Marx, G. A. New York State Agricultural Experiment Station, Geneva, NY U.S.A
and C. Nozzolillo University of Ottawa, Canada
The preceding article provides background information about PI 356980
in relation to its use in studies of basal stem anthocyanin. Although this
accession was found to contain plants with, and plants without, basal stem
anthocyanin (PNL 9:42-45, 1977; PNL 10:63, 1978; PNL 10:64-65, 1978), any
conclusions drawn from studies of this accession are bound to be ambiguous
until the genetic situation is clarified. Accordingly, reciprocal crosses
were made at Geneva using plants of four sublines of PI 356980 provided by
C. Nozzolillo and designated by her as 2-4, 2-8, 5-18, and 10-5. Sublines
2-4 and 2-8 were lines with, whereas 5-18 and 10-5 were lines without, basal
stem anthocyanin. The field-grown F1 plants from these crosses produced
some 3500 seeds, approximately 1400 of which were retained in Geneva and
2200 of which were forwarded to Ottawa (where not all have been analyzed).
When remnant selfed seeds from plants of these four second-generation
(G2) lines were planted, line 2-4 was found to produce plants with and plants
without basal anthocyanin whereas 2-8 produced plants exclusively with and
5-18 and 10-5 produced plants exclusively without basal anthocyanin. Since
the seeds in each instance derived from individual selfed plants, line 2-1
was clearly heterozygous for a gene(s) controlling the presence or absence
of basal stem anthocyanin. Significantly, moreover, the plants of this line
which showed red stems also expressed the "w" allele at D locus whereas the
plants without basal anthocyanin (green stems) manifested the "co" allele
at the same locus. Although the field-grown Fi plants from the crosses
involving 2-4 (Dw) and the other sublines (all Dco) were not scored for basal
anthocyanin, they were scored for Dw and Dco. Eighteen were classified as
Dw and 11 as Dco, clearly showing that at least some of 2-4 plants used as
parents were heterozygous Dw/Dco (Dw is known to be dominant to Dco).
In the F2 of crosses involving 2-4 plants as one parent, progenies
from the 11 F1 plants which were Dco were exclusively green-stemmed (Table 1)
In contrast, the 18 F1 plants scored as Dw produced progenies segregating
both for Dw - Dco and for the red and green stem. In these populations
all 722 (151 at Geneva and 371 at Ottawa) of the F2 plants with Dw exhibited
basal stem anthocyanin whereas all 243 (117 at Geneva, 126 at Ottawa) of
the F2 plants with Dco were without stem anthocyanin. Scoring was easy
and clear except for two plants which showed so little pigment as to be
questionable. Progeny tests of these two plants showed them to be heterozy-
gous for Dw/Dco, the Dw segregants being unmistakably red stemmed, and the
Dco segregants being green stemmed. Thus, without exception, the Dw allele
in this line was found to be associated with red stems whereas the Dco
allele was associated with green stems. The combined total of 965 plants
(468 observed at Geneva and 497 at Ottawa) is large enough to suggest pleio-
tropy rather than tight linkage but even more plants would be desired.
Bearing on the question of linkage vs pleiotropy are observations of
subline 2-8 and progeny from crosses with it. This subline, like 2-4,
shows basal stem anthocyanin but, unlike 2-4, it is Dco rather than Dw.
F2's of crosses between 2-8 and either 5-18 or 10-5 (both green stemmed)
PNL Volume 1 1979
showed a clear 3:1 ratio of red vs green stems but without segregation for
alleles at the D locus since all are Dco. Obviously, therefore, red stems
can occur in association with Dco, suggesting that the association found
in subline 2-4 may be an expression of linkage rather than pleiotropy.
But this cannot yet be accepted as fact since it is also possible that the
basal anthocyanin in sublines 2-4 and 2-8 is determined by entirely different
The cross 2-8 and 2-4 is especially noteworthy in light of the fact that
both lines have red stems but 2-8 is Dco and 2-4 is Dw. Since, however, the
F1 was Dco and the F2 showed a 3 red : 1 green ratio without segregation for
"w" or "co" at the D locus, we can only conclude that the 2-4 parental plant
was heterozygous and that in the only F1 seed obtained, we were unlucky enough
to draw the gamete containing Dco (and also green stem). Thus, this cross
bears repeating.
These results confirm and expand the findings presented in the previous
article by showing that at least one type of stem pigmentation is under
monogenic control and by showing a close association between basal stem
pigmentation and the "w" allele of the D locus. It would appear, therefore,
that a concrete beginning has been made with respect to the genetic control
of stem pigmentation in peas. The type of expression reported here is clear
in the seedling stems immediately upon emergence from the growing medium;
later the pigmentation tends to fade. It is recognized that this may be
only a small part of a larger story since different types and expressions
of stem pigmentation have been encountered by us and by others.
27 RESEARCH REPORTS PNL volume 11 1979
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