PNL Volume 17 1985

RESEARCH REPORTS 57

LINKAGE RELATIONSHIPS AMONG MARKERS IN CHROMOSOME 3 AND En, A GENE

CONFERRING VIRUS RESISTANCE

Marx, G. A., N. F. Weeden, and R. Provvidenti

NYS Agricultural Experiment Station, Geneva, NY USA

Two excellent seedling markers, apu and tac, recently were reported

to be situated in chromosome 3, linked with st (4). The data did not

reveal, however, if apu and tac lie towards the b or the M end of the

chromosome, so new linkage tests were performed using b as an additional

marker. F2 populations from the following crosses were scored in the

field in 1984 ( only the relevant genes being listed):

A B_ St apu Tac x A b st Apu tac (and reciprocal)

a B St apu tac x b st Apu Tac

a B St apu tac x A b st Apu Tac

The analysis (Table 1) confirms the placement of apu and tac in

chromosome 3 and localizes tac in the M end of the chromosome. Although

the orientation of apu is somewhat unclear from these data, the overall

body of evidence indicates that apu also lies on the M side of st.

These findings, together with results from previous studies of

chromosome 3 (2, 3), make it evident that suitable seedling markers are

favorably spaced over nearly the entire length of the chromosome. This

set the stage for three further linkage tests conducted in the green-

house in the fall of 1984. They were designed to establish the rela-

tionships among morphological markers, certain isozyme markers, and the

gene En conferring resistance to pea enation mosaic virus (PEMV).

Initially we determined the relationship between three morpholo-

gical markers, st, apu, and tac, and two isozyme markers, Lap-2 and

Aat-c (Table 2). Gel electrophoresis was performed using methods

previously described (5). Linkage between Aat-c and Lap-2 is evident.

Moreover, both isozyme markers showed linkage with tac and both evi-

dently lie distal to tac.

PNL Volume 17 1985

RESEARCH REPORTS

In the second and third greenhouse experiments the F2 populations

segregated for resistance (En) and susceptibility (en) to PEMV. Gritton

and Hagedorn (1) and D. W. Barton (unpublished) earlier had found lin-

kage between En and st. En was believed to be in the M arm of

chromosome 3, but because there was no certainty of this, we used a line

carrying a morphological marker (chi-6) known to be located distal to b.

(2). The cross had the formula:

En Tac Apu st chi-6 x en tac apu St Chi-6

Seedling F2 plants were inoculated in the two-three leaf stage by rub-

bing the leaves with a known culture of PEMV. A week later the plants

were inoculated again to ensure against escapes. Analysis of this cross

revealed that En and tac are rather tightly linked ( - 8 units),

verifying that the En locus resides in the M end of chromosome 3

(Table.3).

The third of the three greenhouse experiments involved the cross:

Aat-c(S) Lap-2(S) En st chi-6 bulf x Aat-c(F) Lap-2(F) en St Chi-6 Bulf

In this case the M end of chromosome 3 was marked only with isozyme

variants and the b end was marked by morphological genes. The rela-

tionships among the genes segregating in this cross (Table 4) were con-

sistent with expectation. However, the St-En relationship is probably

anomalous because the genes were in repulsion and the population size

was small. Although both Aat-c and Lap-2 showed linkage with En,

neither was sufficiently close to be considered a useful marker for En.

However, as described elsewhere in this volume (6), Adh-1, Acp-3 and

Gal-3 also map to this general region of chromosome 3. One of these

loci should show close linkage with En.

Considering past and present evidence, our best current estimate of

the relative order and distance among the markers discussed herein is as

follows:

Aat-c-ll-Lap-2-17-Tac-8-En-22-Apu-10-St-27-B-20-Bulf-8-Chi-6

1. Gritton, E. T. and D. J. Hagedorn. 1980. PNL 12:26-27.

2. Marx, G. A. 1982. PNL 14:43-46.

3. Marx, G. A. 1983. PNL 15:47.

4. Marx, G. A. 1984. PNL 16:46-48.

5. Weeden, N. F. and G. A. Marx. 1984. J. Heredity 75:365-370.

6. Weeden, N. F., G. A. Marx, and E. Pagowska. 1985. PNL 17:75-76.