Winter curing of <Emphasis Type="Italic">Prunus dulcis</Emphasis> cv ‘Butte,’ <Emphasis Type="Italic">P. webbii</Emphasis> and their interspecific hybrid in response to <Emphasis Type="Italic">Xylella fastidiosa</Emphasis> infections

In traditional quantitative genetics, additive effects of genes acting in a population of biparental homozygous lines are estimated on the basis of the phenotypic observations only, usually by taking a difference between mean values for extreme lines. Current molecular methods allow to estimate the additive effects by additionally taking into account the marker data. In this paper we compare these two methods of estimation of additive gene action effects analytically, by simulations and by analysis of real data sets for doubled haploid lines and recombinant inbred lines. The analytic comparison shows under which conditions an agreement of the two methods can be achieved. In most of the considered experimental data and in simulations we observe that the additive effect calculated on the basis of the marker observations is smaller than the total additive effect obtained from phenotypic observations only. This result is discussed, and a weighted regression approach is proposed as a method which can close the gap between the purely phenotypic and genotypic approaches.     

Nuclear genome size and chromosome analysis in <Emphasis Type="Italic">Chenopodium quinoa</Emphasis> and <Emphasis Type="Italic">C. berlandieri</Emphasis> subsp. <Emphasis Type="Italic">nuttalliae</Emphasis>

Cytogenetic characterization by karyotyping and determination of DNA content by flow cytometry of seven cultivated varieties of Chenopodium was performed. Chenopodium quinoa cultivar Barandales and C. berlandieri subsp. nuttalliae cultigens Huauzontle, Quelite and Chia roja showed 2n = 4x = 36, x = 9. Statistically insignificant genome size differences for studied varieties ranged from 2.96 pg/2C (1 Cx = 724 Mbp) in C. quinoa to 3.04 pg/2C (1 Cx = 743 Mbp) in Huauzontle. Karyotype analyses revealed the presence of nine groups of four metacentric chromosomes, including two pairs of chromosomes with satellites. The first pair of satellites was located on the largest pair of chromosomes and the second on a different pair of chromosomes in all accessions analyzed. Variation among varieties was evident in chromosome size, genome length (GL) and the position of satellites. Chia roja exhibited greatest GL (58.82 μm) and biggest chromosomes (2.04 μm). Huauzontle showed the smallest GL (45.02 μm) and shortest chromosomes (1.60 μm). Comparison of GL in studied taxa was statistically significant and allowed to define three groups according to the use given to these plants. These data indicate that they are small, very stable genomes in terms of DNA content, and they support the allotetraploid origin(s) of C. berlandieri subsp. nuttalliae and C. quinoa.     

Mapping a gene conferring resistance to <Emphasis Type="Italic">Wheat yellow mosaic virus</Emphasis> in European winter wheat cultivar ‘Ibis’ (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Wheat yellow mosaic, caused by Wheat yellow mosaic virus (WYMV), is one of the most devastating soil-borne diseases of winter wheat (Triticum aestivum L.) in Japan. Yellow-striped leaves and stunted spring growth, symptomatic of WYMV infection, result in severe yield loss. A new putative WYMV resistance gene in the European wheat cultivar ‘Ibis’ was mapped in the cluster of microsatellite markers including Xcfd16, Xwmc41, Xcfd168 and Xwmc181 on the long arm of chromosome 2D at the distances of 2.0 cM, 4.0 cM, 7.1 cM and 12.4 cM, respectively. WYMV-resistant cultivars contained a common haplotype of the four markers, whereas moderately susceptible and susceptible cultivars did not. These results should be useful in marker-assisted selection for WYMV resistance in wheat.     

Identification of chromosome regions conferring dry matter accumulation and photosynthesis in wheat (<Emphasis Type="Italic">Triticum</Emphasis> <Emphasis Type="Italic">aestivum</Emphasis> L.)

Dry matter accumulation (DMA) and photosynthetic capacity are important traits that influence biological yield and ultimate grain yield in wheat. In this study, quantitative trait loci (QTLs) analyses for DMA of stem, leaves, total plant and photosynthesis traits (Fv/Fm) at the jointing and anthesis stages were studied, using a set of 168 doubled haploid lines (DHLs) derived from the cross Huapei 3 (HP3)/Yumai 57 (YM57). QTL analyses were performed using QTL-Network 2.0 software based on the mixed linear model approach. A total of 18 additive QTLs and 12 pairs of epistatic QTLs were distributed on 16 of the 21 chromosomes. Most of the additive QTLs associated with DMA co-located in the same or adjacent chromosome intervals with QTLs for grain yield and related traits. A major locus Qculmc.sau-5D.1 (14.2%) close to the molecular marker Xwmc215 detected at the jointing stage was shared by QTLs for heading date and vernalization sensitivity, indicating tight linkages or pleiotropisms. One pair of epistatic QTLs, Qleavesc.sau-4A and Qleavesc.sau- 6B, explained 13.11% of the phenotypic variation at anthesis. All QTL × environment interactions were detected at the jointing stage, showing the importance of the jointing stage in determining the final outcome of plant development.     

Potential new genes for resistance to <Emphasis Type="Italic">Mycosphaerella graminicola</Emphasis> identified in <Emphasis Type="Italic">Triticum aestivum</Emphasis> × <Emphasis Type="Italic">Lophopyrum elongatum</Emphasis> disomic substitution lines

Lophopyrum species carry many desirable agronomic traits, including disease resistance, which can be transferred to wheat by interspecific hybridization. To identify potentially new genes for disease and insect resistance carried by individual Lophopyrum chromosomes, 19 of 21 possible wheat cultivar Chinese Spring × Lophopyrum elongatum disomic substitution lines were tested for resistance to barley yellow dwarf virus (BYDV), cereal yellow dwarf virus (CYDV), the Hessian fly Mayetiola destructor, and the fungal pathogens Blumeria graminis and Mycosphaerella graminicola (asexual stage: Septoria tritici). Low resistance to BYDV occurred in some of the disomic substitution lines, but viral titers were significantly higher than those of two Lophopyrum species tested. This suggested that genes on more than one Lophopyrum chromosome are required for complete resistance to this virus. A potentially new gene for resistance to CYDV was detected on wheatgrass chromosome 3E. All of the substitution lines were susceptible to Mayetiola destructor and one strain of B. graminis. Disomic substitution lines containing wheatgrass chromosomes 1E and 6E were significantly more resistant to M. graminicola compared to Chinese Spring. Although neither chromosome by itself conferred resistance as high as that in the wheatgrass parent, they do appear to contain potentially new genes for resistance against this pathogen that could be useful for future plant-improvement programs.     

Alloplasmic effects of <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">B. juncea</Emphasis> on seed characteristics of <Emphasis Type="Italic">B. carinata</Emphasis>

Effects of Brassica napus (N) and B. juncea (J) cytoplasm on seed characteristics of B. carinata (C) were examined. Alloplasmic lines of B. carinata were produced from N × C and J × C hybrids by recurrent backcrossing to the BC₈ generation. Fourteen sets of reciprocal crosses were used. Compared with their euplasmic sibs, alloplasmic B. carinata line seeds with B. napus cytoplasm showed reduced dormancy, higher seed weight, lower germination rate at high temperatures, higher germination rate at low temperatures, and had lower erucic acid and higher linoleic acid contents. Alloplasmic B. carinata line seeds with B. juncea cytoplasm had higher seed weight but lower germination rate than their corresponding euplasmic sibs. These results showed a cytoplasmic effect on seed development, and an influence on seed weight, dormancy, and fatty acid composition. B. carinata was more deleteriously affected by cytoplasm from B. napus than by cytoplasm of B. juncea.     

Induction of <Emphasis Type="Italic">Fusarium</Emphasis> wilt (<Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">pisi</Emphasis>) resistance in garden pea using induced mutagenesis and in vitro selection techniques

Wilt caused by Fusarium oxysporum f. sp. pisi is a serious production constraint for peas worldwide. An attempt was made to isolate wilt-resistant mutants in two susceptible pea genotypes, Arkel and Azad P-1, employing induced mutagenesis and in vitro selection techniques. Two thousand seeds of each genotype were mutagenized either with ethyl methane sulfonate (EMS, 0.2% and 0.3%) or gamma rays (5-22.5 kR) in ⁶⁰Co gamma cell for three consecutive years. Screening of different mutagenized populations under wilt-sick plots resulted in the isolation of 25 mutants exhibiting complete or enhanced wilt resistance compared to parental genotypes. Five of these wilt-resistant mutants also outperformed the susceptible background genotypes in terms of yield and other horticultural traits. Efforts were also made to isolate wilt-resistant regenerants from callus cultures exhibiting insensitivity to culture filtrate (CF) of F. oxysporum f. sp. pisi. A total of 250 regenerants (R ₀) were obtained from CF-insensitive calli selected from medium supplemented with 20% culture filtrate. When evaluated in artificially inoculated sick plots, only five R ₂ lines obtained from the regenerants exhibited enhanced wilt resistance compared to parental cultivars. However, the selected lines did not exhibit resistance levels equivalent to those shown by wilt-resistant lines isolated through in vivo mutagenesis. To conclude, induced mutagenesis through irradiation and EMS treatments exhibited superiority over in vitro selection for inducing wilt resistance in peas.     

Cytological mechanism of 2<Emphasis Type="Italic">n</Emphasis> pollen formation in Chinese jujube (<Emphasis Type="Italic">Ziziphus jujuba</Emphasis> Mill. ‘Linglingzao’)

In order to elucidate the cytological mechanism of 2n pollen formation in Chinese jujube, a cultivar named ‘Linglingzao’ (2n = 2x = 24) which produces relative more 2n pollens naturally was employed for microsporogenesis analysis. Chromosomes paired in 12 bivalents at diakinesis and the first meiotic division was normal, whereas, the second division was characterized by frequent abnormal spindle orientation (parallel and tripolar spindles). Perpendicular, tripolar, and parallel spindles at metaphase II accounted for 72.93, 17.22, and 9.85% respectively. Perpendicular and tripolar spindles led to tetrads and triads formation respectively. Two types of parallel spindles were observed and a significant association correlation (r = 0.84, P < 0.05) between parallel spindle II (fused spindles) and dyads was found which meant only fused spindles can form dyads, while parallel spindle I with a long distance between two sets of chromosomes seemed to lead tetrad formation. At tetrad stage, the observed frequencies of tetrads, triads and dyads were 80.99, 16.40, and 2.61% respectively. By the rule that each tetrad can form four n pollens, each triad can form one 2n pollen and two n pollens, each dyad can form two 2n pollens, the frequency of 2n pollen estimated was 5.71%. Based on the pollen diameter difference between n and 2n pollens, frequency of 2n pollen observed was 6.15%. No significant difference was observed between frequency of 2n pollen estimated and that of observed. The results indicated that, tripolar spindles and fused spindles at metaphase II followed by more triads and few dyads formation at tetrad stage were responsible for the production of 2n pollen in Chinese jujube. 2n gametes observed correspond to first division restitution. Research on the cytological mechanism of unreduced pollen will provide a platform for unreduced pollen induction and polyploidy breeding in Chinese jujube.     

Fine mapping of the <Emphasis Type="Italic">fg</Emphasis> gene using the interspecific cross of <Emphasis Type="Italic">Gossypium hirsutum</Emphasis> × <Emphasis Type="Italic">Gossypium barbadense</Emphasis>

Frego (fg) bract is an important agronomic trait in tetraploid cotton, which has been widely introduced into several cotton varities or lines in the past several years. In order to help us further understand the underlying molecular mechanism of frego bract development, a map-base cloning strategy was used to localize the fg locus. An F₂ population which comprised of 290 fg individuals derived from a cross of the multiple-marker line T582 (G. hirsutum, carrying the fg gene) with Hai7124 (G. barbadense) was constructed. Genetic linkage analysis was carried out to map of the fg locus with SSR and EST-SSR markers in tetraploid cotton. Genetic linkage analysis showed that the fg locus was flanked by the marker NAU3016 and NAU3172 on the long arm of chromosome 3, with the genetic distance of 0.3 cM and 4.7 cM, respectively. The information of fg locus provided the basic information for the final isolation of this important gene in tetraploid cotton, these marker information could be used in marker-assisted selection in cotton.     

Resistance of barley variety ‘Venezia’ and its reflection in the <Emphasis Type="Italic">Blumeria graminis</Emphasis> f. sp. <Emphasis Type="Italic">hordei</Emphasis> population

Genetic resistance is an efficient and environmentally acceptable way of limiting the damaging effects of plant pathogens on yield and quality of crops. Tests of winter barley variety Venezia revealed an unknown resistance to all tested Blumeria graminis f. sp. hordei isolates. Response type arrays (RTAs) obtained here were created using common avirulent (RT 0) isolates and virulent (RT 4) isolates that first appeared in 2011. RTA of Venezia was identical to RTAs of six other varieties, but differed from RTAs of all other previously tested varieties. Venezia was the first variety to be registered with this resistance, and it is recommended that the resistance be designated Ve. Among 905 isolates randomly collected from the Czech aerial pathogen populations from 2009 to 2015, 13 contained Ve virulence. Each of the isolates differed from the others and thus belonged to different pathotypes. Seven of these 13 pathotypes were collected in the western region of the Czech Republic in an area close to Germany, where Venezia was grown. This finding could support the hypothesis that pathotypes virulent to Venezia have migrated from Germany into the Czech Republic.     

Quantitative trait loci analysis of citrus leprosis resistance in an interspecific backcross family of (<Emphasis Type="Italic">Citrus reticulata</Emphasis> Blanco × <Emphasis Type="Italic">C. sinensis</Emphasis> L. Osbeck) × <Emphasis Type="Italic">C. sinensis</Emphasis> L. Osb

Leprosis, caused by citrus leprosis virus (CiLV) and transmitted by the tenuipalpid mite Brevipalpus phoenicis, is one of the most important viruses of citrus in the Americas. Sweet oranges (Citrus sinensis L. Osb.) are highly susceptible to CiLV, while mandarins (C. reticulata Blanco) and some of their hybrids have higher tolerance or resistance to this disease. The mechanisms involved in the resistance and its inheritance are still largely unknown. To study the quantitative trait loci (QTL; quantitative trait loci) associated with the resistance to CiLV, progeny analyses were established with 143 hybrid individuals of ‘Pêra’ sweet orange (C. sinensis L. Osb.) and ‘Murcott’ tangor (C. reticulata Blanco × C. sinensis L. Osb.) from controlled crossings. Disease assessment of the hybrid individuals was conducted by infesting the plants with viruliferous mites in the field. The experiment consisted of a randomized completely block design with ten replicates. The evaluated phenotypic traits were incidence and severity of the disease on leaves and branches, for a period of 3 years. The MapQTL™ v.4.0 software was used for the identification and location of possible QTL associated with resistance to CiLV on a genetic map obtained from 260 AFLP and 5 RAPD markers. Only consistent QTLs from different phenotypic traits and years of evaluation, with the critical LOD scores to determine the presence or absence of each QTL calculated through the random permutation test, were considered. A QTL was observed and had a significant effect on the phenotypic variation, ranging from 79.4 to 84% depending on which trait (incidence or severity) was assessed. This suggests that few genes are involved in the genetic resistance of citrus to CiLV.     

Identification of molecular markers associated with resistance to squash silverleaf disorder in summer squash (<Emphasis Type="Italic">Cucurbita</Emphasis> <Emphasis Type="Italic">pepo</Emphasis>)

Squash silverleaf (SSL), caused by the silverleaf whitefly [Bemisia argentifolii (formerly known as Bemisia tabaci Gennadius, B strain)], is an important physiological disorder that affects squash (Cucurbita spp.) by reducing yield potential. Breeding squash with resistance to SSL disorder can be facilitated by using marker-assisted selection (MAS). Resistance to SSL disorder, in Cucurbita pepo, is conferred by a single recessive gene (sl). The objective of this study was to identify molecular markers associated with resistance. A zucchini squash, SSL disorder resistant breeding line, ‘Zuc76’ (sl/sl) and a SSL disorder susceptible zucchini cultivar ‘Black Beauty’ (Sl/Sl) were screened with 1,152 randomly amplified polymorphic DNA (RAPD) primers and 432 simple sequence repeat (SSR) markers to identify polymorphisms. Using F₂ and BC₁ progeny segregating for SSL disorder resistance, three RAPD (OPC07, OPL07 and OPBC16) primers and one SSR (M121) marker were found associated with sl. Fragments amplified by RAPD primer OPC07 was linked in coupling phase to sl, whereas RAPD primer OPL07 was linked in repulsion phase. RAPD primer OPBC16 and SSR marker M121 were co-dominant. The allelic order of these loci was found to be M121–sl–OPC07–OPL07–OPBC16. The closest marker to sl is M121 with an estimated genetic distance of 3.3 cM. The markers identified in this study will be useful for breeding summer squash (C. pepo) for SSL disorder resistance derived from zucchini squash breeding line ‘Zuc76’.     

Inheritance of foliar resistance to <Emphasis Type="Italic">Phytophthora</Emphasis> species in cacao (<Emphasis Type="Italic">Theobroma cacao</Emphasis> L.): can it be influenced by CSSV infection?

Studies on quantitative genetics of foliar resistance to black pod disease in cacao could inadvertently use cocoa swollen shoot virus (CSSV) infected leaves which could bias the results especially in West Africa where the virus is prevalent. However, effects of CSSV on inheritance and heritability of foliar resistance to Phytophthora species is not known. Choice of an efficient breeding method requires an accurate estimation of genetic effects in selection schemes for foliar resistance to Phytophthora species in cacao. The objective of this study was to investigate the effect of CSSV infection on quantitative genetic parameters of foliar resistance to cocoa black pod disease in a population of 36 F1 hybrids developed by mating six cacao genotypes using a diallel method. The generated F1s and their parents were evaluated for foliar resistance to P. palmivora and P. megakarya using a randomized complete block design (RCBD) with three replications. 1A CSSV and Nsaba CSSV strains were used to infect the cacao genotypes using the patch graft method. The parents chosen showed significant variations for scores of leaf discs after inoculation with P. palmivora and P. megakarya. The leaf disc scores of CSSV infected crosses were lower than leaf disc scores of CSSV-free crosses. Genetic component analysis showed that the effects of GCA and SCA was significant for both CSSV-free and CSSV-infected crosses in resistance to P. palmivora and P. megakarya. The significant GCA and SCA for both CSSV-free and CSSV-infected crosses strongly suggest that both additive and non-additive genetic effects play an important role in the determination of inheritance of foliar resistance to Phytophthora species in cacao. There was significant variability in mean squares of GCA and SCA of CSSV-free and CSSV-infected crosses indicating that CSSV infection modifies GCA and SCA of affected plants. Narrow sense heritability was relatively low (0.31) for foliar resistance to P. palmivora and P. megakarya under CSSV-free and 1A CSSV strain infected conditions. However, heritability for foliar resistance to P. palmivora (0.43) and P. megakarya (0.36) was significantly higher under Nsaba CSSV infected condition. The modifications of mean squares of GCA and SCA and narrow sense heritability due to CSSV infection could mislead in choice of breeding methods indicating that attention must be paid to the infection status of plants when conducting quantitative genetics studies using diseased and healthy plants. CSSV status of leaf samples should be known before using them in leaf disc test. Genotypes Pa7/808 and Pound 7 had high negative GCA effects and are promising parents for enhancement of resistance to black pod disease in cacao.     

A dominant gene for garnet brown seed coats at the <Emphasis Type="Italic">Rk</Emphasis> locus in ‘Dorado’ common bean and mapping <Emphasis Type="Italic">Rk</Emphasis> to linkage group 1

The color of the seed coats of ‘Dorado’ (Phaseolus vulgaris L.) is garnet brown (dark red kidney bean color) and differs from most other dry bean varieties in the Honduran red bean market class. A genetic investigation of the color of ‘Dorado’ (same as DOR364) and G19833 (Liborino market class) seed coats was conducted. Crosses with genetic tester stocks demonstrated that the gene for garnet brown (GB) in ‘Dorado’ was not allelic with the R gene for dominant red (oxblood) seed coat. An allelism test between the ‘Dorado’ gene for GB seed coat and the rk ^drv gene for recessive expression of GB demonstrated allelism. We propose the gene symbol for Rk ^r for the ‘Dorado’ GB seed coat color gene. Rk ^r expresses partial dominance over Rk, where Rk ^r/Rk expresses a paler and highly variable intermediate red color. The interactions of Rk ^r, rk ^drv, and c ^u are discussed. Segregation analysis in the mapping population made up of DOR364 (same as ‘Dorado’) × G19833 recombinant inbred lines showed that the Rk ^r gene mapped to linkage group 1. The new allele at Rk was located at a distance of 17 cM from the RFLP marker Bng130 with a LOD > 3.0.     

Frequent occurrence of unreduced gametes in <Emphasis Type="Italic">Triticum turgidum</Emphasis>–<Emphasis Type="Italic">Aegilops tauschii</Emphasis> hybrids

Spontaneous chromosome doubling via union of unreduced (2n) gametes has been thought to be the way that common wheat (Triticum aestivum L.) was originated from the hybridization of T. turgidum L. with Ae. tauschii Cosson. Previous works have observed unreduced gametes in F₁ hybrids of Ae. tauschii with six of the eight T. turgidum subspecies. It is not clear, however, whether the formation of these unreduced gametes is a norm in the F₁ hybrids. In the present study, we tried to answer this question by assessing the occurrence frequency of unreduced gametes in 115 T. turgidum–Ae. tauschii hybrid combinations, involving 76 genotypes of seven T. turgdium subspecies and 24 Ae. tauschii accessions. Our data show that these hybrid combinations differed significantly (P ≤ 0.01, F = 11.40) in selfed seedset, an indicator for production of unreduced gametes. This study clearly showed that meiotic restitution genes are widely distributed within T. turgidum. However, significant differences were found between as well as within T. turgidum subspecies and in the interaction of the T. turgidum genotypes with those of Ae. taushii. The possible application of the meiotic restitution genes from T. turgidum in production of double haploids is also discussed.     

Complementary DNA (cDNA) cloning and functional verification of resistance to head smut disease (<Emphasis Type="Italic">Sphacelotheca reiliana</Emphasis>) of an NBS–LRR gene <Emphasis Type="Italic">ZmNL</Emphasis> in maize (<Emphasis Type="Italic">Zea mays</Emphasis>)

The nucleotide-binding site (NBS)-leucine-rich repeat (LRR) gene family comprises the largest number of known disease resistance (R) genes and is one of the largest gene families in plants. In the present study, the full-length cDNA of ZmNL (GenBank Accession Number KF765443) was isolated using Rapid Amplification of cDNA Ends. The nucleotide sequence of ZmNL contains an open reading frame of 3156 bp that encodes the ZmNL protein, which is comprised of 1051 amino acid residues. This putative protein has high homology to other known resistance proteins (84% to Triticum aestivum LR10) and belongs to the CC–NBS–LRR type R gene family. The ZmNL gene was introduced into the maize inbred line of Huangzao4 which was highly susceptible to head smut under the control of the maize ubiquitin promoter by Agrobacterium-mediated transformation. The head smut disease incidence of 3 T₂ transgenic lines was significantly reduced (by 18.38–29.40%) compared with the wild type, which indicated that the overexpression of ZmNL gene in maize enhanced the resistance to the fungus Sporisorium reilianum (Kühn) Clint of these plants.     

Production of aneuploids of the cotton hybrid G. barbadense × <Emphasis Type="Italic">G. hirsutum</Emphasis> L. via intergeneric pollination with <Emphasis Type="Italic">Abelmoschus esculentus</Emphasis>

The possibility to induce embryo development after pollination of F₁ interspecific cotton hybrids (Gossypium barbadense × Gossypium. hirsutum) with pollen from Abelmoschus esculentus was investigated to determine if wide-cross hybrid, haploid or other types of progeny might result. Small numbers of progeny (Pa) were indeed recovered after numerous alien pollinations and in-planta development or in-vitro culture. The Pa plants were characterized phenotypically and studied cytogenetically and microscopically to help establish their origin documenting their reproductive basis. Root-tip chromosome counts and meiotic Metaphase-I analyses revealed that chromosome numbers among cells of the Pa plants ranged from 33 to 44 and that the differences in chromosome number among cells of the same plant ranged from 1 to 3, indicating somatic instability. Flow cytometric analysis also indicated the aneuploid nature of Pa plants. Although the reproductive mechanisms need to be characterized more extensively by cytological and molecular means, the observations suggest that alien pollinations may have resulted in parthenogenetic (Pa) egg cell development, or some other unusual reproductive events. The production of wide-crosses and high degrees of aneuploidy could be of use for several types of genomic studies, e.g., functional genomic characterization of genome shock, deletion mapping, and germplasm introgression.     

Formation of unreduced gametes is impeded by homologous chromosome pairing in tetraploid <Emphasis Type="Italic">Triticum turgidum</Emphasis> × <Emphasis Type="Italic">Aegilops tauschii</Emphasis> hybrids

It is believed that unreduced gametes with somatic chromosome numbers play a predominant role in natural polyploidization. Allohexaploid bread wheat originated from spontaneous hybridization of Triticum turgidum L. with Aegilops tauschii Coss. Unreduced gametes originating via meiotic restitution, including first-division restitution (FDR) and single-division meiosis (SDM), are well documented in triploid F₁ hybrids of T. turgidum with diploid Ae. tauschii (genomic constitution ABD, usually with 21 univalents in meiotic metaphase I). In this study, two T. turgidum lines known to carry genes for meiotic restitution were crossed to tetraploid Ae. tauschii. The resulting F₁ hybrids (genomes ABDD), had seven pairs of homologous chromosomes and regularly formed 14 univalents and seven bivalents at metaphase I. Neither FDR nor SDM were observed. The distribution of chromosome numbers among progeny obtained by self pollination and a backcross to T. turgidum showed the absence of unreduced gametes. These results suggest that high homologous pairing interfered with meiotic restitution and the formation of unreduced gametes. This may be related to asynchronous movement during meiosis between paired and unpaired chromosomes or to uneven distribution of chromosomes in anaphases, resulting in nonviable gametes.     

Novel sources of witches’ broom resistance (causal agent <Emphasis Type="Italic">Moniliophthora perniciosa</Emphasis>) from natural populations of <Emphasis Type="Italic">Theobroma cacao</Emphasis> from the Brazilian Amazon

Witches’ broom is a severe disease of Theobroma cacao L. (cacao), caused by the basidiomycete Moniliophthora perniciosa. The use of resistant cultivars is the ultimate method of control, but there are limited sources of resistance. Further, resistance from the most widely used source (‘Scavina 6’) has been overcome after a few years of deployment. New sources of resistance have been intensively searched for in the Amazon basin. Here, we evaluated for witches’ broom resistance, cacao accessions from various natural cacao populations originally collected in the Brazilian Amazon. Resistance of 43 families was evaluated under nursery and/or field conditions by artificial or natural infection, respectively, based on disease incidence. Screening for resistance by artificial inoculation under nursery conditions appeared to be efficient in identifying these novel resistance sources, confirmed by natural field evaluation over a nine-year period. The increase in natural field infection of ‘Scavina 6’ was clearly demonstrated. Among the evaluated families with the least witches’ broom incidence, there were accessions originally collected from distinct river basins, including the Jamari river (‘CAB 0371’; ‘CAB 0388’; ‘CAB 0392’; and ‘CAB 0410’); Acre (‘CAB 0169’); Javari (‘CAB 0352’); Solimões (‘CAB 0270’); and from the Purus river basin, the two most outstanding resistant accessions, ‘CAB 0208’ and ‘CAB 0214’. The large genetic diversity found in cacao populations occurring at river basins from Acre and Amazonas states, Brazil, increased the chance that the selected resistant accessions would be genetically more dissimilar, and represent distinct sources of resistance to M. perniciosa from ‘Scavina 6’.     

A genetic linkage map of <Emphasis Type="Italic">Populus adenopoda</Emphasis> Maxim. × <Emphasis Type="Italic">P. alba</Emphasis> L. hybrid based on SSR and SRAP markers

Populus adenopoda Maxim. and P. alba L. [section Populus (aspen), genus Populus] are two tree species of ecological and economic value. To date, no high-density genetic maps are available for these two species. In this study, 1100 interspecific hybrids were obtained by controlled crossing and embryo culture. Simple sequence repeat (SSR) and sequence-related amplified polymorphisms (SRAP) were used to genotype 189 F₁ individuals. The genetic linkage map of P. adenopoda × P. alba generated from this study includes 212 markers (192 SSRs and 50 SRAPs) and consists of 26 linkage groups spanning 2178.5 cM, with an average distance of 11.7 cM between markers. This is the first SSR- and SRAP-containing genetic linkage map for aspen. The SSRs on the map will serve both as bridges for comparison with the poplar maps published to date and as a direct link to the Populus genomic sequence. Future studies focusing on the data presented here should enhance the density and precision of the map for identifying and localizing quantitative trait loci and promote genomic research on the genus.