Expressivity of apomixis in 2n + n hybrids from an apomictic and a sexual parent: insights into variation detected in Pilosella (Asteraceae: Lactuceae)

Abstract

Reproductive variation was studied in the tetraploid Pilosella aurantiaca, hexaploid P. rubra (both species with facultative autonomous apospory) and in their 2n + n hybrids, which were obtained by crossing with a sexual pollen parent (tetraploid P. officinarum). The different DNA content in P. aurantiaca and P. officinarum demonstrated the actual 2n + n origin, both spontaneous from the field and through experimental crosses, of their hexaploid hybrids. The octoploid 2n + n progeny were recovered from an experimental cross of P. rubra and P. officinarum. The reproductive pathways operating in two maternal facultatively apomictic species and in the hybrids were quantified using a flow cytometric analysis of seeds obtained from either open-pollinated or emasculated plants. Whereas both maternal species displayed a high penetrance of apomixis, the level of apomixis among the majority of 2n + n hybrids was much lower and variable. Some of the hexaploid hybrids had a reduced seed set. Compared to the respective maternal parents, the decrease in apomixis due to haploid parthenogenesis and/or n + n mating was evident in almost all unreduced hybrids, irrespective of their field/experimental origin and ploidy. Hence, the reproductive behaviour in the apomictic maternal parent was profoundly different from that of the 2n + n hybrids with a sexual parent in spite of the preservation of the complete maternal genome in the hybrids. The regulatory interactions in hybrid genomes, such as effects of modifiers, heterochrony, and epigenetic control, may be consistent with the different expressivity of apomixis observed under different genetic backgrounds.

Evolution of apomixis loci in Pilosella and Hieracium (Asteraceae) inferred from the conservation of apomixis-linked markers in natural and experimental populations

“…Analyses of crosses between sexual and apomictic species demonstrate the variable penetrance of apomixis in progeny suggesting that other, quantitative factors or modifiers may influence the final reproductive behaviour in hybrids (Koltunow et al, 2000;Krahulcová et al, 2011;Tucker et al, 2012). However, to date, apomixis penetrance has not been correlated to the presence of the LOA and LOP loci.…”

“…Low penetrance or expressivity of apomixis has been previously reported within naturally sourced and experimental 2n þ n hybrids arising from crosses between a range of Pilosella apomicts as the maternal parent (P. bauhini, P. floribunda and P. aurantiaca) and sexual P. officinarum as the pollen donor (Krahulec et al, 2008;Krahulcová et al, 2009;Krahulcová et al, 2011). These hybrids arise from the fertilization of a chromosomally unreduced egg.…”

“…This test is an approximation for apomictically reproducing plants (Gadella, 1984) because fertilization-independent seed formation is predominantly coupled with apomeiosis (apospory) in wild-type apomicts of Pilosella (Bicknell and Koltunow, 2004). For a more precise determination of the reproductive pathway, the flow cytometric seed screen method was used (Krahulcová et al, 2011). Seeds were harvested from both decapitated capitula (to remove stigma and anthers) and also uncut capitula for comparative seed screening.…”

Transient Activation of Apomixis in Sexual Neotriploids May Retain Genomically Altered States and Enhance Polyploid Establishment

“…So, synthetic F 1 neotriploid plants were able to bypass the highly disturbed meiosis and produce low rates of eupolyploids (both triploid and tetraploid cytotypes) via apomixis, besides the expected aneuploid sexual progeny. The 2 n + n pathway of seed formation is widely known (as B III hybrids; Rutishauser, 1948 ) and commonly found in different apomictic systems (e.g., Harlan and de Wet, 1975 ; Martínez et al, 1994 ; Burson et al, 2002 ; Ramsey, 2007 ; Krahulcová et al, 2011 ; Schinkel et al, 2016 ). However, the activation of apomixis elements in sexual derived crosses opens the question if this could be a transient but recurrent strategy used by odd polyploids and plants under situations of genomic stress to skip disturbed meiosis and reduced fertility.…”

A reference genetic linkage map of apomictic Hieracium species based on expressed markers derived from developing ovule transcripts

“…Although the three major identified apomixis loci in Hieracium are dominant, other factors are thought to alter the apomixis and/or sexual pathways leading to variable penetrance of each of the three apomixis components. Although incomplete penetrance of apomixis occurs at relatively low levels in wild-type apomictic lines (Koltunow et al, 2011b), reduced apomixis penetrance is typically much more pronounced in naturally sourced and experimental Hieracium hybrids (Krahulec et al, 2008;Krahulcová et al, 2009Krahulcová et al, , 2011Okada et al, 2011;Ogawa et al, 2013). By including sexual R35 mutant m115 in the RNA sequencing and marker design process here, transcripts associated with the sexual reproduction pathway are likely to have also been captured, which may prove useful when mapping suppressive elements in the sexual background that play a role in reducing apomixis penetrance in hybrids.…”