Cytokinesis Defects as a Basis for Genome Doubling

Most if not all plants have undergone one or more polyploidization events throughout their evolutionary history, which may have lead to the emergence of new variation and plant speciation. It is generally considered that polyploidization is occurring through the fertilization of unreduced (2n) gametes, which may generate either triploid (nX2n gamete) or tetraploid (2nx2n gamete) offspring. Little is known about the mechanisms through which unreduced or 2n gametes are formed and what conditions may provoke their production. In the last few years, genetic defects have been ascribed that can lead to meiotic restitution producing diploid gametes. Here I will present evidence that cytokinesis defects are a means to generate polyploid gametes. The mutant ET2 shows defects in cytokinesis due to alteration in the expression of the callose synthase GSL8. Because of incomplete cellular divisions primarily occurring in flower organs, multinuclear cells are formed in ET2, which ultimately fuse to generate cells with increased ploidy. These cells participate in male and female reproduction organ development and produce 5-20 % of the male and female gametophytes, which are diploid. As a result, ET2 produces a mixed population of diploid, triploid and tetraploid offspring.

These observations show that mild cytokinesis defects lead to pre-meiotic doubling of the genome through endomitosis and results in the formation of polyploid gametes and offspring. The capacity of these cytokinesis mutants to produce a mix of diploids and polyploids may contribute to plant evolution and diversity as it leads to a sustained production vehicle for polyploid progeny.