Philouze,J.
Epistatic relations between ls and pat-2.
Crosses were made between Severianin, which develops parthenocarpic fruits as a result of the pat-2 gene (Philouze and Maisonneuve, TGC 28, 1978), and lateral suppressor (ls/ls) lines, with 2 objectives: (1) localization of the pat-2 gene on the chromosome map, is (lateral suppressor) being a marker gene on the chromosome 4, and (2) to obtain ls/ls pat-2/pat-2 recombinants. Are these recombinants interesting from a practical point of view? Is pat-2 able to overcome the very low fertility characteristic of the ls gene?
Two ls/ls lines were used: Primabel ls/ls a spontaneous mutant in the
Primabel cultivar, and Monalbo ls/ls obtained by introduction after 5 backcrosses of the ls allele from LA 329 (received from C.M. Rick) in the Monalbo cultivar (a Moneymaker type homozygous for Ve).
40 F2 ls/ls plants from the cross (Severianin x Primabel ls/ls) and 40 F2 ls/ls plants from the cross (Severianin x Monalbo ls/ls) were grown in a heated glasshouse in spring (planted on February 16), and 220 F2 ls/ls plants from the cross (Severianin x Monalbo ls/ls) were grown in the open field (planted on April 30). No fruits of normal size developed on any of the 300 F2 ls/ls plants. Small, dull, ribbed, seedless fruits, characteristic of ls/ls plants, developed on both non-pollinated emasculated flowers and open-pollinated flowers. It was impossible to screen for pat-2/pat-2 plants under these conditions. Two hypotheses could be formulated: (1) the ls and pat-2 genes are linked; (2) the ls and pat-2 genes are independant, but the ls gene prevents the expression of the pat-2 gene.
To choose between these 2 hypotheses, open-pollinated fruits from 50 F2 ls/ls random plants from the cross (Severianin x Monaiho ls/ls) (F2 plants grown in the open field), were harvested. A few seeds were obtained from 25 plants only. The germination of these seeds was very low, and it was possible to follow the F3 offspring of only 9 F2 plants. One F3 plant issued from each of the 9 F2 plants was used as male parent to make crosses with pat-2/pat-2 ls/ls material. Four “F1” plants with the same male parent (total: 36 “F1” plants), and 2 plants of both of the control varieties Severianin and Monalbo, were grown in a heated glasshouse in spring (planted on March 30). The plants were considered parthenocarpic when 5 to 7 fruits developed on the first truss, from a maximum of 7 non-pollinated emasculated flowers, and when the size of seedless fruits from open-pollinated flowers was comparable to that of seeded fruits on the same plant.
The 9 F3 ls/ls plants could be classed in the following genotypes: 2 ±1±, 5 pat-2/+ and 2 pat-2/pat-2 (Table 1). The expected ratio for independence between ls and pat-2 is 3/8, 2/8 and 3/8. The χ2 : 4.48 is not significative (0.l0<P<0.50). It can be concluded that the ls and pat-2 genes are independent and that, in presence of ls, pat-2 is unable to express its ability to develop parthenocarpic fruits of normal size. In other words, as far as fruit-setting ability is concerned, is is epistatic to pat-2.