| Weeden, N.F. | Department of Plant Sciences, Montana State University |
The allele wsp in a homozygous state produces a plant lacking wax except on the upper surface of leaflets (see reference 1 for history of this gene and synonyms). The phenotype is easy to identify, and the gene has been used in many genetic studies. The location of the locus Wsp, along with linked loci Sil, and Alatc has been problematic. The classical linkage map of pea assembled by Blixt (1) placed Wsp on chromosome 7 (Blixt's nomenclature), distantly linked to Mine and R. However cytogenetic and linkage studies have convincingly placed R and Tl on linkage group V of more recent maps (see reference 4 for most recent map), and the work of Folkeson (2) demonstrated that Wsp was located on a chromosome bearing a satellite (linkage group IV or VII). Further linkage analysis also failed to confirm synteny between Wsp and R (3). The most recent linkage map for pea (4) associated Wsp with linkage group IV, but could not assign the locus to any specific region of that linkage group. Here I report joint segregation analysis between Wsp and Age, the latter locus being recently placed near one end of linkage group IV (5).
In one experiment an F2 population derived from the cross C98-53 (wsp) x C98-1-9 (age) was grown in the greenhouse at Montana State University. A joint segregation analysis of the phenotypes observed in this population is presented in Table 1. Segregation at Wsp and Age, as well as other marker loci not shown in Table 1, was normal, and the fertility of both the F1 and F1 was excellent. The lack of the double recessive phenotype in this population of 92 individuals indicates that the two loci are closely linked. A second, smaller population derived from the cross C98-54 (wsp) x C98-1-12 (age, wlo) and consisting of two progeny from each of 24 F2 plants (four seeds did not germinate) gave only one double recessive plant. The distribution of phenotypes was significantly skewed towards linkage and gave a lower number of double recessives than expected (Table 1).
Table 1. Joint segregation analysis between Wsp and Age
----Phenotype---- |
Expected |
Recombinant |
||||||
Population |
N |
+/+ |
+/- |
-/+ |
-/- |
ratio |
c2 |
Fract + S.E. |
C99-1 |
92 |
41 |
28 |
23 |
0 |
9:3:3:1 |
16.6 |
<15 |
C99-3 |
44 |
13 |
15 |
15 |
1 |
17.2:10.3:10.3:6.2 |
9.67 |
not calculated |
The results confirm that Wsp is on linkage group IV and place the locus close to Age. These findings are particularly important because this region of the pea linkage map is deficient in conveniently scored morphological markers (5). The clear expression of the recessive wsp phenotype in seedlings makes this mutation the most reliable morphological marker yet identified on this arm of linkage group IV. Two other loci, Sil and Alatc, can also be placed near Age, although the order of the loci on the map remains uncertain. In addition, the results confirm that this linkage group represents Folkeson's chromosome 7, as was postulated in reference 4.