Observations indicate epistasis of nipple tip gene n-2 over n-4
University of Florida, IFAS, Gulf Coast Research & Education Center, Bradenton, FL 34203
One method to breed for smooth blossom scars in fresh market tomatoes is to use genes
with pointed blossom-end morphology. These genes are recessive although heterozygotes often
have increased frequencies of pointed fruit than wild types (Barten et al., 1994). In my breeding
program two nipple tip genes are being used for this purpose, n-2 and n-4 (Barten et al., 1994). The
n-2 gene has not been mapped. It is characterized by a rather subtle pinpoint blossom scar that
rarely has persistent pointedness in mature fruit. The gene n-4 is likely the same gene as the n
gene first described as being associated with adaxial leaf curl (Young and MacArthur, 1947;
Gardner and Nash, 1987) and mapped to chromosome 5 (Mutschler et al., 1987). The n-4
designation is used because Barten et al. (1994) found LA 2353, which was supposed to have n,
had no leaf curl and did not fit the original description of n. The leaf curl linked with the n-4 (or
historical n) gene has been associated with increased early blight (Gardner and Nash, 1987) and
bacterial spot (Scott, unpublished data) probably because more moisture from rains or dews is
retained on the curled foliage. Thus, varieties with such foliage may be more prone to foliar
diseases when grown in humid production regions. One also has to be careful in developing
varieties that are homozygous for n-4 as they might have mature fruit that have pointed blossom
ends under some environmental conditions and such fruit cause post harvest damage from bruising
or puncturing of other fruit. One way to avoid such problems is to develop hybrids by crossing a
parent that has n-4 with a parent that has n-2. The double heterozygotes are typically smooth
without persistent nipples in mature fruit (Barten, et al., 1994).
Over the years I have crossed inbreds homozygous for n-2 with inbreds homozygous for n-4
and then made selections in generations segregating for both genes. Several times plants
homozygous for n-4 have been selected that then segregate progeny with n-4 and n-2 phenotypes
in 3:1 ratios (data not shown). This would indicate that the original selections were n-4/n-4,n-2/+ and
when n-2/n-2 recombinants occurred the phenotype appears n-2 like. Thus, there is an epistatic
relationship between the two loci with n-4 being hypostatic to n-2. Breeders should be aware of this
in case one wanted to use such a selection as a hybrid parent for crosses with parents homozygous
for n-4. The parent with the n-2 phenotype would be genotypically n-4/n-4, n-2/n-2 and the n-4
expression and its possible drawbacks will be evident in the hybrids (n-4/n-4,n-2/+).
Barten, J.H.M., Scott, J.W., and R.G. Gardner. 1994. Characterization of blossom-end morphology
genes in tomato and their usefulness in breeding for smooth blossom-end scars. J. Amer. Soc. Hort.
Gardner, R.G. and A.F. Nash. 1987. Observations on the nipple tip (n) trait and associated
characteristics. Rpt. Tomato Genet. Coop. 37:45.
No navigation control above? Click here!