Alien on the loose causes strife for natives
Earlier today, while walking beside a local river, I was
pleased to see Lythrum
salicaria,
purple loosestrife, along the water’s edge. However, for readers in North America, it would be a much less welcome sight. In
many parts of the USA and Canada, L. salicaria has become aggressive, often dominating
the vegetation in wetland habitats, as described by Houghton-Thomson et al. (Michigan State University, USA, pp.
877–885).
Lythrum salicaria was probably introduced accidentally
into North America in the early 1800s but it
was not until the 1930s that it began to become a problem. Since then, the
pattern of invasion has elements that one might meet in a science fiction plot:
in each new area that it reaches, it remains, as the authors say, ‘unobtrusive’
for at least 20 years, after which it becomes dominant in suitable habitats in
less than 3 years. The authors wondered whether this pattern of invasion was at
least in part due to hybridization between L. salicaria and the native North American species, L. alatum (winged loosestrife). To test this idea
they carried out a careful analysis, based both on diagnostic morphological
traits and on an AFLP screen, of populations of L. alatum and of European and North American
populations of L.
salicaria.
The results are clear:
genetically and morphologically, the two species are clearly separate; however,
the North American L.
salicaria form
a distinct subgroup within the L. salicaria populations. Further, some of the characters that mark out
the North American populations are actually from L. alatum. In other words, hybridization between
the two species has occurred at some time in the past but the level of
introgression of L.
alatum genes
into the L.
salicaria genome
is actually very low; too low, the authors suggest, to
explain the invasiveness of L. salicaria. For the present, then, the secret of the alien’s success
remains hidden.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
