Classical models suggest that recombination rates on sex chromosomes evolve in a stepwise manner to localize sexually antagonistic variants in the sex in which they are beneficial, thereby lowering rates of recombination between X and Y chromosomes. However, it is also possible that sex chromosome formation occurs in regions with pre-existing recombination suppression. To evaluate these possibilities, we constructed linkage maps and a chromosome-scale genome assembly for the dioecious plant Rumex hastatulus. This species has a polymorphic karyotype with a young neo-sex chromosome, resulting from a Robertsonian fusion between the X chromosome and an autosome, in part of its geographic range. We identified the shared and neo-sex chromosome using comparative genetic maps of the two cytotypes. We found that sex-linked regions of both the ancestral and the neo-sex chromosome are embedded in large regions of low recombination. Furthermore, our comparison of the recombination landscape of the neo-sex chromosome to its autosomal homologue indicates that low recombination rates mainly preceded sex linkage. These patterns are not unique to the sex chromosomes: all chromosomes were characterized by massive regions of suppressed recombination spanning most of each chromosome. This represents an extreme case of the periphery-biased recombination seen in other systems with large chromosomes. Across all chromosomes, gene and repetitive sequence density correlated with recombination rate, with patterns of variation differing by repetitive element type. Our findings suggest that ancestrally low rates of recombination may facilitate the formation and subsequent evolution of heteromorphic sex chromosomes.

1Summary 6 Classical models suggest recombination rates on sex chromosomes evolve in a stepwise manner to 7 localize the inheritance of sexually antagonistic variation in the sex where it is beneficial, thereby 8 lowering rates of recombination between X and Y chromosomes. However, it is also possible that sex 9 chromosome formation occurs in regions with pre-existing recombination suppression. To evaluate 10 these possibilities, we constructed linkage maps and a chromosome-scale genome assembly for the 11 dioecious plant Rumex hastatulus, a species with a young neo-sex chromosome found in part of its 12 geographical range. We found that the ancestral sex-linked region is located in a large region 13 characterized by low recombination. Furthermore, comparison between the recombination landscape 14 of the neo-sex chromosome and its autosomal homologue indicates that low recombination rates 15 preceded sex linkage. Our findings suggest that ancestrally low rates of recombination have facilitated 16 the formation and evolution of heteromorphic sex chromosomes. 17

An international survey of house dust collected from eleven countries using a modified dilution-to-extinction method yielded 7904 isolates. Of these, six strains morphologically resembled the asexual morphs of Aureobasidium and Hormonema (sexual morphs ?Sydowia), but were phylogenetically distinct. A 28S rDNA phylogeny resolved strains as a distinct clade in Dothideales with families Aureobasidiaceae and Dothideaceae their closest relatives. Further analyses based on the ITS rDNA region, β-tubulin, 28S rDNA, and RNA polymerase II second largest subunit confirmed the distinct status of this clade and divided strains among two consistent subclades. As a result, we introduce a new genus and two new species as Zalaria alba and Z. obscura, and a new family to accommodate them in Dothideales. Zalaria is a black yeast-like fungus, grows restrictedly and produces conidiogenous cells with holoblastic synchronous or percurrent conidiation. Zalaria microscopically closely resembles Hormonema by having only one to two loci per conidiogenous cell, but species of our new genus generally has more restricted growth. Comparing the two species, Z. obscura grows faster on lower water activity (aw) media and produces much darker colonies than Z. alba after 7 d. Their sexual states, if extant, are unknown.