The consequences of using inadequate testers in the simplified triple test-cross.

The genetical consequences of common alleles in the L1 and L2 testers of a simplified version of the triple test-cross which is applicable to populations of inbred lines are examined. The test for epistasis under these circumstances becomes ambiguous and can spuriously detect non-allelic interactions when they may not exist although it still provides a test for epistasis and the adequacy of the testers simultaneously. The tests of significance and the estimates of additive variation are biased to an extent related to the dominance and dominance x additive effects of the common loci while the significance and estimates of dominance variation are deflated because they reflect the dominance effects at the non-common loci only. The covariance of sums and differences is also underestimated for the same reasons. These expectations are illustrated by analysing the 190 simplified triple test-crosses that could be extracted from a 20 x 20 diallel set of crosses between pure-breeding lines of Nicotiana rustica.     

Identification of superior parents in a potato breeding programme

Summary An incomplete diallel cross was used to study components of genetic variation in potatoes for a range of characters after early and late harvest. The progenies were also used to evaluate five predictors of progeny performance, namely the mean seedling performance, the mid-parent value and the means of the selfed progenies, of the diploid progenies and of the test-cross progenies. For almost all characters, the general combining ability effects were predominant, although the specific combining ability effects present were greater at late than at early harvests. The seedling performance for tuber yield, number of tubers and average tuber weight did not show any relevant relationship to the field performance. The midparent value provided, in general, satisfactory predictions of the mean progeny performance obtained in the diallel, except for ware tuber yield. The selfed and the diploid progenies did not improve the prediction of progeny means compared to the mid-parent value. The predictions based on the test-crosses surpassed those of the mid-parent value, particularly for tuber yield at ware potato harvest. Methods to identify superior parents are discussed.     

Selection theory for selfed progenies

Summary The purpose of this article was to extend the model used to predict selection response with selfed progeny from 2 alleles per locus to a model which is general for number and frequency of alleles at loci. To accomplish this, 4 areas had to be dealt with: 1) simplification of the derivation and calculation of the condensed coefficients of identity; 2) presentation of the genetic variances expressed among and within selfed progenies as linear function of 5 population parameters; 3) presentation of selection response equations for selfed progenies as functions of these 5 population parameters; and 4) to identify a set of progeny to evaluate, such that one might be able to estimate these 5 population parameters.The five population parameters used in predicting gains were the additive genetic variance, the dominance variance, the covariance of additive and homozygous dominance deviations, the variance of the homozygous dominance deviations and a squared inbreeding depression term.Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 9971     

RFLP analyses and segregation of molecular markers in plants produced by in vitro anther culture, selfing, and reciprocal crosses of two lines of self-incompatible Solanum chacoense.

RFLP analyses were used to characterize several plant populations of Solanum chacoense Bitt. developed to investigate the generation of new S alleles at the self-incompatibility locus. The plant material consisted of two diploid parental lines, their anther culture derived (AC) progenies, their selfed progenies, and their reciprocal F1 hybrids. The RFLP analyses on the AC plants (121 individuals in total) permitted unambiguous identification of their origin. In particular, a distinction between plants originated from reduced (n) or unreduced (2n) microspores could be made. All the AC plants produced by gametic embryogenesis showed distinct RFLP patterns, whereas a number of clones (i.e., plants with identical RFLP patterns) were found among those regenerated via callus. The analyses conducted on the selfed progenies (69 plants) and the F1 hybrids (66 plants) showed only one case of accidental outcross. Segregation studies of the RFLP markers revealed significant deviations from expected Mendelian ratios in both AC-derived populations, as well as in the selfed progenies. Such deviations, however, were rare in the reciprocal F1 hybrids. These results are discussed in relation to the possible presence of genetic sieves operating during AC, illegitimate selfing, or during normal fertilization.     

Sibling competition, size variation and frequency-dependent outcrossing advantage in Plantago coronopus

Many plants display limited seed dispersal, thereby creating an opportunity for sibling competition, i.e. fitness-determined interactions between related individuals. Here I investigated the consequences of intra-specific competition, by varying density and genetic composition of neighbors, on the performance of seedlings derived by selfing or outcrossing of the partially self-fertilizing plant Plantago coronopus (L.). Seedlings from eight plants, randomly selected from an area of about 50 m² in a natural population, were used in (i) a density series with either one, four or eight siblings of each cross type per pot and (ii) a replacement series with eight plants per pot where selfed and outcrossed siblings were grown intermixed in varying frequencies. Density had a pronounced effect on plant performance. But, except for singly grown individuals, no differences were detected between selfed and outcrossed progenies in vegetative and reproductive biomass. When grown intermixed, selfed offspring were always inferior to their outcrossed relatives. The magnitude of reduction in performance was dependent on the number of outcrossed relatives a selfed seedling had to compete with, giving rise to a frequency-dependent fitness advantage to outcrossed seedlings. The major result of this study is (i) that the relative fitness of inbred progeny is strongly affected by the type of competitors (inbred or outbred) and (ii) that inbreeding depression varies according to the density and frequency of outbred plants and could be considered as a density- and frequency-dependent phenomenon. It is argued that sibling competition, due to the small genetic neighborhood of P. coronopus, might be an important selective force in natural populations of this species.     

苎麻疫霉生物学性状遗传与变异研究

在实验室条件下研究了不同地区和不同寄主来源的苎麻疫霉（PhytophthoraboehmeriaeSawada）的菌落形态、菌丝线性生长速率（以下简称为生长速率）及同宗配合性状的遗传与变异,结果指出苎麻疫霉菌落形态和生长速率的遗传存在3种类型：（1）在单游动孢子后代和自交后代均可稳定遗传;（2）在单游动孢子后代稳定遗传而在自交后代发生变异;（3）在单游动孢子后代和自交后代均发生变异。结果表明,该菌菌落形态和生长速率的遗传具有多样性,上述两性状可以由细胞核杂合基因控制,也可以由细胞核纯合基因控制,还可能由细胞质因子控制。试验结果还指出,苎麻疫霉的同宗配合性状在单游动孢子和单卵孢后代均可稳定遗传,表明在供试的苎麻疫霉菌株中控制该性状的遗传因子是纯合的。     

苎麻疫霉生物学性状遗传与变异研究*

在实验室条件下研究了不同地区和不同寄主来源的苎麻疫霉（PhytophthoraboehmeriaeSawada）的菌落形态、菌丝线性生长速率（以下简称为生长速率）及同宗配合性状的遗传与变异,结果指出苎麻疫霉菌落形态和生长速率的遗传存在3种类型：（1）在单游动孢子后代和自交后代均可稳定遗传;（2）在单游动孢子后代稳定遗传而在自交后代发生变异;（3）在单游动孢子后代和自交后代均发生变异。结果表明,该菌菌落形态和生长速率的遗传具有多样性,上述两性状可以由细胞核杂合基因控制,也可以由细胞核纯合基因控制,还可能由细胞质因子控制。试验结果还指出,苎麻疫霉的同宗配合性状在单游动孢子和单卵孢后代均可稳定遗传,表明在供试的苎麻疫霉菌株中控制该性状的遗传因子是纯合的。     

一个新的小麦黄化突变体的遗传研究

摘要: 通过对冬小麦“西农1718” 自然黄化突变体多年连续自交、与突变亲本回交以及与其他基因型进行正反交, 研究突变性状的遗传规律。观察统计发现, 突变体中金黄株发育至抽穗－开花期前后死亡, 黄-绿株自交后代表现为1金黄株︰2黄-绿株︰1绿株, 绿株自交后代不再分离; 黄-绿株与突变亲本回交及与其他基因型正反交, 后代均表现为1黄-绿株︰1绿株。遗传分析表明, 该小麦黄化突变体是由一对核基因控制的不完全显性遗传, 其中, 金黄株是纯合体(au/au), 表现为致死, 黄-绿株为杂合体(Au/au), 绿株为纯合体(Au/Au)。     

Variation of selfing rate and inbreeding depression among individuals and across generations within an admixed Cedrus population

We investigated the variation and short-term evolution of the selfing rate and inbreeding depression (ID) across three generations within a cedar forest that was established from admixture ca 1860. The mean selfing rate was 9.5%, ranging from 0 to 48% among 20 seed trees (estimated from paternally inherited chloroplast DNA). We computed the probability of selfing for each seed and we investigated ID by comparing selfed and outcrossed seeds within progenies, thus avoiding maternal effects. In all progenies, the germination rate was high (88–100%) and seedling mortality was low (0–12%). The germination dynamics differed significantly between selfed and outcrossed seeds within progenies in the founder gene pool but not in the following generations. This transient effect of selfing could be attributed to epistatic interactions in the original admixture. Regarding the seedling growth traits, the ID was low but significant: 8 and 6% for height and diameter growth, respectively. These rates did not vary among generations, suggesting minor gene effects. At this early stage, outcrossed seedlings outcompeted their selfed relatives, but not necessarily other selfed seedlings from other progenies. Thus, purging these slightly deleterious genes may only occur through within-family selection. Processes that maintain a high level of genetic diversity for fitness-related traits among progenies also reduce the efficiency of purging this part of the genetic load.     

Analysis of Line × Tester Progenies Compared in Orthogonally Supplemented Efficiency Balanced Incomplete Block Design

An analysis of experimental design is suggested for comparing progenies of lineXtester mating system. The line × tester mating system is an expanded top-cross mating system, used frequently in practice. This system is of interest for breeders dealing with estimation and testing of general as well as specific combining abilities. In the suggested design, treatments, i.e. hybrids obtained in crosses, and testers are studied in orthogonally supplemented incomplete block design. Analysis of variance has been presented, estimators of expected values for progenies and for testers have been given, testing of hypotheses concerning contrasts of interest for progenies, lines and testers has been discussed. Estimators of effects of general and specific combining abilities as well as tests permitting for testing the significance of various genetic parameters, have also been given.     

Estimates of marker-associated QTL effects in Monte Carlo backcross generations using multiple regression

Summary The decision of whether or not to use QTLassociated markers in breeding programs needs further information about the magnitude of the additive and dominance effects that can be estimated. The objectives of this paper are (1) to apply some of the Moreno-Gonzalez (1993) genetic models to backcross simulation data generated by the Monte Carlo method, and (2) to get simulation information about the number of testing progenies and mapping density in relation to the magnitude of gene effect estimates. Results of the Monte Carlo study show that the stepwise regression analysis was able to detect relatively small additive and dominance effects when the QTL are independently segregating. When testing selfed families derived from backcross individuals, dominance effects had a larger error standard deviation and were estimated at a lower frequency. Linked QTL require a higher marker mapping density on the genome and a larger number of progenies to detect small genetic effects. Reduction of the environmental error variance by evaluating selfed backcross families in replicate experiments increased the power of the test. Expressions of the number of progenies for detecting significant additive effects were developed for some genetic situations. The ratio of the within-backcross genetic variance to the square of a gene effect estimate is a function of the number of progenies, the heritability of the trait, the marker map density and the portion of the genetic variance explained by the model. Different values (from 0 to 1) assigned to (relative position of the QTL in the marker segment) did not cause a large shift in the residual mean square of the model.     

Assessment of the value of doubled haploids as progenitors in cocoa (Theobroma cacao L.) breeding

In order to evaluate twelve doubled haploids (DHs) of Theobroma cacao L. used as parents, a trial was set up in C?te d'Ivoire. Several traits were observed, such as yield, vigour, yield/vigour ratios, resistance to the black pod disease caused by Phytophthora, percentage of flat beans and mean weight of 100 cocoa beans. Out of the three progenies derived from crosses between two DHs, two showed severe drawbacks. A reduction of the heterogeneity within these progenies was occasionally observed for some of the traits, but failed to be consistent. When tested as female parents in combination with diploid testers, some of the DHs showea significantly higher combining value than their parents for traits such as the mean weight of 100 beans and the yield/canopy surface ratio. The results showed the potential of DHs to improve selected parents in only one cycle of selection but more crosses between two DHs need to be tested in order to evaluate potential of the resulting F(1) progenies.     

Twins and Triplets in Pearl Millet: their Cytology and Origin

Multiple radicles and plumules were produced in high frequenciesin selfed seed of six plants in pearl millet (L.S. 326–3).Ninety-five of these survived to maturity. All were diploids,except in one twin where a trisomic (2n+1) was found. The occurrenceof albino twins in progenies of heterozygous plants, and normalhomozygotes in twins from translocation heterozygotes suggeststheir origin through sexual reproduction. The origin of theremaining twins and triplets is not known with certainty. Pennisetum americanum, twins, triplets, tertiary trisomy, translocation hybridity     

Pollen genotype selection for a simply inherited qualitative factor determining resistance to chlorsulfuron in maize

Pollen genotype selection for genes expressed in both the haploid and diploid phases of the plant life cycle can lead to correlated responses detectable in the sporophyte. A pollen selection was conducted in two genetic backgrounds of maize (Zea mays L.) for chlorsulfuron resistance, conferred by the XA17 allele. Plants of two backcross (BC) lines segregating 1 (heterozygote, resistant) : 1 (homozygote, susceptible) for chlorsulfuron resistance were used as pollen donor. Selection treatment was applied during microspore development, and tassels were cut about 10 days before anthesis and placed into a liquid medium with or without 40 mg l^–1 of chlorsulfuron. Pollen was used to fertilize an unrelated susceptible genotype (tester). The resulting testcrosses (TC) were evaluated in the greenhouse by spraying seedlings with chlorsulfuron at 23 g ha^–1. Non-selected TC progenies derived from heterozygous BC plants showed a proportion of resistant and susceptible plants close to the expected 1:1 ratio, while non-selected TC progenies derived from homozygous BC plants showed susceptible plants only. Selected TC progenies derived from heterozygous BC plants showed a frequency of resistant plants ranging from 89% to 100%. BC plants homozygous for the susceptible allele, subjected to selection treatment, gave poor seed set, and no resistant plants were found in their TC progenies. Resistant TC plants obtained through pollen genotype selection were selfed, and the proportion of resistant plants was close to 3:1 in all selfed families, in accordance with the hypothesis that all of them inherited the XA17 allele through selection. In this study, pollen genotype selection was extremely effective, and its effect persisted in the second sporophytic generation. Received: 19 November 1999 / Accepted: 30 April 2000     

Impact of monoecy in the genetic structure of a predominately dioecious conifer species, Araucaria angustifolia (Bert.) O. Kuntze

Understanding the mating system of a tree species is important in genetic conservation and tree breeding strategies because it affects the inbreeding and genetic diversity of the descendant populations. Araucaria angustifolia (Bert.) O. Kuntze is a mainly dioecious species that reproduces through outcrossing. However, some monoecious trees have been identified and they may reproduce through self-fertilization. The objective of this study was to confirm the expected relatedness of full-sibs in outcrossed hand-pollinated progenies of female seed trees, self-sibs of hand self-pollinated monoecious seed trees, and to investigate the mating system of open-pollinated progenies of female and monoecious A. angustifolia trees. To do this, eight microsatellite loci were used to genotype hand- and open-pollinated progenies. Our results show that the relatedness of outcrossed hand-pollinated progenies are true full-sibs and progenies from a selfed monoecious seed tree are self-sibs, which confirms the hand-pollination method used. Open-pollinated female seed trees reproduced only by outcrossing, generating progenies with a mixture of full- and half-sibs. Monoecious seed trees reproduced mainly by xenogamy, generating progenies with mixtures of self-sibs, full-sibs, half-sibs and self-half-sibs. We also found that an increase in the effective number of pollen donors ( $ N_{\text{ep}} $ ) would lead to an increase in the total number of alleles ( $ K $ ) within progenies. Our results also suggest that monecious trees have limited potential to modify the genetic structure through selfed seed production due to the very low estimated selfing rate in these trees and the rare occurrence of these trees in natural populations.     

Population structure and combining ability of diverse Medicago sativa germplasms

Although unadapted germplasms have been used to improve disease and insect resistance in alfalfa, there has been little effort to use these for improving forage yield. We evaluated genetic diversity and combining ability among two unadapted germplasms (Medicago sativa ssp. sativa Peruvian and M. sativa ssp. falcata WISFAL) and three Northern U.S. adapted alfalfa cultivars. Population structure analyses indicated that the WISFAL and Peruvian germplasms were genetically distinct from the cultivars, although Peruvian was relatively closer to the cultivars. Peruvian and WISFAL germplasms were intermated to generate a novel hybrid population. This population was crossed to the three cultivars as testers, and the testcross progenies were evaluated for forage yield along with the hybrid population, the original germplasms (Peruvian, WISFAL and cultivars), testcrosses of Peruvian and WISFAL to the three cultivars and a three-way hybrid of the cultivars. The experiment was carried out in the field in Temuco, Chile and Arlington, Wisconsin, USA, and forage was harvested during two seasons. Results from these evaluations showed that hybrids between the Peruvian × WISFAL population and the cultivar testers yielded as much as the cultivar testers. Heterosis was observed between Peruvian and WISFAL, and between these germplasms and the cultivar testers, suggesting that each germplasm may contain different favorable alleles. If Peruvian and WISFAL populations contain alleles at different loci that complement cultivar testers, then combining and enriching these alleles in a single population could result in improved combining ability with alfalfa cultivars.     

插入含Ds因子的T-DNA产生的水稻脆秆突变株的遗传和分子分析

在构建由农杆菌介导的玉米Ds转座因子插入的水稻转化群体中,得到了一个茎秆等组织发生脆性突变的株系。理化指标定量测定表明,脆性株系的载荷强度和纤维素含量都比正常植株低很多,可溶性糖含量略有减少。对这个突变株的分子检测结果表明Ds因子在脆性株系中为单位点插入。检测了自前3代（T1,T2,T3)植株中T-DNA(Ds)插入与脆性表型的共分离关系。初步结果表明这个突变是T－DNA（Ds)的插入造成的,这个突变基因可能与水稻纤维素合成有关。     

Application of interspecific sesquiploidy to introgression of PLRV resistance from non-tuber-bearing Solanum etuberosum to cultivated potato germplasm

Summary Hybridization of synthetic allotetraploids of S. pinnatisectum with S. etuberosum (4x-EP) with S. acaule (2n = 4x = 48) resulted in two individuals that were highly fertile, in contrast to all other progenies. The unique individuals are hexaploids, 2n = 72, while the other progenies are tetraploids, 2n = 48. They are thought to be the products of a union between 2n eggs of S. acaule and normal 1n microspores of 4x-EP. The fertile hexaploids (designated 6x-AEP) produced abundant selfed seed and viable hybrids with cultivated diploid potato, S. phureja, when developing embryos were rescued from berries and cultured before transplanting to pot culture. The extreme variability in chromosome constitution of the hybrids with S. phureja and selfed progenies indicates that addition and substitution lines of etb chromosomes bearing genes of interest to breeders could easily be produced from this material. The production of sesquiploids, as the 6x-AEP hybrids are called, is discussed as a useful bridging step in the introduction of alien genes from genomes that share little homology with the cultivated genome.This work was part of the requirement for a Ph.D. degree of the senior author R. Chavez at The University of Birmingham, UK     

Triple test cross analysis in F2 populations of four barley crosses

Summary The triple test cross analysis of Jinks and Perkins (1970) was used to study different components of genetic variation in four barley F₂ populations, C 164x IB 226, C 164xJyoti, IB 226xP 113 and DL 3xP 113, for final plant height, spike length, 100-kernel weight, grain yield per plant and harvest index. The overall epistasis (i type) was, in general, a minor component but the j & 1 type epistasis was an important element for all five characters in cross 3 (IB 226xP 113). Both the additive (D) and dominance (H) components were highly significant for all the five characters in all four crosses. The dominance was directional in all cases except for 100-grain weight in crosses 1 (C 164xIB 226), 2 (C 164xJyoti) and 4(DL 3xP 113).     

The efficiency and optimal size of triple test cross designs for detecting epistatic variation.

The triple test cross and two of its associate designs have been compared for their theoretical and practical efficiency in detecting epistatic variation. The comparisons are made on the basis of optimal experimental sizes required for each of these tests to detect a modest level of epistasis significantly (P less than or equal to 0-05) and with a reasonable certainty (95 per cent). The experimental sizes are determined for various combinations of heritability, dominance ratio and gene association and for both duplicate and complementary epistasis. Two versions of the test epistasis designed by Kearsey and Jinks (1968). Test 1a and Test 1b, do not differ much in their theoretical efficiency for detecting epistasis and the optimal experimental sizes required by them to detect non-allelic interactions significantly are largely impraticable except when dominance and heritability are high and the degree of association is 50 per cent or more. Both the tests require much smaller experiments to detect duplicate epistasis than complementary epistasis of the same magnitude and this difference is more pronounced for lower levels of heritability and dominance. The theoretical efficiency of Test 2 (given by Jinks, Perkins and Breese, 1969), however, does not vary with the type of epistasis but the sensitivity of the test is inversely related to the degree of gene association between the tester parents. The practical implications of the present investigation are discussed and the validity of some of the most important theoretical predictions and assumptions are tested on a triple test cross involving 80 inbred lines of Nicotiana rustica.