| On the use of the 5B genetic system in wheat breeding:
The allosyndetic pairing in nulli-5B amphidiploids J. R. LACADENA and A. AZPIAZU Departamento de Genetica, Facultad de Ciencias, Universidad Complutense de Madrid, Spain It is known that chromosome 5B of common wheat, Triticum aestivum L, carries a genetic system which inhibits or suppresses homoeologous pairing. Meiotic pairing between wheat chromosomes and those of related species (and its consequent interspecific gene recombination) is possible in appropriate hybrids lacking chromosome 5B (RILEY, CHAPMAN and KIMBER 1959). In order to break down the barrier of sterility of hybrids RILEY and CHAPMAN (1963) pointed out the possibility of using nulli-5B amphidiploids in the breeding programs in which is aimed to introduce into common wheat the alien genetic variation. Allosyndetic pairing (and its consequent interspecific gene recombination) in interspecific hybrids is cytologically proved by the occurrence of quadrivalents or higher meiotic associations, while heptavalents or higher associations are the cytological proof in their corresponding amphidiploids. Since homoeologous pairing competes with the pairing between homologous chromosomes in nulli-5B amphidiploids, a reduction in the amount of the interspecific gene recombination can take place and, consequently, in the main aim of the breeding program; i.e. the introduction into wheat of the alien genetic variation. So, for the same combination T. aestivum-Aegilops longissima, RILEY and CHAPMAN (1963) found that the mean values of quadrivalent and heptavalent meiotic associations per pollen mother cell in, respectively, hybrids and amphidiploids were 1.04 and 0.10 in the most favourable case; that is to say, ten-fold higher in mono-5B hybrids than in their corresponding nulli-5B amphidiploids. In the hybrids T. aestivum-Secale cereale and T. aestivum-Aegilops columnaris deficient for chromosome 5B, LACADENA (1967) found quadrivalent and/or higher meiotic associations at first metaphase with a frequency of 0.04 and 0.56, respectively, per pollen mother cell. Nevertheless, on analyzing cytologically at the present time their corresponding nulli-5B amphidiploids we have not found any heptavalent or higher associations among, respectively, 518 and 500 pollen mother cells observed at first metaphase; i.e., it has not been found (but not excluding its possibility) the cytological proof of allosyndetic pairing and subsequent interspecific gene recombination. This is the reason why we consider interesting to emphasize that previously stated elsewhere on the use of the 5B genetic system in order to gain the most profit in wheat breeding. The two following alternatives were proposed: i) to make successive backcrosses of the 5B-deficient hybrids to wheat as the male recurrent parent (RILEY and KIMBER 1966), or ii) to restore the lost fertility of hybrids by duplicating the chromosomes of the offspring obtained from the first backcross and then to backcross successively to wheat as the male recurrent parent (LACADENA 1967). In any of both alternatives hexaploid forms can be obtained in which wild genes have been introduced into the wheat genome as a consequence of the interspecific gene recombination which took place at the merosis of the original hybrid deficient for chomosome 5B. Although the two proposed methods seem to assure the highest level of interpsecific gene recombination, the use of nulli-5B amphidiploids may also be of interest as reported by RILEY, KIMBER and LAW (1967); they were able to transfer the resistance to Puccinia recondita from Aegilops umbellulata to wheat by backcrossing the nulli-5B amphidiploid T. aestivum-Ae.umbellulata to wheat as recurrent parent. Literature cited LACADENA, J. R. 1967. Introduction of alien variation into wheat by gene recombination. I. Crosses between mono-V (5B) Triticum aestivum L. and Secale cereale L. and Aegilops coiumnaris ZHUK. Euphytica 16: 221-230. RILEY, R. and V. CHAPMAN 1963. The effects of the deficiency of chromosome V(5B) of Triticum aestivum on the meiosis of synthetic amphiploids. Heredity 18: 473-484. RILEY, R., V. CHAPMAN and G. KIMBER 1959. Genetic control of chromosome pairing in intergeneric hybrids with wheat. Nature 183: 1244-1246. RILEY, R. and G. KIMBER 1966. The transfer of alien genetic variation to wheat. Rept. Pl. Breed. Inst., Cambridge 1964-65: 6-36. RILEY, R., G. KIMBER and C. N. LAW 1967. Cytogenetics (Recognition of intergeneric chromosomal homoeologies by interference with the 5B system). Rept. Pl. Breed. Inst., Cambridge, 1965-66: 105-116. (Received April 26, 1971) |