Credit: A. DAVIS, JOHN INNES CENTRE

Structures that occur in closely related organisms and that look the same are usually considered to be homologous — their similarity is taken to arise from their common ancestry. Common sense suggests that the more complex such structures are, the less likely they are to have evolved independently and the more valuable they should be for studying systematics. But what if ‘obviously’ identical organs have arisen through two mutually exclusive developmental routes?

Beverley Glover and colleagues have revealed one such case (Gene 331, 1–7; 2004). It occurs in the floral organs of the genus Solanum from the nightshade family. In one group of these species, the anthers — the flower's pollen-producing organs — are arranged as a cone, which functions like a ‘pepperpot’ (see the yellow, cone-like structures in a and b, right). In the pepperpot of bittersweet (S. dulcamara), the anther surfaces are held together by a glue-like secretion (a). In another species from the same group, tomato (S. lycopersicum), they are instead linked by interlocking hairs, or trichomes, along the edges of the anthers (b).

Glover et al. find that tomato trichomes are clearly required for pepperpot formation. In one form, the dialytic mutant, which lacks them, the pepperpot fails to develop (d). In bittersweet, however, trichomes surprisingly prevent pepperpot formation. Glover et al. show this using transgenic plants in which expression of a gene from snapdragon leads to the development of hairs on bittersweet anthers. The hairs push the glue-bearing surfaces apart, preventing pepperpot formation (c).

This result makes it unlikely that the tomato-type pepperpot originated from the bittersweet type, or vice versa, because the development of anther hairs in bittersweet-type cones would probably have caused the cone to fall apart, whereas the addition of glue to tomato-type cones already supported by trichomes would probably have carried no selective advantage. So the most plausible conclusion is that pepperpots originated twice independently in the lineages that led to tomato and bittersweet.

Molecular systematic analysis confirms that tomato and bittersweet are closely related, and the traditional view would be that their pepperpot cones are obviously homologous. But genetic tinkering and mutant analysis show that they probably are not — that they are convergent, having taken different routes to the same end. Life's potential to invent complex structures more than once may worry systematists, who depend on reliable characters to reconstruct relationships between organisms. But it will please anyone who admires nature's innovative power.