Induction and evaluation of colchitetraploids of two species of Tinospora Miers, 1851

Autotetraploidy, both natural and/or induced, has potential for genetic improvement of various crop species including that of medicinal importance. Tinospora cordifolia (Willdenow, 1806) Miers, 1851 ex Hooker et Thomson, 1855 and T. sinensis (Loureiro, 1790) Merrill, 1934 are two diploid species, which are dioecious, deciduous and climbing shrubs with high medicinal importance. Among the three methods used for induction of polyploidy by colchicine treatment, it was cotton swab method which successfully induced the polyploidy in both species. The morphological and cytogenetical features of the synthetic tetraploids were compared with their diploid counterparts. The tetraploids were morphologically distinct from diploid plants. They exhibited larger organs, such as stem, leaves, inflorescence, fruits, flowers and seeds. The tetraploids were characterized by the presence of low quadrivalent frequency and high bivalent average. Unequal distribution of chromosomes at anaphase I was found in 60% cells. The present study provides important information on the superiority of autotetraploids as compared to diploid counterparts in both species.

Cytogenetical investigations in colchicine-induced tetraploids of Cyamopsis tetragonoloba L.

Successful induction of autotetraploidy has been achieved in five accessions of Cyamopsis tetragonoloba L. The diploid seedlings of these accessions were treated with different concentrations of aqueous colchicine using the cotton-swab method for 10–18 hours within 2–3 days. The highest percentage of success was recorded when the seedlings were treated with 0.2% colchicine for 10 h within two days. The synthesized plants showed remarkable enhancement in several morphological and floral characters making them more robust and better plants from the food and feed aspect. Cytologically, quadrivalent frequency ranging from 3.18 to 5.45 and univalent frequency ranging from 0.08 to 1.10 were characteristic of the colchicine-induced tetraploids. Among all the associations, bivalent chromosome associations were observed more frequently (2.95 to 6.04). The anaphase I and II disjunction of bivalents/chromosomes was leading more or less regularly and equally to the formation of at least few seeds from some of the synthesized plants. Significant enhancement in morphological traits as revealed in colchicine-induced tetraploid plants having a better food and feed value and normal meiotic behaviour of synthesized autotetraploids leading to a good seed set may ultimately result in the genetic improvement of Cyamopsis tetragonoloba.

Synapsis in a natural autotetraploid

To test assumptions of the autotetraploid chromosome pairing model regarding events during synapsis, whole-mount spreads of synaptonemal complexes (SCs) of Machaeranthera pinnatifida (=Haplopappus spinulosus) (Asteraceae) (2n = 4x = 16) were analyzed by electron microscopy. On the assumption of one synaptic initiation per chromosome arm, each pachytene quadrivalent is expected to have one partner switch (PS), and the frequency of pachytene quadrivalents for each chromosome is predicted to be 2/3 (or 0.67). However, to the contrary, we observed a range of one to four PSs per pachytene quadrivalent with an overall mean of 1.56. This suggests that the number of synaptic initiations is greater than one per chromosome arm (or >two per chromosome), and the predicted frequency of pachytene quadrivalents should be >8/9 (based on a minimum of three initiations per chromosome). However, in close agreement with the model, the observed pachytene quadrivalent frequency from SCs in this study was 0.69. To explain the apparent discrepancy between the observed frequency of PSs and the observed frequency of quadrivalents, the possibility of nonindependent synaptic initiations and presynaptic alignment are discussed in the context of their potential influence on quadrivalent frequency. Recombination nodules (RNs), which were scored in about half the SC spreads, occurred at a frequency (9.6 per nucleus) comparable with the chiasma frequency at diakinesis (9.3 per nucleus). The frequency of RNs as well as their distribution is consistent with the hypothesis that RNs occur at sites of crossing over and chiasma formation.Key words: autopolyploid, Machaeranthera pinnatifida, meiosis, recombination nodules, synaptonemal complex.

Differences in the synaptic pattern in two autotetraploid cultivars of rye with different quadrivalent frequencies at metaphase I

Synaptic behaviour of the two tetraploids rye cultivars Gigantón (G) and Tetrapico (T) displaying significant differences in their quadrivalent frequencies at metaphase I was analyzed by electron microscopy in surface-spread prophase I nuclei. A different behaviour was observed between the two cultivars; the synaptonemal complex (SC) quadrivalents frequency being significantly higher in G than in T at prophase I. Moreover, the G SC quadrivalents had more synaptic partner exchanges (SPEs) and their location was more distal than the T SC quadrivalents. However, inverse findings were found at metaphase I, the quadrivalent frequency was higher in T than in G. The role that different factors, mainly the number and location of the SPEs and the frequency and distribution of chiasmata, could play in the evolution from prophase I to metaphase I in both cultivars is discussed.Key words: autotetraploid rye, synaptonemal complex, spreading.

Chromosome pairing affinity and quadrivalent formation in polyploids: do segmental allopolyploids exist?

When polyploid hybrids with closely related genomes are propagated by selfing or sib-breeding, the meiotic behaviour will turn into essentially autopolyploid behaviour as soon as the affinity between the genomes is sufficient to permit occasional homoeologous pairing. An allopolyploid will only be formed when the initial differentiation is sufficient to completely prevent homoeologous pairing (in some cases enhanced by specific genes), or when segregational dysgenesis prevents transmission of recombined chromosomes. A new polyploid hybrid may be considered a segmental allopolyploid and may show reduced multivalent formation as a result of preferential pairing between the least differentiated genomes. An established polyploid is either an autopolyploid or an allopolyploid. In exceptional cases it is thinkable that a stable segmental allopolyploid arises, in which some sets of chromosomes are well differentiated and behave as in an allopolyploid, whereas other sets are not well differentiated and behave as in an autopolyploid. No clear cases have been found in the literature so far. Key words : chromosome, pairing affinity, quadrivalent frequency, segmental allopolyploidy.

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum × L. peruvianum hybrid

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.Key words: Lycopersicon, tetraploid hybrid, synaptonemal complex, pairing partner exchange, diakinesis/metaphase I.

Cytogenetics of fertility improvement in artificial autotetraploids of Hyoscyamus niger L.

Genetically stable artificial autotetraploids with over 90% seed set were obtained by colchicine treatment of the solanaceous species Hyoscyamus niger L. (4x = 68). The tetraploids were vigorous and had earlier been shown to yield considerably more tropane alkaloid per individual than the source diploids. The tetraploids had high bivalent frequencies at meiosis, and high fertility as a result of the formation of genetically balanced gametes. There was a systematic and significant decrease in quadrivalent frequency and an increase in bivalent frequency in three subsequent generations tested (C0, C1, C2). Possible causes of high bivalent frequency are discussed. Seed fertility can probably be increased in advanced generations by further selection for fertility and maximization of heterozygosity.Key words: Hyoscyamus niger, autotetraploid, meiosis, bivalents, fertility, selection.

Meiotic chromosome association in diploid and tetraploid Avena strigosa Schrel.

In diploid Avena strigosa the frequency and distribution of open (rod) bivalents suggest that a limited number of chromosomes, possibly two, fail to have a chiasma in one (short) arm in over 60% of the cells. In the tetraploid these are expected to form bivalents instead of quadrivalents at metaphase I of meiosis in a high frequency. In addition, interstitial chiasmata are expected in the long arms of these chromosomes, when involved in quadrivalents. In the tetraploid the frequency of quadrivalents is indeed low, and the configurations with interstitial chiasmata are distributed as expected. However, application of Sybenga's models suggests that the pairing system rather than the chiasma system would be responsible for the low quadrivalent frequency. It is suggested that apparent negative chiasma interference across the point of pairing partner exchange, resulting from variation in its location, invalidates application of these models to the present material. Negative interference results in an even higher than expected open bivalent frequency, in combination with a relatively high ring quadrivalent frequency. In addition, not only the subacrocentric chromosomes but also the more metacentric chromsomes, by this same mechanism, tend to form open bivalents more often than in the diploid.Key words: meiosis, bivalent, quadrivalent, diploid, tetraploid, Avena strigosa

A reassessment of genome relationships between Thinopyrum bessarabicum and T. elongatum of the Triticeae

Chromosome pairing and chiasma frequency in diploid (2n = 2x = 14; JE genomes), amphidiploid (2n = 4x = 28; JJEE), and triploid (2n = 3x = 21; JJE) hybrids between Thinopyrum bessarabicum (2n = 2x = 14; JJ) and T. elongatum (2n = 2x = 14; EE) were analyzed. The diploid hybrids (JE) showed a mean pairing of < 0.01V + 0.30IV + 0.28III + 4.98II + 1.97I with 8.36 chiasmata per cell. The pairing was rather poor, most bivalents being rod-shaped; some were clearly hetero-morphic and loosely paired (probably pseudochiasmate). The diploid hybrids were sterile, showing the reproductive isolation of the parental species. The JJE triploid had a mean chromosome configuration of < 0.01VI + 0.06IV + 1.53III + 5.46II + 5.20I with a chiasma frequency of 13.45 per cell. Chromosomes of the duplicated genome JJ showed preferential pairing, forming mostly ring bivalents with two or even three chiasmata each, as in the T. bessarabicum parent; most chromosomes of the E genome remained as univalents. Thus, the E genome chromosomes offered little synaptic competition to the chromosomes of the duplicated JJ genome. The degree of preferential pairing was even stronger in the JJEE amphidiploids, which predominantly showed bivalent pairing with up to 14 ring bivalents in some cells. They had a mean pairing of 0.01VI + 0.55IV + 0.26III + 11.75II + 1.42I; the mean quadrivalent frequency per cell varied from 0.10 to 1.53. Thus J and E genomes essentially maintained their meiotic integrity at the 4x level. This pattern of chromosome pairing in hybrids at different ploidies and the sterility of diploid hybrids show that J and E are distinct genomes and that there is little justification for merging them, as suggested by previous workers. The J and E are homoeologous at best. The merger of Lophopyrum (E genome) with the genus Thinopyrum (J genome) would be improper. Although the J and E genomes are close enough to permit some intergenomic gene flow, which may be exploited in plant breeding, they are certainly not close enough to have the same genomic designation. The JJEE amphidiploids are meiotically stable and may be a useful source of genes for wheat improvement.Key words: genome, meiosis, chromosome pairing, phylogenetic relationships, Thinopyrum, interspecific hybrid, autoallo-triploid, amphidiploid.

Genetic and cytogenetic analyses of the A genome of Triticum monococcum. IV. Synaptonemal complex formation in autotetraploids

Electron microscopy of synaptonemal complex spreads from autotetraploid Triticum monococcum (2n = 4x = 28) revealed a minimum mean of 3.59 multivalents per zygotene–pachytene nucleus. The range of values was from 1 to 6 multivalents per nucleus. Most of the multivalents were quadrivalents with single, medially located pairing partner switch points. Lateral element pairing switches, particularly the few multiple switches, were often accompanied by extensive asynapsis around the switch point. The synaptonemal complex multivalent frequency is considerably higher than the metaphase I quadrivalent frequency previously reported for the same material. Calculations of expected pachytene quadrivalent frequency from metaphase I data, using several published theoretical models, gave values that did not agree with the results obtained here. The difference between the multivalent frequencies at pachytene and metaphase I does not appear to be the result of a correction process. Instead, it could be caused by a combination of preferential pairing or crossing-over and the effects of the position of partner switches and asynapsis associated with switches. Key words: autotetraploid, multivalents, synaptonemal complex, pairing effects.