4 Molecular Cytogenetics of Arabidopsis

Abstract

Arabidopsis thaliana (L.) Heynh. is widely used for studies of genetics and molecular genetics because of its many advantages: the small plant size, short life cycle, and small genome. The low genome size, measured as nuclear DNA content, is convenient for molecular analysis because some 60% of the genome consists of single-copy DNA sequences, most of which are genes or associated sequences (Leutwiler et al. 1984). However, the small genome makes the examination of the chromosomes, their activity, segregation, pairing, and aberrations in number or structure – that is, cytogenetics – difficult (Fig. 1). Nevertheless, using the highest powers of light microscopy (limited by the wavelength of light), the five pairs of chromosomes in the diploid plant (2n = 10) have been investigated both at mitosis and meiosis for many years (Laibach 1907, named as Stenophragma thalianum; Steinitz-Sears 1963; Ambros and Schweizer 1976).

In other organisms, particularly humans and cereals, cytogenetic analysis has led molecular analysis and has been widely used to understand the structure, organization, and behavior of the genome (Heslop-Harrison 1991). In Arabidopsis, the chromosome size has made such analysis difficult; even for distinguishing between diploid and tetraploid strains, alternative methods such as weighing seeds or measuring pollen grains are widely used. The use of trisomic stocks with an extra chromosome (2n = 2 x + 1 = 11; Sears and Lee-Chen 1970) has been of great value in gene mapping in Arabidopsis (Koornneef and van der Veen 1983; see Koornneef, this volume), as in other species. By examining the...