Numerically unreduced (2n) gametes and sexual polyploidization in Actinidia

This is the first report on the production of numerically unreduced (2n) gametes in Actinidia species. Their occurrence was apparent from direct observations of microporogenesis in 2x, 3x, 4x and 6x plants and was inferred from ploidy analysis of progenies from 4x-2x and 2x-4x crosses. Both male and female plants were found to produce 2n gametes. 2n gamete producers were detected only in interploidy crosses. The observed frequency of 2n gametes was either similar to or towards the lower end of the ranges reported for other genera. The role of sexual polyploidization in the evolution of the genus is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

2n gametes in potato species and their function in sexual polyploidization

Summary The extent and pattern of polyploidy in the tuber-bearing Solanums varies among the many taxanomic series that have been identified in this subsection of Solanum. While several series appear to be entirely diploid, others exhibit a range of ploidy levels from 2x to 6x, and some contain only polyploid species.In many diploid, triploid and tetraploid species 2n gametes (gametes or gametophytes with the sporophytic chromosome number) have been detected. Both 2n eggs and 2n pollen occur. 2n gametes provide the opportunity for unilateral and bilateral sexual polyploidization. The genetic determination and consequences of sexual polyploidization strongly suggest that 2n gametes have been the major instrument in the polyploid evolution of the tuber-bearing Solanums. Somatic doubling of species and interspecific hybrids appears to be of very limited importance.New evidence for the occurrence of 2n eggs and 2n pollen in many species is reported, and data from the literature are added to illustrate the widespread distribution of 2n gametes throughout the subsection. A very high correlation is found between polyploidy and 2n gametes, and its significance is discussed. Proof is presented for the occurrence of alleles governing 2n pollen production in the cultivated tetraploids, providing additional evidence for the hypothesis that 2n gametes have been involved in their origin.Multiple unilateral and bilateral sexual polyploidizations are proposed for the origin of the cultivated tetraploids: this accounts for the large variability encountered in this group, which closely resembles that of the related diploids. Similar evolutionary pathways are hypothesized for the other polyploid complexes in the subsection.A scheme is proposed in which participation of both 2n and n gametes link together all ploidy levels in the tuber-bearing Solanums, thus overcoming the ploidy barriers and providing for gene flow throughout the sympatric species of the subsection.Paper No. 2093 from the Laboratory of Genetics. Research supported by the College of Agricultural and Life Sciences, a gift from Frito-Lay, Inc., and grants from NIH (GM 15422) and The International Potato Center. Plant materials, unpublished chromosome counts and technical help of IR-1 are gratefully acknowledged.     

Cytological mechanisms of 2n egg formation in a diploid genotype of Medicago sativa subsp. falcata

Summary Gametes formed without meiosis in alfalfa would be useful in basic and applied research. Therefore, the cytological analysis of macrosporogenesis of a diploid plant of Medicago sativa subsp. falcata (L.) Arcangeli (named PG-F9), previously selected as a good 2n egg producer, was conducted. A stain-clearing technique was applied which also allowed the analysis of microsporogenesis to be performed. Two mechanisms of 2n egg formation were determined: absence of the second meiotic division (mechanism of the SDR type) and apomeiosis. In particular, it is noteworthy that 2n eggs produced via apomeiosis should retain the same parental genotype, because apomeiotic cells give rise directly to the female gametophyte without undergoing meiosis. The presence of binucleate female gametophytes in the ovules at an early stage of development confirmed the presence of apomeiosis in PG-F9. These abnormalities concerned only macrosporogenesis; in fact, the analysis of microsporogenesis showed the production of normal tetrads of microspores after normal meiosis. Possible application of the discovered mechanisms of 2n egg production in breeding research are discussed.     

Effect of high temperature on the production of 2n pollen grains in diploid roses and obtaining tetraploids via unilateral polyploidization

To integrate the gene pool of a wild species (primarily diploid) into a cultivated pool (primarily tetraploid), a crossing between a dihaploid cultivated rose and a hybrid of Rosa wichurana allowed to obtain interspecific diploid hybrids that produced 2n pollen grains. A return to a tetraploid level sought by breeders can then be considered using sexual polyploidization, obtained by crossing a tetraploid cultivated rose with these hybrids. Application of a high‐temperature regime led to a small but significant increase in the percentage of 2n pollen grains in these hybrids of up to 4.6%. This result was obtained by applying high temperatures close to 32°C during the day to plants cultivated in a glasshouse during recurrent cycles of bloom. Crosses were made between an unreleased tetraploid hybrid tea rose, as a female, and the diploid hybrid that produces the most 2n pollen grains. Tetraploid (42.1%) and triploid (57.9%) offspring were obtained. The use of these 2n pollen grains of the first division restitution type should facilitate the introgression of complex traits of interest.     

Bilateral sexual polyploidization in potatoes

Summary Bilateral sexual tetraploidization was achieved by intercrossing diploid Phureja × haploid Tuberosum hybrids. Both parents contribute gametes with the somatic number of chromosomes (diplogynoids and diplandroids). More than 100 tetraploid progeny appeared in nine 2x–2x families. The tetraploids are generally more vigorous and significantly outyield their diploid full-sibs. The superiority of the tetraploids is interpreted on the basis of the mode of diplandroid (2n pollen) formation by first meiotic division restitution (FDR). Heterozygosis, epistasis and genetic diversity, if present in the parental diploid genotype, are largely maintained in the 2n gametes formed by FDR. and syngamy brings about more opportunities for heterotic responses in conjunction with polyploidization. Differences in tuber yield among tetraploid clones within families were found to be statistically significant (P <0.05). Thus, unlike somatic chromosome doubling, which can merely increase chromosome numbers, or somatic hybridization, which could in addition bring about heterosis, sexual polyploidization is also associated with genetic variability. The degree of inbreeding and the genotypic structure in the population of newly arisen tetraploids is discussed. The relationship among asexual reproduction, preservation of favorable nonadditive genetic effects by FDR. restoration of sexuality, and polyploidization is explored. The possible evolutionary significance of these factors is emphasized.     

Relevance of sexual polyploidization for crop improvement – A review

Colchicine induced polyploids have not directly contributed for crop improvement in the past. On the other hand, the so-called natural polyploids, derived from the functioning of numerically unreduced(2n) gametes have been shown to be more relevant for crop improvement in many cases. Different types of cytological abnormalities during meiosis can give rise to 2n gametes and the genetic composition of these gametes is variable. Depending on the type meiotic abnormalities, various types of 2ngametes, such as first division restitution(FDR), second division restitution (SDR),indeterminate meiotic restitution (IMR) and post meiotic restitution (PMR) gametes,among others, have been described in recent years. For the improvement of autopolyploids such as potato, alfalfa,Vaccinium spp., and some of the fodder grasses, FDR gametes have been proved to be highly useful. However, the use of 2n gametes for the improvement of allopolyploid crops has received much less attention so far. Some of the investigations on allopolyploids, derived from Festuca-Lolium, Alstroemeria and Lilium species hybrids, have revealed that 2ngametes can be most useful for the introgression of alien genes and chromosomes into cultivars. An important feature of using sexual polyploidization in the case of allopolyploids is that introgression can be achieved through recombination due to genetic crossing-over between alien chromosomes as well as addition of alien chromosomes, which is extremely difficult or impossible to achieve in the case of colchicine induced allopolyploids. Because of the recent developments in the field of plant molecular biology, methods have become available for the analysis of 2ngametes and sexual polyploid progenies more accurately and to develop systematic breeding approaches. The methods include DNA in situ hybridization (GISH and FISH)and molecular mapping (AFLP, RFLP, RAPDs).In addition to providing basic information on the genetic and genome composition of the polyploid progenies, these methods can be potentially useful for a more efficient creation of desirable breeding material and cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

The production of 2n pollen in rose

Based on the size differences found between haploid and diploid pollen produced by diploid and tetraploid rose cultivars, respectively, 2n pollen producers were identified in a population of 53 diploid hybrids from a cross between a dihaploid rose, derived from the haploidization of a tetraploid modern cultivar and the diploid species R. wichuraiana. Frequency of 2n pollen producers was estimated in 2002, 2003 and 2004. Highly variable frequencies were found i) within population; ii) during years of observation (between years and between different months in the same year). The variation of 2n pollen production could be related to environmental fluctuations. A cytological analysis of male meiosis was carried out in 10 hybrids randomly chosen. Among meiotic abnormalities leading to 2n pollen formation, triads (containing a 2n microspore at one pole and two n microspores at the other) resulting from abnormal spindle geometry were frequently observed. The mode of 2n pollen formation is genetically equivalent to a First Division Restitution (FDR) mechanism. FDR 2n pollen transmits a high percentage of the heterozygosity from the diploid parent – 2n pollen producer-to the tetraploid offspring.     

Bilateral polyploidization inDactylis glomerata L. subsp.lusitanica: occurrence,morphological and genetic characteristics of first polyploids

Summary InDactylis glomerata L. subsp.lusitanica, triploid and tetraploid plants were obtained by bilateral sexual polyploidization in crosses between diploid parents known to produce 2n gametes. The polyploid and diploid progeny were compared for allozyme diversity (allele number and heterozygosity), phenological (pollen fertility, inflorescence emergence date), cellular (stomatic cell size) and morphological characters (vegetative biomass, seed weight, total seed number per plant, panicle number and seed number per panicle length) in four contrasting growing conditions. Stomatal cell size and the total number of alleles were significantly lower in diploids than in polyploids which sometimes recovered more than two distinct alleles at a locus via 2n gametes possessing heterozygous loci. Pollen fertility was similar in diploids and tetraploids, but significantly lower in the triploids. Seed weight was significantly higher in polyploids, although the average number of seeds per panicle length and the total number of seeds was higher in the diploid sibs. No phenological differences occurred among ploidy levels whereas such differences were observed inDactylis in all the natural areas of sympatry between diploids and their related polyploids. Our results provide evidence for an instantaneous effect of polyploidization on certain traits and suggest that natural selection would act subsequently on several other traits to allow the establishment of new polyploids.     

Discovery of 2n gametes in tetraploid oat Avena vaviloviana

Summary Sexual polyploidization via the action of 2n gametes (gametes with the sporophytic chromosome number) has been identified as the most important evolutionary mode of polyploidization among plant genera. This study was conducted to determine whether 2n gametes are present in the tetraploid level of the genus Avena (2n=4×=28) Twenty tetraploid Avena lines, representing four species and one interspecific hybrid, were screened for pollen grain size in order to differentiate between n and 2n pollen. Avena vaviloviana (Malz.) Mordv. line PI 412767 was observed to contain large pollen grains at a 1.0% frequency. Cytogenetic analyses of pollen mother cells of PI 412767 revealed cells with double the normal chromosome number (i.e., 56 chromosomes at metaphase I and anaphase I). The mode of chromosome doubling was found to be failure of cell wall formation during the last mitotic division that preceded meiosis. The resulting binucleate cells underwent normal meiotic divisions and formed pollen grains with 28 chromosomes. Based on the formation and function of 2n gametes, three models involving diploid and tetraploid oat lines are proposed to describe possible evolutionary pathways for hexaploid oats. If stable synthetic hexaploid oat lines could be developed by utilizing 2n gametes from diploid and tetraploid oat species through bilateral sexual polyploidization, the resulting hexaploids could be used in breeding programs for transferring genes from diploids and tetraploids to cultivated hexaploids.     

Polyploidy and sexual polyploidization in the genus Vaccinium

Genus Vaccinium,consisting of blueberries, cranberries, lingonberries and many related wild species, includes diploid, tetraploid, and hexaploid species. Most evidence indicates that the tetraploid species are autotetraploids, with non-preferential bivalent chromosome pairing. Although homoploid interspecific crosses usually produce numerous fertile hybrids if the parents are from the same section of the genus, inter sectional crosses at the diploid level normally produce no seedlings, weak seedlings, or seedlings that are very low in fertility. There is a strong but not complete triploid block within Vaccinium. Even with insections, tetraploid × diploid (and the reciprocal) crosses normally give only a few tetraploid and a few triploid hybrids. Hexaploid × diploid crosses within sections are very hard to make, and the few hybrids that have been obtained are pentaploid. The frequency of 2n gametes varies,both among genotypes within species and among species. Vaccinium pollen is normally shed in tetrads, and the frequency of large pollen grains shed in dyads can be used to estimate 2n gamete frequency. Cultivated blueberries occur at both the tetraploid and the hexaploid levels, and there are important genetic resources in the diploids. Unreduced gamete production has permitted transfer of genetic material from the diploid to the tetraploid level and from the diploids and tetraploids to the hexaploid level via triploid hybrids. Intersectional crosses can occasionally produce tetraploid Vaccinium hybrids that appear to behave as amphidiploids and have medium to high fertility. CommercialVaccinium cultivars are normally propagated by cuttings. Intersectional hybridization, chromosome doubling, and asexual propagation could permit the production of novel hybrid combinations with value as ornamentals or in fruit production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Increasing 2n gamete production in diploid alfalfa by cycles of phenotypic recurrent selection

Two cycles of selection (3rd and 4th) were carried out in a diploid population of alfalfa to improve the frequency of 2n gametes. All the plants produced unreduced gametes, except one in the 2n egg population. The average seed set, used as a measure of 2n gametes production, increased significantly in the 2n pollen selection. Conversely, the increase was not significant in the case of 2n eggs. The improvement in 2n pollen production was confirmed cytologically. These results, like those of the first two cycles of selection, suggest that 2n pollen production is controlled by few major genes while 2n egg production is controlled by one recessive major gene and influenced by minor genes. The mutants selected have been utilized for producing heterotic tetraploid progenies through Bilateral Sexual Polyploidization (BSP).     

Cytological studies of 2n male gamete formation in sugarcane, Saccharum L.

The germplasm for modern sugarcane cultivars (Saccharum spp. hybrids)has been derived principally from S. officinarum (2n = 80), and S. spontaneum (2n = 40 to 128). Diploid gamete formation has been significant in developing cultivated sugarcane, but the cytological basis for the processes involved is not clearly understood. This research investigated microsporogenesis in nine clones of Saccharum spp. Hybrids and in S. officinarum and S. spontaneum. Diploid gamete formation occurred in all 11 lines, but was least frequent in S. spontaneum and S. officinarum which produced 0.5% and 0.8%2n gametes, respectively. In the hybrid lines, 2n gametes were formed infrequencies ranging from 0.9% to 4.4%. Cytological evidence was obtained for dyad and triad formation during microsporogenesis. Detailed analysis of chromosome behaviour at meiosis indicated that 2n male gamete formation is probably attributable to the absence of cytokinesis rather than a combination of asynchrony and non-disjunction. The clones were ranked on the basis of the frequencies with which they formed 4 × 1n microspores and the data were analysed using χ ² tests for homogeneity. These established that theSaccharum spp. hybrids could be designated as either ‘high’ or ‘low’ frequency haploid gamete producers. Conversely, the latter group, which formed diploid gametes most frequently (2.2%–4.4%), can be described as high frequency diploid gamete producers. The identification of clones most frequently forming diploid gametes may facilitate the more rapid recovery of desirable sugarcane genotypes because such clones could be selected for preferential use in clonal improvement. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reproductive behaviour of tetraploid alfalfa plants obtained by unilateral and bilateral sexual polyploidization

The objective of the present study was to evaluate the fertility of tetraploid plants obtained from diploid mutants that produced 2n gametes via bilateral (BSP) and unilateral sexual polyploidization (USP) schemes. Controlled matings between selected plants from BSP and USP were carried out according to a complete diallel. The level of male and female fertility of each plant was estimated within full-sib, half-sib and non-inbred crosses. Crosses with unrelated self-fertile and male-sterile testers were also performed. Pollen size and stainability were determined by acetocarmine staining. The number of ovules per ovary was counted and the callose deposition within ovules examined to evaluate embryo sac fecundity. Cross-fertility was generally much higher for USP than BSP plants (on average, 0.3 vs. 0.03 seeds per flower pollinated). Both male and female fertility were inversely related to the inbreeding level of cross combinations. The in vitro pollen germinability was positively correlated to male fertility of BSP plants. The level of in vivo embryo sac sterility was not correlated to female fertility in either BSP or USP plants. Female fertility was restored in both BSP and USP groups when plants were crossed with unrelated tetraploid testers. Male fertility also increased in USP plants but remained rather low in the BSP plants. Scaling up the ploidy level by means of 2n gamete union resulted in tetraploid plants with low fertility. In particular, the BSP process yielded virtually self-sterile and highly cross-sterile plants. The well documented positive effect of sexual tetraploidization on forage yield was accompanied by worsening of fertility traits. This revised version was published online in July 2006 with corrections to the Cover Date.     

Occurrence of 2n and jumbo pollen in the Brazilian ginseng (Pfaffia glomerata and P. tuberosa)

An analysis of microsporogenesis was carried out in four populations of Pfaffia tuberosa (2n=4x=68) and in ten populations of P. glomerata(2n=2x=34). Monads, dyads and triads were reported at high frequency in all populations of P. tuberosa. In P. glomerata, on the other hand, only four populations presented monads, and at low frequency. Dyads and triads were observed in both species, albeit at a higher frequency in P. tuberosa populations. Certain abnormalities of spindles in the second division, such as sequential and tripolar spindles, led to triad formation by the rejoining of two chromosome sets. The precise causes of monad and dyad formation have not been found during meiosis, although evidence exists that dyads originated by omission of second meiosis and monads by omission of both meiotic divisions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Occurrence, inheritance and use of reproductive mutants in alfalfa improvement

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.     

A re-examination of the mechanisms of 2n gamete formation in potato and its implications for breeding

Summary The claim that first division restitution gametes are produced through the unique meiotic abnormality of parallel spindle formation at anaphase II of microsporogenesis in Solanum species appears to be not justified. An examination of the phenomenon of meiotic nuclear restitution in fifty-six diploid Solanum clones revealed that the parallel orientation of spindles either at metaphase or anaphae II does not necessarily lead to dyad or unreduced microspore formation. However, the fused spindle formation at metaphase II of microsporogenesis invariably leads to dyad and unreduced microspore formation. The 2n gametes produced through fused spindles may be genetically equivalent to first division restitution gametes, and therefore, they may be useful in potato breeding. In addition to fused spindle formation, at least three other types of meiotic abnormalities that could lead to 2n gamete formation were also observed. Although any one, or two, of these mechanisms were predominantly observed in certain clones, it was by no means certain that other mechanisms did not operate in these clones at the same time, or at a later stage. Because of the great modifying influence of environment on the process of meiosis, and meiotic nuclear restitution, it might be erroneous to conclude that one or the other meiotic nuclear restitution mechanism alone operates at a time in a given clone. In order to elucidate the genetic basis of 2n gamete formation in potato, it is desirable (i) to answer whether there are exclusive genetic mechanisms for the formation of first or second division restitution gametes; (ii) to establish reliable cytological criteria for their detection; and (iii) to determine the effect of environmental factors on the occurrence of different mechanisms.     

The origin of polyploids via 2n gametes in Vaccinium section Cyanococcus

Summary The production of 2n pollen (pollen with the sporophytic chromosome number) was evaluated in 4x and 6x taxa of Vaccinium section Cyanococcus. Mean frequencies of 2n pollen producers were 17.1% and 8.3% in natural 4x and 6x populations, respectively. The frequency of 2n pollen producers in the 4x species ranged from 8.6% (V. angustifolium) to 23.8% (V. pallidum). Level of 2n pollen production was genotypically variable (1% to 37.4%). The widespread occurrence of 2n pollen in 2x, 4x and 6x taxa suggests that sexual polyploidization was widespread and responsible for the origin of the polyploid species found in this genus. The frequency of 2n pollen producers was not significantly different between the 4x species and their putative 2x ancestors. These results support the origin of 4x and 6x taxa as a consequence of sexual polyploidization. Polyploids derived from sexual polyploidization would be expected to have increased fitness and flexibility due to the mode of 2n pollen formation. In blueberry species the predominant mode of 2n pollen formation is genetically equivalent to a first division restitution mechanism (FDR). FDR 2n pollen transmits a high percentage of the heterozygosity and a large fraction of the epistasis from the 2x parent to the 4x offspring.     

Transmission of Verticillium wilt resistance to tetraploid potato via unilateral sexual polyploidization

Summary Verticillium wilt is a serious disease of potato and is caused by the soil-borne fungi Verticillium dahliae and V. albo-atrum. No major cultivar is resistant to this disease. Two diploid interspecific potato clones, C287 and C545, were previously identified with consistently high levels of Verticillium wilt resistance and are thought to have the same genotype for the loci that confer resistance to V. dahliae stem colonization. The purpose of this study was to determine whether resistance to V. dahliae stem colonization could be transferred to the tetraploid level in potato via unilateral sexual polyploidization (USP). Progenies in eight families obtained by crossing C287 and C545 to two tetraploid breeding clones, S438 and S440, and the cultivar ‘Atlantic’ were planted in a V. dahliae-infested field and a field without a significant V. dahliae infestation. Resistance was evaluated relative to C545 and C287. There were differences among families for stem colonization and yield. No differences between the mean stem colonization of C545 and C287 progeny were detected. Family differences due to the tetraploid parents indicate that they contributed resistance to the progeny. Stem colonization data from this experiment were consistent with the proposed complementary two-gene model for Verticillium wilt resistance in the diploid parents. Unilateral sexual polyploidization is an effective method for transferring V. dahliae stem colonization resistance to the tetraploid level.     

Meiotic restitution mechanisms and 2n pollen formation in a Solanum tuberosum dihaploid and in dihaploid × wild species hybrids*

A Solatium tuberosum dihaploid that proved to be a 2n pollen producer, exhibited two types of meiotic abnormality involved in 2n pollen formation: premature cytokinesis at prophase II and parallel/tripolar spindles in meiosis II. For this reason such a dihaploid was preferentially used as a seed parent in crosses with the diploid species S. tarijense, S. sanctae-rosae, and S. phureja. Diploid interspecific hybrids were evaluated for male fertility, for 2n pollen production and stability over three years and for meiotic nuclear restitution mechanisms. Pollen stainability ranged from 6 to 85% and 2n pollen from 5 to 35% among hybrids. The meiotic mechanism generating 2n pollen was that of spindle abnormality in meiosis II and consequent dyad/triad formation. High correlation was found between fused/parallel spindles and dyads.     

Failure of cytokinesis and 2n gamete formation in Brazilian accessions of Paspalum

Cytological analysis on 112 Brazilian accessions of Paspalum showed that 52 were affected by failure of first or second cytokinesis during microsporogenesis. When the failure of cytokinesis occurred only in the first division and in the second division the cytokinesis was normal, a dyad of microspores was formed. In most of them the two cells remained binucleate, but in some a restitutional nucleus was observed. When the first division was normal and the failure of cytokinesis occurred only in the second division, dyads and triads were observed. A restitutional nucleus in one or both cells of the dyad, or in the binucleate cell of the triad was also observed. The percentage of dyads and triads varied among accessions, reaching 15.75 of the sporads in some. The absence of cytokinesis indicates the possibility that the widespread occurrence of polyploidy in the genus Paspalum originates from 2n pollen grains. This revised version was published online in July 2006 with corrections to the Cover Date.