RING‐POSITION AND FREQUENCY OF CHIASMA FAILURE IN RHOEO SPATHACEA

The mean chiasma number per cell was 11.44 + 0.04 in 489 cells. Due to chiasma failures, the ring-of-twelve chromosomes may be broken into two or three chains. Cells with four or more chains were not observed. All six possible two-chain situations and eight different threechain situations were found. All possible lengths of chains from one to all twelve chromosomes were found, with "chain"-of-one inordinately frequent. The overall mean number of chromosomes in 273 chains in 188 cells is 8.26 + 0.31 and 5.38 + 0.31 among 154 chains in the 69 cells that had two or more chains. The mean number of chains per cell among these 188 was 1.45 + 0.13. In 73 cells, 113 chiasma failures were found to be distributed at random among the twelve chromosome arm positions. The absence of association either between length of arm or between presence-absence of secondary constriction and frequency of chiasma failure support the generally accepted theory that, in Rhoeo, synapsis and crossing over are restricted to small terminal segments on all chromosomes. RHOEO SPATHACEA (Swartz) Stearn has priority over the more widely known synonym R. discolor Hance (Stearn, 1957) and is characterized by chromosome structural hybridity to such an extent that no two chromosomes in the diploid plant are genetically identical. The ring-of-twelve chromosomes (Fig. 1), first observed by Belling (1927), is interpreted as having resulted from extensive reciprocal translocation. The two arms of each chromosome synapse, respectively, with an arm of each of two other chromosomes. One, two, or three chains are often observed (Darlington, 1929; Kato, 1930; Sax, 1931; Simmonds, 1945). There may be zero, one, two, or more chiasmata per pair of chromosome arms at diplonema, but only zero to one at diakinesis. Where chiasmata have failed to form in one or more pairs of arms, the ring becomes one or more chains. Because of the presence of complete terminalization (Sax, 1935), twelve terminal chiasmata form a complete ring, eleven form a chain-of-twelve chromosomes, ten form two chains, and so forth. We (Lin and Paddock, 1973) have confirmed Sax's (1931) map of the morphological sequence of chromosomes around the ring. The six most heterobrachial chromosomes conjoin invariably by their short arms. The failure of chiasma formation is reaonsably expected to be more frequent at these relatively shorter arms but we know of no published data on this point. The present report includes the overall chiasma number per cell and the frequency of chiasma failure at each position in the ring. 1 Received for publication 23 February 1973. MATERIALS AND METHODS-Meiosis was examined in PMC of diploid Rhoeo spathacea [2x = 2n = 12, Suessenguth (1921)]. Our method of making slides has been described (Lin and Paddock,1973). A study of early synapsis was prevented because of the sticky early prophase chromosomes. The study was restricted to diakinesis and metaphase I. For overall chiasma frequency and chain frequency, 489 PMC were examined (Table 1). Among 70 MI and three late diakinesis cells, in which the exact arm positions of chiasma failures were ascertained (Table 3), some were from the 489-cell sample. We use Flagg's (1958) symbols for complexes, a and pl, but have changed Sax's (1931) use of symbols for arms of chromosomes. Woolf's (1957) G-test of heterogeneity was used in testing randomness. Photomicrographs were made from permanent preparations. Interpretation of each cell was made while we both looked through a dual-view microscope. Figures 7-10 are our interpretations of Fig. 3-6. Chiasma failures were distinguishable from the numerous instances in which the pressure exerted during squashing broke chiasma connections because, in the latter, the homologous arms were still aligned and/or in proper sequence position. RESULTS AND DISCUSSION-Chiasma frequency -Table 1 is a summary of the different configurations found in MI and diakinesis cells. There were 5595 terminal chiasmata in 489 cells. The mean chiasma number per cell is then 5595/489 = 11.44 + 0.0368. Walters and Gerstel (1948) reported 10.9. From the data in their Table 3 we have calculated a standard error of + 0.1054.