Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

Abstract

The co-occurrence of apomixis (asexual reproduction) and polyploidy in plants has been the subject of debate in regard to the origin and evolution of asexuality. In recent years, polyploidy has been postulated as a maintenance and stabilization factor rather than as a source of apomixis origin. It is assumed polyploidy facilitates the compensation for mutation accumulation, and hence, the rare occurrence of diploid apomixis indirectly supports this finding. Nevertheless, diploid apomicts exist and are successful, especially in the genus Boechera. While comparing phenotypic traits, fitness-related traits and apomixis penetrance between both diploid and triploid apomicts in the genus Boechera, it was expected to find trait variance that can be attributed to ploidy. Surprisingly, little trait variation could be assigned to ploidy, but rather trait variations were mainly genotype-specific. Additionally, it is shown that paternal contribution is very important for trait success, even though all offspring are genetically identical to the mother plant. This harbors implications for the introduction of apomixis into crop plants, considering the effects of paternal contribution during asexual reproduction. Nevertheless, polyploidy is an efficient way to buffer deleterious mutations, but the flexibility of diploid apomicts of the genus Boechera for rare sexual events contributes to their success in nature.

Natural Variation in Epigenetic Pathways Affects the Specification of Female Gamete Precursors in Arabidopsis

“…Several studies have raised the possibility that alternative reproductive pathways, such as tetraspory or asexual reproduction through seeds (apomixis), evolved as a response to hybridization, genomic collisions, or unstable climatic environment (Carman, 1997;Voigt-Zielinski et al, 2012;Lovell et al, 2013;Hojsgaard et al, 2014). Segregating F2 populations of ecotypic crosses showed a continuous distribution of the frequency of ectopic configurations, suggesting that the genetic factors influencing the phenotype are quantitative and conserved among different ecotypes.…”

The Conserved Chimeric Transcript UPGRADE2 Is Associated with Unreduced Pollen Formation and Is Exclusively Found in Apomictic Boechera Species

“…7D). High variability in pollen morphology and unreduced pollen formation, in addition to tolerance to deviations from the sexual endosperm balance number, have been described for some Boechera taxa (Böcher, 1951(Böcher, , 1954Voigt et al, 2007;Aliyu et al, 2010;Voigt-Zielinski et al, 2012). Despite this variability, castration experiments (Böcher, 1951) and extensive flow cytometric analyses of seeds (Aliyu et al, 2010) strongly support selection pressure for the maintenance of unreduced pollen development to fulfill endosperm balance requirements in diploid apomicts.…”

“…Besides the formation of unreduced female gametes, apomictic Boechera spp. also produce unreduced pollen, as demonstrated by the fact that diploids and triploids produce seeds almost exclusively with hexaploid (6C = (Voigt et al, 2007;Aliyu et al, 2010;Voigt-Zielinski et al, 2012). Hence, strong selection pressure to maintain a balanced two maternal-to-one paternal genome ratio (i.e.…”

Depletion of Key Meiotic Genes and Transcriptome-Wide Abiotic Stress Reprogramming Mark Early Preparatory Events Ahead of Apomeiotic Transition

“…Several apomictic Boechera (diploid or polyploid) had been classified under B. holboellii clade and/or several others previously belong to this clade, e.g., B. divaricarpa (used in this study) had been reclassified into “trashcan” hybrids involving sexuals like B. stricta and B. sparsifolia (Rushworth et al, 2011). Diploids and polyploids (e.g., triploids) can be tractable within these “ holboeleii and hybrid” clades (Naumova et al, 2001; Aliyu et al, 2010; Voigt-Zielinski et al, 2012), and current studies in several labs focus on the genome evolution of the identified apomictic diploids and their sexual parents by genome sequencing. Although exhaustive comparative studies have not yet been conducted, Roy concluded that B. holboellii apomictic populations are likely polyphyletic with substantial allelic variation thus greater fixed heterozygosity than what were observed for B. gunnisoniana , a stand-alone monophyletic apomictic triploid (Roy, 1995).…”