Cytotype stability, facultative apomixis and geographical parthenogenesis in Ranunculus kuepferi (Ranunculaceae)

Abstract

Background and aims: Asexual organisms are more widespread in previously glaciated areas than their sexual relatives ('geographical parthenogenesis'). In plants, this pattern is probably dependent on reproductive isolation and stability of cytotypes within their respective distribution areas. Both partial apomixis and introgressive hybridization potentially destabilize the spatial separation of sexual and apomictic populations. The wide distribution of apomicts may be further enhanced by uniparental reproduction which is advantageous for colonization. These factors are studied in the alpine species Ranunculus kuepferi.

Methods: Geographical distribution, diversity and mode of reproduction of cytotypes were assessed using flow cytometry and flow cytometric seed screening on samples from 59 natural populations of Ranunculus kuepferi. Seed set of cytotypes was compared in the wild.

Key results: Diploid sexuals are confined to the south-western parts of the Alps, while tetraploid apomicts dominate in previously glaciated and in geographically isolated areas despite a significantly lower fertility. Other cytotypes (3x, 5x and 6x) occur mainly in the sympatric zone, but without establishing populations. The tetraploids are predominantly apomictic, but also show a partial apomixis via an uncoupling of apomeiosis and parthenogenesis in the seed material. Both pseudogamy and autonomous endosperm formation are observed which may enhance uniparental reproduction.

Conclusions: Diploids occupy a glacial relic area and resist introgression of apomixis, probably because of a significantly higher seed set. Among the polyploids, only apomictic tetraploids form stable populations; the other cytotypes arising from partial apomixis fail to establish, probably because of minority cytotype disadvantages. Tetraploid apomicts colonize previously devastated and also distant areas via long-distance dispersal, confirming Baker's law of an advantage of uniparental reproduction. It is concluded that stability of cytotypes and of modes of reproduction are important factors for establishing a pattern of geographical parthenogenesis.

Fig. 1.

Map of the distribution of Ranunculus kuepferi. Small black squares connected to the pie diagrams indicate the location, population numbers correspond to Table 1. Pie diagrams present the proportions of cytotypes for each population, as indicated. The black line indicates the extension of the last glacial maximum of the Würm glaciation.

Fig. 2.

Linear regression of the relative ploidy level Cx value and the genome size in picograms based on leaf material. The slope of the regression is indicated.

Fig. 3.

(A) Histogram of FCSS with five peaks: 1 and 2, standard Zea mays in the G₁ and G₂ phase, respectively; 3–5, Ranunculus kuepferi (Rk): 3, 3 Cx, embryo G₁ phase; 4, embryo G₂ phase; 5, 10 Cx endosperm (peak G₁ phase). (B) Scheme of the respective embryo sac after fertilization, illustrating a triploid unfertilized embryo corresponding to peak 3, and the 10 Cx endosperm corresponding to peak 5. Since the mother plant was tetraploid, the embryo sac must have developed via a disturbed meiosis. The 3x em developed without fertilization, the endosperm via pseudogamy.

Fig. 4.

Boxplots of the variation of percentages of well-developed achenes per collective fruit for each cytotype. The box shows the 25th and 75th percentile range and the median value. N = number of collective fruits.