II.8 The synaptonemal complex and chromosome pairing in haploid barley.
C. B. Gillies. Department of Physiology, Carlsberg Laboratory, G1. Carlsbergvej 10, DK-2500, Copenhagen Valby, Denmark.
Anthers of haploid barley have been fixed and serially sectioned for electron microscopy. Fixation and staining methods used were the same as previously used for maize anthers (Gillies, 1973). The serial micrographs of PMC's have been used to reconstruct the paired and unpaired lateral components of the chromosomes at stages of meiotic prophase corresponding to pachytene in a diploid.
The haploids were kindly supplied by Dr. C. J. Jensen of the Danish Atomic Energy Establishment at Risø (C. J. Jensen, 1973). They were derived from crosses of Hordeum vulgare and H. bulbosum (both 2n = 14) by the method of Kasha and Kao (1970). The bulbosum chromosomes are selectively eliminated in the early zygote (Subrahmanyam and Kasha, 1973) and the haploid embryos were cultured after about 14 days. Root tip squashes were used to check ploidy level. The two haploid plants used were derived from Sultan, a Dutch cultivar, and from an F1 of Sultan and Carlsberg II mutant 29 (a high lysine line produced by EMS treatment). Both are self-pollinating, two row spring types.
Results and Conclusions
1. Synaptonemal complexes are readily seen in "pachytene" stages. They appear normal in dimension (cf. Westergaard and von Wettstein, 1972, Fig. 22), but there may be more of a lighter staining granular material around the lateral components and between the laterals and the chromatin. This material is also seen in late pachytene in diploid nuclei and could be a breakdown product of the synaptonemal complex (cf. Westergaard and von Wettstein, 1970 in late pachytene of Neottiella).
2. Unpaired lateral components are also seen.
3. Phosphotungstic acid and Bernhard's staining techniques work well, but do not seem to differentiate the centromeres.
4. Using phosphotungstate staining, nuclei of the F1 have been reconstructed. In one nucleus 117mu of paired synaptonemal complex and 160mu of unpaired lateral component were measured. This represents about 60% pairing. In a second nucleus 69mu of synaptonemal complex and 298mu of unpaired lateral component were found (31% pairing). Total haploid lengths were 393mu and 436mu, or 83% and 92% respectively of the pachytene length as determined by Sarvella, Holmgren and Nilan (1958) in diploid barley.
5. The pairing configurations were similar to zygotene stages in maize. The variation in amount of pairing suggests that the reconstructed nuclei were in zygotene/early pachytene, and that more complete synaptonemal complex formation might occur at later stages.
6. Synaptonemal complex formation occurs at many places on each chromosome, but several long stretches of completed synaptonemal complexes were found. Both inter- and intrachromosomal pairing occurs, but it was not possible to completely reconstruct all seven chromosomes from end to end. At least some chromosomes ended with paired lateral components at the nuclear envelope.
7. The two nucleolus organizers were attached to a single nucleolus, with satellites extending into the interior of the nucleolus. However no pairing was seen between the two discrete organizer regions.
8. The extensive amount of inter- and intrachromosomal synaptonemal complex formation suggests that pairing is non-homologous, and not a result of associations of duplicated regions. The chiasma (bivalent) frequency of 0.05 per metaphase I cell (Sadasivaiah and Kasha, 1971) suggests a virtual absence of crossing over, as would be expected if associations at pachytene were non-homologous.
References:
Gillies, C. B. 1973. Maize Genetics Coop. NewsLetter 47: 25-27.
Jensen, C. J. 1973. Barley Genetics Newsletter 3: 23-24.
Kasha, K. J. and Kao, K. N. 1970. Nature 225: 874-876.
Sadasivaiah, R. S. and Kasha, K. J. 1971. Chromosoma 35: 247-263.
Sarvella, P., Holmgren, J. B. and Nilan, R. A. 1958. The Nucleus 1: 183-204.
Subrahmanyam, N. C. and Kasha, K. J. 1973. Chromosoma 42: 111-125.
Westergaard, M. and von Wettstein, D. 1970. Compt. Rand. Trav. Lab. Carlsberg 37: 239-268.
Westergaard, M. and von Wettstein, D. 1972. Ann. Rev. of Genetics 6: 71-110.