A STUDY OF MEIOSIS IN HAPLOPAPPUS GRACILIS (COMPOSITAE)

JACKSON, R. C. (U. Kansas, Lawrence.) A study of meiosis in Haplopappus gracilis (Com. positae). Amer. Jour. Bot. 46(7): 550-554. Illus. 1959.-A study of meiosis in the two-paired Haplopappus gracilis has shown that each pair of chromosomes is easily recognized throughout the various stages of meiosis beginning with pachytene. A comparison of mitotic and meiotic chromosomes has been made, and the data indicate that a conspicuous heteropycnotic knob on chromosome B at pachytene is the nucleolar organizer. Other recognizable chromomeres were observed, but additional study is needed before a cytological map can be drawn to show whether regions other than those described on chromosome A and B may be used as markers. A BRIEF REPORT of the chromosome number and morphology of Haplopappus gracilis (Nutt.) Gray was presented earlier (Jackson, 1957a). The haploid chromosome number of 2 found in this species is the lowest reported for flowering plants. Haploid numbers of 4, 6, and 8 occur in species morphologically related to H. gracilis (Jackson, 1957a,b). Preliminary studies of artificial hybrids between the annual H. gracilis and a perennial species indicate that the low number in H. gracilis probably has been derived from a higher number by an aneuploid reduction process. Thus the situation in Haplopappus may parallel that reported for Crepis by Babcock (1947). In addition to the cytogenetic studies being carried on in H. gracilis by the author, investigators in the fields of biochemistry and radiation biology are using this species as a research tool. It thus becomes important to document the cytological phenomena of this entity for use as a reference in additional work. The low chromosome number in H. gracilis makes possible the clear observation of each pair of chromosomes in the various stages of meiosis beginning with pachytene. MATERIALS AND METHODS.-Immature heads for cytological study were taken from plants growing in the greenhouse. The fresh heads were cut in half and fixed for 48 hr. in a mixture of 3 parts of absolute ethyl alcohol and 2 parts of propionic acid. Material that was to be used for study of early prophase was then placed in 45% propionic acid with sufficient ferric acetate added to mordant the chromosomes. After 24 hr., the disc flowers were dissected out, and their disc corollas con1 Received for publication February 27, 1959. This study has been supported by a grant from the National Science Foundation. taining the anthers were macerated in iron-propionocarmine stain. Material to be used for the study of later stages was fixed as above and then stained according to the Feulgen technique outlined by Johansen (1940). Microsporocytes were used for study of meiosis while observations on mitosis were made from somatic cells of the squashed corolla, stamens, or style tissues. Usable slides stained by both techniques were made permanent by introducing a Venetian turpentine and propionic acid mixture under one edge of the cover glass while withdrawing the stain from the other side with filter paper. OBSERVATIONS.-A11 of the recognized stages of meiosis were studied, but more detailed observations were made of pachytene and later stages. Leptotene.-The microsporocytes at this stage were distinguished from somatic cells by their larger size and circular outline when squashed. At leptotene, the chromosomes were not suitable for study because of their great length. They frequently were intertwined to such an extent that squashing the cells only served to confuse the pattern. In some cells it was possible to observe a slight spiral in parts of the chromosomes. Zygotene.-At the beginning of zygotene, the chromosomes appeared to have contracted from their leptotene length. Pairing of homologues was of the intermediate type described by Swanson (1957) in which synapsis may begin at several regions along the length of the chromosomes. In the paired segments the chromosomes were loosely coiled around one another. At this stage some parts of the chromosomes stained darker than others. Pachytene.-The chromosomes had contracted considerably by pachytene, and in well-stained Fig. 1-6. Photomicrographs of various stages of meiosis.-Fig. 1. Pachytene stage showing completely paired chromosomes with B attached to the nucleolus. The arrow indicates the large chromomere on A.-Fig. 2. Same as fig. 1 except the chromosomes have been darkened with India ink to show the relationship of the strands in more detail. The centromeres are indicated by the light stippled areas.-Fig. 3. Pachytene with the rounded nucleolar knob attached to a part of the nucleolus as indicated by the long arrow. An adjacent chromomere is indicated by the short arrow.-Fig. 4, 5. Diplotene. Coiling can be seen in both figures, and chiasmata are shown in fig. 4.-Fig. 6. Diakinesis. Size difference of A and B bivalents is apparent. All figures X2000.