The Geography of Phytochemical Races

2.7 North and Central America 85
study. Its fl avonoid chemistry is particularly well documented, the enzymology
and genetics of the fl avonoid pathway are well understood, and the molecular biological
tools are available.
2.7.3 Asclepias tuberosa (Asclepiadaceae)
Asclepias tuberosa L., butterfl y weed) consists of two subspecies, based on leaf characteristics,
native to North America, subsp. tuberosa and subsp. interior Woodson.
The latter named taxon enjoys a range that encompasses most of the central part of
the continent, from Ontario west to Minnesota and southwestward to western Utah
and Arizona. Much of what is known concerning the biology of North American
members of this genus has come from the work of R. E. Woodson, Jr. (1947, 1953,
1954, and 1962). The particular study of interest here deals with his study of the
geography of fl ower color variation in subsp. interior (Woodson, 1964, with summarizing
paragraphs by Sewell Wright).
Flower color of butterfl y weed ranges from yellow to reddish orange with a defi -
nite geographic component to the different hues. The yellow color derives from
a background of carotenoids with the range of yellow–orange through orange to
reddish-orange being due to increasing amounts of anthocyanin overlying the yellow
base. Woodson (1964) reported that a crude extract of the anthocyanin in 0.1 N
HCl exhibited an absorption maximum at 515 mµ, which lies within the range of
anthocyanin glycosides. Although no further comment was made concerning the
identity of the pigment, it seems safe to suggest, on the basis of published maxima
for anthocyanins (Harborne, 1967b, p. 17), that the pigment could be a pelargonidin
5-glycoside (λ max = 513 mµ for pelargonidin 5-glucoside).
Extensive sampling of butterfl y weed was conducted along transects designed
to follow the trends already observed with leaf characteristics (Woodson, 1947,
1953). The starting point for the transects was a point in Camden County, Missouri
(his C1 site), with a series of transects radiating at intervals of about 120 miles
northward into Minnesota, northeastward into Ontario, and westward to western
Colorado and Utah. A fourth transect was followed from C1 eastward to the
Atlantic coast through an area occupied by subsp. tuberosa (Fig. 2.53). Samples
collected along the transects within each of the areas defi ned by the radiating
transects were scored for color using a standardized color system. Carotenoid
concentration appeared not to vary, whereas the anthocyanin contribution varied
signifi cantly with the highest concentrations per plant and largest frequencies
of red-pigmented plants seen in plants from Missouri, Illinois, adjacent Kansas,
and southern Iowa. With a few exceptions, anthocyanin level dropped along the
radiating transects, most precipitously along the northern track with nearly pure
yellow-fl owered plants observed in central Minnesota. Pure yellow plants were
observed in the westernmost sites as well. Reduction of anthocyanin concentration
was not uniform, however, as witnessed by the occurrence of populations
in western Texas, New Mexico, and Colorado that featured signifi cant numbers