Institute of Cytology and Genetics, Novosibirsk 630090, Russia

Pea is a convenient model for studying the genetic control of compound leaf development. In addition, the pea has well developed stipules, evident as a pair of large leaf-like appendages at the base of each compound leaf. Superficial similarity of the stipules and leaflets is in accordance with the existence of a number of genes (red, lath, blb, etc.) controlling development of both organs [1, 3, 6]. At the same time, there are two mutations, st and coch, with a specific effect on stipule structure, that is exhibited by transformation of the stipules into very narrow blades. Moreover, both mutations reduce bracts on the inflorescence, suggesting the homology of these organs to stipules but not to leaves. The mutation st has no noticeable effects on the morphology of other organs or on the plant fertility and viability, while the mutation coch causes numerous pleiotropic effects, including diverse anomalies in inflorescence and flower structure [4, 5], associated with a sharp decrease in fertility, to complete sterility in some mutant alleles. The detailed analysis of the coch effects [2, 3, 5] revealed that the mutant stipules are on their path to transformation into the compound leaves, that can sometimes cause three compound leaves to appear at one plant node. These observations suggest that coch belongs to a class of homeiotic genes which comprise the basis of development programs of plants [10].

As a rule, homeiotic genes code for the proteins which act as transcription regulation factors. It is quite possible that the molecular product of the gene Coch represses the program of development of a compound leaf in the stipule primordia

For a more profound understanding of the coch gene action mechanism, it is desirable to study its combinations with other genes rendering effects on development of the compound leaf and other organs. However, negative pleiotropic effects of coch seriously impede construction of such gene combinations. To avoid this difficulty, a semilethal gene, such as coch, may be flanked by a pair of convenient marker genes. Scoring for such markers would facilitate the maintenance of lines with the semilethal gene in a heterozygous state. Earlier we have revealed a close linkage of coch with molecular marker Sca (coding for one of the seed proteins) and His1 (coding for histone H1 subtype 1). Sca is especially useful as a marker because the genotype of a plant with respect to this gene easily can be determined by electrophoresis of material from a small cotyledon fragment of a dry seed [7]. Unfortunately, earlier [9] in the genetic analysis of an F2 progeny we obtained approximately equal recombination frequency values for each pair from three genes, coch, His1 and Sca, preventing us from generating a correct map of this chromosome area.

In this work we have analyzed the F2 progeny of two crosses. In the first cross SG-coch and JI1238 were used as parents, the latter being a multiple-marker line which differed from the SG-coch line at four loci (coch, Sca, r and tl) on linkage group V. Joint segregation analysis indicated that coch was located near Sca, but on the side opposite from r and tl (Table 1).

Table 2. Joint segregation data in F2 progeny of cross line (R, tl-w, His1-1, coch-1, Sca-f, Py1) x (r, Tl, His1-2, Coch, Sca-s, py1)

Gene pair	Phase	Number of progeny with designated phenotypea									Joint c2	Recomb. fraction	Stand. Error
		A/B	A/h	A/b	h/B	h/h	h/b	a/B	a/h	a/b
coch/py	R	49		17	—	—	—	25		0	7.9*	<20	—
coch/Sca	C	18	47	1	—	—	—	0	4	21	68***	5.2	2.4
coch/His1	C	16	45	5	—	—	—	0	5	19	47***	10.7	3.4
coch/gp	C	56		10	—	—	—	10		15	18***	24	5.2
py1/Sca	C	22	45	7	—	—	—	0	6	11	28***	16	4.2
py1/His1	C	22	44	7	—	—	—	2	6	9	18***	23	4.9
py1/gp	R	50		24		—	—	16		1	4.9	24	9.8
Sca/His1	C	15	0	3	1	49	1	0	1	20	139***	5.2	1.7
gp/Sca	C	16	39	11		—	—	2	12	11	8.4	31	5.7
gp/His1	C	13	39	13	—	—	—	3	11	11	5.4	36

^a A,a - first gene; B,b - second gene; h - heterozygous.
        1. Both genes are dominant: capital letter stands for dominant allele.
        2. Second gene is codominant: capital A stands for the dominant allele of the first gene
            and capital B - for an allele of the second gene, being in coupling with A.
        3. Both genes are codominant: capital letter stands for an allele of the first parent.
^* = P < 0.05,
^** = P < 0.01,
^*** = P < 0.001
    Calculations were done with the aid of our original program Cros.

The absence of identifiable recombinants between the genes py1 and coch (which were in repulsion phase) did not allow the determination of the order of these genes, but in the F3 progeny of one plant with genotype (Coch/-, py1/py1, Sca-f/ Sca-s, His1-1/ His1-2), coch/coch homozygotes were detected. This result indicates that the original F2 plant was heterozygous with respect to the gene coch, and thus, placing coch between Sca and py1. The genetic map (in cM) of this region of linkage group V, constructed with the help of computer program JoinMa p [8], is given below.

The location of coch between the genes Sca and py1 allows genetic manipulations with the gene coch in heterozygous plants (Sca-f, coch, Py1 / Sca-s, Coch, py1). We only need to plant seeds heterozygous for Sca and then select plants with phenotype Coch, Py. By selecting the seeds homozygous for Sca-f allele, it is possible to assure that the coch allele was not lost.

This work was partly supported by the Russian State Program "Russian Fund for Fundamental Research."

 

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