The Geography of Phytochemical Races

28 2 Examples Within Continents
preference for the acyanic form of white clover. It is interesting to note that in the
outdoor experiments lasting for 2–3 weeks, animals adjusted their dietary intake of
cyanogenic clover relative to the acyanic form to a sustainable level. The authors
also reported feeding preferences for the acyanic form by two species of slugs,
Arion ater and A. subfuscus.
Additional information relating to cyanogenesis polymorphisms can be found in a
recent paper by Schappert and Shore (1999) who studied the phenomenon in Turnera
ulmifolia L. (Turneraceae), which is used by Euptoieta hegesia (Lepidoptera: Nymphalidae)
as its primary host plant.
A further note pertaining to cyanogenic taxa, although not concerned with geographic
differences per se, could have relevance in discussions of relationships of
species or populations based upon cyanogenesis. It has generally been held that a
positive reaction implies the functioning of the two genes, Ac and Li, required for
synthesis (one that directs making the glycoside and one that codes for the hydrolyase),
and a negative reaction implies the lack of one or both. Kakes and Chardonnens
(2000), working with mixed populations of the related species Trifolium
repens and T. occidentale Coombe in Bretagne, demonstrated that the hydrolyase
linamarase was absent from all individual plants examined (763). The apparent lack
of linamarase from T. occidentale had been reported some years earlier (Gibson
et al., 1972), but only ten plants were used in that work. The more recent results
strongly suggest that cyanogenic frequencies in these two closely related species are
controlled by different mechanisms. In other words, cyanogenesis does not have the
same genetic meaning in these two species.
It is useful to include a recently described example of cyanogenic polymorphism
here, although the species involved is neither related to the clovers nor does it occur
in Europe. The work in question comes from a study of variation in cyanogenesis
in two populations of Eucalyptus polyanthemos Schauer subsp. vestita L. Johnson
and K. Hill from the Brisbane Ranges National Park in Victoria, Australia (Goodger
et al., 2002). Two sets of trees were sampled, 100 from a population at Stony
Creek, and 201 from a population at Sutherland Creek. All trees (100/100) from
the Stony Creek set were cyanogenic with concentrations in the range 0.17–1.98 mg
cyanide per gram dry weight leaves. Six acyanogenic trees were detected within the
Sutherland Creek set (195/201) with maximum concentration of cyanide in the others
reaching 2.07 mg cyanide per gram dry weight leaves. The authors, assuming no
sampling error, speculated that the absence of acyanogenic trees in the Stony Creek
population might be the result of selection against some localized herbivore. Again,
follow-up studies of this system would be welcomed.
2.3.3 Lotus corniculatus (Fabaceae)
A few paragraphs above, we saw the part played by Lotus corniculatus in the cyanogenic
glycoside story. We now turn our attention to another aspect of this species
that has attracted a considerable amount of attention and has, coincidentally,