PNL Volume 18 1986

RESEARCH REPORTS

LINKAGE RELATIONS OF Curl, Orc, AND "Det" WITH MARKERS ON CHROMOSOME 7

Marx, G. A. NYS Agricultural Experiment Station, Cornell University

Geneva, NY USA

Plants possessing the EMS-induced recessive mutant curl (6) bear a

clear and distinctive phenotype early in the seedling stage and throughout

their ontogeny. Though curl segregants are weaker and less productive than

Curl/- counterparts, the mutant is a favorable marker for genetic studies.

Preliminary observations made several years ago indicated that curl might

belong to chromosome 7 (data not shown).

In 1984-1985 I constructed populations to verify and refine the

linkage relationship among curl, r, and bt. The results of these crosses,

three in all, are given in Table 1. For one population the F1 seeds were

separated into round (R/-) and wrinkled (r/r) classes before planting but

each F2 plant was also classified for R-r again at the dry seed stage to

ensure the accuracy of the classification.

The data argue persuasively that curl is linked with r and bt in what

is still generally accepted as chromosome 7, the Indicated order being

curl-r-bt. Since, however, there still is controversy concerning the

placement of the r-tl-bt group (2,3) curl is an additional clear marker

which may help to resolve the controversy. Another unsettled matter con-

cerns the placement of wsp on chromosome 7. I have not succeeded in

confirming the reported linkage relationship between wsp and chromosome 7

markers (4). The fact that curl appears to be located toward the wsp end

of chromosome 7 on the current map should aid in the ongoing quest to

settle this issue as well. The linkage relations of two other newly dis-

covered markers may also contribute to a resolution.

Orc, described by Blixt and Swiecicki (1,7), is a dominant mutant that

causes orange cotyledons containing high amounts of lutein (5). My own

experience with Ore indicates that the lower leaves of mutant plants become

orangy yellow about the time the plants begin to flower, thus providing

another diagnostic criterion for scoring Or c plants in segregating

populations. A moderate sized F2 segregating simultaneously for Bt-bt,

Orc-orc, and R-r indicates that the three genes are linked (Table 2).

Accordingly, Orc and curl should show strong linkage, a supposition we are

in the process of testing.

Another mutant that has recently come into my possession, and which I

will provisionally designate det� , also shows linkage with R and Bt. Seed

of the mutant was provided through the courtesy of Dr. Peter Matthews; the

mutant line is maintained in the John Innes Institute Pisum collection

under the number J.I. 1358/B. The mutant behaves as a monogenic recessive

(Table 3a) and is distinguished from normal, wild type plants by its deter-

minate reproductive behavior. The term "determinate" has been used in

1/ It falls to Drs. Matthews and Blixt to accept or reject the gene name and

symbol.

PNL Volume 18 1986 RESEARCH REPORTS

various contexts in the pea literature so it is important to make clear

that the present mutant confers a determinate habit in the botanical sense

i.e. reproductive growth is terminated with an inflorescence. These plants

typically produce two sets of inflorescences on the main stem. The first

peduncle is borne in the leaf axis of the first reproductive node but the

second and final reproductive node bears two peduncles. Further reproduc-

tive activity may occur but, if so, it arises from axillary branches lower

down the stem.

Analysis of various crosses between J.I. 1358 and several marker lines

revealed evidence of linkage between det and r and bt (Table 3b,c).

The det mutant may have important implications for applied breeding as

a means to regulate the balance of reproductive and vegetative growth.

There may be an advantage associated with the limits imposed on the maximal

number of pods at the top of the main stem axis. As a result, photosyn-

thates may be preferentially partitioned to the fruits and seeds because

there is no longer a commitment, or even a potential, to continued apical

development. Because pods borne above the third or fourth reproductive

node on conventional varieties rarely, if ever, contribute significantly to

crop yield under commercial field conditions, continued pod and seed forma-

tion puts a drain on plant metabolites but that drain is not translated

into economic yield. Pods produced on det plants tend to be borne within a

relatively narrow time span and consequently the pods and seeds are rela-

tively uniform in maturity. Pods and seeds borne on axillary branches may

or may not contribute to field yield depending on the level of intra- and

inter-plant competition and on the level of available environmental

resources. It may be possible to enhance yield potential still more by

incorporating genes for multiple podding. Finally, the trait may

facilitate hand harvesting in lines intended for the home and market

garden. The validity, if any, of these speculations must await extensive

breeding, selection, and evaluation.

1. Blixt, S. and W. K. Swiecicki. 1983. PNL 15:9-10.

2. Folkeson, D. 1985. PNL 17:14-15.

3. Lamm, R. 1978. PNL 10:32-33.

4. Lamprecht, H. 1954. Agri Hort. Genet. 12:115-120.

5. Ludwicki, J. and w". K. Swiecicki. 1985. PNL 17:51-53.

6. Sidorova, K. K. and L. P. Uzhlntzeva. 19/5. PNL 7:56.

7. Swiecicki, W. K. and S. Blixt. 1984. PNL 16:70-72.

PNL Volume 18 1986 RESEARCH REPORTS

Table 1. (a-c) Analysis of joint segregation in F2, for genes Curl, R,

and Bt in three different three-point crosses (Curl r bt x

curl R Bt); (d) Segregation for Curl and curl in F₂ of a

population not segregating for linked markers.__

PNL Volume 18 1986 RESEARCH REPORTS

Table 3. (a) Single gene segregation for indeterminate (Det ) and deter-

minate (det) in two F2 populations; and (b) analysis of joint

segregation of Det with Bt and with R.

(a) Number of normal (Det) and determinate (det) segregants reproductive

habit in two segregating F2 populations.

(b) Analysis of F2 populations derived from three two-point crosses

involving Det, Bt, and R.

EDITOR'S NOTE:

Four articles submitted by W. K. Swlecickl arrived too late to be

included in the present volume. Although the articles were posted on 19

February, they did not arrive in Geneva until 24 April, at which time

the volume had already been delivered to the printer. The envelope

containing the manuscripts was marked "Air Mail", "express",

"Certified", so clearly Dr. Swiecicki was not responsible for the ex-

traordinary delay in getting the manuscript to me.

In one article Dr. Swiecicki provides good evidence for linkage

between det and r and tl. He therefore rightfully shares equal credit

for being first to establish the linkage relations or det. Moreover,

his article also takes note of the possible applied implications of the

det mutant.

In another article, Swiecicki's data led him to conclude that Orc

is located on chromosome 1 and that the yellow orange foliage is caused

by a separate but linked gene, designated orl. However, my evidence

(above) suggests linkage of Orc on chromosome 7 and that the foliage

color is a pleiotropic effect of Orc. Thus, in this case, our conclu-

sions conflict. If Swiecicki's view prevails then he deserves priority.

The third and fourth articles, co-authored with B. Wolko, consider

the use of isozyme markers in identifying cultivars and confirm the

linkage of Aat-p and A on chromosome 1.