PRODUCTION OF VARIABLE ANEUPLOID NUMBERS OF CHROMOSOMES WITHIN THE ROOT TIPS OF PAPHIOPEDILUM WARDII

THE CHROMOSOME number has been found to differ from cell to cell within the root tips of Paphiopedilum Wardii Summerhayes, one of the mottledleaved Burmese lady slipper orchids. Lack of constancy as regards somatic chromosome number within an individual is not unknown in plants. An occasional cell, for example, or a number of scattered cells, or a group of associated cells within a root may be tetraploid or octaploid. Langlet's (1927) term "polysomaty" is commonly applied in these cases. There are, however, species or varieties in which the somatic complement of chromosomes fluctuates in the vicinity of the diploid number with no evidence of duplication of the entire set. Apparently these variations have diverse histories and are to be explained differently. The chromosome or chromosomes contributing to variability may be supernumerary to the normal complement and much smaller than its members. The B chromosomes of maize are an example (Kuwada, 1919; Fisk, 1927; Longley, 1927) as are the-centric fragments surviving from the last previous meioses referred to by Darlington (1937). The variability, on the other hand, may involve replication of members of the complement. There has been little observation on mitoses leading to varying numbers other than through polysomaty. Since the mechanism by which the change in number is brought about can be followed with considerable ease in P. Wardii, a -brief investigation was made. The process involves only certain members of the chromosome set and leads to cells polysomic, in Blakeslee's terminology, for those members of the complement. The causal mechanism bears close resemblance to that of polysomaty. These facts lead to the adoption of the word "aneusomaty"2 as a name for the phenomenon. Root tips were fixed in Randolph's modification of Navashin's fluid, imbedded in paraffin, sectioned, and stained with crystal violet-iodine. Counts of chromosomes were made in serial sections from the root tip through the embryonic region. Although the plants supplying the root tips came from two different sources, they may be members of a single clone since the species is rare in cultivation and is propagated vegetatively. The idiogram (fig. 6) shows the complement to contain seven pairs of atelomitic chromosomes of graduated lengths, two shorter pairs having about the same length. Each member of one pair of chromosomes has a secondary constriction which delimits a region similar in size to the smallest chromosomal type of the complement. The remainder of the chromosomes are rod-shaped; they vary 1 Received for publication March 19, 1945. 2 The author wishes to express his appreciation for Prof. C. E. Allen's advice in selecting an appropriate name. from 27 to 31 in number according to the present observations. Their lengths are graduated so that the longer and shorter rod-shaped chromosomes can be distinguished easily. Some of the types of intermediate length have morphological peculiarities which are sufficient to distinguish them but this group is generally difficult to assort. The rod-shaped chromosomes are not all strictly telomitic; the locations of their centric regions vary from apparently terminal to sufficiently subterminal so that an extremely short second arm is visible. The somatic number as determined in equatorial plates ranges from 41 to 45 (fig. 3 and 2, respectively). The shortest type of chromosome (no. 20) is almost spherical; it possesses two horn-like proj ections, one to a chromatid, at the end anterior in polar congression. Because of its shape and the fact that it is from one-half to two-thirds the length of the next shortest chromosome, it is recognized easily. Two to six (fig. I and 5) chromosomes of this type are present in various plates; they may occur in several kinds of associations lying on the periphery of the spindle or they may be scattered throughout the plate. The latter condition is more frequent. The total number of chromosomes, omitting those of type 20, varies from 38 to 40 (fig. 4 and 1). Two additional types of chromosomes have different frequencies of occurrence in equatorial plates. One of these (no. 19) likewise belongs to the class of smaller chromosomes. It has a swollen midregion and tapers toward each end, the end anterior in polar migration sometimes being stretched. It is almost as easily distinguishable as the shortest chromosomal type. Two, three, or four (fig. 4, 2, and 1) may be present in somatic plates. The other variant (no. 10) is one of the medium-sized chromosomal types, being roughly three times as long as type 20. Like chromosomes of type 20 and 19, it has a single projection or two small projections, depending upon the view and state of division, at its forward end. Two or three of these (fig. 5 and 3) may be present in an equatorial plate. Table 1 presents an analysis of the equatorial plates shown in figures 1-5. The variable number of occurrences of three chromosomal types allows similar total number of chromosomes to be reached by different means. The data in table 1 dealing with figures 2 and 5 demonstrate this point. Chromosomes of type 20 are the most variable in numbers. So frequent are misdivisions of chromosomes of this type that some stages in their progress have been observed. The apparent explanation is that the two daughter chromatids of a type 20 chromosome may dissociate at the end opposite the centric region before or during the early stages