Comparative Mapping Reveals Autosomal Origin of Sex Chromosome in Octoploid Fragaria virginiana

Abstract

Recent evolution of separate sexes in flowering plants provides unparalleled opportunities for understanding the early stages of sex chromosome evolution, including their origin from autosomes. Moreover, the transition from combined to separate sexes can be associated with speciation via polyploidization in angiosperms, suggesting that genome doubling/merger may facilitate sterility mutations required for sex chromosome formation. To gain insight into the origin of sex chromosomes in a polyploid plant, we doubled the simple sequence repeat (SSR) density and increased genome coverage in a genetic map of octoploid Fragaria virginiana, a species purported to have a "proto-sex" chromosome, where limited recombination occurs between 2 linked "loci" carrying the male- and female-sterility mutations. Incorporation of almost 3 times the number of SSR markers into the current map facilitated complete characterization of the F. virginiana proto-sex chromosome, revealing its largely autosomal nature and the location of the sex-determining region toward the distal end. Furthermore, extensive synteny between our genetic map and a map involving diploid hermaphroditic congeners allowed assignment of linkage groups to homeologous groups, identification of the proto-sex chromosome's autosomal homoeolog, and detection of a putative rearrangement near the sex-determining region. Fine mapping and additional comparative work will shed light on the intriguing possibility that rearrangements during polyploidization were involved in the evolution of sex chromosomes in Fragaria.

Comparative Genetic Mapping Points to Different Sex Chromosomes in Sibling Species of Wild Strawberry (Fragaria)

“…DNA was diluted to 0.03 ng/ml with distilled deionized water for PCR reactions. Because we were interested in testing synteny, rather than creating a genetic map of the whole genome, we used 56 primer pairs for DNA markers (SSRs or genes) that have mapped either to the homeologous group (HG) in F. virginiana to which the protosex chromosome belongs (i.e., HG VI; Spigler et al 2010) or to the homeologous linkage group (LG) in a diploid interspecific cross (i.e., LG 6; Sargent et al 2006Sargent et al , 2008. PCR was performed using the Poor Man's PCR protocol (Schuelke 2000) as previously described (Spigler et al 2008).…”

“…To identify which products of a given primer pair represented alleles at a single locus, we first mapped products from a given primer pair in JoinMap 4.0 (Van Ooijen 2006). We then retained one member of each allelic pair for creating the full map, consistent with the singledose marker approach of Wu et al (1992) (also see Spigler et al 2010).…”

“…Graphical maps were generated in MapChart 2.1 (Voorrips 2002). Once linkage groups in HG VI in F. chiloensis were assembled, we named them on the basis of LG-specific markers shared with the F. virginiana map in Spigler et al (2010). Thus, LG designations presented here for F. chiloensis represent hypotheses about homeology with F. virginiana.…”

“…Later, selection for recombination suppression between these loci to reduce the production of neuters as well as the gradual degeneration of the nonrecombining chromosome eventually lead to heteromorphism (reviewed in Charlesworth 2008). Detailed studies in a number of species have provided convincing evidence for these initial steps, including linked sterility loci, recombination suppression, and its genomic consequences (e.g., Westergaard 1958;Bachtrog 2005;Telgmann-Rauber et al 2007;Filatov 2008;Uno et al 2008a;Yu et al 2008b;Spigler et al 2010), whereas comparative studies have exposed additional complexity in the evolutionary dynamics of sex-determining chromosomes. For example, sex chromosomes not only have arisen de novo from different autosomes and with different dominance relations (i.e., XY or ZW) in related species (e.g., fish: Cnaani et al 2008;Ross et al 2009;frogs: Ogata et al 2007;Uno et al 2008b), but also well-established sex chromosomes are subject to change, e.g., the fusion of an ancestral sex chromosome to an autosome leading to the formation of neo-sex chromosomes in Drosophila miranda (White 1973;Steinemann and Steinemann 1998;Bachtrog and Charlesworth 2000).…”

“…Recent genetic maps have revealed macrosynteny between the diploid and octoploid genomes (Rousseau-Gueutin et al 2008;Sargent et al 2009;Spigler et al 2010), allowing the autosomal homeolog of the proto-sex chromosome of subdioecious F. virginiana to be identified . Our study takes advantage of this published work to begin to answer the overarching question: Is the sex-determining region of dioecious F. chiloensis syntenous with that of its sibling species, subdioecious F. virginiana?…”