJ-1 appeared to be promising and need the attention of the breeder for their exploitation.

A new explanation for egg production heterosis in grosses between white leghorns and australorps

Abstract: 1. Present population genetics theory treats heterosis as the reversal of inbreeding depression. This theory is not generally applicable to heterosis occurring in crosses between different breeds or between unrelated strains of the same breed. 2. Heterosis occurring in such crosses is explained in terms of different complementary epistatic gene pathways homozygous in the parental breeds. 3. In the absence of linkage, these epistatic gene pathways segregate in the second and subsequent cross‐bred generations leading to the loss of the heterosis observed in the first cross‐bred generation.

The relationship between heterozygosity and heterosis in reproductive traits in mice.

Abstract: Continuous reciprocal backcrossing results in individuals which are 67% heterozygous, and which, assuming a linear relationship between heterozygosity and heterosis, would be expected to exhibit 67% of the heterosis found in Fi hybrids An experiment was undertaken to investigate this relationship in the reproductive traits of mice By crossing and repeated backcrossing in both directions using two unrelated strains of outbred mice, females were produced which were 25, 50, 75 and 100% heterozygous The crossbred females and the two purebred strains (922 females in all) were mated to a sire line of a genetically distinct strain to standardize foetal heterosis The reproductive performance of females with different percentage heterozygosity gave estimates of maternal heterosis A significant positive linear relationship was found between heterozy-gosity and heterosis in litter size, total weight of litter and individual weight of progeny both at birth and at weaning The relevance of these results for livestock breeding is discussed

Are 'multiple cross-multiple pollen hybrids' an answer for productive populations in Brassica campestris (var.) Brown Sarson? : Part 2: evaluation of 'mucromphs'.

Abstract: A set of 167 ‘mucromphs’ was evaluated in their F1 generation for seven ‘plant’, four ‘plot’ and five ‘ratio’ characters. Methods of characterizing the parents and the hybrids on the basis of combining ability effects described in the first part were found to be efficient in identifying desirable cross combinations for pedigree breeding or generating composite populations. Order effects of multiple-pollen combinations were found to be potent for a number of character components. High x Low cross-combinations were found to produce a high frequency of heterotic crosses followed by High x High. Selected biparental progenies, single crosses, three-way crosses and elite varieties were successful female parents and combinations of pollen from two varieties, one variety and three varieties were successful male parents, in that order, in producing significant heterosis in mucromphs. The relationship between general and specific combining ability on the one hand and realised heterosis on the other was discussed.

Heterosis and combining ability studies in grain sorghum

Abstract: Heterosis over superior parent indicated that grain yield was most heterotic character followed by the number of grains per panicle. The yield superiority of hybrids resulted mainly from the increase in number of seeds per panicle. The hybrids were earlier and taller.

Heterosis for chiasma frequency and quantitative traits in common beans (Phaseolus vulgaris L.).

Abstract: Ten genotypes, including inbreds, hybrids, and advanced populations, were examined in order to elucidate the relationship between position and frequency distribution of chiasmata and quantitative traits, including yield heterosis in common beans. The hybrid and advanced population groups were determined to possess 83% and 54% increased chiasma frequency, respectively in contrast to inbred lines. The increase in chiasma frequency of these populations was further manifested in a high number of interstitial chiasmata. The regular and superior chromosome behaviour of the hybrids was found to be positively associated with quantitative measures on bean yield, harvest index and bean yield efficiency. The results were discussed from the point of view that: a) increased interstitial chiasmata may provide an effective mechanism for maintaining genetic diversity and heterosis in hybrid populations; and b) heterosis for chiasma frequency and quantitative traits may be due to dispersed genes on the chromosomes having combined intra-and interallelic interactions. The data provide evidence for the existence of positive associations between interstitially localized chiasmata with its recombination potential and regular chromosome behaviour to bean yield heterosis. The role of enhanced interstitial chiasmata to promote higher levels of genetic variation and heterozygous advantage is discussed.

F1 hybrid versus 32 selected F7 lines performance of common winter wheat (Triticum aestivum ssp. vulgare).

Abstract: One F1 common wheat hybrid (‘Zlatna Dolina’ x ‘Primepi’) which had exhibited significant heterosis in a previous study was compared with 32 F7 lines selected from the same cross. The yield test was planted in the 1976–1977 season at two locations: Zagreb, Yugoslavia (optimal environment) and Hutchinson, Kansas, USA (stress environment). The purpose of this experiment was to determine if a homozygous segregant superior to the F1 hybrid could be obtained from the same cross combination.

Components of fitness and frequency-dependent overdominance in hermaphrodite plants.

Abstract: Heterosis occurs when outbred individuals show greater viability, fertility or vigor than inbred ones. Recent theoretical studies of fertility variation in partially selfing heterotic populations have used two models of heterosis. The first model is overdominance, such that the heterozygote for any locus controlling the fertility variation may be fitter than either homozygote (e.g., Jain, 1968; Ho and Ross, 1973; Ross, 1978). The second model is inbreeding depression, such that the offspring of selfings are less fit than offspring of crosses, regardless of genotype (e.g., Valdeyron et al., 1973; Lloyd, 1975; B. and D. Charlesworth, 1978). In the first model fitness expressions are readily obtained, but in the second it is less clear how fitness should be measured. In this paper fecundity is defined as the quantity of ovules or pollen produced by a hermaphrodite plant, and fertility is the number of offspring per individual, regardless of viability. This paper has four aims. First, it aims to obtain a fitness measure which is suitable for hermaphrodite (or monoecious) flowering plants which show inbreeding depression. Second, the paper emphasizes that fitness values of hermaphrodites cannot usually be represented by constants, but instead are frequency dependent. The third aim is to show that fecundity variation in hermaphrodites can lead to overdominance, and that this overdominance may be present at some genotype frequencies only. The fourth aim is to emphasize that many fitness-related measures, previously studied in sex-polymorphic populations (e. g., Lloyd, 1974; Ross and Weir, 1975), are probably found in most hermaphrodite populations also. For example, a genotype may have different ovule and pollen outcrossing rates (Horovitz and Harding, 1972b; Ross, 1977a, 1977b). Different genotypes may have different outcrossing rates, or different degrees of function as male or female parents (Ross, 1977a, 1977b, and unpubl.; Lloyd, unpubl.). Most of these measures are frequency dependent. Although this paper briefly considers sex polymorphisms, this is for comparison with hermaphroditism, and it is emphasized that this paper is concerned essentially with hermaphroditism.