Selection in autotetraploids

Summary Theoretical studies indicated that response to selection would always be greater in diploid than in autotetraploid populations when gene frequency was the same in both, and that situations in which little or no response to selection could be expected would be more frequent in autotetraploids. Interpretation of the coefficient of selection in terms of escape from infection in a program of selection for disease or insect resistance indicated that moderate levels of escape from infection can drastically reduce response to selection in some cases.The zygotic constitution of an autotetraploid population will change as it approaches a new random mating equilibrium once selection pressure is relaxed. The changes will result in no change in the population mean if the trait under selection exhibits no dominance, but the mean will decrease slightly if there is dominance.     

<Emphasis Type="Italic">In vitro</Emphasis> induction and identification of autotetraploids of <Emphasis Type="Italic">Dioscorea zingiberensis</Emphasis>

Dioscorea zingiberensis is an important medicinal plant and a source of diosgenin in China. We report research on the induction, characteristics, and chemical assays of polyploid plants of D. zingiberensis. Immersing calli in 0.3% colchicine solution for 16 h prior to culture induced a high number of autotetraploid plants. The induction rate reached as high as 36.7% of treated calli. More than 50 lines of autotetraploid plants were obtained. All tetraploid plants showed typical polyploidy characteristics. Twenty selected tetraploid lines were transferred to the field for determination of morphological characteristics and for chemical assays. Six elite lines have been selected for further selection and breeding into new varieties for commercial production.     

四倍体青花菜低稔性胚胎学研究

以二、四倍体青花菜为材料,在现蕾期分别套袋自交,对四倍体与二倍体的胚囊结构、受精以及受精后胚胎发育过程进行比较研究.结果表明:四倍体青花菜胚囊发育完整,与二倍体相似.四倍体花粉萌发率较二倍体低,花粉管生长较慢且不整齐.四倍体和二倍体青花菜自交均能正常受精,四倍体青花菜在受精后胚胎发育的进程上均较其二倍体滞后约2 d,且胚胎发育在进度上不一致,并存在胚胎败育现象,正常胚胎只占25.8%,导致四倍体结实率比二倍体降低22.6%.     

Improving the analysis of low heritability complex traits for enhanced genetic gain in potato

Key message

Best linear unbiased prediction (BLUP), which uses pedigree to estimate breeding values, can result in increased genetic gains for low heritability traits in autotetraploid potato.

Abstract

Conventional potato breeding strategies, based on outcrossing followed by phenotypic recurrent selection over a number of generations, can result in slow but steady improvements of traits with moderate to high heritability. However, faster gains, particularly for low heritability traits, could be made by selection on estimated breeding values (EBVs) calculated using more complete pedigree information in best linear unbiased prediction (BLUP) analysis. One complication in applying BLUP predictions of breeding value to potato breeding programs is the autotetraploid inheritance pattern of this species. Here we have used a large pedigree, dating back to 1908, to estimate heritability for nine key traits for potato breeding, modelling autotetraploid inheritance. We estimate the proportion of double reduction in potatoes from our data, and across traits, to be in the order of 10 %. Estimates of heritability ranged from 0.21 for breeder’s visual preference, 0.58 for tuber yield, to 0.83 for plant maturity. Using the accuracies of the EBVs determined by cross generational validation, we model the genetic gain that could be achieved by selection of genotypes for breeding on BLUP EBVs and demonstrate that gains can be greater than in conventional schemes.     

Aneuploidy and inbreeding depression in random mating and self-fertilizing autotetraploid populations

Summary Almost all autotetraploids produce aneuploid progeny because of irregularities at meiosis. Aneuploid plants produce high frequencies of aneuploids. If it were not for selection against aneuploid gametes and sporophytes the amount of aneuploidy would increase every generation. Most experimental and theoretical studies on population genetics and heterosis in autotetraploids have neglected aneuploidy as a factor. To take aneuploidy into account experimentally requires the cytological identification of all chromosomes and to consider it theoretically requires a huge amount of computations. Consequently, microcomputer programs have been devised to show the effects of random mating and self-fertilization in autotetraploid populations. According to the model aneuploidy rapidly increases in randomly mated and self-fertilized autotetraploid populations until they achieve an equilibrium where the amount of aneuploidy introduced into the population is balanced by the amount of aneuploidy removed from the population by selection. The model suggests that self-fertilized populations have greater frequencies of aneuploid gametes and zygotes than do randomly mated populations and therefore aneuploidy may be a significant cause of the great inbreeding depressions found in autotetraploids.Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 9998     

白花除虫菊同源四倍体株系花期除虫菊酯含量及农艺性状分析

采用高效液相色谱法测定了16个白花除虫菊[Pyrethrum cinerariifolium（Trev．）Vis．]同源四倍体株系干花中的总除虫菊酯、总除虫菊酯Ⅰ（PyⅠ）和总除虫菊酯Ⅱ（PyⅡ）含量,并分析了16个同源四倍体株系花中总除虫菊酯含量的动态变化规律以及花期的农艺性状。结果表明,管状花开放初期,同源四倍体株系干花中的除虫菊酯含量最高,其中11个株系的总除虫菊酯含量高于二倍体株系且7个株系干花中的总除虫菊酯含量高于1．4％,达到一级花的质量标准。同源四倍体株系的花薹和花序的农艺性状表现出明显的多倍体性状,花薹低,花盘直径大,干花产量高,通过合理密植可以提高同源四倍体株系的干花产量。     

Population genetic evidence for rapid changes in intraspecific diversity and allelic cycling of a specialist defense gene in Zea

Two patterns of plant defense gene evolution are emerging from molecular population genetic surveys. One is that specialist defenses experience stronger selection than generalist defenses. The second is that specialist defenses are more likely to be subject to balancing selection, i.e., evolve in a manner consistent with balanced-polymorphism or trench-warfare models of host-parasite coevolution. Because most of the data of specialist defenses come from Arabidopsis thaliana, we examined the genetic diversity and evolutionary history of three defense genes in two outcrossing species, the autotetraploid Zea perennis and its most closely related extant relative the diploid Z. diploperennis. Intraspecific diversity at two generalist defenses, the protease inhibitors wip1 and mpi, were consistent with a neutral model. Like previously studied genes in these taxa, wip1 and mpi harbored similar levels of diversity in Z. diploperennis and Z. perennis. In contrast, the specialist defense hm2 showed strong although distinctly different departures from a neutral model in the two species. Z. diploperennis appears to have experienced a strong and recent selective sweep. Using a rejection-sampling coalescent method, we estimate the strength of selection on Z. diploperennis hm2 to be approximately 3.0%, which is approximately equal to the strength of selection on tb1 during maize domestication. Z. perennis hm2 harbors three highly diverged alleles, two of which are found at high frequency. The distinctly different patterns of diversity may be due to differences in the phase of host-parasite coevolutionary cycles, although higher hm2 diversity in Z. perennis may also reflect reduced efficacy of selection in the autotetraploid relative to its diploid relative.     

甜叶菊同源四倍体离体诱导及鉴定

用不同浓度秋水仙素溶液处理甜叶菊不定芽,诱导同源四倍体,并进行解剖学、染色体鉴定和流式细胞仪鉴定倍性。结果表明:(1)用0.20%的秋水仙素溶液浸泡甜叶菊不定芽12h,同源四倍体诱导率最高,可达32.14%。(2)同源四倍体植株与二倍体(对照)相比,其气孔、叶片等均表现巨大性,且叶片变厚、叶色浓绿、叶片皱缩。(3)对照植株染色体2n=2x=22,四倍体植株染色体2n=4x=44;流式细胞仪倍性鉴定结果显示,对照DNA相对含量为100,四倍体DNA相对含量为200。(4)该研究共鉴定出48株甜叶菊同源四倍体植株,为进行倍性植株的诱导奠定了技术基础,为进一步开展甜叶菊同源四倍体新品种的选育提供了实验材料。     

不同倍性水稻亚种间杂种小孢子发生的细胞学观察

利用塑料半薄切片对水稻同源四倍体亚种间杂种F1及其对应的二倍体杂种F1小孢子母细胞减数分裂过程的细胞学变化进行观察研究.结果表明,同源四倍体水稻亚种间杂种造孢细胞期和小孢子母细胞期已经表现出较高频率的异常;减数分裂过程中,小孢子母细胞出现异常更加复杂,主要包括小孢子母细胞液泡化和退化两大类,这些异常是导致杂种花粉败育和花粉低育性的重要原因之一;此外,绒毡层异常也是导致杂种的花粉育性降低的因素.二倍体杂种小孢子母细胞在减数分裂过程的异常类型与水稻同源四倍体亚种间杂种基本相似,其绒毡层异常频率较低,对其花粉低育性影响不大.     

Effect of gametic disequilibrium on selection in an autotetraploid population

Summary The effects of a gametic disequilibrium (DSE) in an autotetraploid population on response to selection as measured by the covariance of selection were investigated. The theoretical responses were calculated for mass selection [Mass (1)] and half-sib progeny test selection (HSPT) in a two-allele (B and b), single locus, autotetraploid population. The complexity of calculations precluded analytical expressions for the covariances so numerical analysis was used assuming the following genetic models: monoplex dominance, partial monoplex dominance, duplex dominance, partial duplex dominance, and additive gene action.The results indicated the DSE could greatly affect the covariance of selection. For a constant allele frequency the DSE might double the covariance expected with selection in a population at random mating equilibrium (RME) of gametes, but in other instances approach zero. For all genetic models and the two breeding methods investigated the covariance of selection was always increased when the frequency of BB gamete exceeded p² (where p is frequency of allele B) and decreased when the frequency of BB gamete was less than p². The possible incorporation of this information into a long term breeding program and some other ramifications were briefly discussed.With the DSE the covariances of selection with HSPT and Mass (1) had a proportionality of 1:2, respectively, with the additive genetic model, but this relationship rarely occurred for other genetic models. The deviations from this ratio were not large in comparison to differences between selection in populations in DSE and RME.Cooperative investigations of the Alfalfa Production Research Unit, United State Department of Agriculture, Agricultural Research Service, and the Nevada Agricultural Experiment Station, Reno, Nevada. Paper No. 512. Scientific Journal Series, Nevada Agricultural Experiment Station     

同源四倍体水稻受精与胚胎形成过程的观察

激光扫描共聚焦显微术具有“组织与细胞CT”的功能,可以对整体组织进行扫描并构建三维结构。在水稻胚囊发育研究上建立的整体染色透明激光扫描共聚焦显微术辅助D IC法,对同源四倍体水稻广陆矮4号-4x和L202-4x受精和胚胎发育过程进行了研究,描述授粉后不同时段胚囊发育的特点,发现同源四倍体水稻受精、胚胎和胚乳发育过程及特点与正常的二倍体水稻的基本一致,但在不同发育时段中存在无胚、无胚乳胚囊、胚乳吞噬胚、胚胎发育停滞、胚囊退化等异常。2份材料的异常情况存在差异。这些异常均可能导致结实率降低。     

Microsatellite Analysis of Genetic Variation and Population Genetic Differentiation in Autotetraploid and Diploid Rice

Genetic diversity and population genetic structure of autotetraploid and diploid populations of rice collected from Chengdu Institute of Biology, Chinese Academy of Sciences, were studied based on 36 microsatellite loci. Among 50 varieties, a moderate to high level of genetic diversity was observed at the population level, with the number of alleles per locus (A _e) ranging from 2 to 6 (mean 3.028) and polymorphism information content ranging from 0.04 to 0.76 (mean 0.366). The expected heterozygosity (H _e) varied from 0.04 to 0.76 (mean 0.370) and Shannon’s index (I) from 0.098 to 1.613 (mean 0.649). The autotetraploid populations showed slightly higher levels of A _e, H _e, and I than the diploid populations. Rare alleles were observed at most of the simple sequence repeat loci in one or more of the 50 accessions, and a core fingerprint database of the autotetraploid and diploid rice was constructed. The F-statistics showed genetic variability mainly among autotetraploid populations rather than diploid populations (F _st = 0.066). Cluster analysis of the 50 accessions showed four major groups. Group I contained all of the autotetraploid and diploid indica maintainer lines and an autotetraploid and its original diploid indica male sterile lines. Group II contained only the original IR accessions. Group III was more diverse than either Group II or Group IV, comprising both autotetraploid and diploid indica restoring lines. Group IV included a japonica cluster of the autotetraploid and diploid rices. Furthermore, genetic differences at the single-locus and two-locus levels, as well as components due to allelic and gametic differentiation, were revealed between autotetraploid and diploid varieties. This analysis indicated that the gene pools of diploid and autotetraploid rice were somewhat dissimilar, as variation exists that distinguishes autotetraploid from diploid rices. Using this variation, we can breed new autotetraploid varieties with some important agricultural characters that were not found in the original diploid rice varieties.     

Different drought-stress responses in photosynthesis and reactive oxygen metabolism between autotetraploid and diploid rice

Photosynthetic light curve, chlorophyll (Chl) content, Chl fluorescence parameters, malondialdehyde (MDA) content, phosphoenolpyruvate carboxylase (PEPC) activity and reactive oxygen metabolism were studied under drought stress in two autotetraploid rice lines and corresponding diploid rice lines. Net photosynthetic rate decreased dramatically, especially under severe drought stress and under high photosynthetic active radiation in diploid rice, while it declined less under the same conditions in autotetraploid lines. Compared with the corresponding diploid lines, the Chl content, maximum photochemical efficiency of photosystem (PS) II, and actual photochemical efficiency of PSII were reduced less in autotetraploid lines. PEPC activities were higher in autotetraploid rice lines. PEPC could alleviate inhibition of photosynthesis caused by drought stress. The chromosome-doubling enhanced rice photoinhibition tolerance under drought stress. The lower MDA content and superoxide anion production rate was found in the autotetraploid rice indicating low peroxidation level of cell membranes. At the same time, the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were higher in autotetraploid rice lines. SOD, POD, and CAT could effectively diminish the reactive oxygen species and reduced the membrane lipid peroxidation.     

Sequence diversity of five <Emphasis Type="Italic">Medicago sativa</Emphasis> genes involved in agronomic traits to set up allele mining in breeding

Lucerne (Medicago sativa) is an autotetraploid forage legume, whose breeding mainly relies on phenotypic recurrent selection to create synthetic populations. Allele mining could be a strategy to take advantage of the large genetic diversity of the species. This paper reports the polymorphism of five genes impacting the phenotype for selected traits: CAD1 and CCoaOMT (digestibility), CONSTANS-like (forage yield), NHX1 (salt tolerance), and WXP1 (drought tolerance). Complete genes were sequenced for 387 genotypes of 43 cultivated accessions and 20 genotypes of wild accessions. Wild versus cultivated polymorphism were compared, adaptive evolution was evaluated by comparing M. sativa and Medicago truncatula sequences and variants of the cultivated pool were characterized. We showed that the wild pool was more variable than the cultivated pool, with 36.6% and 8.4% of the variants that were specific to the wild pool and the cultivated pool, respectively. This result confirmed a bottleneck effect during domestication and selection. We also found that CAD1, CCoaOMT, and NHX1 were under a strong purifying selection and contained few non-synonymous variants (8, 5, and 8, respectively), while CONSTANS-like and WXP1 were under a less pronounced purifying selection and were more polymorphic (45 and 91 non-synonymous variants, respectively). This result suggests that adaptive evolution could be an indicator of expected polymorphism. The application of allele mining strategy for plant breeding in autotetraploid species is discussed.     

Association between chemical and genetic variation in Calophyllum inophyllum, a medicinally important tree of the Western Ghats of India

Intersubspecific autotetraploid rice hybrids have high heterosis in both vegetative and reproductive growth, but low seed set hinders commercial utilization of autotetraploid rice. Autotetraploid rice hybrids with high and low pollen fertility were used in the present study to compare microtubule distribution patterns and chromosome behavior during pollen mother cell (PMC) meiosis, using indirect immunofluorescence laser scanning confocal microscopy. Microtubule distribution patterns of autotetraploid hybrids were similar to diploid rice, but many different kinds of abnormalities were found in the hybrid with low pollen fertility and seed set. Abnormal microtubule organization including structurally distorted microtubules at pachytene, loosely knitted perinuclear microtubules at diakinesis during prophase?I, and abnormal spindles, viz. multipoles, loss of spindle pole focus, abnormal size of spindles, and so on, were found at metaphase?I and metaphase?II. Some cells developed into triad with no formation of tetrad. Abnormal chromosome behaviors included high percentage of multivalents, chromosome lagging, chromosome bridges, and micronuclei. All these abnormalities were found more frequently in low-fertile hybrid than in high-fertile hybrid. These results suggest that abnormal microtubule distribution pattern is an important factor which affects pollen fertility and percentage seed set in autotetraploid rice, and may also have a close relationship with chromosome behavior.     

The effects of migration load,selfing, inbreeding depression,and the genetics of adaptation on autotetraploid versus diploid establishment in peripheral habitats

The distribution and abundance of polyploids has intrigued biologists since their discovery in the early 20th century. A pattern in nature that may give insight to processes that shape the distribution and abundance of polyploids is that polyploid populations are sometimes associated with peripheral habitats within the range of a species of mixed ploidy. Here, adaptation and competition of a diploid versus an autotetraploid population in a peripheral habitat are examined theoretically. It is shown that a nascent autotetraploid population adapts to and outcompetes a diploid population in the periphery when the rate of gamete dispersal is high, and when the mode of gene action is recessive for moderate to high rates of selfing. With additive or dominant modes of gene action, the conditions for an autotetraploid to outcompete a diploid in the periphery appear determined more by the rate of selfing and less by gamete dispersal. All of these results are based on empirical work that suggests inbreeding depression is higher in diploids versus autotetraploids. Generally, the results indicate that, although autotetraploids incur minority cytotype exclusion, diploids face burdens themselves. In the case of adaptation to a peripheral habitat, this burden is migration load from gamete and propagule dispersal.     

采用秋水仙碱创制优质、抗热同源四倍体不结球白菜

以不同浓度秋水仙素处理二倍体不结球白菜(Brassica campestrisssp.chinensisM ak ino)子叶生长点,对变异株进行了形态解剖学、农艺学、细胞学及营养品质鉴定。结果表明:0.1 mol/L秋水仙素处理6次的效果最佳。与二倍体相比,四倍体植株、气孔、花器官均表现巨大性;气孔密度、结实率显著降低;四倍体白菜蛋白质、可溶性糖和维生素C含量比二倍体分别增加15.69%、71.25%和22.18%;夏季高温条件下四倍体表现出良好的丰产性和抗热性。     

Breeding for high production of leaves of baobab (Adansonia digitata L) in an irrigated hedge system

This study presents an evaluation of an irrigated “market garden” trial established with 3-month-old seedlings of baobab. The trial included offspring from 59 open-pollinated family lots from ten provenances and four bulked provenance sample lots. Leaf productivity and seedling growth were evaluated monthly from the third month after establishment. We found lowered leaf productivity during the dry season despite the plants being irrigated. We provide the first estimates of heritability for leaf production and growth of the species. We assumed that the families of seed from open-pollinated single trees consist of true half-sibs but also provided adjusted estimates assuming a high level of selfing. Differences among provenances and families within provenances were highly significant (p?<?0.01) with respect to leaf productivity. Heritability estimates (adjusted and non-adjusted) for leaf productivity were moderate to low, predicting moderate genetic gain from selection. Strong genetic and phenotypic correlations were estimated between diameter and leaf production, indicating that simple selection for diameter can efficiently increase leaf production. The dry weight/fresh weight ratio was not significantly different among provenances or families within provenances. Based on the findings, we discuss how breeding can increase the total leaf production and its seasonal distribution.     

Selection of a high and stable pigment-producing strain in cultured Euphorbia millii cells

Summary A high pigment-producing strain of cultured Euphorbia millii cells was isolated by clonal selection. The pigment obtained was red and consisted mainly of anthocyanin. The amount of this pigment obtained after 24 selections was seven times that found in the original cells. Statistical and cell-pedigree analyses proved that this cell strain has stable productivity for this red pigment.     

Allozyme diversity in the tetraploid endemic Thymus loscosii (Lamiaceae)

BACKGROUND AND AIMS: Thymus loscosii (Lamiaceae) is a tetraploid perennial species endemic to the Ebro river basin (north-eastern Spain), which is included in the National Catalogue of Endangered Species. It is a tetraploid species (2n = 54), presumably an autotetraploid originated by the duplication of a 2n = 28 genome and the subsequent loss of two chromosomes. Allozyme electrophoresis was conducted to survey the levels and distribution of genetic diversity and to test the previous autopolyploid hypothesis for its origin. In addition, both in situ and ex situ conservation measures are proposed. METHODS: Eight populations were sampled for analysis by standard methods of starch gel electrophoresis, and six putative enzymatic loci were resolved (five consistently and one only partially). KEY RESULTS: Banding patterns exhibited no evidence of fixed heterozygosity and showed both balanced and unbalanced heterozygotes. In addition, most individuals showed a pattern consistent with the presence of three or four alleles at a single locus. High levels of genetic variability were found at population level (P = 85 %, A = 3.0, He = 0.422), in addition to a trend of an excess of heterozygotes. CONCLUSIONS: Allozyme data support the hypothesis that T. loscosii is an autotetraploid, and the high number of alleles at some loci may be due to repeated polyploidization events. The high values of genetic variation found in this species agree with those expected for tetraploids. The excess of heterozygotes may be due to some barriers to inbreeding (e.g. occurrence of gynodioecy) and/or selection for heterozygosity.