Submitted to: Journal of Heredity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 11, 2010
Publication Date: March 8, 2010
Citation: Spigler, R., Lewers, K.S., Johnson, A., Ashman, T. 2010. Autosomal origin of sex chromosome in a polyploid plant. Journal of Heredity. 101:S107–S117.
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 (femaleness) and the genes that cause lack of fruit set (maleness). Previously, we created a map showing the position of 217 DNA sequences in relation to each other and to the genes controlling maleness and femaleness. We also showed that these genes were right next to each other on the same chromosome, and that we had thereby identified this wild strawberry species as the only known representative of the earliest stage of sex chromosome evolution. In this recent work, we added 174 new DNA sequences to the previous map and compared the resulting new map with the published map of a strawberry ancestral species with only one fourth the number of chromosomes of the cultivated strawberry. The comparison enabled us to identify the ancestral chromosome from which the pre-sex-chromosome had evolved. This information is of significant interest to scientists studying genome evolution and useful to scientists developing improved strawberry varieties.
While theory on sex chromosome evolution is well developed, evidence of the early stages of this process remains elusive, in part because this process unfolded in many animals so long ago. The relatively recent and repeated evolution of separate sexes (dioecy) and sex chromosomes in plants, however, provides novel insight into the early stages of sex chromosome evolution. Moreover, in plants the transition to dioecy from hermaphroditism is often associated with speciation via polyploidization, suggesting that polyploidy may facilitate the evolution of separate sexes. To address the gap in our understanding of the earliest stages of sex chromosome evolution and the positive association between polyploidy and dioecy, we identified the autosomal homoeolog of the proto-sex chromosome in a subdioecious octoploid plant (Fragaria virginiana) via comparative analysis of a well-resolved SSR-based genetic map and a published map of a diploid hermaphroditic putative ancestor. We assessed levels of macrosynenty and evaluated whether rearrangements were involved in the evolution of the protosex chromosome. Our results clearly support of the hypothesis that the proto-sex chromosomes in Fragaria virginiana arose from a single pair of autosomes in the diploid, with few genomic changes, and provide first evidence of such an event in an octoploid plant.