In higher plants, tandemly repeated DNA sequences appear as distinct, heteropycnotic regions located at certain sites on specific chromosomes and look much like beads on a string. They are called knobs by maize cytogeneticists. The genetic effects of knobs in maize include: (1) increased recombination (McClintock, CIW Yearbook 42:148-152, 1943; Rhoades and Dempsey, Genetics 53:989-1020, 1967); (2) neocentromere activity (Rhoades and Vilkomerson, PNAS 28:433-436, 1942); (3) preferential segregation (Longley, Genetics 30:100-113, 1945; Dempsey and Rhoades, MNL 44:56-61, 1970); (4) chromosome breakage and chromatin loss (Rhoades et al., PNAS 57:1626-1632,1967; Rhoades and Dempsey, MNL 46:48-51, 1972); (5) sex differences in recombination (Rhoades, J. Amer. Soc. Agron. 33:603-615, 1941; Phillips, MNL 45:123-125, 1971).
In wild plants, knobs are typically telomeric; whereas, in cultivated species, they move to internal positions on chromosomes. In maize and annual teosinte, approximately 23 intercalary knob positions have been identified. In diploid perennial teosinte, Zea diploperennis, and Guatemalan teosinte, the knobs are telomeric. A cytological study of knobs in Fl progeny of crosses using maize as the pollen parent and diploperennis as the female parent demonstrated that the number of terminal knob sites is less than would be expected in Mendelian inheritance, and the number of intercalary knobs is above that expected to be inherited from the male maize parent (Eubanks, MNL 60:103, 1986). The phenomenon of knob transposition and chromosome reorganization as a result of interspecific hybridization is suggested by these data. It contrasts with the basic assumption in maize cytogenetics that knobs, like mutant genes, are stable, heritable characters. Evidence from a recent study of crosses between diploperennis and annual teosinte indicates that the number of knobs in the F1 progeny is dependent upon the direction of the cross.
Annual teosinte plant material for the experiment was grown from seed provided by the Southern Regional Plant Introduction Station, Experiment, Georgia (Zea mexicana, P.I. # 331779, origin: Miraflores, Mexico), and diploperennis plants were grown from seed provided by Professor Hugh Iltis at the University of Wisconsin-Madison (Zea diploperennis, Guzman # 777, origin: Jalisco, Mexico). The first phase of the research was carried out at Indiana University in Bloomington in 1985, and the second phase at Vanderbilt University in Nashville in 1986. The cross was made both ways with each species serving as male and female parent. Sporocytes from individual parent plants and their F1 progeny were collected and examined cytologically. The number of knobs observed at pachytene was recorded (Table 1).
The diploperennis parent had 6 small terminal knobs. The annual teosinte parent had 4 intercalary knobs. If knobs are inherited in accordance with Mendelian genetics, the number of knobs in the F1 progeny should range from a minimum number of 4 to a maximum number of 10. Interestingly, however, when annual teosinte was the female parent, only 3 knobs were observed on the chromosomes of the F1 progeny. This is 1 knob less than would be expected of the minimum number. These plants had a 25% germination rate, were weak, spindly and male sterile. When annual teosinte was the male parent, 14 knobs were observed, 4 above the predicted maximum number. These plants had a 75% germination rate, were vigorous, highly tillered and male fertile.
These results raise three questions. I. Do the knob data reveal an amplification of repetitive DNA in some crosses and a loss of knob heterochromatin in other crosses when the sexes of the parent plants are reversed? II. Is there transposition of knob heterochromatin in progeny of interspecific crosses? III. Is knob satellite DNA genetically inert or is it transcribed and does it play a role in gene regulation? More work on the inheritance and expression of knob heterochromatin is needed to gain a better understanding of its genetic functions and evolutionary role in the origin of maize.
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