The genetic location of vp1

vp1 is an interesting pleiotropic mutation which besides causing premature germination of the embryo: fails to synthesize anthocyanin pigments in the aleurone. Anthocyaninless vp1 aleurones have been found to be deficient in at least 3 enzymes of flavonoid biosynthesis (PAL, chalcone synthase and UFGT), as well as in several other metabolically unrelated enzymes that show pronounced increases in late stages of aleurone development, such as ADH and catalase (Dooner, Plant Physiol., in press). Though vp1 has been the subject of rather extensive physiological studies, we only know its location to chromosome arm. Its precise genetic location has not been established, mostly because of the difficulty of maintaining homozygous vp1 stocks, which have to be grown continuously due to the absence of dormancy in the mutant stock. However, Robertson has described a vp1 allele extracted from a stock of Ken McWhirter that shows considerable dormancy, making it possible to maintain a homozygous vp1 stock (MNL 35:104-105, 1965). We have taken advantage of the existence of this allele to map vp1.

Two sets of crosses were performed in order to map vp1. The first set of crosses was aimed at obtaining the approximate location of vp1 in 3L. The heterozygote (Vp1) a1 sh2/(vp1) A2 Sh2 was backcrossed to (Vp2) a2 sh2, and from that cross the infrequent crossovers A1 sh2 and a1 Sh2 were selected and selfed in order to score vp1. Both A1 sh2 individuals segregated vp1; of the two a1 Sh2 individuals, one was Vp1/Vp1 and the other one segregated vp1. On this basis, vp1 was placed proximal to the a1 sh2 pair in 3L. A random sample of parental a1 sh2 and A1 Sh2 types from the above cross were also planted and selfed to classify their vp1 constitution. The following classes were obtained: 37 Vp1 a1 sh2; 28 vp1 A1 Sh2; 22 vp1 a1 sh2 and 25 Vp1 A1 Sh2. Thus, there is very loose linkage between vp1 and the a sh2 pair. From the above data, vp1 would map 42 map units proximal to the a1 sh2 pair.

Based on the above results, a second three-point cross was set up as follows. gl6 (Vp1) 1g2l Gl6 (vp1) Lg2 heterozygotes were backcrossed to gl6 (Vp1) lg2. Progeny from the backcross were scored for gl6 and lg2 and selfed to classify vp1. They fell into the following classes:
 
gl6 Vp1 lg2 31
Gl6 vp1 Lg2 36
Gl6 Vp1 lg2 3
gl6 upl Lg2 4
Gl6 vp1 lg2 3
gl6 Vp1 Lg2 7

From the above results the following maps can be drawn:

Figure.

A comparison of distances in the distal (righthand) side of vp1 reveals reasonable agreement with the current linkage map for the lg2-a1 interval, and places vp1 between gl6 and lg2.

Hugo K. Dooner


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