A KARYOTYPE OF THE ARABIDOPSIS-THALIANA GENOME DERIVED FROM SYNAPTONEMAL COMPLEX-ANALYSIS AT PROPHASE-I OF MEIOSIS

Synaptonemal complex (SC) complements prepared for electron microscopy allow high-resolution chromosome analysis. SC preparations were used to determine the relative lengths of the chromosomes of the Arabidopsis thaliana genome. This complements the extensive genetic and molecular analyses of these chromosomes. The two smallest SCs (chromosomes) carry a nucleolus-organizing region (NOR) at or near one end, and at prophase I of meiosis both have a relative SC length of about 16-17% of the total complement. The three longest SCs have relative lengths of 19, 22.5 and 25% and can be identified on this criterion. Fluorescent in situ hybridization with nonradioactively labelled ribosomal RNA confirmed that there are two pairs of NOR chromosomes and also showed that the ribosomal genes are localized with the nucleolus. SC formation at NORs is often interrupted and this may reduce recombination in these regions of the genome.