of their DNA sequences and production of iridoid glycosides and caffeoyl phenylethyl
glycosides such as verbascoside (Grayer et al., 1999). However, the Nepetoideae
are chemically a highly derived subfamily in the Lamiaceae, whereas the
Scrophulariaceae, which normally produce monoterpenoid-derived iridoid glycosides
instead of essential oils, are chemically more similar to the other subfamilies
of the Lamiaceae, which produce the same classes of compounds (Grayer et al.,
1999). This means that the ability to produce essential oils and 6-,8-oxygenated
surface flavones has probably originated separately in both families (parallel evolution).
Perhaps all plant families that produce iridoids have the potential to alter
the production of monoterpenoid iridoids to that of monoterpenoid-containing
essential oils. This biosynthetic change also seems to have happened independently
to certain genera of the related family Verbenaceae. The development of the production
of surface flavones in plants that produce essential oils may therefore be
another example of parallel evolution.
References