The Geography of Phytochemical Races

26 2 Examples Within Continents
Fig. 2.13 Generalized reaction sequence from starting amino acid [65] to product cyanogenic
glycoside [66]
L. corniculatus in southern England, and demonstrated that presence of cyanogenic
glycosides is dominant over their absence. Further work on inheritance of cyanogenesis
in this species was complicated by the fact that this clover is a tetraploid.
Work with other species led to more defi nitive results, however.
The genetics of cyanogenic glycoside formation were determined using Trifolium
repens L. (Corkill, 1942; Atwood and Sullivan, 1943). Two loci were identifi ed, one
that controls formation of the cyanogenic glycoside (Ac) and one, (Li), that controls
synthesis of the glycohydrolase (also referred to as linamarinase). The reactions are
represented in the conversion of generalized amino acid [65] to the cyanogenic glucoside
[66] (see Fig. 2.13), which requires several chemical steps, with subsequent
breakdown of the latter to HCN, glucose, and an aldehyde. In order for a plant to
be cyanogenic, it must have dominant alleles at both loci, that is, Ac__Li__. Three
different combinations yield acyanogenic plants: Ac_lili, which has cyanogenic glycosides
but lacks the glycohydrolase, acacLi_, which has the enzyme but lacks the
glycoside; and acaclili, which lacks both. In the case of Lotus corniculatus, which
exhibits the same phenomenon, plants that were cyanogenic were referred to as
variety (or forma) amara, whereas acyanogenic plants were referred to as variety
dulcis, which language scholars will recognize as the terms, respectively, for bitter
and sweet.
Attention was refocused on cyanogenic polymorphism in the early 1950s, owing
to extensive studies of the phenomenon in clover (Trifolium repens L.) in Europe
by Daday (1954a, b). In the fi rst of a series of papers, he reported that populations
of clover in extreme southern and southwestern Europe had much higher frequencies
of HCN-positive individuals than populations located in northeastern Europe,
some of which lacked cyanogenic glycosides altogether (Fig. 2.14). Populations in
central Europe exhibited intermediate frequencies. The second report described the
frequency of occurrences as a function of elevation, wherein one sees the frequency
of cyanogenic individuals in populations decrease as one progresses to higher and
higher sites in the Alps. Whereas a population situated at 580 m had predominantly
HCN-positive individuals, plants in a population at 1950 m were acyanogenic.
de Araujo (1976) reported a similar relationship between elevation and cyanogenesis
in clover growing in Wales.
Further confi rmation that the HCN polymorphism is a general phenomenon came
from the work of a colleague at the University of British Columbia, Fred Ganders
(1990), who studied white clover from six sites in southwestern British Columbia
and adjacent Washington. In all but one of the sites (the exception being a particularly