There are a growing number of examples where naturally occurring mutations disrupt an established physiological or developmental pathway to yield a new condition that is evolutionary favored. Asexual reproduction by seed in plants, or apomixis, occurs in a diversity of taxa and has evolved from sexual ancestors. One form of apomixis, diplospory, is a multi‐step development process that is initiated when meiosis is altered to produce an unreduced rather than a reduced egg cell. Subsequent parthenogenetic development of the (...) unreduced egg yields genetically maternal progeny. While it has long been apparent from cytological data that meiosis in apomicts was malfunctional or completely bypassed, the genetic basis of the phenomenon has been a long‐standing mystery. New data from genetic analysis of Arabidopsis mutants1 in combination with more sophisticated molecular understanding of meiosis in plants indicate that a weak mutation of the gene SWI, called DYAD, interferes with sister chromatid cohesion in meiosis I, causes synapsis to fail in female meiosis and yields two unreduced cells. The new work shows that a low percentage of DYAD ovules produce functional unreduced egg cells (2n) that can be fertilized by haploid pollen (1n) to give rise to triploid (3n) progeny. While the DYAD mutants differ in some aspects from naturally occurring apomicts, the work establishes that mutation to a single gene can effectively initiate apomictic development and, furthermore, focuses efforts to isolate apomixis genes on a narrowed set of developmental events. Profitable manipulation of meiosis and recombination in agronomically important crops may be on the horizon. BioEssays 30:798–801, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

We use genomic information to tell us stories of evolutionary origins. But what does it mean when different genomes report wildly different accounts of lineage history? This genomic “discordance” can be a consequence of a fascinating suite of natural history and evolutionary phenomena, from the different inheritance mechanisms of nuclear versus cytoplasmic (mitochondrial and plastid) genomes to hybridization and introgression to horizontal transfer. Here, we explore how we can use these distinct genomic stories to provide new insights into the maintenance (...) of sexual reproduction, one of the most important unanswered questions in biology. We focus on the strikingly distinct nuclear versus mitochondrial versions of the story surrounding the origin and maintenance of asexual lineages in Potamopyrgus antipodarum, a New Zealand freshwater snail. While key questions remain unresolved, these data inspire multiple testable hypotheses that can be powerfully applied across a broad range of taxa toward a deeper understanding of the causes and consequences of mitonuclear discordance, the maintenance of sex, and the origin of new asexual lineages. (shrink)

The unusual occurrence and developmental diversity of asexual eukaryotes remain a puzzle. De novo formation of a functioning asexual genome requires a unique assembly of sets of genes or gene states to disrupt cellular mechanisms of meiosis and gametogenesis, and to affect discrete components of sexuality and produce clonal or hemiclonal offspring. We highlight two usually overlooked but essential conditions to understand the molecular nature of clonal organisms, that is, a nonrecombinant genomic assemblage retaining modifiers of the sexual program, and (...) a complementation between altered reproductive components. These subtle conditions are the basis for physiologically viable and genetically balanced transitions between generations. Genomic and developmental evidence from asexual animals and plants indicates the lack of complementation of molecular changes in the sexual reproductive program is likely the main cause of asexuals' rarity, and can provide an explanatory frame for the developmental diversity and lability of developmental patterns in some asexuals as well as for the discordant time to extinction estimations. (shrink)