The morphology of the pachytene chromosomes of Ricinus communis L. (2n = 20), the castor plant, was examined and compared, using light microscopy, in two ...
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PACHYTENE CHROMOSOME MORPHOLOGY WITH REFERENCE TO SEX INSTABILITY IN RICINUS COMMUNIS L. HARRY S . PARIS Division of Vegetable Crops, Agricultural Research Organization, Newe Ya'ar Experiment Station, P .O . Haifa, Israel Received 2 May 1980
INDEX WORDS
Ricinus communis L ., castor, chromosome morphology, sex instability . SUMMARY
The morphology of the pachytene chromosomes of Ricinus communis L . (2n = 20), the castor plant, was examined and compared, using light microscopy, in two sex forms, one having an apical distribution of pistillate flowers (wild type), the other having a uniform distribution of pistillate flowers (mutant) . Nonreverted mutants resulting from sex instability of the wild type did not display microscopically discernable alterations in chromosome morphology . INTRODUCTION
The ten pachytene bivalents of Ricinus communis L . are easily distinguishable from one another by their relative lengths and the size, shape, and distribution of their heterochromatic blocks . Chromosome lengths, as well as lengths and positions of heterochromatic blocks, segments composing these blocks, heterochromatic chromomeres, and primary, secondary, and tertiary constrictions have been quantified and depicted in an idiogram (PARIS et al ., 1978) . The idiogram thus serves as a tool for detecting cryptic differences in pachytene morphology (PARIS et al ., 1980) . In wild type castor, the pistillate flowers are restricted to the apical region of the inflorescence, and staminate flowers are borne in the basal region . However, in a number of populations and races, mutants have been observed in which the primary raceme exhibits a uniform distribution of pistillate flowers, from base to apex (SHIFRISS, 1956) . These mutants are valuable agriculturally for their concentrated yield, and with incorporation of genes conditioning the presence of interspersed staminate flowers, for hybrid seed production (SHIFRISS, 1960 ; ZIMMERMAN & SMITH, 1966) . Except for a rare recessive mutant (CLAASSEN & HOFFMAN, 1950), the mutants result from a form of genetic instability (SHIFRISS, 1956, 1960 ; JAKOB & ATSMON, 1965 ; SHIFRISS & GEORGE, 1966) whose mechanism is poorly understood . It has been hypothesized that instability brings about a chromosomal rearrangement, though this may be submicroscopic (SHIFRISS, 1960) . The objective of the present study was to determine if mutants occurring in genetically unstable, wild type material exhibit alterations in the morphology of the pachytene chromosomes . 1 93
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MATERIALS AND METHODS
Four plants of race A39 were studied . The details of the origin of this race were given elsewhere (PARIS et al ., 1980) . A39 is highly inbred . It is also highly unstable genetically with regard to sex expression ; in self-pollinated progenies of plants having apical distribution of pistillate flowers throughout life, the frequency of mutants having a uniform distribution of pistillate flowers in the primary raceme is 3 % or more . A39 also carries genes id, and id, for,presence of interspersed staminate flowers (GEORGE, 1966 ; GEORGE & SHIFRISS, 1967) . Two of the four plants had apical distribution of pistillate flowers throughout life . They were indistinguishable phenotypically, and had identical morphology of pachytene chromosomes . The idiogram of the castor chromosome complement was constructed using these same two plants . The other two plants, designated 39-104-1 and 39-104-3, had uniform distribution of pistillate flowers throughout life (i .e . they were non-reverted) and were indistinguishable phenotypically . They occurred spontaneously in a self-pollinated progeny of A39 (0 . Shifriss, personal communication) . Pachytene analysis of microsporocytes was possible due to the presence of interspersed staminate flowers . Techniques of fixation and enzyme treatment of staminate flower buds, staining of chromosomes, light microscopy, phase contrast optics, and photomicrography were the same as described earlier (PARIS et al ., 1978) . OBSERVATIONS AND DISCUSSION
The pachytene complement in a representative pollen mothercell of the uniform plants is presented in Fig . 1 . Many of the chromosome landmarks, i .e . heterochromatic segments, chromomeres, and primary, secondary, and tertiary constrictions, are discernable in this cell . The chromosomes of both uniform plants were observed in numerous such cells . No differences in pachytene morphology were detected between the two plants . For comparing apical and uniform, the procedure was to first use the idiogram as a standard for comparison with chromosome morphology in the uniform plants . Each time a particular chromosome landmark defined in the idiogram was observed in the uniform plants, it was scored . Each landmark was scored at least twelve times . Chromosomes of the uniform plants were then used as a standard of comparison against those of the apical plants, as follows . Chromosomal sites at which chromomeres and constrictions appeared that were not depicted in the idiogram were scored . These chromomeres and constrictions invariably were minute, and appeared only infrequently . Sites scored repeatedly were subsequently examined in chromosomes of the apical plants . In all cases, these minute chromomeres and constrictions were occasionally found at the same chromosomal sites in the apical plants . It is therefore concluded that pachytene chromosome morphology in the apical and uniform plants was identical . It should be mentioned that sex-reversals, i .e . plants which are uniform early in life but which partially or completely revert to apical later, were not examined cytologically . However, plants 39-104-1 and 39-104-3 represent a more extreme manifestation of sex instability, having been non-reverted throughout life, even after producing well over 100 racemes (see SHIFRISS, 1960) . 1 94
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SEX INSTABILITY IN CASTOR
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Fig . 1 . Photomicrograph and interpretive drawing of the pachytene chromosomes in a microsporocyte of uniform mutant 39-104-3 (x 2300) .
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From the observations reported here, the possibility that sex instability of castor involves chromosomal anomalies of some sort cannot be excluded . However, it has been shown that the most extreme end products of instability in the wild type, the nonreverted mutants, do not differ from the wild type in chromosome morphology, as discernable at the level of resolution of the light microscope and with current techniques . ACKNOWLEDGEMENTS
This work was conducted while the author was a graduate student in the Department of Horticulture and Forestry, Cook College, Rutgers - The State University, New Brunswick, New Jersey, USA . The author expresses his thanks to Dr Oved Shifriss for provision of the plant material and his encouragement and to Dr Gojko Jelenkovic for provision of laboratory facilities . REFERENCES
CLAASSEN, C . E . & A . HOFFMAN, 1950 . The inheritance of the pistillate character in castors and its possible utilization in the production of commercial hybrid seed . Agron . J. 42 : 79-82 . GEORGE, W . L. JR ., 1966 . Genetics of interspersed staminate flowers in Ricinus communis L . Ph .D . Thesis, Rutgers - The State University . GEORGE, W . L. JR . & 0 . SHIFRISS, 1967 . Interspersed sexuality in Ricinus. Genetics 57 : 347-356 . JAKOB, K. M . & D . ATSMON, 1965. Sex inheritance in Ricinus communis L . : Evidence for a genetic change during the ontogeny of female sex reversals . Genetica 36 : 253-259. PARIS, H . S ., O . SHIFRISS & G . JELENKOVIC, 1978 . Idiogram of Ricinus communis L . J . Hered . 69 : 191-196 . PARIS, H . S ., O . SHIFRISS & G . JELENKOVIC, 1980 . Nucleolar organizing chromosomes of Ricinus. Theor . Appl. Genet . 57 : 145-152 . SHIFRISS, 0 ., 1956 . Sex instability in Ricinus . Genetics 41 : 265-280 . SHIFRISS, 0 ., 1960. Conventional and unconventional systems controlling sex variations in Ricinus. J. Genet. 57 : 361-388. SHIFRISS, 0 . & W . L . GEORGE JR ., 1966 . Effect of nutrition on incidence of females in Ricinus communis. Bot. Gaz . 127 : 242-245 . ZIMMERMAN, L . H . & J . D. SMITH, 1966 . Production of F, seed in castorbeans by use of sex genes sensitive to environment . Crop Sci . 6 : 406-409 .
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