Sexual devolution in plants: apomixis uncloaked?

Abstract

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 mutants 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.