1. Univalent formation in progeny from a diploid branch of a haploid individual ofCrepis capillaris belonging to theX strain is caused through failure of chiasma formation between homologous chromosomes which are shown to be regularly associated at pachytene.
2. A significant difference in the amount of univalent formation is found between two plants, and between two buds on the same plant.
3. Chiasma frequency is reduced in all three kinds of bivalents, and there is an almost direct relation between the amount of chiasma formation and the size of chromosomes. This is shown by the percentage of univalents and the mean number of chiasmata per bivalent.
4. Univalent homologous chromosomes frequently lie in juxtaposition at diakinesis. This is considered to be due to their previous association and coiling at pachytene and early diplotene.
5. TheA bivalents in a single plant ofCrepis capillaris have a reduced frequency and an abnormal distribution of chiasmata. In bivalents with single interstitial chiasmata, 95 per cent. have the chiasma in a subterminal position at the end of the long arm. Univalents of these chromosomes are occasionally found, and unpaired threads are observed at pachytene. Chiasma formation and possibly pairing are therefore restricted in one region of the chromosome.
KeywordsHomologous Chromosome Chiasma Frequency Chiasma Formation Spindle Attachment Restitution Nucleus
- Beadle, G. W. (1930). “Genetical and cytological studies of Mendelian asynapsis inZea mays.”Mem. Cornell agric. Exp. Sta. No. 129, 1–23.Google Scholar
- -- (1931). “A gene in maize for supernumerary cell divisions following meiosis.”Ibid. No. 135.Google Scholar
- —— (1932c). “A gene inZea mays for failure of cytokinesis during meiosis.”Cytologia,3, 142–55.Google Scholar
- —— (1933). “Further studies of asynaptic maize.” Ibid.,4, 269–87.Google Scholar
- Blakeslee, A. F. (1928).Yearb. Carneg. Instn. 27, 42.Google Scholar
- Darlington, C. D. (1932).Recent Advances in Cytology.Google Scholar
- Heilborn, Otto (1930). “Temperature und Chromosomenkonjugation.”Svensk bot. Tidskr. 24, 12–25.Google Scholar
- Hollingshead, Lillian (1930a). “Cytological investigations of hybrids and hybrid derivatives ofCrepis capillaris andCrepis tectorum.”Univ. Calif. Publ. agric. Sci. 6, 55–94.Google Scholar
- —— (1930b). “A cytological study of haploidCrepis capillaris plants.” Ibid.6, 107–34.Google Scholar
- —— (1932). “The occurrence of unpaired chromosomes in hybrids between varieties ofTriticum vulgare.”Cytologia,3, 119–41.Google Scholar
- Moffett, A. A. (1932). “Chromosome studies inAnemone. I. A new type of chiasma behaviour.”Cytologia,4, 26–37.Google Scholar
- Richardson, M. Margaret (1935). “Meiosis inCrepis. I. Pachytene association and chiasma behaviour inCrepis capillaris (L.) Wallr. andC. tectorum L.”J. Genet. 31, 101–17.Google Scholar
- Sax, K. (1931). “Chromosome ring formation inRhoeo discolor.”Cytologia,3, 36–53.Google Scholar