Natural variation in flowering time
There is large variation in the timing of the floral transition between different accessions of Arabidopsis. For example, accessions collected from mountainous regions or higher latitudes are often winter annuals, flowering only after vernalisation stimuli, whereas ecotypes collected from milder climates do not require a prolonged exposure to winter conditions in order to flower (Simpson et al., 1999; Henderson et al., 2003) .
Flowering Locus
Crossing winter and summer annuals demonstrated that these differ at two major effect loci, called FLOWERING LOCUS C (FLC) and FRIGIDA (FRI) (Clarke and Dean, 1994; Clarke et al., 1995; Alonso-Blanco et al., 1998; Michaels and Amasino, 1999; Sheldon et al., 1999) . Winter annuals contain dominant alleles of both loci, which delay flowering until vernalisation (Simpson et al., 1999 ; see section 1.4.3.3).
Surprisingly crossing summer annual accessions also demonstrated the existence of natural-genetic variation between the accessions. For example, the Landsberg erecta (Ler) and Columbia ( Col ) accessions are both early flowering and show similar flowering responses, but in F 2 populations created by intergressing these accessions late flowering plants segregate. This suggests that these two accessions contain different combinations of alleles that confer flowering time variation (Lister and Dean, 1993) . Similar results were obtained in crosses between the Cape Verde islands and Ler accessions (Alonso-Blanco et al., 1998).
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