FORTY YEARS O F GENECOLOGY
19 1
The comparison of the performance of cloned F, individuals from the
reflexa x subalpine nevadensis cross at the different transplant stations
revealed notable differences in the effect of environment upon gene
expression. Clausen and Hiesey refer to “qualitative” characters, mostly
expressed in the flowers, which although subject to some modification at
the different stations are yet relatively stable, and to “quantitative”
characters modifiable by environment to the extent that genic differences may be totally masked. These modifiable characteristics mostly
relate to growth properties and developmental periodicity. In stem
growth, a high correlation (r = 0.48) was observed between performance
at Stanford (30 m altitude) and at Mather (1400 m), and a lower one
(r=0.12) between Stanford and Timberline (3050 m). In respect to
flowering time, the F, progeny showed scarcely any significant correlation between the stations, although at each the order of flowering between the different individuals was consistent from year to year. It may
be surmised that both penetrance and expressivity of the genes segregating in the F, of the reflexa x subalpine nevadensis cross vary in the
environments of the three transplant stations, but Clausen and Hiesey
comment that the genic systems controlling the quantitative characters
have proved too complex for any satisfactorily comprehensive interpretation of the effects of the diverse environments on gene expression.
Another feature of great interest in the work of the Carnegie group is
the analysis of the correlations between the characters distinguishing
the ecological races in F, progeny of inter-racial crosses, (Clausen and
Hiesey, 1958a, 1960). These are expressed diagrammatically in Figs. 2a
and 2b for the two fully analysed crosses, typica x alpine nevadensis and
reJlexa x subalpine nevadensis. In the first of these crosses, of the 91
possible combinations of the 14 characters, 67 showed some degree of
association. In the second, the 12 characters permitted 66 combinations,
of which 38 were significantly correlated.
Correlations of the type observed could conceivably arise at a physiological level, and it may well be that some of those illustrated in Fig. 2
do represent different expressions of single basic growth processes.
However, Clausen and Hiesey consider that evidence points to some
form of genetical association between the characters, which they refer to
as coherence. The chromosome number of P. glandulosa is low (n=7),
and they attribute the coherence of racial characters to the high probability that, with so small a number, some at least of the genes governing
any pair of quantitatively varying, polygenically governed characters
will be linked. They have shown very convincingly that the coherent
constellations of characters studied are either themselves concerned
with survival in the habitats of the different ecological races, or cohere
also with unobserved physiological properties which determine survival.
19 1
The comparison of the performance of cloned F, individuals from the
reflexa x subalpine nevadensis cross at the different transplant stations
revealed notable differences in the effect of environment upon gene
expression. Clausen and Hiesey refer to “qualitative” characters, mostly
expressed in the flowers, which although subject to some modification at
the different stations are yet relatively stable, and to “quantitative”
characters modifiable by environment to the extent that genic differences may be totally masked. These modifiable characteristics mostly
relate to growth properties and developmental periodicity. In stem
growth, a high correlation (r = 0.48) was observed between performance
at Stanford (30 m altitude) and at Mather (1400 m), and a lower one
(r=0.12) between Stanford and Timberline (3050 m). In respect to
flowering time, the F, progeny showed scarcely any significant correlation between the stations, although at each the order of flowering between the different individuals was consistent from year to year. It may
be surmised that both penetrance and expressivity of the genes segregating in the F, of the reflexa x subalpine nevadensis cross vary in the
environments of the three transplant stations, but Clausen and Hiesey
comment that the genic systems controlling the quantitative characters
have proved too complex for any satisfactorily comprehensive interpretation of the effects of the diverse environments on gene expression.
Another feature of great interest in the work of the Carnegie group is
the analysis of the correlations between the characters distinguishing
the ecological races in F, progeny of inter-racial crosses, (Clausen and
Hiesey, 1958a, 1960). These are expressed diagrammatically in Figs. 2a
and 2b for the two fully analysed crosses, typica x alpine nevadensis and
reJlexa x subalpine nevadensis. In the first of these crosses, of the 91
possible combinations of the 14 characters, 67 showed some degree of
association. In the second, the 12 characters permitted 66 combinations,
of which 38 were significantly correlated.
Correlations of the type observed could conceivably arise at a physiological level, and it may well be that some of those illustrated in Fig. 2
do represent different expressions of single basic growth processes.
However, Clausen and Hiesey consider that evidence points to some
form of genetical association between the characters, which they refer to
as coherence. The chromosome number of P. glandulosa is low (n=7),
and they attribute the coherence of racial characters to the high probability that, with so small a number, some at least of the genes governing
any pair of quantitatively varying, polygenically governed characters
will be linked. They have shown very convincingly that the coherent
constellations of characters studied are either themselves concerned
with survival in the habitats of the different ecological races, or cohere
also with unobserved physiological properties which determine survival.