| Variation of chromosome pairing in hexaploid wheat
7. A model and simulation for the chromosome arrangement in premeiotic nucleus
T. KATO Hiroshima Agricultural College, Higashihiroshima 724, Japan In hexaploid wheat, anyone of those three factors, extra dosage of 5BL, colchicine and high temperature treatments, reduced the degree of homologous pairing and in the same PMCs it induced homoeologous pairing (Feldman 1966 ; Driscoll et al. 1967; Kato and Yamagata 1980). This peculiar pairing behavior can reasonably be explained by a hypothesis that a chromosome is situated closely to its homologous chromosome but is separated from its homoeologous ones in premeiotic nucleus (Feldman 1966). In premeiotic nucleus, however, direct observation on the chromosome condition was very difficult and did not contribute enough information to the precise analysis on this problem. The present study was undertaken to analyse the chromosome arrangement in premeiotic nucleus from other lines of approach. First, topographical model was applied to the chromosome arrangement and its variation. Then, the simulation which was based on the model was carried out to examine whether it could reproduce the peculiar pairing behavior as described above. The details of the model were as follows (see Fig. 1); 1) The premeiotic nucleus is spherical in shape. 2) On the nuclear envelop (spherical surface), there exist three circular regions. Each region corresponds to the individual genome. This region has a radius, rho and is separated from other regions by a distance, gamma. 3) Every chromosome belonging to the same genome attaches its one particular part to the corresponding region. The chromosome part attached to that region is unique point from where synapsis starts. 4) When the distance between homoeologues (between attached sites) is less than the constant, Pd', such homoeologues can form synapsis at zygotene stage, otherwise they do not succeed in pairing. The other constant, Pd, is provided for the case of homologues. 5) The above three factors causing the peculiar pairing affect the chromosome arrangement with increasing rho and gamma. After setting adequate values in parameters (see Fig. 2), rho and gamma were increased by deltarho and deltagamma, respectively. In each increase step, chromosomes (attached sites) were scattered randomly 1000 times within the region, and the distances between homeologues or homologues were measured and compared with Pd' or Pd, respectively. Figure 2 shows a typical result from the simulation. In this figure, it is clearly found that the pairing frequency of homoeologues increased with rho and gamma. Figure 2 indicates also that the decrease of deltagamma/deltarho made homoeologous pairing occur at earlier increase step of rho. The increase of rho obviously resulted in the decrease of the pairing frequency of homologues. Hence, this result means that when deltagamma is sufficiently small compared with deltarho, homoeologous pairing can be induced even in the PMCs Showing higher degree of homologous pairing. This situation may correspond to the actual result from high temperature treatment. |