The effect of genes on A and B chromosomes of a number of Triticinae on meiotic behaviour in Triticum aestivum and its hybrids with related species

Results obtained and hypotheses that have been put forward to integrate them, are summarized in this section.1. Studies of chromosome association in F 1 hybrids Triticum aestivum X Secale cereale with and without chromosome 5B and in the presence or in the absence of B-chromosomes of rye, showed, as expected. that in the absence of chromosome 5B a high degree of pairing was observed. The presence of B-chromosomes only slightly reduced chromosome association, whether chromosome 5B was present or not.2. In F 1T. aestivum X S. cereale hybrids, at 20°C, a significant increase in the mean chiasma frequency per cell in nulli 5D + B's hybrids was observed. This interaction of genes carried by B-chromosomes of rye with genes carried by A-chromosomes was only detected in the absence of chromosome 5D of wheat.3. A substantial rise in chromosome association was also observed in an inter-varietal T. aestivum hybrid Chinese spring X Lindström when chromosome 5D of wheat was absent and B-chromosomes of rye were present, both at 20°C and at 10°C. This was mainly due to a shift in the relative numbers of rod and ring. bivalents, since the number of univalents was very low and similar. No change in chromosome association due to B-chromosomes was detected when 5D was present.4. Bayliss and Riley (1972) found in nullisomic 5D plants of T. aestivum a higher frequency of ring bivalents (with more than two chiasmata) than in disomic 5D plants. being however, the values of mean chiasmata per cell very similar in both plants. Mello-Sampayo and Miller (1979) detected a reduction in the number of ring bivalents with an increasing dosage of the long arm of chromosome 5D. These data together with our results led us to suggest the existence of a desynaptic gene in chromosome 5D which directly interacts with gene(s) carried by B-chromosomes of rye.5. It was also found that in F 1 hybrids T. aestivum X S. cereate , in the absence of chromosome 5D, synchronization within PMC's was reduced, which was not affected by the presence of B-chromosomes neither by differences in temperature (22°C and 15°C). In the absence of chromosome 5B no alteration In the synchronization of meiosis was detected, but the variation between plants covered the entire range observed between types in the experiments with nulli 5D.6. Asynapsis at low temperature is prevented by a stabilizer gene of chromosome association (Ltp) located in the long arm of chromosome 5D of T.aestivum. Studies in the detection of compensatory genes, at 10°C. for Ltp in diploid relative species of wheat were performed using F 1 hybrids T. aestivum X S. cereale. T. longissimum (=Ae. sharonensis) and T. speltoides, as well as T. durum X T. longissimum (= Ae. sharonensis) in which chromosome 5D was absent (nullisomic 5D). Results obtained demonstrated the existence of a Ltp -like gene in T. speltoides and T. durum, which compensates to a certain degree the absence of chromosome 5D. No compensation for the absence of Ltp was. however, found in S. cereale and in T.longissimum (= Ae. sharonensis).7. Asynapsis has also been detected in plants of T. aestivum which were disomic for a special isochromosome of the long arm of chromosome 5D (iso 5DLM). This suppressive effect in chromosome association was even more detectable in F 1 hybrids T. aestivum (di-isosomic 5DLM) X S. cereale, T. longissimum (= Aegilops sharonensis) and T. longissimum (= Ae. longissima, intermediate pairing line).8. Meiotic isochromosome configurations in several lines of T. aestivum which were disomic for different isochromosomes, were studied. A higher number of univalent isochromosomes was observed in di-isosomic 5DLMand 5B Lplants than in other di-isosomic combinations. The results demonstrated that (a) gene(s) of 5DLMaffecting chromosome pairing can be considered as (a) general suppressor(s) of chromosome association rather than a specific inhibitor of homoeologous pairing.9. Several hypotheses were put forward to explain the appearance of this suppressor gene(s) in isochromosome 5DLM.Although we cannot discard the hypothesis that it derives from a translocation with 5B Lit seems, however, more likely that it arose by mutation or duplication of a pre-existing but otherwise undetectable gene.10. Besides the described effects dependent on chromosome 5D of T. aestivum, it is also known that this chromosome carries in its short arm a nucleolar organizer. In ditelosomic 5D Lplants the total number of rDNA copies decreases substantially (-32%) and this was also observed in plants carrying four doses of chromosome 5D (tetrasomic 5D, -30%). In a disomic 5D plant which had been derived from a tetrasomic 5D the number of rDNA copies was, however, similar to that observed in euploids. Mono-isosomic 5D Land di-isosomic 5D Lplants showed a reduction of only 5% and 16%, respectively, in the total number of rDNA copies.11. In tetraploid wheats it was also observed that the introduction of a segment of the long arm of chromosome 5D in the complement (as in "Resende") or the addition of two entire chromosomes 5D (as in "AAL9") increased in 19% and 33%. respectively, the number of rRNA genes.12. These results show that chromosome 5D plays an important role on the control of the total number of rRNA genes although only a few rDNA genes are carried by its nucleolar organizer. The existence of a regulatory mechanism located in 5D Lwhich modulates the number of rRNA genes is suggested.13. A higher transcriptional rate for rRNA in root-tip cells of di-isosomic 5DLMplants has been measured (Viegas and Mello-Sampayo. 1975). In these same plants a high degree of asynapsis was detected both in hexaploid wheat and F 1 hybrids with diploid relative species. These observations taken together may mean a functional correlation between rDNA multiplicity and the control of meiotic behaviour.The results obtained throughout this work contributed especially to a better understanding of the role of chromosome 5D, since several new aspects of this chromosome in different cellular mechanisms, were detected It was suggested that a new desynaptic gene and a gene to control synchronization within PMC's in meiosis should exist in 5D. Moreover, the long arm of chromosome 50 should carry a suppressor of pairing (in a mutated isochromosome 5D L) and gene(s) regulating the total number of rDNA copies. Also the existence of the Ltp gene in this arm has been confirmed.The way in which genes carried by B-chromosomes interact with genes carried by A-chromosomes has also been a goal of this work. When an A-chromosome carrying a promoter of pairing is absent from the genetic complement, the addition of B-chromosomes of rye increased chromosome association. On the contrary. in the absence of a suppressor gene, B-chromosomes decreased chromosome association. This seems to suggest a regulating role for the genes carried by B- chromosomes of rye in what concerns the control of chromosome association.To construct a more descriminative map of gene location for chromosome 5D, subsequent crosses between ditelosomic 5D L, 5D Splants and the diploid relative species would be performed. In the same way, the future study of chromosome association in plants carrying simultaneously the isochromosome 5D Land 5DLMwould give more information about the suppressor gene existing in 5DLM. It would also be possible to evaluate. if the parallel made between the effect of this gene and Ph gene carried by 5D Lis legitimate by the study of nulli 5B-iso 5DLMplants. A better understanding of the control of rDNA copies in T. aestivum will be achievable through the future use of in situ hybridization DNA/rRNA techniques, complementary to the results already obtained by in vitro techniques.