Development of colchicine‐induced tetraploid St. Augustinegrass (Stenotaphrum secundatum) lines

St. Augustinegrass is well suited for lawns and commercial landscapes. While many genotypes are cross‐fertile, all cultivars are propagated vegetatively in sod production. To ensure varietal purity, development of sterile triploid hybrids by crossing tetraploid and diploid genotypes has been successfully used in other warm‐season turfgrasses. Applying this model in St. Augustinegrass would be beneficial to sod producers and turf managers who require purity for certification and uniformity for performance, respectively. This study was conducted to develop colchicine‐induced tetraploid lines of St. Augustinegrass. Seeds of cultivar ‘Raleigh’ were treated with four colchicine concentrations at four exposure times. A non‐treated control was included among the treatments. Seedlings that germinated were screened for genome size changes using flow cytometry. Line DSA 13005 and two progeny lines derived through selfing, DSA 16001 and DSA 16016, were corroborated as tetraploids (2n = 4x = 36) through chromosome counts. These lines will be used in future breeding efforts to attempt development of sterile triploid cultivars of St. Augustinegrass.     

Improved techniques for the induction and isolation of polyploids in the genus Fragaria

Summary The sensitivity of Fragaria seedlings to colchicine is dependent on the plant organ that is treated. Complete immersion of seedlings in a 1.5% colchicine solution results in total lethality, whereas the survival rates were more than 75% even at concentrations of 3.0% when only shoot apices were treated. High proportions of polyploids were isolated by treating shoot apices of seedlings with a 2.0% colchicine solution for 24–28 h. The dropper method is preferred to the tube method as it involves a minimum of manipulation and requires simple equipment. A differential response to colchicine was observed within and between different diploid species, diploid and tetraploid hybrids.     

A comparative investigation of seed germination, metabolism and seedling growth between two polyploid Triticum species

The embryo and seedling growth patterns of two Triticum polyploid species, namely the tetraploid, T. durum (AABB), and the hexaploid, T. aestivum (AABBDD), were compared. Two developmental phases were identified, an initial lag phase during embryo growth and an exponential phase during seedling growth. These developmental phases were distinctly different in the two species. The initiation of mitosis prior to the inception of the exponential phase, occurred at 16 hours after imbibition in the hexaploid and 20 hours after imbibition in the tetraploid. All the growth rates of the hexaploid, except that of the shoot, showed higher activities – food reserves were mobilized much faster, root growth was faster, number of cells in mitosis over 24 hours higher: for the hexaploid primary root (7.0%) and first pair of lateral roots (6.5%), while for the tetraploid it was found to be 5.9% in the primary root and 5.7% in the first pair of lateral roots. The primary root and first pair of lateral roots of the hexaploid emerged respectively six hours and four hours earlier in comparison to those of the tetraploid. The inception time of the exponential phase in seedling growth and mobilization of food reserves were observed 12 hours earlier in the hexaploid although no differences were observed in the inception times for the shoot growth and respiration rate. It can therefore be concluded that the genomic composition of the hexaploid promotes a higher growth rate and therefore a higher metabolic rate than the tetraploid. This revised version was published online in July 2006 with corrections to the Cover Date.     

A cytogenetic study of bilateral sexual polyploidization in cassava (Manihot esculenta Crantz)

Microsporogenesis and megasporogenesis were cytogenetically and histologically analysed in three cassava clones:‘Rayong 1′,‘Rayong 60′,‘M. mga’ and two hybrid lines,‘OMR 3641‐1’ and ‘OMR 3641‐1’ to elucidate the evolution of sexual polyploids in cassava. At telophase II, formation of 17‐21 micronuclei per pollen cell plate was observed in 16 out of 351 cell plates in ‘M. mga’. Micronuclei were observed at low (0.3‐2.3%) frequencies, at the sporad stage in all clones. Monads, dyads, triads and tetrads, which are established sources of high ploidy levels were observed at low (2.6%) and high (22.2%) frequencies. Megasporogenesis in ‘Rayong 1’ and ‘Rayong 60’ showed a lack of second meiotic divisions after a successful first division that resulted in partly unreduced embryo sacs with 2n eggs, suggesting another unrecognized and, as yet, unreported source of sexual polyploid formation in cassava. Meiotic abnormalities during microsporogenesis and megasporogenesis are implicated as being responsible for the formation of mixoploids (triploids and tetraploids) in cassava breeding programmes. A cytogenetic mechanism resulting in bilateral sexual polyploids through different gametic fertilization pathways in cassava is suggested and its role in breeding is briefly discussed.     

A technique for the production of polyploids in grasses

Summary A satisfactory technique has been developed whereby vegetative grass tillers can be successfully treated with colchicine to produce polyploid sectors. It is particularly useful for producing polyploids in sterile hybrid material and as an alternative technique to seedling treatment if the number of seeds available is in limited supply.     

Exploitation of the incompatibility mechanism for the production of F1 hybrid forage grasses

Summary The utility of the two locus (SZ) incompatibility system for the production of F₁ hybrids in forage grasses is examined in relation to its mode of action at the tetraploid level and in dihaploids. Schemes are proposed for producing tetraploid populations with a high level of within population incompatibility or of single or double cross hybrids from the dihaploids.     

Cytological behaviour of the somatic hybrids Passiflora edulis f. flavicarpa + P. cincinnata

Meiosis and pollen viability of interspecific somatic hybrids, i.e. Passiflora edulis f. flavicarpa , the yellow passion fruit +  P. cincinnata , a wild species, were examined and compared with their corresponding diploid fusion parents. Firstly, the cybrid nature of these hybrids was confirmed by leaf-stomatal measurements and RFLP analyses using four mitochondrial genes as probes. The meiotic behaviour revealed relatively high stability, with most of the hybrid cells showing 18 bivalents. Some instability, such as a quadrivalent configuration was also recorded which has been interpreted as an interchange that occurred in the progenitors more than as a result of in vitro culture or chromosome reorganization in the new genome. Even in low frequencies, the occurrence of univalents resulted in mis-division, laggard and micronucleus formation. High values of pollen viability (>70%) were found in the diploid parents as well as in the hybrid plants. The results are discussed in relation to the possibilities of applying somatic hybridization for improving passion fruit varieties.     

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.     

60Co-γ辐射对黄花苜蓿出苗和染色体的诱变效应

用不同剂量(600～1 400 Gy,5个梯度,每梯度增加200)的60Co-γ射线辐照吸胀和未吸胀的黄花苜蓿(Medicago falcata L.)种子,通过对出苗、根尖体细胞染色体变异情况的观察,分析辐照对出苗率、叶形态、多倍体诱变率等的影响.结果表明,同剂量辐照下,未吸胀种子的出苗率明显低于吸胀种子出苗率;随辐射剂量增强,第1片真叶长出的时间延长,幼苗生长变缓;60 Co-γ射线辐照黄花苜蓿种子其染色体加倍的适宜剂量为1 200 Gy.