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.     

自交作物群体的遗传组成分析

用数学方法推导出了自交作物群体遗传组成的分析公式。根据所推出的公式,可以计算出任何自交世代群体的基因型频率和同一表现型中各基因型的频率。对自交群体遗传组成的分析可用于质量性状的选择,确定选择时需要的有效群体大小和选出有效群体的大小,亦有助于确定选择的最佳世代。     

Chloroplast and nuclear DNA variation among homozygous plants in a population of the autogamous annualMicroseris douglasii (Asteraceae,Lactuceae)

The autogamous diploid annualMicroseris douglasii of California occurs in many isolated populations. The populations consist of one to many highly inbred biotypes. Morphological variation among populations usually is greater than within populations. In spite of the virtual absence of gene flow even within populations, genetically determined character differences are randomly distributed and associated throughout the range of the species. Recent evidence even suggests introgression of chloroplasts from the relatedM. bigelovii. Offspring families from 25 plants of a very variable population were raised and examined for segregation of morphological and molecular (RAPD) markers. All 25 original plants were completely homozygous for all markers, but each differed from all others at least in some markers. The population consisted of two genetically isolated groups of plants: a distinct inbred line (3 plants) and 22 plants with random associations of a common set of markers and characters, possibly recombinant inbreds from a past hybridization event. One of these 22 plants contained a chloroplast genome found inM. bigelovii, the other 24 plants a chloroplast genome found only inM. douglasii.     

Dissection of conjugants and mating type plus and minus cells in selfing clones of the isogamous green alga Closterium ehrenbergii

In the sexual reproduction of the green alga Closterium ehrenbergii, two sexually competent cells that are morphologically indistinguishable from the vegetative cells first come close to each other to form a sexually interacting pair. Each then divides into two gametangial cells. Isogamous conjugation occurs between nonsister gametangial cells of the two resulting pairs. With unusual selfing clones derived from a certain cross of heterothallic strains, we dissected apart a pair of gametangial cells that had already been united together by a delicate transparent tube, into which each gametangial cell was going to develop its conjugation papilla. In spite of such a degree of differentiation, when each was cultured in fresh medium, individual gametangial cells could dedifferentiate into vegetative cells and form subclones. By crossing such subclones with standard stable heterothallic mating-type strains, we show that each selfing clone of this alga actually produces both stable mt ⁺ and stable mt ^- cells, in addition to unstable mt ^- cells with selfing potency, during its mitotic vegetative growth. Although the selfing in C. ehrenbergii studied here differs in certain points from true homothallism, the results of the present study provide insight into how homothallism might have evolved from heterothallism.     

A Markov chain Monte Carlo strategy for sampling from the joint posterior distribution of pedigrees and population parameters under a Fisher-Wright model with partial selfing

A simple population genetic model is presented for a hermaphrodite annual species, allowing both selfing and outcrossing. Those male gametes (pollen) responsible for outcrossing are assumed to disperse much further than seeds. Under this model, the pedigree of a sample from a single locality is loop-free. A novel Markov chain Monte Carlo strategy is presented for sampling from the joint posterior distribution of the pedigree of such a sample and the parameters of the population genetic model (including the selfing rate) given the genotypes of the sampled individuals at unlinked marker loci. The computational costs of this Markov chain Monte Carlo strategy scale well with the number of individuals in the sample, and the number of marker loci, but increase exponentially with the age (time since colonisation from the source population) of the local population. Consequently, this strategy is particularly suited to situations where the sample has been collected from a population which is the result of a recent colonisation process.     

Outcrossing increases infection success in the holoparasitic mistletoe <Emphasis Type="Italic">Tristerix aphyllus</Emphasis> (Loranthaceae)

Most studies on the fitness advantage of outbreeding in host–parasite systems have been assessed from the host rather than the parasite perspective. Here, we performed experimental pollination treatments to evaluate the consequences of outbreeding on fitness-related traits in the holoparasitic mistletoe Tristerix aphyllus in a 2-year field study. Results indicate that self-pollinated plants had a lower fruit production than outcrossed plants (20.4% and 29.5% reduction in 2002 and 2003, respectively), and resulting inbred fruits were smaller than outcrossed fruits in both years. No effect was detected for seed mass. The percentage of germination of inbred seeds was 15.1% and 6.0% lower than outcrossed seeds in 2002 and 2003, respectively. Inbred seedlings had shorter radicles, which translated to a 71.6% and 60.0% reduction in infection success compared with outcrossed plants in 2002 and 2003, respectively. Overall, our results revealed significant inbreeding depression on almost every trait that was examined. Although the mean value of traits varied from a year to another, the magnitude of inbreeding depression did not change significantly between years. Our findings constitute the first evidence that outcrossing increases infection success and probably virulence in parasitic plant populations.     

Selfing and inbreeding depression in seeds and seedlings of Neobalanocarpus heimii (Dipterocarpaceae)

We evaluated the degree of selfing and inbreeding depression at the seed and seedling stages of a threatened tropical canopy tree, Neobalanocarpus heimii, using microsatellite markers. Selection resulted in an overall decrease in the level of surviving selfed progeny from seeds to established seedlings, indicating inbreeding depression during seedling establishment. Mean seed mass of selfed progeny was lower than that of outcrossed progeny. Since the smaller seeds suffered a fitness disadvantage at germination in N. heimii, the reduced seed mass of selfed progeny would be one of the determinants of the observed inbreeding depression during seedling establishment. High selfing rates in some mother trees could be attributed to low local densities of reproductive individuals, thus maintenance of a sufficiently high density of mature N. heimii should facilitate regeneration and conservation of the species.     

Extinction and fixation times with dominance and inbreeding

The effect of partial inbreeding on extinction and fixation times of a selected allele with partial dominance is studied using a diffusion model. Asymptotic approximations are obtained for large populations and the accuracy of the approximations was found to increase with inbreeding level. They show that inbreeding reduces extinction and fixation times compared to random mating at least by a factor 1+F, where F is Wright’s fixation index. The reduction of extinction and fixation times due to inbreeding is stronger for strong selection and if alleles are either highly recessive or highly dominant. This bears implications for the effect of inbreeding on the signature of selective sweeps. These findings extend previous results obtained for random mating populations and help clarifying previous simulation and numerical results on the effect of inbreeding on the dynamics of selected alleles.     

Fine-scale analysis of genetic structure in the brooding coral Seriatopora hystrix from the Red Sea

The dispersal of gametes and larvae plays a key role in the population dynamics of sessile marine invertebrates. Species with internal fertilisation are often associated with very localised larval dispersal, which may cause small-scale patterns of neutral genetic variation. This study on the brooding coral Seriatopora hystrix from the Red Sea focused on the smallest possible scale: Two S. hystrix stands (~100 colonies each) near Dahab were completely sampled, mapped and analysed at five microsatellite markers. The sexual mode of reproduction, the likely occurrence of selfing and the level of immigration were in agreement with previous studies on this species. Contrary to previous findings, both stands were in Hardy–Weinberg proportions. Also, no evidence for spatially restricted larval dispersal within the sampled areas was found. Differences between this and previous studies on S. hystrix could reflect variation in life history or varying environmental conditions, which opens intriguing questions for future research.     

Quantitative studies on the mating system of opium poppy (Papaver somniferum L.)

Summary Nearly 400 individuals at two locations and over a number of years were crossed and subsequently scored for selfing versus outcrossing in eight monohybrid populations of opium poppy (Papaver somniferum). Two different marker loci, petal colour (R/r) and capsule size (B/b) were used to determine the male gametes that had effected fertilizations in F₂ recessives (rr and bb). The estimates of the outcrossing parameter were found to vary with year, location and for the marker locus used ( range: 0.0988–0.3704). Study of two dihybrid crosses involving the two loci simultaneously, further confirmed that outcrossing at the R/r locus was significantly greater than that at the B/b locus. The nature of the outcrossing was, in general, nonrandom. Selfmg predominated in this species; however, there was a high frequency of natural outcrossing for generating variations in P. somniferum.CIMAP publication No. 1086