40 PNL Volume 19 1987 RESEARCH REPORTS
ADDITIONAL EVIDENCE PLACING Rb IN CHROMOSOME 3
Marx, G. A. NYS Agricultural Experiment Station
Geneva, New York 14456 USA
In 1971 (1) Gritton reported data indicating loose linkage of
ruftosus (rb) and st_ on chromosome 3. Unfortunately, neither the
raw data nor the chi-square values were provided. My efforts to
corroborate this report using st_ and chi-6 in the _b end of
chromosome 3 were unavailing (2). Therefore I set out to deter-
mine if _rb is situated at the _M end of chromosome 3. Since tac
and M are closely linked (4) and since tac is a favorable seedling
marker and M is a seed gene, I developed a tac line homozygous for
R j^b; this line also carried apu which is located near jst (4,5).
The tac apu line was crossed with one of my lines (A686-156) known
to be JR _b. Fi's were grown in the field where abundant seed
was produced; all were wild-type with respect to the Tac and Apu
loci. The F2 seed borne on the F\ plants segregated for
Rb-rb. The round (Rb/-) and wrinkled (rb/rb) seeds were separated
prior to planting and then planted in separate groups in green-
house flats containing builders sand (63 seeds/flat). A total of
503 seedlings were classified.
As expected, tac showed evidence of linkage with apu, the
estimated recombination frequency being 23.6 + 2.2% (Table 1).
This result is consistent with earlier findings (3,5). The data
also reveal evidence of a loose linkage between tac and r_b (39.0 +
3.7). Past results indicated that apu is situated between _s_t and
tac, i.e. apu is closer to tac than is st^. Thus, apu should lie
closer to _rb than st^. Yet the present data show no evidence oi
linkage between apu and _rb. In fact, the data indicate that r_b
lay distal to tac, and thus presumably more distal to st_ than to
apu. Ordinarily it would be difficult to reconcile Gritton \s
evidence of linkage between st_ and _rb and the present findings.
However, inasmuch as estimates of linkage intensity in Pisum are
notoriously variable, the present data lend support to the con-
clusion that jr_b resides on chromosome 3. Since rb_ is an important
gene in biochemical studies and since isozyme markers have been
mapped to the pertinent region in chromosome 3, there is reason to
suppose that an isozyme marker will be found in close proximity
with rb.
1. Gritton, E. T. 1971. PNL 3:15-16.
2. Marx, G. A. 1982. PNL 14:43-46
3. Marx, G. A. 1984. PNL 16:46-48.
4. Marx, G. A. 1986. PNL 18:49-52.
5. Marx, G. A., N. F. Weeden, and R. Provvidenti. 1985.
PNL 17:57-60.

PNL Volume 19 1987 RESEARCH REPORTS 41
Table 1. F2 analysis of a three-point cross:
tac apu Rb x Tac Apu rb .
Tac
Apu
Rb
Tot.
Gene Pair
Chi-square
Recomb.
fract.
S.E.
+
+
+
228
+
+
-
94
X
Y
Linkage
+
-
+
30
Tac - Apu
0.56
0.08
101.84**
23.6
2.2
+
-
-
18
Tac - Rb
0.56
0.72
10.02**
39.0
3.7
-
+
+
51
-
+
-
7
Apu - Rb
0.08
0.72
0.00
-
-
-
-
+
60
(Pop. C286-543-550)
-
-
-
15
503
*****