Ploidy-Dependent Effects of Light Stress on the Mode of Reproduction in the Ranunculus auricomus Complex (Ranunculaceae)

Abstract

Polyploidy in angiosperms is an influential factor to trigger apomixis, the reproduction of asexual seeds. Apomixis is usually facultative, which means that both sexual and apomictic seeds can be formed by the same plant. Environmental abiotic stress, e.g. light stress, can change the frequency of apomixis. Previous work suggested effects of stress treatments on meiosis and megasporogenesis. We hypothesized that polyploidy would alter the stress response and hence reproductive phenotypes of different cytotypes. The main aims of this research were to explore with prolonged photoperiods, whether polyploidy alters proportions of sexual ovule and sexual seed formation under light stress conditions. We used three facultative apomictic, pseudogamous cytotypes of the Ranunculus auricomus complex (diploid, tetraploid, and hexaploid). Stress treatments were applied by extended light periods (16.5 h) and control (10 h) in climate growth chambers. Proportions of apomeiotic vs. meiotic development in the ovule were evaluated with clearing methods, and mode of seed formation was examined by single seed flow cytometric seed screening (ssFCSS). We further studied pollen stainability to understand effects of pollen quality on seed formation. Results revealed that under extended photoperiod, all cytotypes produced significantly more sexual ovules than in the control, with strongest effects on diploids. The stress treatment affected neither the frequency of seed set nor the proportion of sexual seeds nor pollen quality. Successful seed formation appears to be dependent on balanced maternal: paternal genome contributions. Diploid cytotypes had mostly sexual seed formation, while polyploid cytotypes formed predominantly apomictic seeds. Pollen quality was in hexaploids better than in diploids and tetraploids. These findings confirm our hypothesis that megasporogenesis is triggered by light stress treatments. Comparisons of cytotypes support the hypothesis that ovule development in polyploid plants is less sensitive to prolonged photoperiods and responds to a lesser extent with sexual ovule formation. Polyploids may better buffer environmental stress, which releases the potential Frontiers in Plant Science | www.frontiersin.org

Chasing the Apomictic Factors in the Ranunculus auricomus Complex: Exploring Gene Expression Patterns in Microdissected Sexual and Apomictic Ovules

“…However, an increase in copies of A − would be restricted by its dosage effects, as increasing expressivity (i.e., level of apomixis) will concomitantly reduce levels of residual sexuality, thus making it increasingly impossible to attain an obligate (100% expressivity) apomictic hexaploid individual via recombination. This explains observations in Ranunculus , Paspalum and many other apomictic genera in which no fully apomictic plants had been so far recovered ([ 46 , 65 , 80 , 81 ]; although, see [ 82 ] for cases of obligate apomixis in Boechera ). The opposite situation is also possible, with occasional recombinant individuals having no copies of A − that have lost their capacity to reproduce apomictically.…”

“…However, the high dosage of the wild-type A + from the tetraploid sexual parent might explain the relatively high frequencies of sexual ovule (69%) and seed formation (29%) in hexaploid apomicts [ 44 ] compared to other polyploid apomicts having divergent evolutionary histories in which sexuality is found at residual levels (±5%, e.g., [ 75 , 76 ]). Likewise, Ranunculus variabilis , a naturally-related tetraploid apomict of the R. auricomus complex, showed overall higher frequencies of apospory and apomictic seed formation than our hexaploid hybrids used here [ 46 ]. These results suggest that the dosage of apomixis-factors depends on the parentage and evolutionary origin of plants, and does not necessarily increase linearly with the level of ploidy.…”

“…The opposite situation is also possible, with occasional recombinant individuals having no copies of A − that have lost their capacity to reproduce apomictically. Even though it has not been observed in natural apomictic hexaploid Ranunculus auricomus individuals [ 44 , 45 , 46 , 48 ], a reversal to sex in an apomictic lineage has been rarely observed [ 81 , 83 ] but is theoretically expected in apomictic populations [ 67 , 84 , 85 ].…”

“…Analyses based upon AFLPs and SSRs, combined with DNA sequence [ 39 , 40 ] and RNAseq SNPs [ 41 ], support a hybrid origin for hexaploid apomicts resulting from a cross between a putative 4x R. cassubicifolius and a 2x R. carpaticola sometime during the last glacial period (60,000–80,000 years ago). Apomictic Ranunculus species are aposporous ( Hieracium -type), pseudogamous and express apomixis facultatively [ 36 , 37 , 42 , 43 , 44 , 45 , 46 ]. Aposporous initial (AI) cells arise from a somatic cell of the nucellus during the time in which the megaspore mother cell (MMC) starts meiosis [ 35 , 43 , 44 ], and competes for the formation of the female gametophyte within the ovule.…”

Phenotypic Responses, Reproduction Mode and Epigenetic Patterns under Temperature Treatments in the Alpine Plant Species Ranunculus kuepferi (Ranunculaceae)

“…Experimental research suggests that abiotic conditions like temperature and light [ 41 , 53 , 54 , 55 ] can directly alter the mode of reproduction in plants. Specifically, low temperature may trigger apomictic seed formation, albeit at low frequencies [ 41 ].…”