Triploid Bridge and Role of Parthenogenesis in the Evolution of Autopolyploidy

Abstract

Autopolyploidization is considered to play an important role in plant evolution. In polyploidization, the polyploid evolves from the original diploid cytotype, in which the triploid state is considered to mediate the process (triploid bridge). Nevertheless, the fitness of triploid individuals seems to be too low to facilitate the polyploidization process (triploid block). The evolutionary condition of autopolyploidy was analyzed using a mathematical model focusing on the role of parthenogenesis in triploid and tetraploid individuals. In addition, offspring were assumed to arise by sexual reproduction by conjugations between haploid, diploid, and triploid gametes produced by diploid, tetraploid, and triploid individuals. According to the analysis, even if triploid block suppresses the fitness of sexually produced triploids, the polyploidization process can proceed when parthenogenesis occurs frequently. If only triploids frequently reproduce parthenogenetically, the evolutionary consequences tend to depend on the fitness of the tetraploid individuals. On the basis of a predetermined parameter set, if tetraploid fitness is relatively low, all three ploidies can coexist. Otherwise, tetraploidization occurs. In this case, triploid parthenogenesis promotes not only triploidization but also tetraploidization. However, if both triploids and tetraploids frequently reproduce parthenogenetically, the ploidy levels with the highest fitness are likely to dominate in the population through direct competition among cytotypes.

Ploidy frequencies in plants with ploidy heterogeneity: fitting a general gametic model to empirical population data

“…Thus, we do not assume any a priori minority cytotype disadvantage. The progeny of each individual are thus fully determined by the probabilities of individual gametic types (see [12]). (We refer to this later as the 'gametic process'.)…”

“…In this paper, we develop a simple and general gametic model of ploidy-level frequencies, on the basis of the models of Husband [10] and Yamauchi et al [12]. Such models predict equilibrium frequencies of individual ploidy levels using a few basic parameters ( probabilities of formation of unreduced gametes and fertilities of higher ploidy levels).…”

Production of Diploid Male Gametes in Arabidopsis by Cold-Induced Destabilization of Postmeiotic Radial Microtubule Arrays

“…As a result, cold-induced SDR 2n gametes can provide some heterozygosity and associated allelic diversity to its polyploid progeny. Moreover, as most natural polyploid lineages are thought to originate from a unilateral polyploidization event, in which the fusion of reduced and unreduced gametes results in the formation of unstable triploid intermediates (triploid bridge hypothesis; Yamauchi et al, 2004;Considine et al, 2012), the genetic influence of both types of 2n gametes on the evolutionary success of the resulting polyploid is less relevant and depends more on the fitness of the triploid intermediate.…”

The “Polyploid Hop”: Shifting Challenges and Opportunities Over the Evolutionary Lifespan of Genome Duplications

“…In addition, direct competition between the parental diploid and its derivative autopolyploid can hinder the establishment of a nascent polyploid as predicted by theory (Rodríguez, 1996;Yamauchi et al, 2004). This of course depends on niche overlap between the diploid and polyploid.…”

“…Even more, polyploid establishment is substantially constrained by overall low chances of polyploid mutants persisting among their diploid progenitors (Levin, 2002;Ramsey and Schemske, 2002). This stems from both direct competition between the two cytotypes (Yamauchi et al, 2004) and frequency dependent selection (Levin, 1975), which suggests most autopolyploids are likely to go extinct before establishment (Levin, 1975;Husband, 2000).…”