As Japanese knotweed took over the U.K. in the late 1970s, widespread sampling began turning up several different forms of the plant. To this point, botanists like Anne Connolly and John Bailey who studied knotweed assumed that the few "aberrant" male-fertile plants being found were far outnumbered by the ubiquitous male-sterile female clone loosed from British gardens some 130 years prior. When someone got the notion to do chromosome analysis, however, a more complex picture emerged.
The original clone — identified by typically small leaves and a truncate leaf-base morphology — was actually an octoploid, carrying eight sets of the same 11 chromosomes, written by biologists as 2n = 88, where "n" equals the contribution of one putative parent. With the normal condition of most organisms being diploid (e.g., in humans the diploid number is 2n = 46, with 23 chromosomes from mom, and 23 from dad), and with a basic number of 11, the ancestor of this knotweed clone would have been 2n = 22, meaning that two doubling events (first to 44, then to 88) had occurred over its evolutionary history. In contrast, other knotweed forms — male-fertile and a litany of larger-leafed females — proved to be a mix of tetraploid, hexaploid, octoploid and decaploid (you do the math) resulting from crosses and backcrosses between either of the two known Japanese knotweeds (F. japonica var. japonica and var. compacta) and another introduced species, giant knotweed (F. sachalinensis). These hybrids were collectively labelled Bohemian knotweed (F. x bohemica); the etymology was complicated but the implications clear: in addition to its prodigious spread by vegetative means, knotweed was also reproducing sexually and had been for a while, generating both viable seeds and new clonal lineages, a phenomenon Bailey and Connolly now traced, through examination of plant collections, back to the late 1800s.
Yes, plant genetics is an exhausting topic that can quickly lose even the most biologically savvy reader (or writer). So let's distill: rather than simply being overrun by descendents of the Japanese knotweed clone plucked from his Leiden nursery by Dutch explorer Philipp von Siebold and gifted to London's Kew Gardens, the U.K.'s knotweed mess was a mix of that lineage plus a growing "swarm" of interspecies and intervariant hybrids. The same thing was occurring simultaneously throughout Europe (Bohemian knotweed so-named for its original 1983 description from Czechoslovakia) and pretty much everywhere else knotweed was found outside its native range — which, by this point, comprised most of the Northern Hemisphere.
On the one hand, this was good to know given knotweed's hitherto inexplicable success. On the other hand, it was troubling given what was known about heterosis, or hybrid vigor. Heterosis references the phenomenon of improvement or increased function in any biological trait derived from mixing different parental lineages (think how much better your no-name mutt is at everything than that neurotic, vacuous, but lovely purebred Irish Setter next door). In other words, Japanese knotweed was a formidable colonist, and giant knotweed somewhat less so though still capable, but their hybrid put both to shame: Bohemian knotweed was an invader on both steroids and speed. Experimental studies have quantified this, repeatedly showing "... that invasive knotweed hybrids are indeed more competitive than their parents and that hybridization increased the invasiveness of the exotic knotweed complex."
Squamish, Britannia Beach and Vancouver residents won't be pleased to know that, at minimum, 80 per cent of the knotweed tearing up their communities is Bohemian.
As any gardener knows, when you throw a mix of flower seeds into the ground, some do better than others depending on how well-adapted each species is at the genetic level to the soil type, location, latitude, and climate in which the garden lies. Similarly, when differing plant forms meet to hybridize widely and generate an ongoing range of genotypes, each variant is challenged by the same classic tenets of natural selection: certain genotypes will do better than others in a particular place, and those will come to dominate that environment. With a range of environments and a constant supply of new genotypes searching them out, eventually each finds its match: it's Plenty of Fish for plants. Most weeds — and no shortage of crop species — find their ideal mates this way. Using the many forms of knotweed as example, our horrified eyes see the process selecting for ever-better invasion success, though the plant is simply doing what it does anywhere: passively sorting through colonization scenarios involving whatever mix of vegetative and sexual reproductive modes are available to it, in this case turbocharged by our addition of several easily crossed forms that otherwise wouldn't be found together. As Bailey summarized: "... a massive inadvertent breeding experiment on a world scale!"
The revelation of numerous forms of knotweed increased public demand for serious action. That arrived when a duty-of-care obligation was finally placed on rhizome disposal by Britain's 1990 Environmental Protection Act, legislation that birthed a nationwide industry dedicated to eradicating knotweed from development sites. Indeed the militant chemical and mechanical warfare that ensued has, as of 2015, totalled the equivalent of $3 billion USD; among other high-profile projects, the 2012 London Olympics famously suffered costs associated with knotweed removal from building sites to the tune of $130 million USD.
Help, however, may finally be at hand: scientific expeditions to Japan in the new millennium were successful in pinpointing the precise locale from which the original Kew clone and other knotweed variations were obtained, with the added bonus of identifying an indigenous insect biocontrol candidate. The psyllid sap-sucker Aphlara itadori, was, after extensive testing, approved for release in carefully monitored U.K. sites in 2010 — the first release of a biocontrol organism in the history of the British Isles. Controlled releases are now also underway in B.C., promising knotweed relief — but not freedom — for beleaguered Lower Mainlanders.
And what of the fate of Siebold's infamous Garden of Acclimatisation at Leiden? In the year of his death, 1866, it boasted nearly a thousand different plant species and varieties. By the time the eminent English horticulturists F.W. Burbidge and P. Barr showed up for a tour in 1883, it was naught but a tangle of neglected jungle, long-since overrun by a single energetic inhabitant — Japanese knotweed.
Leslie Anthony is a Whistler-based author, editor, biologist and bon vivant who has never met a mountain he didn't like.
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