1. ¹Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, USA
2. ²Genetic Improvement of Fruits and Vegetables Lab, USDA-ARS, Beltsville Agricultural Research Center, Beltsville, MD, USA
3. ³Department of Horticulture and Landscape Architecture, Washington State University, Pullman, WA, USA

Correspondence: Dr T-L Ashman, Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260-3929, USA. E-mail: tia1@pitt.edu; Dr KS Lewers, Genetic Improvement of Fruits and Vegetables Lab, USDA-ARS, Beltsville Agricultural Research Center, Beltsville, MD 20705-2350, USA. E-mail: kim.lewers@ars.usda.gov

Received 1 May 2008; Revised 25 July 2008; Accepted 8 August 2008; Published online 17 September 2008.

Abstract

The evolution of separate sexes (dioecy) from hermaphroditism is one of the major evolutionary transitions in plants, and this transition can be accompanied by the development of sex chromosomes. Studies in species with intermediate sexual systems are providing unprecedented insight into the initial stages of sex chromosome evolution. Here, we describe the genetic mechanism of sex determination in the octoploid, subdioecious wild strawberry, Fragaria virginiana Mill., based on a whole-genome simple sequence repeat (SSR)-based genetic map and on mapping sex determination as two qualitative traits, male and female function. The resultant total map length is 2373 cM and includes 212 markers on 42 linkage groups (mean marker spacing: 14 cM). We estimated that approximately 70 and 90% of the total F. virginiana genetic map resides within 10 and 20 cM of a marker on this map, respectively. Both sex expression traits mapped to the same linkage group, separated by approximately 6 cM, along with two SSR markers. Together, our phenotypic and genetic mapping results support a model of gender determination in subdioecious F. virginiana with at least two linked loci (or gene regions) with major effects. Reconstruction of parental genotypes at these loci reveals that both female and hermaphrodite heterogamety exist in this species. Evidence of recombination between the sex-determining loci, an important hallmark of incipient sex chromosomes, suggest that F. virginiana is an example of the youngest sex chromosome in plants and thus a novel model system for the study of sex chromosome evolution.