Genetic mapping of sex determination in a wild strawberry, Fragaria virginiana reveals earliest form of sex chromosome

Authors: SPIGLER, RACHEL - UNIV OF PITTSBURGH; Lewers, Kimberly; MANN, DORRIE - WASHINGTON STATE UNIV; ASHMAN, TIA-LYNN - UNIV OF PITTSBURGH

Submitted to: Heredity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2008
Publication Date: 9/18/2008
Citation: Spigler, R., Lewers, K.S., Mann, D., Ashman, T. 2008. Genetic mapping of sex determination in a wild strawberry, Fragaria virginiana reveals earliest form of sex chromosome. Heredity. 101:507-517.

Interpretive Summary: Scientists who develop improved strawberry varieties sometimes use a wild strawberry as a parent, because the wild strawberry may be the only source of the trait, such as resistance to a new disease. The particular wild strawberry needed may be one whose flowers have no male parts (anthers and pollen) or whose flowers fail to set fruit even when pollinated. Therefore, additional information is needed about the inheritance of the genes that cause lack of male parts and the genes that control lack of fruit set. We made a large family of 184 seedlings from cross-pollinating two wild strawberry plants, one lacking male parts, and one that has, in the past, produced seedlings that produced no fruit. We characterized all 184 for the presence of male parts and for fruit set, as well as the presence or absence of 217 specific DNA sequences. We found that the gene controlling production of male parts is on the same chromosome as the gene(s) controlling fruit set. We also found a specific DNA sequence very close to them both on that same chromosome. This information is useful to scientists developing improved strawberry varieties.

Technical 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. However, we are now just beginning to gain insight into the initial stages of sex chromosome evolution via studies in species with transitional breeding systems. Here we inform on the genetic mechanism of sex determination in the octoploid subdioecious wild strawberry, F. virginiana Mill. by creating a SSR-based genetic map and mapping sex determination as two qualitative traits pertaining to male and female function. The resultant map was comprised of 217 markers arranged into 42 linkage groups with an average marker spacing of 13.4 cM. Total map length was 2399.3 cM. We estimated that approximately 70% and 91% 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 along with two SSR markers. These traits were separated by approximately 6 cM. Our phenotypic and genetic mapping results support a model of gender determination with at least two linked loci (or gene regions) with major effects. Evidence of recombination between the loci indicates both female and male heterogamety exist in this species. And some, but not all, of the hallmarks of incipient sex chromosomes, suggest that F. virginiana perhaps represents a species with the youngest example of a sex chromosome in plants and thus a novel model system for the study of sex chromosome evolution.