The study of two variable amplicons of rye indicates that RYS1, a mobile 
element, is activated during tissue culture. We propose that RYS1 could be a 
foldback (FB) transposon. The FB transposons have been rarely reported in 
plants; RYS1 is the first described in rye and also the first active plant FB 
transposon reported. Preferential integration points in the rye genome exist, 
because the new insertions seem to be located, in all studied cases, in the same 
genome positions. We assume that RYS1 became active in rye very recently, as 
different plants from in vivo-growing cultivars showed that these elements were 
present or absent in the same genomic position in which the in vitro-activated 
element was found. This high rate of modification in these particular loci, both 
in the in vivo and in vitro populations, could indicate that probably the 
mechanisms promoting genetic variability in nature are the same that induce 
variation in vitro, and the modifications induced by somaclonal variation could 
be already present in vivo populations.