A CYTOGENETIC ANALYSIS OF CERTAIN POLYPLOIDS IN GRINDELIA (COMPOSITAE)

Two diploid taxa, Grindelia procera and G. camporum, and 3 tetraploid ones, G. camporum, G. hirsutula, and G. stricta, have been studied to ascertain their interrelationships. Meiosis in diploid parental strains was regular, the common chromosome configuration being 5 rod bivalents and 1 ring bivalent. The average chiasmata frequency per chromosome was 0.60. Pollen fertility was about 90% in all strains examined. Diploid interspecific hybrids had normal meiosis with an average chiasmata frequency of 0.56 per chromosome. No heterozygosity for inversions or interchanges was detected, and pollen fertility was above 85%. Meiosis in parental tetraploid strains was characterized by the presence of quadrivalents in addition to a complementary number of bivalents. The average chiasmata frequency per chromosome was 0.59 and pollen fertility was generally about 80%. Tetraploid interspecific hybrids also had quadrivalents, normal meiosis, and high pollen fertility. Close genetic relationships between the diploids and between the tetraploids are indicated, and geographical, ecological, and seasonal barriers to gene exchange exist. Attempts to obtain hybrids between diploids and tetraploids were successful in a few cases. The hybrids were tetraploid and had normal meiosis and fertility similar to parental and F₁ tetraploids. Their origin was by the union of unreduced gametes of the diploid female parent and normal pollen from the tetraploid parent. On the basis of chromosome homology, normal meiosis, plus high fertility exhibited in the diploid, tetraploid, and diploid X tetraploid interspecific hybrids, these species of Grindelia are considered to be a part of an autopolyploid complex. Gene exchange between diploids and diploids, tetraploids and tetraploids, and diploids and tetraploids is possible. Tetraploid G. camporum may have originated by hybridization between G. procera and diploid G. camporum with subsequent doubling of chromosomes and selection for the combined characteristics of the diploids.     

Desynapsis in diploid and tetraploid clones of ryegrass

Desynapsis is described in diploid and tetraploid plants of perennial ryegrass. The plants were derived by repeated cloning of a single mixoploid (2n=14 and 28) detected among colchicine-treated seedlings. The diploid and tetraploid clones varied in degree of desynapsis, chiasma number, and fertility. The variation among the clones was probably environmental. The progeny of the mixoploid parent included dipoids, tetraploids, and an aneuploid. One diploid and the aneuploid were desynaptic and originated perhaps by selfing. Apparently a single recessive gene determined desynapsis. The role of synaptic genes in controlling the chemical structure and function of nucleoprotein macromolecules is discussed.     

Fertility and selection in tetraploid Lolium

Selection over five generations increased the fertility (seed set) in a population of tetraploid Lolium perenne from 67.57 per cent in the second (C ₂) to 73.95 per cent in the seventh (C ₇) generation. The increased fertility is associated with a high quadrivalent frequency and a low frequency of bivalents, trivalents and univalents. The high quadrivalent frequency in the selected population was achieved partly by a slight overall increase in chiasma frequency but mainly by a redistribution of chiasmata.     

Morphological and cytological studies of colchicine-induced tetraploids inTorenia fournieri lind

The present paper concerns a morphological and cytological comparison between the naturally occurring diploid and the colchicine induced tetraploid ofTorenia fournieri Lind.Treatment of growing points of young seedlings with 0.20% aqueous colchicine for 10 hours on two successive days was most effective in inducing tetraploidy. The induced tetraploids showed the usual gigas characters associated with autotetraploidy. There was a significant reduction in pollen fertility and seed setting of tetraploids as compared to their diploid progenitors.The chromosome numbers of the diploids and the induced tetraploids were determined as n=9 and n=18, respectively. The diploids showed normal meiosis whereas tetraploids were characterized by a high frequency of unequal distribution of chromosomes at anaphase. The induced tetraploids possessed larger flowers. The variability in flower size of the tetraploids offers a good scope for evolving a better horticultural variety of this species.     

The Distribution of Chromosome Races of Ranunculus ficaria L. in the British Isles

The chromosome numbers of 889 plants from 222 British populationsof Ranunculus ficaria were counted. 66.1 per cent of the populationscontained diploids (49.1 per cent of plants), 4.1 per cent containedtriploids (5.5 per cent of plants) and 43.2 per cent tetraploids(45.4 per cent of plants); 13.1 per cent of the populationscontained two or more polyplotypes. At the fruiting stage, tetraploidplants can be reliably distinguished from diploids by the presenceof axillary bulbils in the tetraploids and their absence inthe diploids. The production of well-developed heads of achenes,which is more characteristic of the diploids than of the tetraploids,is a less reliable character. Diploid plants are widespreadthroughout the British Isles but tetraploids are more commonin the east than in the west; 17.6 per cent of the diploid plantshad B-chromosomes and these plants were virtually confined tosouthern England and the Midlands. The occurrence of triploidsand of mixed populations of diploids and tetraploids are bothshown to be more common than previously supposed.     

Nucleotypic influences on chromosome-specific chiasma variation in Crepis capillaris. I. Responses to early colchicine treatment and chromosome doubling.

Additive and multiplicative effects of colchicine treatment at the seedling stage and of chromosome doubling on chromosome-specific chiasma frequency at metaphase I have been measured in comparisons between C0 and untreated diploids and between C0 autotetraploids and C0 diploids. Early colchicine treatment increases the frequency of chromosome C univalents to 1.8% but has no similar effect on chromosomes A and D. Colchicine treatment has little net effect on mean chiasma frequency, deducting an average of 0.204 chiasmata per set but otherwise multiplying the mean by a factor of 1.182. These additive and multiplicative effects represent averages of six phenotypes. Chromosome doubling in a tetraploid-diploid chimaera subtracts an average of 0.265 chiasmata per set but otherwise doubles the numbers of chiasmata at the diploid level (x 2.134). Comparison of six diploids and tetraploids reveals modest average additive (+ 1.103) and multiplicative effects (x 1.190). The implications of these findings are discussed in the light of new analyses of previously published data. Key words : chiasmata, Crepis, colchicine, meiosis, polyploidy.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

Fertility and meiotic chromosome behaviour in autotetraploid pearl millet

Summary Tetraploidy was induced in outbred pearl millet and selection for high and low seed set was started in the C₁ generation. Segregation in the C₃ generation was observed for fertility and also for meiotic features: per cent seed set in selfed earhead, chiasma frequency, chromosome association and chromosome distribution in pollen mother cells were all affected. However, variation in seed set was observed even between samples not differing in meiotic features. It is apparent that factors regulating seed set in autotetraploid pearl millet were genic as well as chromosomal.A high frequency of univalents and trivalents was the main cause of sterility; quadrivalent misdisjunction was not a significant factor. As univalency decreased with increased chiasma formation, the gain was in the form of quadrivalents. However, individuals not differing in chiasma frequency did differ in chromosome association frequencies, indicating that the dependence of chromosome pairing behaviour on chiasmata was subject to genotypic influence.     

A comparison of the nodulation of diploid and tetraploid varieties of red clover inoculated with different Rhizobial strains

Summary The effectiveness of three different strains ofRhizobium trifolii on a diploid and a tetraploid variety of red clover (Trifolium pratense) grown in aseptic mineral-agar culture was compared. It was found that with two of the rhizobial strains tested, the tetraploid plants were slower to nodulate than the diploid plants, but that a hundred per cent nodulation was obtained in both cases. With the third strain, however, a sixty-six per cent nodulation of the diploid plants was reduced to an eight per cent nodulation of the tetraploids.In general, over a given period, fewer nodules were formed on the tetraploid plants, but these tended to be larger than on the diploids. The fresh weight of the tetraploid plants always exceeded that of the diploids, but less so in cases of poorly nodulated plants.     

Formation of first division restitution (FDR) 2n-megaspores through pseudohomotypic division in ds-1 (desynapsis) mutants of diploid potato: routine production of tetraploid progeny from 2xFDR × 2xFDR crosses

Summary The level and mode of 2n megaspore formation was studied in full-sib diploid potato clones with either normal or desynaptic (ds-1ds-1) meiosis. Cytological analysis revealed that functional 2n megaspores produced by normal and desynaptic clones originate exclusively from second division restitution (SDR) and first division restitution (FDR), respectively. SDR 2n megaspores resulted from the omission of the second meiotic division following chromosome doubling after anaphase I, whereas FDR 2n megaspores resulted from a direct equational division of univalent chromosomes at anaphase I (pseudohomotypic division). Comparative data strongly indicated that the observed mechanisms of SDR and FDR 2n megaspore formation are extremes of a continuum that is being brought about by common genes for precocious chromosome division. Depending on the relative timing of cell cycle and chromosome division, this precocious chromosome division may impose postreductional (SDR) or prereductional (FDR) restitution of the sporophytic chromosome number under normal synaptic and desynaptic conditions, respectively. The observed frequencies of 2n megaspores closely correlated with seed set, following pollination by tetraploid varieties and by desynaptic diploid clones with exclusive FDR 2n pollen formation. Up to 54.0 and 21.5 seeds/ fruit were obtained from normal synaptic (SDR) and desynaptic (FDR) progeny, respectively. The high frequency of segregants with either SDR or FDR 2n megaspore formation (78.0 and 45.2%, respectively) supports the hypothesis that sexual polyploidization is the driving force behind the origin and evolution of polyploid Solanum species. The present identification of diploid potato clones with consistent FDR 2n megaspore formation extends the opportunities for direct transfer of enhanced diploid germ plasm to tetraploids, and particularly advocates the feasibility of 2x(ds-1; FDR)×2x(ds-1; FDR) breeding schemes in cultivar development and the production of relatively vigorous and uniform true potato seed (TPS) varieties. Its potential value and limitations for breeding and the experimental induction of diplosporic apomixis are discussed.     

In vitro induction of tetraploid in pomegranate (Punica granatum)

Tetraploid plants were obtained in pomegranate (Punica granatum L. var. `Nana') by colchicine treatment of shoots propagated in vitro. Shoots cultured on MS medium supplemented with 10 mg l^–1 colchicine, 1.0 mg l^–1 BA and 0.1 mg l^–1 NAA for 30 days produced tetraploids at a high frequency of 20%. No tetraploids were detected by treating the shoots in 5000 mg l^–1 colchicine for 114 h. Shoots treated by 5000 mg l^–1 colchicine for 96 h produced three morphological mutants with narrow leaves, which were later confirmed as mixoploids that separated into diploids and tetraploids after further subculture. In vitro tetraploid plants had shorter roots, wider and shorter leaves than the diploid ones. Tetraploid pomegranate plants grew and flowered normally in pots, but possessed flowers with increased diameter and decreased length compared to diploids. The number of pollen grains per anther was higher in tetraploids, but the viability of pollen decreased significantly.     

Meiosis and pollen mitosis in diploid and triploid Colocasia antiquorum Schott.

The relationship between diploid and triploid forms of Colocasia antiquorum Schott. was assessed through comparative meiotic and pollen mitotic studies. Owing to poor spreading of the chromosomes of both materials, karyological observations on pachytene nuclei were limited to a few chromosomes. Among the two nucleolar chromosomes and a metacentric, telochromomere-bearing chromosome of the diploid, the latter and one of the nucleolar chromosomes characterized by a heteropycnotic short arm were identified in both bivalent and trivalent associations in the triploid. The homologues in these cases were homomorphic and intimately paired. Two types of heteromorphic bivalents exhibiting partial pairing of homomorphic segments were also recorded in the triploid. Among the 14 bivalents of the diploid at diakinesis, two were nucleolus-associated. In the triploid, chromosomal associations at diakinesis included trivalents (2 to 9), bivalents and univalents, and the chiasma frequency per paired chromosome was lower than in the diploids. In 21.6 percent of the PMCs at this stage intragenomic pairing of one or two chromosomes was observed. Post-diakinesis stages in the diploid were regular while in the triploid they were marked by various irregularities in a majority of the cells. However, fertility (stainability), size and divisional frequency of pollen in both materials were remarkably similar. Chromosome numbers in pollen nuclei in the triploid ranged from 8 to 25. Based on these data an autopolyploid origin for the triploid Colocasia and a lower base number than the gametic chromosome number for this genus are advanced.     

In vitro induction and identification of tetraploid plants of <Emphasis Type="Italic">Paulownia tomentosa</Emphasis>

Polyploidization is a major trend in plant evolution that has many advantages over diploid. In particular, the enlargement and lower fertility of polyploids are very attractive traits in forest tree breeding programs. We report here a system for the in vitro induction and identification of tetraploid plants of Paulownia tomentosa induced by colchicine treatment. Embryonic calluses derived from placentas were transferred to liquid Murashige and Skoog (MS) medium containing different concentrations of colchicine (0.01, 0.05, or 0.1%) and incubated for 24, 48, or 72 h on an orbital shaker at 110 rpm. The best result in terms of the production of tetraploid plantlets was obtained in the 48 h + 0.05% colchicine treatment, with more than 100 tetraploid plantlets being produced. The ploidy level of plantlets was verified by chromosome counts, flow cytometry, and morphology. The chromosome number of tetraploids was 2n = 4x = 80 and that of diploid plantlets was 2n = 2x = 40. The relative fluorescence intensity of tetraploids was twofold higher than that of diploids. The tetraploid and diploid plantlets differed significantly in leaf shape, with those of the former being round and those of the latter pentagonal. The mean length of the stomata was longer in tetraploid plants than diploid plants, and stomatal frequency was reduced with the increased ploidy level. The tetraploids had large floral organs that were easily distinguishable from those of diploid plants.     

A new cytotype of Jacobaea vulgaris (Asteraceae): frequency,morphology and origin

Jacobaea vulgaris subsp. vulgaris (syn. Senecio jacobaea subsp. jacobaea) constitutes an intricate polyploid complex distributed in Europe. Four cytotypes have been reported in this species, three with euploid (diploid, tetraploid and octoploid; 2n=20, 40 and 80) and one with aneuploid (2n=32) chromosome numbers. Here we report that the diploid chromosome number (2n=20) reported from Bulgaria is due to misidentification with Jacobaea aquatica. On the other hand, we have discovered a new, hexaploid (2n=6x=60) cytotype within J. vulgaris subsp. vulgaris using flow cytometry. The new cytotype occurs within four sympatric populations of otherwise tetraploid and octoploid plants in Pannonia (one locality in the eastern Czech Republic and two localities in southwestern Slovakia) and in Podillya (one locality in western Ukraine). The frequency of hexaploid individuals within 76 studied populations is very low (only 10 of 693 analysed plants), and hexaploids probably represent hybrids between tetraploid and octoploid plants. Three mixed populations with hexaploid plants were subjected to detailed morphological and pollen fertility analyses. Multivariate morphometric analysis reveals partial separation of tetraploid and octoploid plants, whereas hexaploid individuals are similar in morphology to octoploids. In comparison with tetraploids, octoploids and hexaploids exhibit slightly longer ray florets, involucral bracts and tubular florets and more hairy outer achenes. Hexaploid plants display larger pollen grains and lower pollen fertility compared to tetraploids and octoploids.     

Pollinator-mediated assortative mating in mixed ploidy populations of Chamerion angustifolium (Onagraceae)

Establishment of polyploid individuals within diploid populations is theoretically unlikely unless polyploids are reproductively isolated, pre-zygotically, through assortative pollination. Here, we quantify the contribution of pollinator diversity and foraging behaviour to assortative pollen deposition in three mixed-ploidy populations of Chamerion angustifolium (Onagraceae). Diploids and tetraploids were not differentiated with respect to composition of insect visitors. However, foraging patterns of the three most common insect visitors (all bees) reinforced assortative pollination. Bees visited tetraploids disproportionately often and exhibited higher constancy on tetraploids in all three populations. In total, 73% of all bee flights were between flowers of the same ploidy (2x–2x, 4x–4x); 58% of all flights to diploids and 83% to tetraploids originated from diploid and tetraploid plants, respectively. Patterns of pollen deposition on stigmas mirrored pollinator foraging behaviour; 73% of all pollen on stigmas (70 and 75% of pollen on diploid and tetraploid stigmas, respectively) came from within-ploidy pollinations. These results indicate that pollinators contribute to high rates of pre-zygotic reproductive isolation. If patterns of fertilization track pollen deposition, pollinator–plant interactions may help explain the persistence and spread of tetraploids in mixed-ploidy populations.     

Cytological relationships in theConvolvulus pluricaulis complex

InConvolvulus pluricaulis Chois. two forms (2n=18, 36) along with one colchicine-autotetraploid have been investigated morphologically and cytologically. The diploid regularly formed 9_II although in the “off-season” plants certain irregularities were observed including formation of unreduced pollen grains. The natural tetraploid and the colchicine-autotetraploid had mean frequencies of configurations of 1.56_IV+14.04_II+1.48_I and 2.95_IV+0.56_III+11,43_II+0.8_I, respectively. The mode of chromosome association in the two types of tetraploids was comparable. A comparison of the morphological characters of the two tetraploid types, further, suggested close similarity. In addition they were indistinguishable from the diploid from except being gigas. From these data it is inferred that the natural tetraploid presumably arose as a result of direct duplication of the diploid form, through the chance fusion of unreduced spores. The lower quadrivalent frequency of the natural polyploid is ascribed to a gradual shift towards bivalent association accompanying natural selection for fertility. The taxonomic status of the two forms (2x, 4x) is discussed and the varietal status accorded to the tetraploid form is supported.     

Hybridization between diploid Centaurea pseudophrygia and tetraploid C. jacea (Asteraceae): the role of mixed pollination,unreduced gametes,and mentor effects

We studied hybridization between the diploid Centaurea pseudophrygia and the tetraploid C. jacea by performing crossing experiments and screening natural populations using flow cytometry. The experiments confirm that the studied species exhibit strong reproductive isolation. Interspecific hybrids were formed at a low frequency, including triploids (originating from reduced gametes) and tetraploids (involving unreduced gametes of the diploids). In contrast, hybrids were almost absent among seeds and adult plants of natural mixed populations and among the offspring from experimental pollinations with a mixture of pollen of both ploidy levels. We found that mixed pollination is an important mechanism for preventing hybridization between plants of different ploidy levels and sustaining the reproduction of the tetraploids. A mentor effect (induced selfing in the presence of pollen of different ploidy levels) was observed in both diploids and tetraploids, reinforcing the reproductive isolation between cytotypes. Higher ploidy levels (pentaploid, hexaploid) involving unreduced gametes of the tetraploid species were identified. Notably, pentaploids were discovered for the first time in Centaurea sect. Jacea. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 93–106.     

Induced allopolyploidization of autotetraploidBrassica campestris

Seeds of a variety of allogamous Brown Sarson (Brassica campestris) were irradiated with gamma rays either before or after duplication of their chromosome number by colchicine. Tetraploids from seeds irradiated before duplication had mean quadrivalent and chiasma frequencies of 1.85 and 28.35 respectively. Those from seeds irradiated after duplication had frequencies of 2.82 and 29.94, while unirradiated tetraploids had 2.77 and 30.53 respectively. Small structural aberrations inducing preferential pairing are assumed to have caused the lower quadrivalent frequency after the first treatment. The results suggest that it may be more expedient to induced, allopolyploidlike chromosome behaviour by irradiation at the diploid than at the tetraploid level.     

Chromosome association and pollen fertility of parental and interspecific hybrids of Lycopersicon esculentum X L. hirsutum and L. pennellii

Cytological studies were carried out on two wild species (L. hirsutum and L. pennellii) and the cultivated species (L. esculentum) of tomato and their F1 hybrids. Both parents and hybrids show a diploid chromosome number of 2n=24. The meiotic behaviour of the cultivated species showed a high degree of chromosome homology resulting in a high level of chiasmata frequency per bivalent. In contrast, the two wild species showed a slight increase in uniyalent frequency and a decrease in bivalent formation and chiasmata frequency. The meiotic behaviour of the hybrids showed a high level of univalents and low levels of bivalents as well as trivalents. Highly significant decreases in chiasmata frequency and increases in meiotic abnormalities, especially in the L. esculentum X L. pennellii hybrid, also were detected. The high meiotic irregularity and low chiasmata frequency recorded in the second hybrid indicated the disharmony and difference between its parental genomes and also served to predict its sterility. With regard to degree of pairing recorded in the hybrids, there is a possibility that sterility in such cases may refer to genetic factors in addition to the previously mentioned reasons. Pollen fertility showed no great difference between L. esculentum and L. hirsutum and their F1 hybrid, but a significant decrease was recorded in the L. esculentum X L. pennellii hybrid, which was clearly associated with high meiotic irregularity, low chiasmata frequency and chromosome association.