Cytological aberrations in relation to wheat improvement

Cytological irregularities were found to be occurring in all three of the varieties studied and no plants failed to show at least a small percentage of cytological aberrations during maturation of their pollen mother cells. As measured by cytological aberrations, Marquillo was found to be more unstable than either Marquis or Minn. No. 2303, whereas Marquis and Minn. No. 2303 were about equal in this respect, although Minn. No. 2303 gave a lower percentage of non-conjunction. Since Minn. No. 2303 is the result of an attempt to combine the desirable characters of the durum and vulgare groups of wheat and therefore shows, as far as practical purposes are concerned, that a germinally stable variety can be obtained from crosses involving these two species, it is of special interest to the agronomist and plant breeder. The correlations found to exist between the different aberrations strongly indicate that the occurrence of micronuclei may be taken as a criterion of the frequency of occurrence of non-orientation and non-conjunction. If such proved to be the case, the work and time involved in determining the germinal instability of a variety or cross may be greatly reduced, and, consequently, the number of plants that can be studied materially increased. It is comparatively easy to find the tetrad stage of microsporogenesis at which time the percentage of cells showing micronuclei are determined, but it is rather difficult to find the proper stage for determining the percentage of pollen mother cells that exhibit non-orientation and non-conjunction. The cytological aberrations in the parent plants of Marquillo were found to be positively associated with variability of some characters of the progeny. These aberrations were also found to be negatively associated with the means of the progeny of the Marquillo plants studied cytologically. The latter relationship was found to be more marked for Marquis than for Marquillo. No significant association could be demonstrated between either the different cytological aberrations in Minn. No. 2303 or the cytological aberrations and the variability or means of the characters of the progeny. These results with Minn. No. 2303 may be accounted for by assuming that it is essentially a homogeneous population so far as cytological aberrations are concerned. This assumption is not without some foundation as Minn. No. 2303 is not as far removed from an individual plant selection as either Marquillo or Marquis. The results with Marquillo and Marquis are explainable if these two varieties are composed of a heterogeneous population as far as cytological aberrations are concerned. In this case, selection within either Marquillo or Marquis would be expected to produce a strain with a lower percentage of cytological aberrations than the parents.