THE AMERICAN SPECIES of Paeonia, P. calilornica and P. brownii, show in natural populations a high degree of meiotic irregularity. They have evolved a cytogenetic system of which the most prominent feature is permanent translocation heterozygosity, comparable to that of Oenothera (Stebbins and Ellerton, 1939; J. Walters, 1942). They show in addition a great deal of inversion hybridity, revealed by the familiar bridge-and-fragment configurations of heterozygous paracentric inversions, and by the presence of heteromorphic chromosome pairs which have been interpreted as the result of heterozygosity for pericentric inversions (J . Walters, 1952). In the course of the previous studies of structural hybridity, occasional cells were noted which showed fragmentation in meiosis occurring in various degrees. A range of breakage has been observed from cells with a single fragment to cells in which not a single intact chromosome is present. As spontaneous fragmentation in meiosis is not common, and has not previously been reported to occur in a structural hybrid, a study was undertaken to determine the extent to which fragmentation occurs in P. calijornica, and whether any relationship can be shown with the other kinds of meiotic irregularity. The results of this study form the subject of the present paper.MATERIALS AND METHODS.-Flower buds of P. calilornica were collected in the field, fixed in Bradley's (1948) fixative, and stored in the fixative under refrigeration. The studies and photomicrographs were made from temporary and permanent iron-acetocarmine smears. In order to investigate the relationship between the extent of translocation heterozygosity and that of breakage, the plants selected for study included four pairing types: the translocation homozygote, with 5[]; an intermediate translocation heterozygote, with 06+2[]2; and two types of extreme heterozygote, with 010 and 06 +04 respectively. In addition to the material collected in the wild, fixations were made from two plants that had been transplanted several years earlier into the Santa Barbara Botanic Garden, to observe possible effects on breakage of growing a plant in other than its native habitat. In five plants, four stages of meiosis (metaphase and anaphase of the first and second divisions) were studied, to 1 Received for publication October 25, 1955. This work has been supported by grants from the Committee on Research of the University of California, Santa Barbara College. Much of the research was done in the laboratory of the Santa Barbara Botanic Garden. The author acknowledges with thanks the support of these institutions, and wishes also to thank Dr. Marta S. Walters for helpful suggestions and for critical reading of the manuscript.2 The symbol 0 means "ring of." determine the frequency and behavior of fragments and related phenomena throughout the course of meiosis. In an additional 11 plants, only anaphase I was studied. One hundred cells of each stage studied in each plant were selected at random. TYPES AND FREQUENCIES OF FRAGMENTs.-Meiotic fr...
