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With their extraordinary diversity in sexual systems, flowering plants offer unparalleled opportunities to understand sex determination and to reveal generalities in the evolution of sex chromosomes. Comparative genetic mapping of related taxa with good phylogenetic resolution can delineate the extent of sex chromosome diversity within plant groups, and lead the way to understanding the evolutionary drivers of such diversity. The North American octoploid wild strawberries provide such an opportunity. We performed linkage mapping using targeted sequence capture for the subdioecious western Fragaria virginiana ssp. platypetala and compared the location of its sexdetermining region (SDR) to those of 2 other (sub)dioecious species, the eastern subspecies, F. virginiana ssp. virginiana (whose SDR is at 0-5.5 Mb on chromosome VI of the B2 subgenome), and the sister species F. chiloensis (whose SDR is at 37 Mb on chromosome VI of the Av subgenome). Male sterility was dominant in F. virginiana ssp. platypetala and mapped to a chromosome also in homeologous group VI. Likewise, one major quantitative trait locus (QTL) for female fertility overlapped the male sterility region. However, the SDR mapped to yet another subgenome (B1), and to a different location (13 Mb), but similar to the location inferred in one population of the naturally occurring hybrid between F. chiloensis and F. virginiana (F. ×ananassa ssp. cuneifolia). Phylogenetic analysis of chromosomes across the octoploid taxa showed consistent subgenomic composition reflecting shared evolutionary history but also reinforced within-species variation in the SDR-carrying chromosome, suggesting either repeated evolution, or recent turnovers in SDR. [ABSTRACT FROM AUTHOR]

Summary: Recombination in ancient, heteromorphic sex chromosomes is typically suppressed at the sex‐determining region (SDR) and proportionally elevated in the pseudoautosomal region (PAR). However, little is known about recombination dynamics of young, homomorphic plant sex chromosomes.We examine male and female function in crosses and unrelated samples of the dioecious octoploid strawberry Fragaria chiloensis in order to map the small and recently evolved SDR controlling both traits and to examine recombination patterns on the incipient ZW chromosome.The SDR of this ZW system is located within a 280 kb window, in which the maternal recombination rate is lower than the paternal one. In contrast to the SDR, the maternal PAR recombination rate is much higher than the rates of the paternal PAR or autosomes, culminating in an elevated chromosome‐wide rate. W‐specific divergence is elevated within the SDR and a single polymorphism is observed in high species‐wide linkage disequilibrium with sex.Selection for recombination suppression within the small SDR may be weak, but fluctuating sex ratios could favor elevated recombination in the PAR to remove deleterious mutations on the W. The recombination dynamics of this nascent sex chromosome with a modestly diverged SDR may be typical of other dioecious plants. [ABSTRACT FROM AUTHOR]

Gynodioecy, the coexistence of females and hermaphrodites, occurs in 20% of angiosperm families and often enables transitions between hermaphroditism and dioecy. Clarifying mechanisms of sex determination in gynodioecious species can thus illuminate sexual system evolution. Genetic determination of gynodioecy, however, can be complex and is not fully characterized in any wild species. We used targeted sequence capture to genetically map a novel nuclear contributor to male sterility in a selfpollinated hermaphrodite of Fragaria vesca subsp. bracteata from the southern portion of its range. To understand its interaction with another identified locus and possibly additional loci, we performed crosses within and between two populations separated by 2000 km, phenotyped the progeny and sequenced candidate markers at both sex-determining loci. The newly mapped locus contains a high density of pentatricopeptide repeat genes, a class commonly involved in restoration of fertility caused by cytoplasmic male sterility. Examination of all crosses revealed three unlinked epistatically interacting loci that determine sexual phenotype and vary in frequency between populations. Fragaria vesca subsp. bracteata represents the first wild gynodioecious species with genomic evidence of both cytoplasmic and nuclear genes in sex determination. We propose a model for the interactions between these loci and new hypotheses for the evolution of sex determining chromosomes in the subdioecious and dioecious Fragaria. [ABSTRACT FROM AUTHOR]