3. Further studies on the behavior of the abnormal tenth chromosome. (See last News Letter)
Plants heterozygous for a normal chromosome 10 and an abnormal chromosome 10, differing from the normal in that it has a piece of chromatin attached to the distal end of the long arm as described by Longley (1937, 1938), give an unusual type of behavior for these two homologues. When used as the female parent the percentage of the basal megaspores receiving the abnormal chromosome 10 is approximately 67 percent instead of the expected 50 percent. The R locus was found to lie extremely close to the end of the short arm; there being one percent of recombination between R and the distal end of the short arm. This placing of R would mean that d7 does not lie beyond R as Singh's data indicated. Crossover studies in the g R region showed no reduction in plants heterozygous for the abnormal chromosome so it is likely that the low recombination value between R and the end is the true distance and is not due to a reduction from the true value caused by the presence of the redundant piece of chromatin. Earlier, it was suggested that competition among megaspores might account for the excess number of eggs carrying the abnormal chromosome 10, i.e. in a considerable number of cases non-basal megaspores with the abnormal chromosome would develop into the embryo sac at the expense of basal megaspores with normal chromosomes 10. Examination of 200 young embryo sacs showed, however, that the embryo sac always arose from the basal megaspore so the above hypothesis can be definitely ruled out. The alternative explanation is that selective segregation during the two meiotic divisions results in the abnormal chromosome passing to the basal megaspore more frequently than expected on a random basis. This explanation is being tentatively accepted. There is no sterility on the ears so abortion of ovules with the normal chromosome 10 does not account for the discrepancy. Since the R locus is so close to the end of the long arm it may be used to mark the normal and abnormal chromosomes thus making it possible to collect a large amount of data. When pollen from a plant heterozygous for the two chromosome types is used it is found that pollen carrying the abnormal chromosome is at a disadvantage when competing with grains carrying the normal chromosome. Using the R alleles to mark the two chromosomes it was found in one experiment that 59.7 of the functioning pollen grains carried the normal chromosome. Since comparable results were obtained when different normal chromosomes 10 were used against the abnormal chromosome 10 it may be argued that the redundant piece of chromatin is not wholly inert but possesses some genetically active material.
If selective segregation is the correct explanation of the unusual results obtained on the female side it is of some interest to give the following results. In the summer of 1939, 75.7 percent of the individuals in a population of 4,501 coming from female plants heterozygous for the two chromosome types carried the abnormal chromosome 10. A duplicate of the seed planted in 1939 was planted in 1940 but only 62.8 percent of the individuals in a population of 4,922 possessed the abnormal chromosome. Since the two lots of seed were identical it appears that environmental conditions influence the segregation of the heteromorphic bivalent. This behavior is similar to that found in certain insects where temperature differences determine whether the X or Y chromosome is extruded into the polar bodies.