Half-sib Family Means Research Articles

Diallel crosses in Picea abies IV. Genetic variation and inheritance patterns in short-term trials

Abstract The complete diallel cross is the only mating design that provides estimates of variance components of general combining (GCA), specific combining ability (SCA), maternal and reciprocal effects, in addition to heritabilities and genetic correlations. To obtain such estimates, complete diallels were made among 10 trees in each of three natural Norway spruce populations from altitude 300 and 500 m in southern Norway. Seedlings from families from the diallels and open pollinations were tested in short-term tests on agricultural soil at one site at altitude 85 m until age 10 years from seed. Tree height at ages 7 and 10 years and diameter at age 10 had strongly significant GCA variance components within each population. The components for SCA and maternal effects were small and not significant, indicating low levels of non-additive genetic variation. For the days of initiation and cessation of the shoot elongation period the GCA components were dominating and had the highest heritability estimates in two of the diallels. Estimates of genetic correlations between traits measured in earlier nursery trials and height and diameter in the short-term trials had low and not consistent values in the three diallels. The duration of the shoot growth period and rate of growth showed positive relationships with height and diameter. Strong relationships were present between half-sib family means from the diallels and open-pollinated families for height, diameter and phenology traits. Progeny trials testing open-pollinated half-sib families from natural populations can be used for selection of candidates for the initial breeding populations.

QuLinePlus: extending plant breeding strategy and genetic model simulation to cross-pollinated populations\u2014case studies in forage breeding

Plant breeders are supported by a range of tools that assist them to make decisions about the conduct or design of plant breeding programs. Simulations are a strategic tool that enables the breeder to integrate the multiple components of a breeding program into a number of proposed scenarios that are compared by a range of statistics measuring the efficiency of the proposed systems. A simulation study for the trait growth score compared two major strategies for breeding forage species, among half-sib family selection and among and within half-sib family selection. These scenarios highlighted new features of the QuLine program, now called QuLinePlus, incorporated to enable the software platform to be used to simulate breeding programs for cross-pollinated species. Each strategy was compared across three levels of half-sib family mean heritability (0.1, 0.5, and 0.9), across three sizes of the initial parental population (10, 50, and 100), and across three genetic effects models (fully additive model, a mixture of additive, partial and over dominance model, and a mixture of partial dominance and over dominance model). Among and within half-sib selection performed better than among half-sib selection for all scenarios. The new tools introduced into QuLinePlus should serve to accurately compare among methods and provide direction on how to achieve specific goals in the improvement of plant breeding programs for cross breeding species.

Prediction of wood density breeding values of Pinus taeda elite parents from unbalanced data: A method for adjustment of site and age effects using common checklots

• Wood density of elite parents of loblolly pine (Pinus taeda L.) was investigated in 6 to 18 year-old progeny trials. The sampling was carried out separately in seven testing regions in the southeastern US. A checklot was the only connection between elite parents planted at different trials in a testing region. • We used a data normalization method suggested for unbalanced designs in cDNA microarray experiments to remove confounding site and age effects using the checklot as a reference sample. Wood density breeding values of parents were predicted by fitting a linear mixed model to the normalized data. • Using the reference samples to remove site and age effects appears to be an effective method for analysis of unbalanced progeny tests data. In general, wood density (kg/m3) decreased from coastal to inland plantings and from the southern to the northern planting. Considerable genetic variation for wood density was detected among these fast-growing elite parents in six of seven testing regions, with half-sib family mean heritabilities ranging from 0.71 to 0.97 within a testing region. With the exception of two regions, checklots were stable across trials in a region, based on regressing the checklot means on trial means.

Dominance, epistasis, heritabilities and expected genetic gains

Although epistasis is common in gene systems that determine quantitative traits, it is usually not possible to estimate the epistatic components of genotypic variance because experiments in breeding programs include only one type of progeny. As the study of this phenomenon is complex, there is a lack of theoretical knowledge on the contribution of the epistatic variances when predicting gains from selection and on the bias in estimating genetic parameters when fitting the additive-dominant model. The objective of this paper is to discuss these aspects. Regarding a non-inbred population, the genetic value due to dominance and the epistatic components of the genotypic value are not indicators of the number of favorable genes present in an individual. Thus, the efficiency of a selection process should be based on the narrow-sense heritability, a function only of additive variance. If there is no epistasis, generally it is satisfactory to assess the selection efficiency and to predict gain based on the broad-sense heritability. Regardless of the selection unit or type of epistasis, the bias in the estimate of the additive variance when assuming the additive-dominant model is considerable. This implies overestimation of the heritabilities at half sib family mean, plant within family and plant levels, and underestimation if the selection units are full sib progenies. The predicted gains will have a bias proportional to that of the heritability.

The genetic correlation of panicle shattering with the intact rachilla form of seed retention in Phalaris aquatica L.

Panicle shattering and seed retention are distinct characters that influence seed dispersal in Phalaris aquatica L.(phalaris). One form of seed retention in phalaris is the result of the thickening of the rachilla (intact rachilla) and disruption of the rachilla-break age mechanism. This form of seed retention results in seed dispersal as seed-retaining panicle fragments, providing an opportunity to reduce ant predation of seed, and thereby increase the potential for seedling recruitment. A genetic analysis of panicle shattering and the intact rachilla form of seed retention was conducted on parental clones and derived half-sib families of across between the seed retaining cultivar Atlas PG and the non-seed retaining cultivar Sirocco, grown as spaced plants over two seasons in the field. A simulation of panicle shattering of the parents and families was also devised under glasshouse conditions. The narrow-sense heritability of panicle shattering in the field was 51 ± 10% and 58 ± 12%,on the basis of half-sib family means and parent-offspring regression, respectively,and its genetic correlation with panicle shattering under simulated conditions was24 ± 8%. The narrow-sense heritability of the intact rachilla form of seed retention was 70 ± 17% on a half-sib family mean basis. Seed retention had a negative genetic correlation of 71 ± 19% with panicle shattering, posing a difficulty for breeding seed-retaining cultivars with readily shattering panicles. However, it was considered likely that the two traits were controlled by independent sets of genes and that this association could be broken. This association is discussed in the context of breeding cultivars with seed dispersal characteristics that promote seedling recruitment under conditions of ant predation of the seed bank.

Potential genetic gains for carbon sequestration: a preliminary study on radiata pine plantations in New Zealand

Preliminary estimates of gains in carbon sequestration, obtainable by applying genetic improvement, were calculated for radiata pine plantations in New Zealand. Data on height, diameter and wood density from New Zealand progeny trials were used to estimate family-mean heritability ( h F 2) and the phenotypic variance of half-sib family means for stem dry-weight production. Estimates of h F 2 ranged from 0.15 to 0.59 for the different trials. Based on unweighted averages of these genetic parameter estimates, the predicted gain for selecting the best 50 of 500 parents for dry-weight production, and the best 10 of 1000 parents, and making crosses between these parents, was 14.6 and 22.2%, respectively. The amount of carbon sequestered in the stem, branches and roots of typical stands of unimproved radiata pine was simulated using the C_Change option in the stand growth simulator stand treatment and growth simulation (STANDPAK). The amounts of carbon sequestered at the end of a rotation, and the long-term average (carbon held on site by growing radiata pine over successive rotations) were estimated. Carbon sequestered under a standard (‘direct sawlog’) management regime would differ among regions, from 211.2 tonnes at the end of a 28-year rotation in Canterbury to 322.3 tonnes on the East Coast. A ‘plant-and-leave’ low-cost regime sequestered more carbon than the direct sawlog regime with long-term averages at Kaingaroa of 102.9 tonnes carbon per ha and 85.3 tonnes per ha, respectively. The highest simulated genetic gain in long-term carbon sequestration for the standard regime was 29.3 tonnes per ha, for a stand grown in the East Coast region, using seed from the best 10 of 1000 ranked parents. Based on the highest estimated rate of carbon sequestration (East Coast, best 10 of 1000), and a value of 20 NZ$ per tonnes of carbon, the extra carbon credits attributable to genetic improvement, under the direct sawlog regime, would be worth 307 NZ$ per ha; the corresponding figure for the Canterbury region would be 151 NZ$ per ha. Owners of large post-1990 radiata pine plantings on converted farmland or pasture could gain large financial benefits attributable to genetic improvement, if trade in carbon credits becomes established. Assumptions for this study included extrapolating genetic parameter estimates to rotation age, and a similar allocation of biomass (to stem, branches and roots) for both genetically improved and unimproved radiata pine. Now that the potential has been established, more definitive estimates of genetic parameters, and better understanding of the effect of genetic improvement on carbon dynamics, are indicated.

Inheritance of Resistance to Spittle bug (Homoptera: Cercopidae) in Inter specific Brachiaria spp. Hybrids

Apomictic cultivars of several Brachiaria Griseb. spp., selected directly from collections of natural germ plasm, are the most widely sown forages in the tropics. They are particularly well adapted to acid, infertile soils typical of the vast South American savannas. Species of several genera of spittle bugs (Cereopidae) cause widespread and severe damage to pastures of susceptible Brachiaria cultivars. Highly antibiotic genotypes have been identified. The potential for genetic manipulation of resistance in highly antibiotic genotypes was assessed. A positive correlation between topcross parent and F1 family mean for percentage of adult emergence of Aeneolamia varia F. on artificially infested plants in the greenhouse indicated that resistance is under genetic control and should be easily manipulated in a plant breeding program. In a field study, heritability of spittle bug damage score on naturally infested plants was estimated from replicated half-sib families. The estimate of heritability of half-sib family means was 0.44 (±0.098) 5 mo after transplanting, further supporting the conclusion that resistance can be readily manipulated in a breeding program.

SELECTION FOR RESISTANCE TO SWEETPOTATO WEEVIL

Narrow-sense heritability and selection response estimates for weevil resistance were higher based on half-sib family means than on individual plants. Resistance levels in available sources are low and evaluation with low weevil numbers is essential for differentiation. Consequently, highest performing families produce multiple plants with no injured roots and separation of resistance levels among those plants is impossible Since a large amount of additive variance is contained among plants within families and early reduction of plant numbers would increase breeding efficiency, a study was conducted to determine the effectiveness of individual plant selection in combination with half-sib families. Two plants were selected from each of the 10 highest families based on weevil resistance and yield. Selected clones were compareds for resistance levels to means of their families when tested over 2 years. Fifty percent of selected clones had higher resistance levels than their family means indicating that individual plant selection was effective. Optimum methods of combining family and individual plant selection will be discussed.

INHERITANCE OF WEIGHTS OF LAMBS REARED ARTIFICIALLY OR WITH THEIR DAMS

This study involved 250 lambs reared artificially with milk replacer and solid diet from 2 to 70 days and 316 lambs creep-fed solid diet and reared with their dams from birth to 70 days. Thereafter, all lambs were retained on solid diet to 140 days of age. Heritability estimates for weights of lambs reared artificially tended to be larger than the corresponding estimates for lambs reared with their dams (P > 0.05). Phenotypic variances of lambs reared with their dams were larger at birth and at 35 days, whereas lambs reared artificially had larger phenotypic variances at 70 and 140 days of age. Estimates of genetic correlation between half-sib family means of lambs reared artificially with corresponding half-sibs reared with their dams ranged from 0.15 to 0.91, suggesting the absence of sire × rearing system interaction. We conclude that greater genetic response to selection for lamb weights at various ages could be achieved from direct selection among lambs reared artificially compared to those reared with their dams. However, in the absence of artificial rearing, prospective breeding stock reared with their dams should be tested for postweaning performance to permit full expression of transmitted sire differences. Key words: Sheep, lamb rearing, heritability estimates, genetic correlation

Response to Selection for Resistance to Gamma Radiation in the Cotton Boll Weevil1

A genetic selection experiment in the cotton boll weevil, Anthonomus grandis Boheman, was used to evaluate the degree to which variation in postirradiation survival may be under genetic control. Ten generations of family selection for 14-day postirradiation survival increased survival from 35 to almost 70% and increased life expectancy after irradiation by about 4 days. Estimates of realized heritability of half-sib family means of near 0.3 were required for this magnitude of response. The results indicate that selection for genetic improvement has merit in the development of a mass-rearing stock for release in a sterile insect program.

Selection for Increased Leanness of Yorkshire Swine. I. Experimental Procedures and Selection Applied

The procedures utilized and selection applied were described for an experiment designed to evaluate alternative techniques of selection for increased leanness of Yorkshire swine. Two measures of leanness were defined as percentage lean cuts at 81.6 kg live weight (PCLC) and weight of lean cuts at 160 days of age (WLC). These traits were estimated by utilization of ultrasonics on the potential breeding stock. Select lines were established for each trait in addition to a contemporary control. Selection within paternal half-sib families was based on a combined selection index utilizing individual and litter performance. Four generations of selection have been completed. The conventional selection differential was partitioned into three component deviations measured as the deviation of individual phenotype from full-sib family mean (1), full-sib family mean from half-sib family mean (2) and half-sib family mean from line mean (3). The corresponding midparent weighted partitioned selection differentials per generation were 2.79, 1.68 and −.04 kg for the WLC line and .722, .511 and .127% for the PCLC line, respectively. Unintentional positive selection in the control line was about 5% of direct selection in the WLC line and 20% of direct selection in the PCLC line, but for lower PCLC. The intentional selection of boars accounted for approximately 80% of the selection practiced in each select line. Extreme sires and dams of the WLC line produced more offspring than less extreme parents, while no consistent differences were observed in the PCLC line. As indicated by the index in retrospect technique, individual performance received slightly more attention, relative to litter performance, than was intended.

MASS SELECTION FOR POST-WEANING GROWTH IN SWINE. IV. SELECTION RESPONSE AND CONTROL POPULATION STABILITY

Recurrent mass selection for post-weaning average daily gain was practiced for 11 generations in a population of Lacombe swine. Response to selection was assessed from predicted response, 0.241 ± 0.023 kg; observed response, 0.146 ± 0.018 kg; estimated heritability, 0.327 ± 0.031; and realized heritability, 0.198 ± 0.016. Observed response was estimated to be 0.61 of the predicted. Examination of additive genetic variance, selection differentials, drift variance, inbreeding and regression of post-weaning average daily gain on generation time indicated that the pedigreed Yorkshire control population served as an adequate control. Gene effects were not found to be dependent on the sex of the organism as assessed by the genetic correlation between sexes, covariance between half-sib family means and the relative magnitude of parent–offspring-based estimates of heritability.

Predicted response to selection for crossbred performance in mice.

Compared in this study were predicted responses of three crossbred selection methods, using estimates of genetic parameters for seven quantitative traits from two genetically diverse mouse populations, designated C2 and ICR, and their reciprocal interpopulation crosses. The methods of crossbred selection were mass selection (MS), half-sib family selection (HS) and reciprocal recurrent selection (RRS). Traits studied were number born, 12-day litter weight, individual weights at birth, 21 (weaning), 42 and 56 days of age and gain from 21 to 42 days. Expected efficiency of MS was larger than HS for all traits. Expected efficiency of RRS was greater than MS for number born only. Average genetic and average maternal differences between the two populations were significant for all traits. Heterosis, measured as the mean difference between reciprocal F1 crosses and midparent values was also significant for all traits, varying from 3 to 6% of the respective trait means. Heritabilities in the purebred populations estimated from half-sib covariances were small for number born and weaning weight and moderately high for 12-day litter weight, post-weaning gain and adult body weights. Ratios of the sum of both purebred half-sib covariances to both crossbred half-sib covariances were greater than one for 12-day litter weight and the postweaning weight traits, which is compatible with a partial dominance gene model. This ratio was less than one for litter size, suggesting a possible overdominance situation. Genetic covariances and correlations between purebred and crossbred half-sib family means for the same trait generally agreed with the ratio estimates regarding inference to the type of gene action present.