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. 2018 Feb 28;285(1873):20180004.
doi: 10.1098/rspb.2018.0004.

Polygamy or Subdioecy? The Impact of Diallelic Self-Incompatibility on the Sexual System in Fraxinus excelsior (Oleaceae)

Free PMC article

Polygamy or Subdioecy? The Impact of Diallelic Self-Incompatibility on the Sexual System in Fraxinus excelsior (Oleaceae)

Pierre Saumitou-Laprade et al. Proc Biol Sci. .
Free PMC article


How flowering plants have recurrently evolved from hermaphroditism to separate sexes (dioecy) is a central question in evolutionary biology. Here, we investigate whether diallelic self-incompatibility (DSI) is associated with sexual specialization in the polygamous common ash (Fraxinus excelsior), which would ultimately facilitate the evolution towards dioecy. Using interspecific crosses, we provide evidence of strong relationships between the DSI system and sexual phenotype. The reproductive system in F. excelsior that was previously viewed as polygamy (co-occurrence of unisexuals and hermaphrodites with varying degrees of allocation to the male and female functions) and thus appears to actually behave as a subdioecious system. Hermaphrodites and females belong to one SI group and functionally reproduce as females, whereas males and male-biased hermaphrodites belong to the other SI group and are functionally males. Our results offer an alternative mechanism for the evolution of sexual specialization in flowering plants.

Keywords: dioecy; functional gender; plant reproductive system; polygamy/trioecy.

Conflict of interest statement

We have no competing interests.


Figure 1.
Figure 1.
Results of interspecific stigma tests between Fraxinus excelsior and Fraxinus ornus/Phillyrea angustifolia individuals previously assigned to either G1 or G2 SI group. Photos (b,c,f,g) show typical compatibility reactions, with several pollen tubes growing in the stigma tissue and converging towards the style. Photos (a,d,e,h) show typical incompatibility reactions. In this example, Orleans_34 was assigned to the G1 group, and Orleans_35 was assigned to G2. (Online version in colour.)
Figure 2.
Figure 2.
Distribution of sexual phenotype scores from 1 (pure male) to 7 (pure female) in (a) Orléans and (b) Cevennes according to the assigned SI group (black: G1, white: G2) for each studied individual. Bars in (a) are for standard errors among clones and year replicates. In Cevennes (b), most individuals were phenotyped only in 2016.

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