Segmental allopolyploidy and paleopolyploidy in species of Leucaena benth: evidence from meiotic behaviour analysis

In this paper we report meiotic behaviour in 28 accessions of the tetraploid (2n = 4x = 104 or 112) Leucaena confertiflora, L. diversifolia, L. involucrata, L. leucocephala, L. x spontanea and the diploid (2n = 2x = 52 or 56) L. shannonii and L. macrophylla. We compare and discuss our data with that on literature about polyploidy in the genus. Despite the general predominance of bivalent formation, quadrivalents and other associations were found in all the taxa analysed. In the diploid species. multiple associations were found in up to 62% of the cells in L. shannonii and 97.6% in L. macrophylla. In the tetraploid taxa irregularities such as univalents, trivalents and other multivalents were observed in varying proportions, up to 55% in L. involucrata. Mean meiotic indexes per accession in the diploids and tetraploids were over or near 90%, but pollen fertility varied from 54.2% to 87.3%. The rather frequent presence of quadrivalents in the diploid species supports a paleopolyploid origin. For the tetraploid taxa, the presence of quadrivalents reflect chromosomal homology due to polyploid origin. Even if an autotetraploid origin cannot be ruled out by cytological data alone, considering other existing evidence, it is probable that L. confertiflora, L. pallida, L. leucocephala and L. involucrata are segmental allopolyploids. However, an autopolyploid origin for L. diversifolia cannot be ruled out nor cytologically nor by other existing data.     

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.     

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.     

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.     

The origin of natural tetraploid loach Misgurnus anguillicaudatus (Teleostei: Cobitidae) inferred from meiotic chromosome configurations

In the loach, or Oriental weatherfish Misgurnus anguillicaudatus (Teleostei: Cobitidae), diploid (2n = 50) and tetraploid individuals (4n = 100) are often sympatric in central China. The evolutionary mechanism of this tetraploidization was analyzed with the observation of meiotic behavior of chromosomes in both the germinal vesicles of mature oocytes and the primary spermatocytes in diploid and tetraploid loaches. Whereas diploid specimens usually showed 25 bivalents in meiotic cells, tetraploid loaches exhibited 0-6 quadrivalents and 38-50 bivalents in both sexes, with the modal number of quadrivalents as three in females and four in males. In the diploid specimens, the two largest metacentric chromosomes bearing nucleolar organizing regions (NORs) identified by chromomycin A(3) staining and fluorescence in situ hybridization with a 5.8S + 28S rDNA probe formed one bivalent with terminal association. In the tetraploids, four NOR-bearing chromosomes never formed a quadrivalent, but were organized into two terminally-associated bivalents. These findings suggest an autotetraploid origin of the natural tetraploid loach and subsequent rediploidization of whole genome. The latter process, however, seems still in progress as inferred from the concurrence of up-to several quadrivalents and the majority of bivalents.     

Cytological analysis of tetraploid hybrids between sweet potato and diploid Ipomoea trifida (H. B. K.) Don.

Summary Tetraploid F₁ hybrids between Ipomoea batatas, sweet potato (2n = 6x = ca. 90), and diploid (2n = 2x = 30) I. trifida (H. B. K.) Don. showed various degrees of fertility reduction. The present study aimed to clarify its causes by cytological analysis of meiotic chromosome behavior in the diploid and sweet potato parents and their tetraploid hybrids. The diploid parents showed exclusively 15 bivalents, and the sweet potato parents exhibited almost perfect chromosome pairing along with predominant multivalent formation. Their hybrids (2n = 4x= 57–63) formed 2.6–5.0 quadrivalents per cell, supporting the autotetraploid nature. The meiotic aberratios of the hybrids were characterized by the formation of univalents, micronuclei, and abnormal sporads (monad, dyad, triad, and polyad). The causes underlying these aberrations were attributed in part to the multivalent formation, and in part to a disturbance in the spindle function. Three hybrids showing serious meiotic aberrations were very low in fertility. The utilization of the sweet potato-diploid I. trifida hybrids for sweet potato improvement is described and, further, the role of interploidy hybridization in the study of the sweet potato evolution is discussed.     

Cytological techniques for use in apple breeding

Details are given of improved cytological squash techniques for mitotic and meiotic counts which are especially useful in apple breeding. The cytological characteristics of scion varieties (including triploids such as the recently named Suntan), apple rootstocks (including tetraploids), and second backcross progenies of the mildew resistant species Malus hupehensis (2n= 51) to the cultivated apple (in which most seedlings were found to be diploid) are discussed with reference to their implications for apple breeders.     

Thinopyrum distichum addition lines: production,morphological and cytological characterisation of 11 disomic addition lines and stable addition-substitution line

Plants of the partial amphiploid Inia 66/Thinopyrum distichum (2n = 70)//Inia 66 (2n = 56) were used as male parents in crosses with the monosomic series in the common wheat cultivar Inia 66. The genome and homoeologous group of the monosomic used in the cross affected the distribution of chromosome number of the progeny plants in the F₂ and F₄. Meiosis in the pollen mother cells of the B₁F₇ partial amphiploids was not stable, and not different from that of the B₁F₁ in which univalents and multivalents were observed. Disomic addition lines were selected on the basis of morphology and meiotic stability in the F₂, F₄ and F₅. Eleven of the fourteen possible wheat-Th. distichum disomic addition lines were identified using chromosome C-band pattern, as well as size and arm ratio, as genetic markers. Addition of T. distichum chromosome J ^dll produced a phenotype indicating homoeology with wheat group-2 chromosomes. Clear indications of homoeology based on morphological characteristics were not obtained in any of the other addition lines, probably due to the mixed homoeology of the Th. distichum chromosomes relative to wheat. The addition lines were all susceptible to leaf rust, unlike the germplasm-line Indis which carries a leaf rust resistance gene on a translocation segment derived from Th. distichum. Instability of meiotic pairing was observed in all addition lines. The stability, or not, of progeny chromosome counts did not reflect the level of chromosome pairing instability in the parental plants. SDS-PAGE for gliadin-type seed proteins revealed two addition lines which expressed seed storage proteins uncommon to Inia 66 but typical of Th. distichum.     

AUTOTETRAPLOIDY IN HAPLOPAPPUS SPINULOSUS HYBRIDS: EVIDENCE FROM NATURAL AND SYNTHETIC TETRAPLOIDS

Haplopappus spinulosus (Asteraceae) is a herbaceous, perennial weed common throughout the western Great Plains of North America and includes both diploid and tetraploid populations. A number of populations in southeastern Colorado were analyzed cytogenetically and morphologically for two reasons. First, initial observations during a routine population survey showed they were morphologically intermediate between the diploid subspecies glaberrimus and spinulosus, suggesting they might have arisen via hybridization. Second, cytological examination revealed that they were tetraploid. Because there was indication of hybrid origin, it was of interest to determine whether the populations were behaving as autopolyploids or segmental allopolyploids. The distinction between these two polyploid types is not easily made since both are likely to form multivalents at meiosis, but equations derived from a model proposed by Jackson and Hauber (1982) have made it possible to determine statistically whether a tetraploid individual is behaving meiotically as an autotetraploid. Meiotic configuration frequencies at diakinesis were determined for each tetraploid plant sampled, and observed frequencies were compared to those expected for an autotetraploid having the same maximum number of chiasmata per bivalent, chiasma frequency and chromosome number. In general, the meiotic behavior of the tetraploids was no different from that expected for autoploids. The initial hypothesis that the populations were derived from hybridization was tested by a detailed cytogenetic and morphological study of the presumed parental subspecies, F, hybrids, and natural putative hybrids. The evidence supports the hypothesis that the natural autotetraploids arose from the hybridization of ssp. glaberrimus and ssp. spinulosus.     

Chromosome association and fertility in tetraploid ryegrass

In colchicine treated diploid ryegrass,Lolium perenne L. (2n=14), in addition to normal diploids, tetraploids and mixoploids, cytologically aberrant plants were observed. The latter included one aneuploid (2n=25), four translocation heterozygotes (diploid and mixoploid), and one mixoploid desynaptic plant which was a chimera of normal and desynaptic tissues as shown by successive clonal propagations. The normal diploids had chromosome association of 7 II, with an average chiasmata frequency of 12 per pollen mother cell and had 85.6 per cent pollen fertility. The tetraploids showed a mean chromosome association of 0.88 I, 4.83 II, 0.29 III and 3.91 IV, with an average chiasmata frequency of 25 per microsporocyte, and had 83.8 per cent fertile pollen. In the tetraploids, pollen fertility was negatively correlated with quadrivalent frequency and positively so with bivalent and trivalent frequencies, there being no significant correlation with univalent frequency. Among the most frequent types of quadrivalents, only the alternate chain quadrivalent frequency was positively correlated (r=+.9297) with the plllen fertility. On this basis, an increased pollen fertility in the succeeding generations of tetraploids could result from a decrease in the frequency of quadrivalent types other than the alternate chains.     

The fate of multivalents during meiotic prophase in the hybrid Gibasis consobrina x G. karwinskyana Rafin. (Commelinaceae)

The chromosomes of the two closely related diploid species, Gibasis consobrina and G. karwinskyana (Commelinaceae; 2n=2x=10), are morphologically alike, yet form few chiasmate associations at metaphase I in the f₁ hybrid. During meiotic prophase, however, synaptonemal complexes join the majority of the chromosomes of the complement in complex multiple pairing configurations. The F₁ hybrid between different tetraploid genotypes of the same two species similarly forms multivalents during meiotic prophase, which are subsequently eliminated in favour of strictly homologous bivalents before metaphase I. One quadrivalent comprising interchange chromosomes inherited from one of the parents, usually persists to first metaphase. Evidently the resolution of multivalents to bivalents at first metaphase, which accounts for diploidisation, is not attributable to the elimination of multivalents per se, but of multivalents comprising chromosomes of limited homology.     

Cytogenetic analysis of plants regenerated from colchicine-treated callus cultures of an interspecificHordeum hybrid

Summary Hybrids of Hordeum vulgare (HV) x H. jubatum (HJ) were synthesized for purposes of introgressive breeding, but were sterile and the recovery of pure diploid tillers by colchicine applications in vivo was difficult. Plant regeneration from colchicine-treated callus cultures of the hybrid (HV x HJ) was investigated as a means to produce high numbers of pure diploid, fertile intermediates. 10 of 50 plants regenerated in this manner exhibited variable chromosome numbers with means of approximately 37 (expected diploid number = 42). Cytological examinations of microsporogenesis in all such plants revealed a high incidence of bivalent formation at metaphase I (as compared to nearly complete asynapsis in the F₁), but spindle and chromosome abnormalities in later meiotic stages led to complete sterility. Approximately 40% of HJ plants regenerated from colchicine-treated calli appeared to be pure tetraploids of high fertility. These techniques are hence useful for high frequency production of diploid or polyploid plants.     

A single dominant gene for Fusarium wilt resistance in protoplast-derived tomato plants

Summary Autotetraploids were established from 8 diploid wild species of section Arachis. In all the autotetraploids the chromosomes paired largely as bivalents even though they possess the ability to pair as multivalents. Pollen and pod fertility in the C₁ generation were not directly associated with chromosome pairing. The C₂ generation autotetraploids showed a gradual increase in bivalent associations and pollen and pod fertility. The identification of two genomes, A and B, in the diploid species and in the tetraploid, A. hypogaea, of the section Arachis, a fairly good crossability, and the type of chromosome associations observed in hybrids between A. hypogaea and the autotetraploids of wild Arachis species indicated good prospects of utilizing autotetraploids as genetic bridges in transferring desired traits from these taxa into groundnut.Submitted as Journal Article No. 516 by International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

Cytogenetics of double cross hybrids between Pennisetum americanum — P. purpureum amphiploids and P. americanum x Pennisetum squamulatum interspecific hybrids

Summary Pearl millet, Pennisetum americanum L. Leeke-napiergrass, Pennisetum purpureum Schum. amphiploids (2n=42) were crossed with pearl millet X Pennisetum squamulatum Fresen. interspecific hybrids (2n=41) to study the potential of germplasm transfer from wild Pennisetum species to pearl millet. These two interspecific hybrids were highly cross-compatible and more than two thousand trispecific progenies were produced from 17 double crosses. All doublecross hybrids were perennial and showed a wide range of morphological variations intermediate to both parents in vegetative and inflorescence characteristics. Some crosses resulted in sublethal progenies. Chromosomes paired mainly as bivalents (¯x15.88) or remained as univalents. At metaphase I, trivalents, quadrivalents, an occasional hexavalent and a high frequency of bivalents indicated some homeology among the genomes of the three species. Delayed separation of bivalents, unequal segregation of multivalents, lagging chromosomes, and chromatin bridges were observed at anaphase I. Although approximately 93% of the double-cross hybrids were male-sterile, pollen stainability in male-fertile plants ranged up to 94%. Seed set ranged from 0 to 37 seed per inflorescence in 71 plants under open-pollinated conditions. Apomictic embryo sac development was observed in double-cross progenies when crosses involved a pearl millet x P. squamulatum apomictic hybrid as pollen parent. These new double-cross hybrids may serve as bridging hybrids to transfer genes controlling apomixis and other plant characteristics from the wild Pennisetum species to pearl millet.     

Chromosome differentiation in diploid species of Lotus (Leguminosae)

Summary Meiotic chromosome behavior was studied in seven diploid species of Lotus (L. alpinus Schleich., L. japonicus (Regel) Larsen, L. filicaulis Dur., L. schoelleri Schweinf., L. krylovii Schischk. and Serg., L. tenuis Waldst. et Kit., L. corniculatus var. minor Baker) and in 51 interspecific hybrids from 16 different crosses. Meiosis in the diploid species was quite regular. In a high proportion of the PMC's of the hybrids there was close chromosome homology with a normal association of 6 II's. However, meiotic irregularities including bridges, lagging chromosomes, univalents, and quadrivalents, occurred in a small percentage of the cells. The late separation of bivalents, the presence of quadrivalents, and inversion bridges with fragments, would indicate for some hybrids that certain chromosomes were structurally differentiated. The large number of rod bivalents observed at diakinesis was also highly suggestive that genetic nonhomology in one chromosome arm could contribute to the frequency of this type of bivalent. Therefore, the maximum number of 6 II's which occurred in a high percentage of cells may be misleading in that cryptic structural differences between chromosome arms, or segments, are not revealed. Pollen fertility in the species and hybrids was not correlated with meiotic irregularities suggesting that pollen fertility is genotypically controlled.     

Meiotic studies of the F1 hybrid between rice bean (Vigna umbellata) and its wild relative V. minima

The interspecific F₁ hybrid V. umbellata x V. minima, together with the parental species was investigated cytogenetically. Both parents showed normal meiosis, with 11 bivalents at diakinesis. The hybrid was completely pollen sterile. The cytological causes underlying sterility in the hybrid include an array of meiotic abnormalities ranging from non-pairing of chromosomes through the presence of univalents, chain and ring multivalents and anaphase bridges to the formation of micronuclei, during microsporogenesis. The presence of chain and ring multivalents at diakinesis not only demonstrates the structural hybridity of the F₁ hybrid, but also suggests that small interchanges and inversions played a significant role in the speciation within the genus Vigna.     

Quantitative analysis of diploid translocation heterozygotes: test of models and equations

Summary Equations have been derived for two different models of chromosome pairing and chiasmata distribution. The first model represents the normal condition and assumes complete synapsis of homologous bivalents and the arms of interchange quadrivalents. This is followed by a nonrandom distribution of chiasmata among bivalents and multivalents such that each bivalent or bivalent-equivalent always has at least one chiasma. Univalents occur only as part of a III, I configuration at diakinesis or metaphase I. The second model assumes that a hologenomic mutation is present in which all chromosomes of a genome are equally affected. Two different assumptions can be made for such a mutation, and both give the same results: (1) homologous or homoeologous chromosome arms may be randomly paired or unpaired, but synapsis always leads to a crossover; (2) homologous or homoeologous arms always pair, but chiasmata are randomly distributed among the arms. The meiotic configurations at diakinesis or metaphase I are the same for both assumptions. Meiotic configurations of normal diploid interchange heterozygotes show good agreement with numbers predicted by the equations for nonrandom chiasmata distribution among configurations. Inter-specific hybrids with supernumerary chromosomes produced meiotic configurations frequencies in agreement with predictions of equations for random chiasmata distribution, but a hybrid without supernumeraries fitted the nonrandom expectations.     

Studies on male and female meiosis in Indian Allium

Male meiosis in autotetraploid Allium tuberosum (4×=32) is fairly regular, keeping in view its cytological status, with 81 percent of the chromosomes associated in quadrivalents and trivalents. About 5% of the cells have 32 univalents. Anaphase segregation is slightly irregular. While 48% of the pollen mitoses show 16 chromosomes, 87% of the mature pollen is viable as indicated by carmine or iodine staining. — Megaspore mother cells have 64 chromosomes associated in 32 bivalents at metaphase I. Anaphase segregation is normal. In three out of 56 cells studied multivalents, bivalents and univalents are observed as in male meiosis. — It is concluded that the species reproduces by pseudogamous parthenogenesis made possible by meiotic modification. This modification is almost perfect and almost completely specific for female meiosis. Slight effects are observed in male meiosis.     

Further insights on chromosomal pairing of autopolyploids: a triploid and tetraploids of rye

Chromosomal pairing of one triploid and three tetraploid plants of rye, Secale cereale, was analyzed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I cells. Prophase I is characterized by: (i) the weak alignment showed by the three or four unsynapsed or partially homologous synapsed axes; (ii) the low number ber of pairing partner switches (PPSs) displayed by both trivalents and quadrivalents; and (iii) the existence of complex multivalents in which up to 13 chromosomes in the triploid and 22 chromosomes in the tetraploids were involved. However, only few heterologous chromosomal associations were maintained at metaphase I. The results obtained are discussed under the assumptions of the random end pairing model with some modifications.     

Study of a translocation in tetraploid rye

Colchicine induced tetraploids (4x=28) from diploidSecale cereale heterozygous for a translocation showed a strong tendency of non-preferential pairing for the interchanged chromosomes. The normal chromosomes associated in configurations up to quadrivalents, and the translocation complex formed multivalents up to octavalents. Most of the interchanged chromosome associations were characterized by their heteromorphic nature. The percentage of the chromosomes in the interchange complex forming multivalent associations was far higher than that of the remaining twenty chromosomes. Abnormalities were observed at anaphase I and II in the pollen mother cells. The tetraploids appeared to be completely sterile. It is suggested that the high frequency of multivalent formation may be explained on the basis that the interchange might have involved a region of localized chiasmata. The absence of polyploidy in the genusSecale as against its widespread occurrence in the related grass genera may be accounted for, in part, on the basis of non-preferential pairing.