2. Monosomic Maize. A plant monosomic for one of the shorter chromosomes (undetermined) appeared as a parthenogenetic diploid in a tetraploid stock of maize. A detailed study of meiosis with special reference to the behavior of the univalent was made. The univalent in fifty percent of the 770 cases observed was found to go to one of the poles in division I. In the remaining cases the univalent was not included in the daughter nuclei of division but remained in the cytoplasm forming a micro-nucleus. Most (74%) of these free dyad univalents were apparently reincorporated into the spindle of division II. This was indicated by a marked reduction in number of free dyad groups in metaphase II as compared with the frequency of micronuclei at interkinesis. Such cells produce microspores with a normal chromosome complement. In cases where the dyad univalent fails to be reincorporated in the spindle of division II, it often forms an independent spindle and divides. As a result of this, microspores containing a micronueleus in addition to the macronucleus are formed (in 1.9% of the microspores). The univalent was observed to divide in 10% of the first division figures. The resultant chromatids do not divide again in the following division but lag or move to one of the poles giving a 10-9 distribution in anaphase II. Pollen examination shows that 54% of the grains are abortive, due presumably to lack of a full chromosome complement. Selfing of the monosomic plant resulted only in diploid progeny and the same result was obtained when it was used as a pollen parent with normal diploid plants.
Harold E. Fischer and John Einset