Abstract
It is estimated that 5 million years of evolution separate Arabidopsis thaliana from its close relative Arabidopsis lyrata. The two taxa differ by many characteristics, and together they exemplify the differentiation of angiosperms into self-fertilizing and cross-fertilizing species as well as annual and perennial species. Despite their disparate life histories, the two species can be crossed to produce viable and vigorous hybrids exhibiting heterotic effects. Although pollen sterile, the hybrids produce viable ovules and were used as female parent in backcrosses to both parental species. The resulting backcross plants exhibited transgressive variation for a number of interesting developmental and growth traits as well as negative nuclear/cytoplasmic interactions. Moreover, the genesis of a fertile amphidiploid neospecies, apparently by spontaneous somatic doubling in an interspecific hybrid, was observed in the laboratory. The mechanisms responsible for the generation of amphiploids and the subsequent evolution of amphiploid genomes can now be studied through direct observation using the large arsenal of molecular tools available for Arabidopsis.