Genetic Linkage between Runnering and Phosphoglucoisomerase Allozymes, and Systematic Distortion of Monogenic Segregation Ratios in Diploid Strawberry

As part of a strawberry (Fragaria sp.) genome mapping project, we studied the linkage relationship between runnering and phosphoglucoisomerase PGI-2 allozymes in diploid strawberry. The respective r and Pgi-2 loci were found linked with a recombination frequency of 18.1% ± 1.6%(a map distance of 18.9 ± 1.6 cM). This is the second reported linkage in strawberry. The linkage between runnering and phosphoglucoisomerase allozymes, if conserved at the octoploid level, might provide a means of marker-assisted selection for the nonrunnering and bushy branching growth habits in cultivated strawberry. Severe distortion of monogenic segregation ratios was observed for runnering and PGI- 2, and also for an unlinked locus for shikimate dehydrogenase allozymes. Alleles from the perpetual flowering (alpine F. vesca) parents were favored in this distortion. This phenomenon should be considered in future genetic studies using crosses between alpine and nonalpine strawberries. The strawberry (Fragaria sp.) is an excellent subject for genetic and evolutionary studies for many reasons. It is a popular fruit of significant economic importance. The genus has widespread dis- tribution and high genetic diversity in its natural habitats across Asia, Europe, and the Americas (Hancock and Luby, 1993; Luby et al., 1992). There are at least 20 different species and four ploidy levels (Staudt, 1962, 1989). It frequently and spontaneously hy- bridizes to produce natural hybrids, including interspecific hybrids across ploidy levels (Bringhurst, 1990). The hybrid forms are preserved by vegetative propagation. However, genetic research has been hampered by the genetic complexity stemming from polyploidy, particularly in octoploid cultivated strawberry (F. ×ananassa Duch., 2n = 8x = 56). Several authors (Arulsekar and Bringhurst, 1983; Brown and Wareing, 1965; Davis and Pollard, 1991; Williamson et al., 1995) point out that, since there is a similar spectrum of character variation (e.g., seasonal vs. perpetual flow- ering, runnering vs. nonrunnering) and intercrossability between polyploid and diploid species, the most common diploid species, F. vesca L. (2n = 2x = 14), can be used as a model system to study strawberry genes and their linkage relationships. Only a few morphological traits of Mendelian inheritance have been studied in strawberry (Galletta and Maas, 1990). Of these, two are perpetual flowering (overbearing) and nonrunnering hab- its associated with the so-called alpine varieties, a group of F. vesca of European origin. Brown and Wareing (1965) found that each of these two traits was controlled by a single recessive gene (s/s and r/r, respectively). Their results indicated no genetic linkage between flowering and runnering, although the wild type (seasonal flowering, runnering) ceased runner production when