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1.
Sexuality is correlated with diploidy and apomixis with polyploidy in the Brachiaria genus. Brachiaria ruziziensis is a key species in Brachiaria breeding due to its obligate sexuality and intrinsic agronomic qualities. Interspecific crosses in the genus became feasible only when a few diploid accessions of B. ruziziensis were artificially tetraploidized and remained sexual. Hybridization has been done since, using natural tetraploid apomictic accessions of B. brizantha or B. decumbens as pollen donors. Twenty two accessions of B. ruziziensis from the Embrapa Beef Cattle germplasm collection (Campo Grande, MS, Brazil) were cytologically analyzed: 16 are natural diploids (2n = 2x = 18) and six are artificially induced tetraploids (2n = 4x = 36). The meiotic behavior in the 16 diploid accessions varied. The mean of meiotic abnormalities per accession ranged from zero to 24.46%. Meiotic behavior in the induced tetraploid accessions also varied with the mean of meiotic abnormalities ranging from 5.20% to 54.71%. The most common abnormalities observed in both the diploid and the tetraploid accessions, were those related to irregular chromosome segregation. In one tetraploid accession, with a high frequency of those, other irregularities involving chromosome orientation at metaphase plate and chromosome convergence to the poles, a meiotic mutation known as divergent spindle, were recorded. Meiotic behavior should be considered in selecting potential genitors for breeding.  相似文献   

2.
In the genus Brachiaria, genetic variation can be exploited directly from germplasm collections or released using sexual reproduction in normally apomictic polyploids. The discovery of a natural sexual polyploid accession H031 of Brachiaria humidicola collected in Africa, opened new opportunities to exploit the genetic variation in this species. This accession was crossed with an apomictic cultivar BRS Tupi with the same chromosome number (2n = 36) and 361 F1 hybrids were obtained. Following visual selection for leafiness, vigor, growth habit, and the mode of reproduction, 50 hybrids were selected for further agronomic evaluation. The parents and 45 of the 50 selected hybrids were evaluated for the regularity of meiosis. In the female parent (H031), meiosis was somewhat irregular, with 16.3% of abnormal tetrads, whereas the male (cv. BRS Tupi) meiosis was very regular, with only 3.1% of abnormal tetrads. Among hybrids (sexual and apomictic), the percentage of abnormal tetrads ranged from 15.8 to 98.3%. The abnormalities included irregular chromosome segregation, chromosome stickiness and the absence of cytokinesis. Considering that apomixis in the genus Brachiaria is pseudogamic, and that meiotic aberrations can compromise pollen viability, the results of this study present another parameter to aid selection for more stable microsporogenesis. Apomictic derivatives with stable meiosis are candidates for new cultivars whereas sexual hybrids can be retained in breeding for another round of recombination.  相似文献   

3.
New tri-species hybrids (GOS) in the genus Pennisetum involving the cultivated species pearl millet (P. glaucum L.) and two wild species, viz. P. squamulatum Fresen and P. orientale L. C. Rich, are reported. Six hybrid plants were recovered after crossing a backcross hybrid (2n = 3x = 23, GGO) between P. glaucum (2n = 2x = 14, GG) and P. orientale (2n = 2x = 18, OO) with F1s (2n = 6x = 42, GGSSSS) between P. glaucum (2n = 4x = 28, GGGG) and P. squamulatum (2n = 8x = 56, SSSSSSSS). The hybrids were perennial, morphologically intermediate to their parents, and represented characters from the three contributing species. The hybrids contained 2n = 44 chromosomes (GGGSSO) representing 21, 14 and nine chromosomes from P. glaucum, P. squamulatum and P. orientale, respectively. Meiotic and flow-cytometric analysis suggested origin of these hybrids from unreduced female and reduced male gametes. Average chromosome configuration (8.42I + 14.32II + 1.62III + 0.52IV) at Meiosis showed limited inter-genomic pairing indicating absence of significant homology between the three genomes. The hybrids were male sterile (except one) and highly aposporous. P. orientale was identified to induce apospory in hybrid background with P. glaucum at diploid and above levels, though it was quantitatively affected by genomic doses from sexual parent. A case of inducible and recurrent apospory is presented whereby a transition from Polygonum-type sexual embryo-sacs to Panicum-type aposporous embryo-sacs was observed in diploid interspecific hybrids. Results supported independent origin and partitioning of the three apomixis-components (apomeiosis, parthenogenesis, and functional endosperm development), reported for the first time in Pennisetum. Potential utilization of GOS hybrids in understanding genome interactions involved in complex traits, such as perenniality and apomixis, is discussed.  相似文献   

4.
Two diploid accessions of wild oat, CIav6956 and CIav7233, were identified as carrying seedling resistance to oat crown rust (caused by Puccinia coronata f. sp. avenae; Pca). Two vigorous interploidy F1 hybrids were generated from crosses involving the hexaploid oat cultivar Wintaroo and the diploid oat Avena strigosa Schreb. accession CIav6956. An additional interploidy F1 hybrid, designated “F1-Aa1”, was produced from a cross of Wintaroo and the diploid oat accession CIav7233. All three hybrids were more vigorous and taller than the cultivated parent Wintaroo. The three F1 hybrids contained full chromosome complements from both parents (2n = 4x = 28), but no seeds were obtained when the three F1 hybrids were selfed. Meiotic analyses of the hybrids indicated that they exhibited a high degree of inter-genome and intra-genome pairing. Trivalent configurations were detected in 95–96% of meiotic cells and a minimum of three bivalents was present in all cells. An average chiasma frequency of 7.2–7.9 per cell was observed for the three F1 hybrids. A fourth F1 hybrid was subsequently generated from a cross between the diploid oat accession CIav7233 and Wintaroo. One octaploid (2n = 8x = 56) was generated from this hybrid and progeny were resistant to two Pca races. The chromosome number of the octaploid progeny varied between 51 and 54 chromosomes. Development of a chromosome addition line(s) with the crown rust resistance should be possible from these partial-octaploids.  相似文献   

5.
The genus Brachiaria, native to the African tropical savannas, has achieved significance as a pasture grass in many tropical and subtropical countries, including Brazil. Many species and accessions are polyploid and apomictic, which complicates the improvement of breeding stocks through hybridization. In support of breeding programs, cytogenetic characterization, including chromosome counts and evaluation of the meiotic behavior in the accessions of the Brachiaria has been undertaken at the Embrapa Beef Cattle Center. In this study, 22 accessions of B. brizantha were analyzed of which one was found to be diploid (2n = 2x = 18), 18 were tetraploid (2n = 4x = 36) and three were hexaploid (2n = 6x = 54). The meiotic chromosome behavior was slightly irregular in the diploid and in some tetraploid accessions, and highly irregular in most tetra- and hexaploid accessions. Meiotic abnormalities were those common to polyploidy, i.e., multivalent chromosome association at diakinesis and irregular chromosome segregation leading to micronuclei formation in the tetrad stage. Low frequencies of multivalent chromosome associations among polyploids suggest that they may be segmental allopolyploids. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

6.
Collection of wild species is critical to increase crop germplasm gene pools. Gaura coccinea has been identified as an important source of cold tolerance (winter hardiness) for hybridization with non-cold tolerant G. lindheimeri. Identification of species in the wild is often complicated by diagnostic, morphological characteristics that may not be present at the time of collection. G. drummondii, however, can be misidentified as G. coccinea even when using some diagnostic and quantitative traits. Therefore, additional methods of distinguishing between the species are needed. Fifteen populations of both G. coccinea and G. drummondii collected for potential breeding purposes in Texas, California, and Minnesota were used to identify species-specific molecular markers and morphological characteristics. A dendrogram based on randomly amplified polymorphic DNA (RAPD) markers separated the genotypes into two groups, G. coccinea and G. drummondii. One unique RAPD marker was identified in all G. drummondii genotypes, but absent in G. coccinea. Dendrograms based on morphological data separated genotypes into three distinct groups: G. coccinea, G. drummondii, and G. sp. (393-1). Petal/sepal lengths and leaf width were smaller in G. coccinea, while fruit/stipe lengths were smaller in both G. coccinea and genotype 393-1. Cytological analysis of G. coccinea confirmed that most populations were tetraploid (2n = 4x = 28) with one hexaploid (2n = 6x = 42) genotype from Texas (1000-1). G. drummondii were primarily tetraploid (2n = 4x = 28). Occasional triploids were found. These markers, along with screening for cold tolerance, will allow for accurate identification of G. coccinea genotypes for hybridization with G. lindheimeri.  相似文献   

7.
Meiotic nondisjunction during microsporegenesis can lead to aneuploid gametes formation and reduced pollen fertility in plants. This paper reports the prevalence of meiosis I nondisjunction in a resynthesized Brassica napus (AACC, 2n = 38) and its use for aneuploid production. Meiosis in the amphidiploids was characterized by high frequencies of univalents and multivalents per PMC at diakinesis/metaphase I and notably unbalanced chromosome segregations at anaphase I (AI). In all the plants observed, 18.95–44.3% of PMCs exhibited a segregation of 18:20 (n − 1:n + 1) at AI which was caused by nondisjunction of one bivalent or the distribution of two homologous univalents to the same pole. Meiosis proceeded normally after AI then, thus led to the formation of viable n − 1 and n + 1 gametes and high pollen fertility of these plants. Microspore culture was subsequently carried out using these plants in an attempt to isolate Brassica nullisomics. Four nullisomics (2n = 36), two nullihaploids (2n = 18) and one tetrasomic haploid (2n = 20) were identified cytologically and characterized morphologicaly and physiologically. Amplified fragment length polymorphism (AFLP) survey suggested that of the six nullisomics/nullihaploids, one nullihaploids lost one A-genome chromosome and the other five lost C genome chromosome(s). Altogether, different C-genome chromosomes were thought to have been lost in the nullisomics/nullihaploids. The mechanisms underlying the meiotic abnormalities and the implications of these B. napus nullisomics are discussed.  相似文献   

8.
Helleborus species are members of the family of the Ranunculaceae. These popular perennials are all diploids (2n = 2x = 32). This study investigates polyploidy induction by different antimitotic agents. Colchicine, oryzalin and trifluralin were tested in vitro on shoots of Helleborus niger, H. orientalis and H. × nigercors. Furthermore the effect of the antimitotic agents on the viability and the multiplication rate of cultured plantlets were analyzed. Flow cytometry demonstrated that polyploidisation was genotype dependent: using H. niger, tetraploids were obtained using either oryzalin (3 μM) or trifluralin (3 or 10 μM), whereas for H. × nigercors only trifluralin (3 or 10 μM) induced polyploidisation. For H. orientalis neither treatment was effective to produce tetraploids or mixoploids. For these three species, colchicine (100 μM) was ineffective. The polyploidisation events in H. niger and H. × nigercors were confirmed by chromosome counts of mounted nuclei derived from root tips (2n = 4x = 64).  相似文献   

9.
The Narcissus variety, ‘Tête-à-Tête’, has been the most popular variety since 1949, and a well known allotriploid (2n = 3x = 24 + B) of spontaneous origin. Because the identity of one of the parents of this variety was uncertain, the genome constitution of ‘Tête-à-Tête’ was investigated by using genomic in situ hybridization (GISH) and NBS profiling. Both of these techniques confirmed that two different species of Narcissus, viz., N. tazetta (2n = 2x = 20) and N. cyclamineus (2n = 2x = 14) are the parents. GISH clearly identified 10 chromosomes of N. tazetta and 14 chromosomes of N. cyclamineus, the former has contributed one and the latter has contributed two genomes. One B chromosome was identically labelled as those of N. cyclamineus indicating the affinity of the special chromosome to this species. Due to its male and female sterility ‘Tête-à-Tête’ is unsuitable as a parent for utilizing in further breeding, it might be possible to re-synthesize a ‘Tête-à-Tête’ like variety using the now known parents and the original pathway.  相似文献   

10.
Sexual hybridization is an important generator of biodiversity and a powerful breeding tool. Hybridization can also overcome ploidy barriers when it involves 2n gametes, as in the case of unilateral sexual polyploidization (USP) that has been utilized in several crops, among which alfalfa. This research was aimed at gaining insights into the effects of USP on genome methylation and on phenotypic traits in alfalfa, an important forage species. The Methylation-Sensitive Amplified Polymorphism technique was used to estimate the cytosine methylation changes occurring in a tetraploid (2n = 4x = 32) USP progeny from crosses between a diploid Medicago sativa subsp. falcata genotype that produces 2n eggs and a cultivated tetraploid Medicago sativa subsp. sativa variety. De novo methylation or demethylation in the USP progeny were observed for 13% of the detected genomic sites, indicating that methylation changes can be relevant. USP plants showed larger surface area of the leaf epidermis cells than both parents, but this did not result in larger leaf size or higher plant biomass. They displayed significant higher ovule sterility than the tetraploid parent, but normal fertility was observed in crosses with unrelated male testers. We conclude that hybridization and sexual polyploidization resulted in novel variation in terms of remodeling of the methylation landscape as well as changes in phenotypic traits in alfalfa.  相似文献   

11.
Intraspecific somatic hybrids have been produced by protoplast fusion in eight combinations involving 10 dihaploids (2n= 2x= 24) in an attempt to provide new material for potato breeding. Cytological analysis revealed extensive variation in chromosome number, such as aneuploid, aneusomatic and mixoploid hybrids. Most of the hybrids represented the expected chromosome number of 48; however, the frequency of aneuploids reached 50% in some combinations. Some hybrids carried structurally rearranged chromosomes and exhibited a high frequency of aberrant anaphases. Isozyme and random amplified polymorphic DNA (RAPD) patterns of the hybrids from the same fusion combination were uniform. In the field, somatic hybrids showed wide phenotypic variation in 20 morphological characters. There was a significant correlation between certain leaf characters and the ploidy level, which may be used to distinguish the tetraploid hybrids from hexaploids and octoploids. Tuber yield and flowering intensity were highest in tetraploid hybrids (2n= 4x= 48). Eighteen of the 73 hybrids reached higher yields than the standard variety ‘Adretta’. Floral development and fertility were restored in hybrids derived from fusions between non-flowering or sterile dihaploids.  相似文献   

12.
The widespread occurrence of 2ngametes (i.e. gametes with the somatic chromosome number) in the Medicago sativa-coerulea-falcata complex supports the concept that gene flow from diploid to tetraploid species occurs continuously in nature and plays a key role in alfalfa evolution. Breeders realized early that gene transfer between ploidy levels via 2n pollen and 2n eggs would have had potential use in cultivated alfalfa improvement. Cytological investigations provided insights into the types of meiotic abnormalities responsible for the production of 2n gametes. Alterations were defined as genetically equivalent to first (FDR) or second division restitution(SDR) mechanisms. For breeding purposes,data have proven that 2n gametes of the FDR type are more advantageous than those obtained by SDR for transferring parental heterozygosity and retaining epistatic interactions. The use of diploid meiotic mutants that produce 2ngametes is now recognized as one of the most effective methods available for exploiting heterosis and introgressing wild germplasm traits into cultivated tetraploid alfalfa via unilateral (USP) and bilateral sexual polyploidization (BSP) schemes. Both2n egg and 2n pollen producers could be used for direct gene transfer from wild diploid relatives into cultivated alfalfa by means of 2x-4x and4x-2x crosses. Although data have shown that forage yield improvement can be achieved when plants are sexually tetraploidized, problems related to reduced plant fertility and seed production remain largely unexplained. Apomixis has the potential of cloning plants through seed and thus provides a unique opportunity for developing superior tetraploid cultivars with permanently fixed heterosis and epistatic effects. A main goal in alfalfa breeding could be the introduction of functional apomixis (i.e. Apomeiosis and parthenogenesis) in cultivated alfalfa stocks. In the future, the efficiency of alfalfa breeding programs based on the use of reproductive mutants could be improved by direct selection at the genotype level using RFLPs and PCR-based markers. Suitable DNA markers and detailed linkage maps of alfalfa mutants should help to discover apomictic mutants and address basic genetic issues such as the extent of genomicre combination in polyploid hybrids and the effect of sexual polyploidization on heterosis. Molecular markers have recently been used in alfalfa for studying the inheritance of 2n gamete formation and identifying polymorphisms associated to genes involved in meiotic abnormalities. Molecular tagging of 2n egg and 2n pollen formation not only should explain the genetic control and regulation of these traits, but may also be an essential step towards marker-assisted selection of 2n gamete producers and implementation of USP and BSP breeding schemes. Future perspectives include strategies for the map-based cloning of genomic DNA markers,and screening of EST mini-libraries related to flowers at different developmental stages from meiotic mutants and wild-type scan lead to the identification of mRNAs and thus of candidate genes that control 2n gamete formation in alfalfa. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
Interspecific hybridization is an important approach to broaden the genetic base and create novel plant forms in breeding programs. However, interspecific hybridization in Ipomoea is very difficult due to the cross incompatibility. Here we report two novel interspecific F1 hybrids between I. batatas (L.) Lam. (2n = 6x = 90) and two wild species, I. grandifolia (2n = 2x = 30) and I. purpurea (2n = 2x = 30). Hybridization was stimulated by applying plant growth hormones. Morphological, molecular and cytological tests were conducted to confirm their hybridity. We found that the two hybrids were quite distinctive in leaf color and morphology, and yielded intermediate sizes of storage roots compared to their respective parents. Inter-simple sequence repeat analysis showed that the unique DNA bands from the wild parents could be detected in these two hybrids. The cluster analysis also showed that the two F1 hybrids were closer to I. batatas in phylogeny relationship. The number of chromosomes of the two hybrids was both 60, indicating that the hybrids were tetraploid. The meiotic configuration analysis of the H1 of I. batatas × I. grandifolia revealed the occurrence of 17.58I + 14.28II + 1.36III + 2.48IV at metaphase I in average chromosome association per pollen mother cells (PMCs), 4.26I + 18.32II + 2.56III + 3.12IV was average meiotic configuration in the H2 of I. batatas × I. purpurea. Both hybrids appeared to be polyads and multi-microcytes at tetrad phase and differed in their pollen fertility.  相似文献   

14.
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.  相似文献   

15.
Ploidy level and intergenomic recombination was studied in interspecific hybrids between Longiflorum × Asiatic lilies (LA hybrid) backcross to Asiatic parents in order to assess the possibility for analytic breeding in lily. By backcrossing the diploid (2n = 2x = 24) F1 interspecific hybrid between Longiflorum × Asiatic lilies to Asiatic parents, 104 BC1 progeny plants were produced. Among these, there were 27 diploids, 73 triploids (2n = 2x = 36) and 4 aneuploids (2x − 1, 2x + 2 or 2x + 3). In addition, by backcrossing triploid BC1 (LAA) plants to diploid Asiatic parents in 2x − 3x and reciprocal combinations, 14 diploid BC2 progenies were produced. Genomic in situ hybridization (GISH) was performed to study the intergenomic recombination and karyotype composition. GISH indicated extensive intergenomic recombination among the chromosomes in LA hybrids. A large number of Longiflorum chromosomes were transmitted to the BC1 progenies from LA hybrids. However, very few Longiflorum chromosomes were transmitted from the BC1 triploid (LAA) plants to the BC2 progenies. The occurrence of diploid plants in the BC progenies of LA hybrids has opened the prospects of analytic breeding in lilies. In this approach, the selection of superior genotypes can be carried out at the diploid level and polyploid forms are synthesized from superior diploid parents. The advantages of analytic breeding are evident: (a) a maximum level of heterozygosity can be attained in the synthetic polyploids and (b) introgression can be achieved with a minimum of linkage drag. Based on GISH results the potential application of analytic breeding in lily allopolyploids has been discussed.  相似文献   

16.
Psathyrostachys huashanica Keng ex Kuo (2n = 2x = 14, NsNs), a source of wheat stripe rust, take-all fungus, and powdery mildew resistance with tolerance to salinity and drought, has been successfully hybridized as the pollen parent to bread wheat without using immature embryo rescuing culture for the first time. All of the CSph2b × P. huashanica hybrid seeds germinate well. Backcross derivatives were successfully obtained. F1 hybrids were verified as intergeneric hybrids on the basis of morphological observation, cytological and molecular analyses. The results obviously showed the phenotypes of the hybrid plants were intermediate between bread wheat and P. huashanica. Chromosome pairing at MI of PMCs in the F1 hybrid plants was low, and the meiotic configuration was 26.80 I + 0.60 II (rod). Cytological analysis of the hybrid plants revealed the ineffectiveness of the ph2b gene on chromosome association between the parents. Eight RAPD-specific markers for Ns genome were selected for RAPD analysis, and the results indicated that F1 hybrids contained the Ns genome of P. huashanica. Furthermore, the significance of the finding for bread wheat improvement was discussed.  相似文献   

17.
Triticum monococcum L. (2n = 2x = 14, AmAm genome) is one of the most ancient of the domesticated crops in the Middle East, but it is not the ancestor of the A genome of durum wheat (T. durum Desf. 2n = 4x = 28, genomes BBAA) and bread wheat (T. aestivum L., 2n = 6x = 42, genomes BBAADD). It has been suggested that some differentiation has occurred between the Am and A genomes. The chlorina mutants at the cn-A1 locus located on chromosome 7AL have been described in T. aestivum L. and T. durum, and a chlorina mutant has been found in T. monococcum. The aims of our study were to establish linkage maps for chlorina mutant genes on chromosome 7A of T. aestivum and T. durum and chromosome 7Am of T. monococcum and to discuss the differentiation that has occurred between the A and Am genomes. The chlorina mutant gene was found to be linked with Xhbg234 (8.0 cM) and Xgwm282 (4.3 cM) in F2 plants of T. aestivum ANK-32A/T. petropavlovskyi k54716, and with Xbarc192 (19.5 cM) and Xgwm282 (12.0 cM) in F2 plants of T. durum ANW5A-7A/T. carthlicum #521. Both the hexaploid and tetraploid wheats contained a common marker, Xgwm282. In F2 lines of T. monococcum KT 3-21/T. sinskajae, the cn-A1 locus was bracketed by Xgwm748 (25.7 cM) and Xhbg412 (30.8 cM) on chromosome 7AmL. The distal markers, Xhbg412, Xgwm282, and Xgwm332, were tightly linked in T. aestivum and T. durum. The common marker Xhbg412 indicated that the chlorina mutant genes are located on chromosome 7AL and that they are homoeologous mutations.  相似文献   

18.
2n Gamete formation in the genus Brachiaria (Poaceae: Paniceae)   总被引：1，自引：0，他引：1  
Microsporogenesis of several Brachiaria species of the Brazilian collection at Embrapa Beef Cattle has been analyzed in detail. This paper reports abnormal cytokinesis in three accessions of three different species (Brachiaria humidicola, 2n = 4x = 36, Brachiaria decumbens, 2n = 4x = 36, and Brachiaria dura, 2n = 6x = 54). Chromosomes paired in bi-, tri-, and quadrivalents in these accessions, whereas chromosome segregation at meiosis I was characterized by exclusion of laggards as micronuclei. In a high number of meiocytes, the first sign of cytokinesis appeared only in metaphase II and did not divide the meiocyte into a dyad. Total absence of cytokinesis was also detected among meiocytes in the second division. Since in both cases the two metaphase plates were very close, they favored the rejoining of chromosome sets after anaphase II and formed a restitutional nucleus in telophase II. Second cytokinesis occurred after telophase II in most meiocytes. Monads, dyads, and triads with n or 2n nuclei were observed among meiotic products. The 2n gametes observed correspond to the first division restitution (FDR). The number of affected cells in each accession was variable, but the number of microspores with restitutional nucleus, including those scored in tetrads and the released ones, did not exceed 9%. Although polyploidy is common in the genus Brachiaria, its origin is still unclear. Current results suggest that 2n gametes may have contributed to the evolutionary history of the genus.  相似文献   

19.
The meiotic behavior of two half-sib interspecific tetraploid (2n = 4x = 36) promising hybrids, a sexual and an apomictic one, from crosses B. ruziziensis and B. brizantha, was evaluated. Although chromosome paired predominantly as bivalents, a few tri- and quadrivalents were recorded. Results suggest that B. brizantha and B. ruziziensis are closely related and genetic recombination is expected in hybrids. Introgression of specific target genes from B. ruziziensis into B. brizantha and vice-versa may be foreseen. However, abnormalities such as irregular chromosome segregation, chromosome stickiness and abnormal cytokinesis reported in these hybrids affect pollen fertility. More than 65% of pollen grains are sterile. Since the distinctive cytological feature of these hybrids is abnormal cytokinesis, this fact suggests that both parental genomes are unable to coordinate their activities with regard to this cytological phenomenon. Deployment of such hybrids in the process of developing varieties is discussed.  相似文献   

20.
The genetic constitution and diversity of four relictual redwoods are discussed in this review. These include monotypic genera of the family Cupressaceae: coast redwood (Sequoia sempervirens), giant sequoia (Sequoiadendron giganteum), dawn redwood (Metasequoia glyptostroboides), and alerce (Fitzroya cupressoides). All four species are narrow endemics, share a number of common phenotypic traits, including red wood, and are threatened species. Fossil history suggests that the ancestors of redwoods probably originated during the Cretaceous and Tertiary periods and flourished thereafter for millions of years. Towards the end of the Tertiary period began their decline and struggle for existence that continued during the subsequent geologic upheavals and climate changes, until the survival of the present-day redwoods in the current restricted locations in the world (USA, China, and South America). Although two species, Sequoiadendron and Metasequoia, are diploids (2n = 22), and the other two are polyploids: Fitzroya a tetraploid (2n = 4x = 44), and Sequoia a hexaploid (2n = 6x = 66); they all share the same basic chromosome number x = 11. The genome size in the hexaploid Sequoia is one of the largest (31,500 MB) in the conifers, while the genome sizes of diploid Metasequoia and Sequoiadendron are about one-third (~10,000 MB) of Sequoia. Genetic diversity in the redwoods is lower than most other gymnosperms, except in Sequoia, which seems to rank near the upper quarter of the coniferous forest trees. Genomic research is sparse in the redwoods, and should be pursued for a better understanding of their genome structure, function, and adaptive genetic diversity.  相似文献   

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