Sexual polyploidization in plants – cytological mechanisms and molecular regulation

Abstract

In the plant kingdom, events of whole genome duplication or polyploidization are generally believed to occur via alterations of the sexual reproduction process. Thereby, diploid pollen and eggs are formed that contain the somatic number of chromosomes rather than the gametophytic number. By participating in fertilization, these so-called 2n gametes generate polyploid offspring and therefore constitute the basis for the establishment of polyploidy in plants. In addition, diplogamete formation, through meiotic restitution, is an essential component of apomixis and also serves as an important mechanism for the restoration of F1 hybrid fertility. Characterization of the cytological mechanisms and molecular factors underlying 2n gamete formation is therefore not only relevant for basic plant biology and evolution, but may also provide valuable cues for agricultural and biotechnological applications (e.g. reverse breeding, clonal seeds). Recent data have provided novel insights into the process of 2n pollen and egg formation and have revealed multiple means to the same end. Here, we summarize the cytological mechanisms and molecular regulatory networks underlying 2n gamete formation, and outline important mitotic and meiotic processes involved in the ectopic induction of sexual polyploidization.

The Fate of Chromosomes and Alleles in an AllohexaploidBrassicaPopulation

“…Several mutations resulting in high frequencies of unreduced gamete formation have been identified in Arabidopsis (D 'Erfurth et al 2008;De Storme and Geelen 2011), and genetic factors have been implicated in other species (Tavoletti et al 1991). However, it is probable that univalent-induced meiotic restitution [the spread of univalents across the metaphase plate resulting in a restitution nucleus (Catcheside 1934;De Storme and Geelen 2013)] was responsible for the unreduced gamete that contributed to the allohexaploid plant A1. The absence of similarly high frequencies of unreduced gamete production in the SP population may support a mode of bivalent formation for most plants, as suggested by the molecular karyotyping results (Figure 3).…”

Male Parent Identification of Triploid Rubber Trees (Hevea brasiliensis) and the Mechanism of 2n Gametes Formation

“…For example, high temperature can induce 2n female gametes in Populus [39,40]. However, gamete formation is controlled mainly by genes [32,41]. Some genes, such as AtPS1 [42] and JASON [43], were reported to be involved in 2n gamete formation in Arabidopsis.…”

Genetic Structure of Diploid Gametes for the Production of Triploid Citrus Hybrids

“…Little variation can occur along the chromosome due to double reduction events. In the case of post-meiotic doubling, haploid gametes undergo an extra round of genome duplication, leading to the formation of fully homozygous 2n gametes (Bastiaanssen et al, 1998;Ramanna and Jacobsen, 2003;De Storme and Geelen, 2013b;. Thus, 100% homozygosity for all loci is expected among the 2n gametes (Ramanna and Jacobsen, 2003).…”

“…Even the first polyploid was discovered over a century ago (Strasburger, 1910), the genetic and evolutionary implications of polyploidy are still being studied and discussed (De Storme and Geelen, 2013b). Many cultivated species are polyploid; potato varieties include triploids, tetraploids and pentaploids.…”

“…When crossing over occurs, the parental heterozygosity restitution (PHR) rates vary from 100% for loci close to the centromere to 60-70% for loci far from the centromere, depending on the level of chromosome interference (Cuenca et al, 2011). For SDR, the 2n gametes contain two sister chromatids, which reduces the parental heterozygosity level (Bastiaanssen et al, 1998;Cuenca et al, 2011;De Storme and Geelen, 2013b). When crossing over occurs, the PHR rate varies from 0% for loci close to the centromere to 60-75% for loci far from the centromere, depending on the level of chromosome interference (Cuenca et al, 2011).…”