The frequency of crossing over per unit of physical length of the pachytene chromosome is not the same in all parts of the maize genome. In 1968, we studied a transposition involving the transfer of a segment consisting of about 10% of the long arm of chromosome 3 to a new location in the short arm of chromosome 9 (Rhoades, in Replication and Recombination of Genetic Material, Canberra, 1968). This segment of chromatin undergoes little if any recombination either in its original location or in its new position in chromosome 9. Comparisons of Sh Wx recombination in plants homozygous for the transposition with that in plants having only normal chromosomes 9 showed no differences even though the physical distance between the Sh and Wx marker genes was considerably increased in the transposition homozygotes by the inserted chromatin. Similarly, crossing over between the Lg2 and A markers spanning the segment in question in normal chromosome 3 was not affected in plants homozygous for deficient chromosomes 3, in spite of a decrease in the cytological length. We concluded that "no exchanges occur in the transposed segment when it is a part of chromosome 3 or when placed in the Tp9 chromosome."
The differential segment of K10-I appears to be another example of chromatin which has a low rate of crossing over. Df(C), derived from K10-I by loss of the terminal knob and the distal euchromatic segment with the L 0 W markers, possesses the differential segment of K10-I. In Df(C)/N10 sporocytes, the 2 chromosomes 10 are of equal length. In N10/N10 plants the distance from R to the most distal marker, Sr2, is about 35 map units and a comparable genetic length might be anticipated for the region from R to the end of Df(C) when Df(C) is combined with a homologous Df10 chromosome, such as Df(H). This segment can be measured in the absence of a distal marker gene because Df(C) is not male transmissible. Therefore, when Df(C) R/Df(H) r plants are used as male parents in testcrosses, the percentage of R progeny measures the recombination from R to the end of Df(C). Df(H) is longer than Df(C) since it possesses the euchromatic region with the L 0 W markers in addition to the differential segment but in all other respects it is completely homologous to Df(C). Instead of the 35% R Sr2 observed in N10/N10 plants, the frequency of recombination in Df(C)/Df(H) heterozygotes was 8.9%, of which about 3-4% can be attributed to the region from R to the start of the differential segment. Thus, recombination in the differential segment itself must be very low.
The absence of recombination potential in the 2 segments of chromosome 3 and chromosome 10 described above may result from the lack of Chi sites in the DNA or may be due to sequestering of these sites so they are not available to recombination enzymes. In the case of the transposed segment of chromosome 3, addition of B chromosomes to the complement caused at least a twofold increase in recombination in the C Wx region of chromosome 9. Perhaps the Chi sites in the transposed segment were exposed by some activity of B chromosomes. We have not yet tested the response of the differential segment to B chromosomes.
M.M. Rhoades and Ellen Dempsey
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