Six cultivars and five plant introductions of Glycine rnax L. Merr. varied in picograms of DNA per cell from B .$4 to 2.61, and in number of ribosomal-RNA genes per 2 C nucleus from 968 to 243 1 (Laboratory 1, Ames) and from 768 to 2095 (Laboratory 2, Rhinelander). The parameters did not correlate with seed protein content, which varied from 32.8% to 52.6%. With these materials, use of the rRNA locus size as a breeding tool for increasing percentage protein content by use of cytogenetic and genetic manipulations is not warranted.
SUMMARYThe interspecific hybrid between Crepis capillaris and C. dioscorides exhibits suppression of the secondary constriction in the chromosome derived from C. dioscorides; each species normally has one chromosome pair with secondary constrictions. Such structures are known to be associated with the formation of nucleoli and are the probable sites ofribosomal-DNA (rDNA). The two species and the hybrid were studied with respect to DNA and rDNA variation to determine whether reduction of the highly redundant rDNA might occur as a consequence of nucleolar organiser suppression.The DNA content of C. dioscorides was three times greater than of C. capillaris. This is consistent with trends in the genus Crepis. Evolutionary advancement is correlated with a reduction of both chromosome number (C. dioscorides, 2n = 8; C. capillaris, 2n = 6) and chromosome size. The chromosomes of the more advanced species, C. capillaris, are smaller than those of C. dioscorides. No alterations of the parental contributions of rDNA were observed in the hybrid, however; the two species and the hybrid each contained about 5000 rRNA genes. It was concluded that nucleolar organiser suppression must be at the transcription level.
Dotted (Dt) is the regulatory element of a two-unit controlling system in maize. Dt causes the inherited change from the recessive a 1 (colorless) to its dominant allele, A 1 (anthocyanin production), during the development of the stalk, leaves, and endosperm. The mutation events are observed as sectors of color in an anthocyaninless background.One of the most puzzling, but perhaps significant, aspects of controlling elements in maize is that they originate in conjunction with chromosome or chromatid breaks. This fact invokes a requirement that either an existing regulatory mechanism is disturbed by the breakage or that a foreign element is incorporated before fusion of the broken chromatids.Experimental crosses were made between Dt tester stocks and a pollen parent, a large proportion of whose chromosomes 9 were undergoing the chromatid type of bridge-breakage-fusion cycle. New Dt's were induced in endosperm sectors of 250 of 154,422 kernels tested (1/600); among these, two germinal Dt's (Dt (crown) 4 and Dt 5) were recovered, presumably due to chromatid breaks during meiosis or the first microspore division. Dt 5 produces a mutation pattern very similar to the original Dt 1 and is located 0.33 crossover units away from the yg 2 locus. This is close to the known location of Dt 1 (7 crossover units distal to the yg 2 locus) and is suggestive of a specific site for Dt inductive breaks. Dt (crown) 4, on the other hand, is inherited independently of the yg 2 locus and does not support this contention. Dt (crown) 4 represents a new "state" causing a high concentration of fine dots in the crown of the kernel, with little or no dotting at the base.The phase variation of Dt (crown) 4 is discussed together with the tissue-dependent expression of Dt (in-ac) 1 (Dotted, inactive-active). Dt (in-ac) 1 is a new "state" of Dt 1 and shows inactive (no a 1 to A 1 mutations) and active (a 1 to A 1 mutations) phases in the endosperm, whereas it is only in the active phase in the diploid scutellum. The observed phase variation was shown to be a property of the regulatory elements, Dt, responding to differences in the cellular environment.
Dotted (Dt) is the regulatory element of a two-unit controlling system in maize. Dt causes the inherited change from the recessive a (colorless) to its dominant allele, A (anthocyanin production), during the development of the stalk, leaves, and endosperm. The mutation events are observed as sectors of color in an anthocyaninless background.Since its discovery over 40 years ago, Dt has always been found in the terminal knob of the short arm of chromosome 9. This is puzzling because controlling and regulatory elements in general are not located permanently, but change positions (transpose) within the chromosomal complement. To resolve this seeming discrepancy, transpositions were looked for in a homozygous a Dt stock. Because the frequency of aleurone mutations is exponentially related to Dt dosage, a Dt transposition would result in a greatly increased number of dots if the egg or sperm nucleus contained both the transposed Dt and the Dt remaining on chromosome 9. A total of 6 transposed Dt's (Dt-T) were recovered in this manner. Dt-TA was found linked to the gene Y ("yellow endosperm") of chromosome 6. Dt-TB no longer showed linkage with yg2 of chromosome 9, but remains unlocated (the original Dt in this stock is separated from yg2 by 6 or 7 cross-over units.). The remaining transpositions (C-F) assorted independently of Dt on chromosome 9.The transposed Dt's had the same effect as Dt on the frequency and timing of aleurone mutations. An increase in transposition frequency and losses of Dt-T's was characteristic of several of the transposed Dt's. Dt-T's B-F transposed so frequently that testcross ratios of 7∶1 (three Dt' s) and 15∶ 1 (four Dt' s) were observed. No secondary transpositions or losses of Dt-TA were detected. Thus, Dt-TA resembles the original Dt with regard to its transposition frequency and stability.
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