2. Translocations involving B chromosomes. Eight translocations between A and B chromosomes have been obtained from bearing pollen treated with Xrays. The A chromosome of six of these has been identified and the approximate position of breakage points determined, as follows:
|Translocation||A chromosome||B chromosome|
|T6-B||S (dividing nucleolar
*This is the junction of the euchromatic region and the large heterochromatic region.
All of these except T7a-B were tested for male and female transmission. The female transmission was quite normal but the male transmission was distinctly low. For example, a plant heterozygous for T2-B in which the translocation was marked by V4 and the normal chromosome by v4, when used as the male parent on homozygous v4, gave 80-V4 : 164 v4 F1 seedlings. There is considerable crossing over between V4 and the point of breakage so that the frequency with which the translocation is transmitted is less than the ratio indicates. Similar crosses with T4-B, in which the translocation was marked by Su and the normal chromosome 4 by su, when crossed on su gave 253 Su : 797 su. Since very little, if any, crossing over occurs between Su and the point of breakage the ratio of Su : su probably represents a close approximation of the frequency with which T4-B is transmitted.
Evidence that a heterozygous A-B translocation when used as the male parent produces hypo- and hyperploid F1 plants suggested that the low male transmission was a result of nondisjunction in the second microspore division. Hyperploid plants from T1-B, T2-B, T4-B, T7a-B, and T7b-B were identified cytologically and were found to contain the heterozygous translocation plus an extra translocation chromosome. Thus the extra chromosome must have resulted from non-disjunction either at meiosis or elsewhere. In every case the extra chromosome was the translocation chromosome which possessed the B chromosome centromere.
The production of hypoploids was demonstrated when plants heterozygous for an A-B translocation and carrying only dominant factors were crossed on plants carrying appropriate recessives. The data from this type of cross are given in the following table.
appearing in F1
|Su su × T4-B/normal, Su Su||52 su/423||25*|
|su × T4-B/normal, Su Su||31 su/92||34|
|02 gl × T7b-B/normal, O2 O2 Gl Gl||O o2/63||0|
|IJ ij Gl gl × T7b-B/normal, IJ IJ Gl Gl||6 ij gl/42||28*|
*These values have been corrected for the fact that the female parent was heterozygous rather than homozygous recessive.
The appearance of the recessive character in the F1 is due to the loss of the translocation chromosome bearing the factor for the corresponding dominant. Since Gl is nearer the end of the long arm of chromosome 7 than O2, the loss of Gl without the loss of O2 must mean that the absent chromosome is the one possessing the B chromosome centromere.
Proof that non-disjunction occurs at the second microspore division,was obtained from a cross using a hyperploid plant from T2-B as the male parent. Twenty-three F1 plants were examined cytologically. Of the twenty-three, twelve were hyperploid like the male parent; seven were euploid, heterozygous for the translocation; and four were euploid, homozygous normal. The occurrence of twelve hyperploid plants, which could have resulted only from non-disjunction, and the absence of other classes that would be expected with the same frequency from non-disjunction elsewhere show that non-disjunction occurs only at the second microspore division.
The frequency with which non-disjunction occurs may be roughly estimated from the data in the table demonstrating hypoploidy. The maximum frequency with which the recessive may appear is 25% (corresponding to 100% non-disjunction) if the hypoploid plants are viable (as they certainly are in the case of T7b-B and probably also in T4-B). The fact that the observed frequencies equal and exceed this value cannot be taken too seriously since these data were obtained from a limited series of crosses and may be effected by the presence of associated transmission factors. It is known from cytological evidence that the frequency of non-disjunction is not 100%. But the data do suggest a very high frequency and further experiments to determine this with accuracy in each of the A-B translocations are in progress.
Will non-disjunction account for the anomalous male transmission of the intact B chromosome? The combined data of Longley and Randolph, from a cross of a 1B male on a OB female, gave 108 plants with no B chromosomes, 35 with 1, 20 with 2, and 2 with 3 B chromosomes. We should expect, from 50% nondisjunction, 103 plants with no B's, 41 with 1, 21 with 2, and none with 3 B chromosomes. The observed 3 B chromosome plants may be accounted for in other ways. The close fit indicates that the mechanism for the aberrant male transmission of A-B translocations is identical with that of the intact B chromosome.
Can we localize the cause of non-disjunction within the B chromosome? The heterochromatic region may be excluded as a factor in non-disjunction for in T7b-B the chromosome undergoing non-disjunction does not contain this region. Furthermore, non-disjunction is not related merely to the shortness of the chromosome for in the case of Tl-B the translocation chromosome undergoing non-disjunction is longer than the normally behaving short A chromosomes. Consequently, the cause of non-disjunction is related to the position or the special nature of the B chromosome centromere or to some factor in the proximal portion of the euchromatic region of the chromosome.