Knotty problem of hybrid invaders
'Building work halted by knotweed' declared
a headline in my local newspaper. During site clearance prior to construction
work, a building company had discovered a stand of Japanese knotweed (Fallopia
japonica). Rather than attempt to remove the plants themselves, the
builders had called in experts to deal with the problem. So, what is the
problem? Several members of the genus Fallopia are both invasive and
persistent, as described by Tiébré et al. (Gembloux, Belgium, pp. 193–203).
These authors have focused on the breeding systems of these species and on the
formation of interspecific hybrids. In general, plants may be either
hermaphrodite or male-sterile, with variations in the proportion of the two
forms in different parts of the range. For example, in the UK only male-sterile examples of F.
japonica are known, which means that spread from the initial sites of
colonization is by vegetative methods. However, the interfertility of different
members of the complex means that male-sterile plants can set seed if
pollinated by a male-fertile species. Thus the authors confirmed that
male-sterile F. japonica, with F. sachalinensis as the male
partner, forms the hybrid F. x bohemica. Fallopia japonica
can also backcross with F. x bohemica and hybridize with an
ornamental species, F. aubertii. Indeed, in this paper it was these
latter crosses for which the greatest seed set was reported. However, variation
in germinability between different species/hybrids and from year to year means
that seed set is not necessarily an indicator of reproductive success.
Nevertheless, the ready hybridization and the formation of polyploids (as shown
by flow cytometry) clearly give potential for this group to exploit further
niches and to become more of a nuisance. Of particular concern is the use of F.
aubertii as an ornamental, giving ready access of the highly invasive but
male-sterile F. japonica to a source of pollen. Will this cause further
problems? Watch those open spaces!
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
