This contribution is a progress report of our cytogenetic studies on the eleven Df K10 chromosomes described in our article in the 1980 Maize News Letter. Five of the eleven are simple terminal deficiencies; the remaining six have a translocated chromosome involving abnormal 10 and a heterologous chromosome. All eleven are deficient for the Sr2 locus since loss of the dominant allele was used in screening for the deficient K10 chromosomes and all had lost the K10 knob. Of the five simple terminal deficiencies, two, Df K10(H) and Df K10(K), had the W2 locus while the remaining three, (C), (F), and (I), were deficient for both W2 and Sr2. The cytological length at pachynema of the two which were not deficient for the W2 locus was greater than that of the three deficient for W2 and Sr2. The (F) deficiency was not as extensive as (C) or (I). There was no transmission through the pollen of the (C) and (I) deficiencies while the (F), (H), and (K) deficiencies were transmitted although with reduced rates; as expected, the pollen transmission of (F) was lower than that of (H) or (K) since it was a longer deficiency.
Arm 10S and the portion of 10L from the centromere to a position just distal to the R locus are structurally similar in normal 10 and abnormal 10. The euchromatic distal 1/5 of the long arm of N10 to the right of R carries the W2, L13, and Sr2 loci with Sr2 closest to the tip of 10L. The portion of 10L in abnormal 10 corresponding in position to the distal 1/5 of N10 comprises what we have called the differential segment. It has three small knobs, the terminal one lying opposite the telomere of N10 in K10/N10 microsporocytes. No known genes have been assigned to the differential segment. Further, in K10/N10 heterozygotes there are no exchanges within the limits of the differential segment. In abnormal 10, adjacent to the right end of the differential segment (marked by the third of the small knobs) lies a euchromatic stretch of chromatin whose length approximates that of the differential segment. This euchromatic segment, extending beyond the tip of the N10 in K10/N10 pachytene bivalents, is homologous to the distal 1/5 of the long arm of N10. We have shown that it possesses the W2 and Sr2 loci and that their linear order in the transposed segment with respect to the R locus is unchanged. The genetic determiners in K10 responsible for such unique phenomena as preferential segregation and neocentromere formation have been shown by J. Miles and by M. H. Emmerling to be localized in the large heterochromatic knob, which is next in linear sequence along the long arm of K10.
Recombination studies with the Df K10 chromosomes provided some interesting data. G. Y. Kikudome (Genetics, 1959) showed that in K10/N10 compounds the R Sr2 recombination was reduced from the normal frequency of 35% to 1-2% and all exchanges occurred in the segment between the R locus and the first of the three small knobs in the differential segment. Low R Sr2 values were also found in our Df K10/N10 heterozygotes. Although we attributed the great reduction in crossing over distal to R in K10/N10 heterozygotes to genetic dissimilarity of the differential segment with its three small knobs, an alternative explanation ascribes the reduction in crossing over to the large heterozygous heterochromatic K10 knob. We know from Kikudome's work that crossing over in the terminal segments of 9S is reduced in K9/k9 plants. A similar situation for chromosome 10 might obtain in K10/N10 sporocytes although we considered this eventuality unlikely since the K10 knob is known to enhance crossover values, particularly in structural heterozygotes. Discrimination between the two explanations came from crossing Df K10(F) R -/N10 r W2 heterozygotes with r W2/r w2 pollen parents. The Df K10(F) chromosome lacks the K10 knob as well as the W2 and Sr2 loci. The expected classes from noncrossover and crossover gametes in the female parent are given below:
Df K10(F) R - X
N10 r W2 r W2
|Female gametes||r W2||r w2|
|nco R -||colored aleurone green seedlings||R-r spotted aleurone white seedlings|
|nco r W2||colorless aleurone green seedlings||colorless aleurone green seedlings|
|co R W2||colored aleurone green seedlings||colored aleurone green seedlings|
|co r -||colorless aleurone green seedlings||colorless aleurone white seedlings|
As shown in the Punnett square, the expected proportion of colored:R-r spotted: colorless kernels is 1:1:2 from nonrecombinant gametes. Recombination distal to R gives colored kernels producing green seedlings which cannot be distinguished from noncrossovers. Recombination distal to R does not lead to a corresponding increase in the R-r spotted aleurone-white seedling class. The excess of the colored aleurone-green seedling class over the R-r spotted aleurone-white seedling class reflects the amount of recombination distal to R but deviations from expected ratios give an unreliable estimate of recombination. A much better determination of crossing over distal to R comes from a consideration of the kernels with colorless aleurone. With no crossing over distal to R, all colorless kernels produce green seedlings. White seedlings from colorless kernels arise only by crossing over. However, the percentage of recombination distal to R calculated from the relative proportion of green and white seedlings from colorless kernels is only one half the true value since in half of the cases a crossover gamete would be fertilized by a r W2 sperm and produce a green seedling. When 1936 colorless (r) kernels from the above cross were planted, 1862 green and 13 white seedlings (0.69%) were obtained. Twice this value gives 1.38% recombination distal to R in Df K10(F)/N10 heterozygotes, a percentage very close to that found in K10/N10 plants. Clearly, the low frequency of recombination distal to R cannot be ascribed to the K10 knob since comparable values were found when the K10 knob was present and when it was absent. The marked reduction must be caused by genic differences in the differential segment.
There are no mutant alleles distal to R in the K10 chromosome and this would ordinarily mean that crossover studies distal to R in K1O/Df K10 compounds could not be carried out. However, all of our Df K10 chromosomes lack the Sr2 locus and since hemizygous Df/sr2 plants express the recessive striate phenotype, the deficiency for Sr2 can be used to genetically mark the terminus of the Df K10 chromosome. Df K10(C) pollen cannot compete successfully against N10 or K10 pollen. In K10 r Sr2/Df K10(C) R df plants, diagrammed below, the percent of R kernels found in pollen testcrosses is a measure of the amount of crossing over distal to R.
All single exchanges in this structurally homozygous region transfer the R allele to a pollen-transmissible chromosome. The frequency of R kernels was seven percent. The R-sr2 recombination value also measures the crossover frequency between R and the tip of Df K10(C). The two values proved to be identical. Approximately two percent of the observed seven percent should be ascribed to exchanges between R and the differential segment. We have no estimate of the frequency of crossing over in the short region distal to the differential segment. Even if it be zero, there would be only five percent recombination within the homozygous differential segment which is equal in length at pachynema to the distal fifth of the long arm of N10 where there is 35-36% recombination in N10/N10 sporocytes. It may be concluded that crossing over per unit length of chromatin is low in the differential segment. Indeed, our studies with K10 deficiencies (H) and (K) suggest that this region may be devoid of exchanges.
In testcross data from plants heterozygous for K10 r Sr2 and either Df K10(H) or Df K10(K) R -, there was 16% recombination between R and the end of the Df K10 chromosome. This interval includes the homozygous differential segment and the greater part of the adjacent euchromatin, which is homologous in gene content and linear order to the distal fifth of the long arm of N10. However, less than half as much crossing over took place in this stretch of chromatin as in the R-sr2 interval of N10 which is physically approximately only one half as long. We have interpreted these data as indicating that exchanges in K10/Df K10 pairs take place only between R and the first of the three small knobs of the differential segment and in the euchromatin to the right. We suggest that there is no or little recombination in the differential segment in either structurally homozygous or heterozygous bivalents.
M. M. Rhoades and Ellen Dempsey
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