VARIABILITY OF ACCESSORY CHROMOSOMES IN NATIVE POPULATIONS OF ALLIUM CERNUUM

GRUN, PAUL. (Penn. State U., University Park.) Variability of accessory chromosomes in native populations of Allium cernuum. Amer. Jour. Bot. 46(3): 218-224. Illus. 1959.-Small euchromatic accessory chromosomes, which lack visible constrictions, have become established in native populations of nodding onion, Allium cernuum Roth. Variant types, including large accessories, and a form having a visible metacentric constriction, also occur, although more locally restricted in distribution. The accessories show a strong tendency towards non-disjunction in mitotic divisions, resulting in a variability of number in different cells within and between root tips and in different microsporocytes of each plant. The metacentric chromosome undergoes a normal disjunction. Accessories are not paired at first metaphase of meiosis, and drift to poles at random during first anaphase, a few being lost as micronuclei. During second anaphase they go through the normal equational division. In spite of this mitotic and meiotic irregularity, plants of 11 of 14 separate populations collected in Pennsylvania and West Virginia contained accessories in up to a maximum of 50% of the individuals in the population. THE NODDING ONION, Allium cernuum Roth, is a species whose cells contain 14 somatic chromosomes plus a variable number of small accessory or "B" chromosomes (Levan, 1932). A preliminary study of these B chromosomes from populations native to Pennsylvania and West Virginia showed that they are present in variable numbers both within and between plants. These chromosomes are either telocentric or so nearly telocentric that no second arm could be seen. Since the centromere position as well as the variability of expression within a plant are both unusual characteristics to find in chromosomes established in nature, it was decided to carry out an analysis of the frequency of these chromosomes, their behavior in mitosis and meiosis, and their distribution in nature as a background to determination of their possible role in the evolution of the populations. MATERIALS AND METHODS.-Plants were dug from natural populations and raised in pots in a greenhouse at University Park, Pa. Root tips treated with oxyquinoline were smeared in propiono-orcein using the technique of Tjio and Levan (1950). Buds were fixed in a Carnoy solution of 4 parts chloroform: 3 absolute ethanol: 1 glacial acetic acid. Anthers of buds about 2 mm. long were smeared in propiono-carmine. Preparations were studied in temporary mounts and slides made permanent by the technique of 'Sparrow and Spar'Received for publication Sept. 29, 1958. Authorized for publication Sept. 8, 1958 'as paper No. 2290 in the Journal Series of the Pennsylvania Agricultural Experiment Station and Contribution No. 237 from the Department of Botany and Plant Pathology, The Pennsylvania State University. The author wishes to extend his sincere thanks to Drs. H. A. Wahl, P. Lilly, E. M. McNeill, and E. L. Core for their great help in the location and collection of plants. He also wishes to thank Drs. D. F. Mitchell, J. E. Wright, Jr., and H. L. Carnahan for their critical reading of the manuscript. Thanks are also due to Miss Dolores McHugh who made some of the chromosome determinations reported here. row (1949). A Bausch and Lomb DDE microscope with apochromatic oil immersion objective having a numerical aperture of 1.30 was used for all observations. The light source was a ribbon filament lamp set up for K6hler illumination. RESULTS.-The B chromosomes of this species are small and somewhat variable in appearance, though one typical or "standard" form was most frequently seen. The typical form differs from that described by Levan (1932) in that it has no visible constriction (fig. 1). It is extremely difficult to prove whether a given chromosome is telocentric or acrocentric, but it is the author's opinion, for the following reason that these are telocentric chromosomes. Treatment with oxyquinoline before smearing spreads chromosome arms apart. With the aid of this treatment, the regular constrictions of the normal chromosomes of the karyotype (A chromosomes) as well as that of a metacentric B chromosome found in one population, were made clearly evident (fig. 2). If the standard B chromosomes are not telocentric, either the centromere or the second arm must be so short as to be below the resolving power of the light microscope. The length of the different B chromosomes varies from about half that of those in fig. 1 to larger forms as much as twice the length of those illustrated. The abnormally large and small B chromosomes were found, however, only in a few populations. Accessories having a constriction (fig. 2) were found only in the cells of the Alexandria, Pa. population. None of the accessories showed evidence of having heterochromatin. Behavior during mitosis.-The regularity of mitotic division of the B chromosomes was determined by counting those present in a number of cells of different root tips and in different microsporocytes of each plant (table 1). In no cases where an adequate sample of cells has been examined have the standard B chromosomes been