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.

The application of a new gene-based strategy for sequencing the wheat mitochondrial genome shows its structure to be a 452 528 bp circular molecule, and provides nucleotide-level evidence of intra-molecular recombination. Single, reciprocal and double recombinant products, and the nucleotide sequences of the repeats that mediate their formation have been identified. The genome has 55 genes with exons, including 35 protein-coding, 3 rRNA and 17 tRNA genes. Nucleotide sequences of seven wheat genes have been determined here for the first time. Nine genes have an exon-intron structure. Gene amplification responsible for the production of multicopy mitochondrial genes, in general, is species-specific, suggesting the recent origin of these genes. About 16, 17, 15, 3.0 and 0.2% of wheat mitochondrial DNA (mtDNA) may be of genic (including introns), open reading frame, repetitive sequence, chloroplast and retro-element origin, respectively. The gene order of the wheat mitochondrial gene map shows little synteny to the rice and maize maps, indicative that thorough gene shuffling occurred during speciation. Almost all unique mtDNA sequences of wheat, as compared with rice and maize mtDNAs, are redundant DNA. Features of the gene-based strategy are discussed, and a mechanistic model of mitochondrial gene amplification is proposed.

The class B floral homeotic genes from the higher eudicot model systems Arabidopsis and Antirrhinum are involved in specifying the identity of petals and stamens during flower development. These genes exist in two different types termed DEF- and GLO-like genes. The proteins encoded by the class B genes are stable and functional in the cell only as heterodimeric complexes of a DEF- and a GLO-like protein. In line with this, heterodimerization is obligate for DNA binding in vitro. The genes whose products have to heterodimerize to be stable and functional are each other's closest relatives within their genomes. This suggests that the respective genes originated by gene duplication, and that heterodimerization is of relative recent origin and evolved from homodimerization. To test this hypothesis we have investigated the dimerization behavior of putative B proteins from phylogenetic informative taxa, employing electrophoretic mobility shift assays and the yeast two-hybrid system. We find that an ancestral B protein from the gymnosperm Gnetum gnemon binds DNA in a sequence-specific manner as a homodimer. Of the two types of B proteins from the monocot Lilium regale, the GLO-like protein is still able to homodimerize, whereas the DEF-like protein binds to DNA only as a heterodimeric complex with the GLO-like protein. These data suggest that heterodimerization evolved in two steps after a gene duplication that gave rise to DEF- and GLO-like genes. Heterodimerization may have originated after the gymnosperm-angiosperm split about 300 MYA but before the monocot-eudicot split 140–200 MYA. Heterodimerization may have become obligate for both types of flowering plant B proteins in the eudicot lineage after the monocot-eudicot split.