Asymmetric reproductive interference: The consequences of cross-pollination on reproductive success in sexual-apomictic populations of Potentilla puberula (Rosaceae)

Abstract

Apomixis evolves from a sexual background and usually is linked to polyploidization. Pseudogamous gametophytic apomicts, which require a fertilization to initiate seed development, of various ploidy levels frequently co-occur with their lower-ploid sexual ancestors, but the stability of such mixed populations is affected by reproductive interferences mediated by cross-pollination. Thereby, reproductive success of crosses depends on the difference in ploidy levels of mating partners, that is, on tolerance of deviation from the balanced ratio of maternal versus paternal genomes. Quality of pollen can further affect reproductive success in intercytotype pollinations. Cross-fertilization, however, can be avoided by selfing which may be induced upon pollination with mixtures of self- and cross-pollen (i.e., mentor effects). We tested for reproductive compatibility of naturally co-occurring tetraploid sexuals and penta- to octoploid apomicts in the rosaceous species Potentilla puberula by means of controlled crosses. We estimated the role of selfing as a crossing barrier and effects of self- and cross-pollen quality as well as maternal: paternal genomic ratios in the endosperm on reproductive success. Cross-fertilization of sexuals by apomicts was not blocked by selfing, and seed set was reduced in hetero- compared to homoploid crosses. Thereby, seed set was negatively related to deviations from balanced parental genomic ratios in the endosperm. In contrast, seed set in the apomictic cytotypes was not reduced in hetero- compared to homoploid crosses. Thus, apomictic cytotypes either avoided intercytotype cross-fertilization through selfing, tolerated intercytotype cross-fertilizations without negative effects on reproductive success, or even benefitted from higher pollen quality in intercytotype pollinations. Our experiment provides evidence for asymmetric reproductive interference, in favor of the apomicts, with significantly reduced seed set of sexuals in cytologically mixed populations, whereas seed set in apomicts was not affected. Incompleteness of crossing barriers further indicated at least partial losses of a parental genomic endosperm balance requirement.

Keywords: apomixis; crossing barrier; genomic endosperm balance; pollen; polyploidy; selfing.

Figure 1

The study system Potentilla puberula, a hemicryptophte of typically xeric mountainous habitats

Figure 2

Assessment of the mode of mating (cross‐fertilization versus induced selfing) in heteroploid crosses of apomictic high polyploids upon self‐incompatible, tetraploid sexuals without emasculation based on the presence/absence of changes in the paternal genomic contribution to the embryos (and the endosperm) and changes in reproductive success compared to (tetraploid) homoploid crosses. Intercytotype cross‐fertilization will lead to a change in the paternal genomic contribution, whereas no change is indicative of progeny derived from selfing. (i) No differences neither in the paternal genomic contribution nor the reproductive success indicate full self‐compatibility (i.e., selfing of tetraploids), whereas (iv) an increase in the number of parental genomes without reduction in the reproductive success suggests full compatibility of cytotypes (i.e., relaxation of genomic endosperm balance requirement). In contrast, a reduction in reproductive success indicates selection against selfed and/or cross‐fertilized progeny (leading to abortion of seeds), that is, entails an ambiguous inference: (ii) reduced reproductive success, but no change in parental genomes indicates either complete selection against cross‐fertilized progeny (i.e., only selfed progeny developed into seeds) or selfing occurred and some selfed progeny was lost due to inbreeding depression. In contrast, (iii) change in the paternal genomic contribution accompanied by a reduction in reproductive success either indicates some selection against cross‐fertilized progeny or complete selection against selfed progeny

Figure 3

Number of paternal genomes p in the endosperm (expressed as the multiple of the holoploid genome of the pollen recipient) of seeds obtained in hetero‐ and homoploid crosses of Potentilla puberula. Seeds obtained in crosses upon tetraploid pollen recipients were derived by sexuality, those obtained in the other crosses originated from apomixis. Significant deviation of the observed paternal genomic contribution to the endosperm in the homoploid treatments from the values theoretically expected (1p for sexual, 1p and 2p for apomicts) due to inherent systematic methodological error is indicated. Analogously deviation of the paternal genomic contribution observed for the heteroploid treatments from the values observed in the homoploid crosses is indicated. Pollen recipients are represented by the x‐axis; pollen donors by the y‐axis. Tetraploids were crossed with tetra‐, penta‐, and heptaploids only. **p < .01, ***p < .001

Figure 4

Seed set of Potentilla puberula derived from a common garden crossing experiment. 4x, 5x, 6x, 7x, and 8x refer to the ploidy level of the pollen recipients (tetra‐ to octoploid). Asterisks indicate significant differences in selfed individuals (self) from homoploid crosses (homo). Sample size was 3 populations/12 individuals for tetraploids, 11/59 for pentaploids, for 5/20 hexaploids, for 5/19 heptaploids, and 4/15 for octoploids. Boxes span the range between the 25th and 75th percentile with indicated median, and whiskers extend to 1.5‐fold the interquartile range. Outliers are represented by open circles

Figure 5

Number of paternal monoploid genomes x in the endosperm of seeds obtained from crosses of tetraploid sexual pollen recipients (N = 209) with tetra‐ (a), penta‐ (b), and heptaploid pollen donors (c)

Figure 6

Examination of the requirement for a balanced maternal: paternal genomic ratio in the endosperm of seeds produced by tetraploid sexual individuals of Potentilla puberula. Generalized linear mixed models revealed a significant (p < .001) and nonsignificant (p = .570) relation between seed set and germination rate, respectively, and Δ p (averaged for each flower), that is, the deviation of the observed number of parental genomes (p) in the endosperm from their number in endosperms with balanced parental genomic ratios (2m:1p). Black and white dots illustrate flowers subjected to homoploid and heteroploid crosses, respectively

Figure 7

Examination of the requirement for a balanced maternal: paternal genomic ratio in the endosperm of apomictically derived seeds in four cytotypes of Potentilla puberula. Generalized linear mixed models revealed a significant relation between seed set and Δ p (averaged for each flower), that is, the deviation of the observed number of parental genomes in the endosperm from the balanced number (4m:2p), for pentaploids (p = .002), but not for hexa‐, hepta‐, and octoploids (p = .915, .364 and .518). Headings indicate the ploidy level of the pollen recipient. Black and white dots illustrate flowers subjected to homoploid and heteroploid crosses, respectively