Sex chromosomes evolved from autosomes many times across the eukaryote phylogeny. Several models have been proposed to explain this transition, some involving male and female sterility mutations linked in a region of suppressed recombination between X and Y (or Z/W, U/V) chromosomes. Comparative and experimental analysis of a reference genome assembly for a double haploid YY male garden asparagus (Asparagus officinalis L.) individual implicates separate but linked genes as responsible for sex determination. Dioecy has evolved recently within Asparagus and sex chromosomes are cytogenetically identical with the Y, harboring a megabase segment that is missing from the X. We show that deletion of this entire region results in a male-to-female conversion, whereas loss of a single suppressor of female development drives male-to-hermaphrodite conversion. A single copy anther-specific gene with a male sterile Arabidopsis knockout phenotype is also in the Y-specific region, supporting a two-gene model for sex chromosome evolution.

Tomato powdery mildew caused by Oidium neolycopersici has become a globally important disease of tomato (Lycopersicon esculentum). To study the defense responses of tomato triggered by tomato powdery mildew, we first mapped a set of resistance genes to O. neolycopersici from related Lycopersicon species. An integrated genetic map was generated showing that all the dominant resistance genes (Ol-1, Ol-3, Ol-4, Ol-5, and Ol-6) are located on tomato chromosome 6 and are organized in three genetic loci. Then, near-isogenic lines (NIL) were produced that contain the different dominant Ol genes in a L. esculentum genetic background. These NIL were used in disease tests with local isolates of O. neolycopersici in different geographic locations, demonstrating that the resistance conferred by different Ol genes was isolate-dependent and, hence, may be race-specific. In addition, the resistance mechanism was analyzed histologically. The mechanism of resistance conferred by the dominant Ol genes was associated with hypersensitive response, which varies in details depending on the Ol-gene in the NIL, while the mechanism of resistance governed by the recessive gene ol-2 on tomato chromosome 4 was associated with papillae formation.

One Sentence Summary: The Y chromosome in garden asparagus determines sex via two Y-linked genes, one that suppresses female pistil development and another that promotes male anther development. AbstractThe origin and early evolution of sex chromosomes has been hypothesized to involve the linkage of factors with antagonistic effects on male and female function. Garden asparagus (Asparagus officinalis L.) is an ideal species to investigate this hypothesis, as the X and Y chromosomes are cytologically homomorphic and evolved from an ancestral autosome pair in association with a shift from hermaphroditism to dioecy. Mutagenesis screens paired with single-molecule fluorescence in situ hybridization (smFISH) directly implicate Y-specific genes that respectively suppress female (pistil) development and are necessary for male (anther) development. Comparison of contiguous X and Y chromosome assemblies shows that hemizygosity underlies the loss of recombination between the genes suppressing female organogenesis (SUPPRESSOR OF FEMALE FUNCTION, SOFF) and promoting male function (TAPETAL DEVELOPMENT AND FUNCTION1, aspTDF1). We also experimentally demonstrate the function of aspTDF1. These findings provide direct evidence that sex chromosomes can function through linkage of two sex determination genes.

Tomato (Lycopersicon esculentum) is susceptible to the powdery mildew Oidium lycopersici, but several wild relatives such as Lycopersicon parviflorum G1.1601 are completely resistant. An F2 population from a cross of Lycopersicon esculentum cv. Moneymaker x Lycopersicon parviflorum G1.1601 was used to map the O. lycopersici resistance by using amplified fragment length polymorphism markers. The resistance was controlled by three quantitative trait loci (QTLs). Ol-qtl1 is on chromosome 6 in the same region as the Ol-1 locus, which is involved in a hypersensitive resistance response to O. lycopersici. Ol-qtl2 and Ol-qtl3 are located on chromosome 12, separated by 25 cM, in the vicinity of the Lv locus conferring resistance to another powdery mildew species, Leveillula taurica. The three QTLs, jointly explaining 68% of the phenotypic variation, were confirmed by testing F3 progenies. A set of polymerase chain reaction-based cleaved amplified polymorphic sequence and sequence characterized amplified region markers was generated for efficient monitoring of the target QTL genomic regions in marker assisted selection. The possible relationship between genes underlying major and partial resistance for tomato powdery mildew is discussed.

Lycopersicon peruvianum LA2172 is completely resistant to Oidium neolycopersici, the causal agent of tomato powdery mildew. Despite the large genetic distance between the cultivated tomato and L. peruvianum, fertile F1 hybrids of L. esculentum cv. Money maker x L.peruvianum LA2172 were produced, and a pseudo-F2 population was generated by mating F, half-sibs. The disease tests on the pseudo-F2 population and two BC,families showed that the resistance in LA2172 is governed by one dominant gene, designated as 01-4. In the pseudo-F2 population, distorted segregation was observed, and multi-allelic, single-locus markers were used to display different marker-allele configurations per locus. Para-meters for both distortion and linkage between genetic loci were determined by maximum likelihood estimation, and the necessity of using multi-allelic, single-locus markers was illustrated. Finally, a genetic linkage map of chromosome 6 around the 01-4 locus was constructed by using the pseudo-F2 population.