Mode of reproduction in Arabidopsis suecica

摘要

Arabidopsis suecica (2n=26) is an allopolyploid with A. thaliana, (2n=10) and A. (Cardaminopsis) arenosa (2n=16, 32) as its parental species (Hylander 1957; Mummenhof and Hurka 1994; Price et al. 1994; O′Kane et al. 1996). It has recently attracted interest since one of its parents, A. thaliana, is the leading model organism in plant biology (Chen et al. 1998, 2004; Comai et al. 2000). Thus A. suecica is an obvious candidate model species for polyploidy research. It shares several features with its parent A. thaliana: it has a short generation time and it can be kept at a limited size. In addition, A. suecica has a limited overall genome size of approximately 280 Mb (unpubl. results) and has a single origin less than 60?000 years ago, with respect to both the chloroplast (S?ll et al. 2003) and the nucleus (unpubl.).Whether A. suecica is used for evolutionary or functional investigations, it is of considerable interest to extend our knowledge of its reproductive system and population structure. The parents of A. suecica have widely different breeding systems. A. thaliana is a selfer with all the ‘classical’ correlates that normally accompany this breeding system; high individual homozygosity, a relatively high level of linkage disequilibrium, a low within-population variation and a relatively high between-population differentiation (Bergelson et al. 1997, Hagenblad and Nordborg 2002). On the other hand A. arenosa is, according to our observations in the greenhouse, self-incompatible and thus a strict outbreeder. Active pollination between individuals is, in our experience, necessary to achieve seed formation. In addition, A. arenosa produces a distinct scent and its flowers are clearly visible also at a distance. Both these features are typical of insect pollinated cross-breeders (Richards 1986). A. thaliana and A. suecica differ from A. arenosa in both these traits: neither has a scent and the flowers are much less conspicuous.A. suecica has not so far been investigated with respect to its breeding system. Our experience shows that it is fully self-fertile when grown in isolation (unpubl. results). A limited investigation of variation and population structure using RAPDs (Lind-Halldén et al. 2002) indicated a low level of variation in A. suecica. This is compatible with its single origin and short time of existence. The limited variation was mostly distributed between populations which is typical for effective inbreeders. However, our observations so far do not definitely exclude apomixis which also tends to show low within- and relatively large between-population variation (Loveless and Hamrick 1984). The a priori probability of a single species in the family Brassicaceae being apomictic may be low; still, apomixis occurs in Arabis holeboellii (B?cher 1951; Sharbel and Mitchell-Olds 2001), and for future interpretation of the evolution of A. suecica the difference between any sexual system and apomixis is crucial. In this study we investigate controlled crosses as well as single mother families in order to establish the mode of reproduction and the population structure of A. suecica.