Abstract
Most heterostylous species show self- and intramorph incompatibility and models established for such taxa have traditionally been applied to the evolution of stylar dimorphism and heterostyly in Narcissus, a genus with late-acting self-incompatibility. The model of Lloyd and Webb (1992a,b) proposed that, in an approach-herkogamous ancestor, stylar dimorphism and heterostyly appeared consecutively as a result of two single mutations selected positively to enhance cross-pollination. Most polymorphic Narcissus are stylar dimorphic with two anther whorls, the lower positioned in the middle of a narrow floral tube, and style lengths that locate the stigmas above or below the low-level anthers. Here, I propose that in an ancestor with open-tubed flowers, late-acting self-incompatibility and variable style length, the narrowing of the floral tube increased self-pollination and ovule discounting in individuals with the stigma at the same height as the low-level anthers, imposing disruptive selection against this phenotype and causing the bimodal distribution of style lengths. This hypothesis stresses the need of avoiding self-interference for the selection of stylar dimorphism. It does not exclude the promotion of cross-pollination as a force for subsequent evolution of heterostyly in the genus nor the need of inter-morph pollination for the maintenance of polymorphism.
Highlights
And stylar dimorphism are similar sex polymorphisms of hermaphroditic flowers whose characteristics and frequency of occurrence differ sharply among certain angiosperm families
Floral morphs differ in the position of the stigma while anther position remains similar, with low reciprocity among morphs
Both evolutionary models were developed for heterostyly with heteromorphic selfincompatibility system (HetSI), the genus Narcissus has been frequently used to illustrate the model of Lloyd and Webb because it appears to embody the initial stages
Summary
Logical outcomes of the proposed evolutionary pathway include, first, that the stylar length may still present evident variation (i.e. a subdued bimodal distribution) rather than two absolutely discrete classes. Re-exploring the raw data of Arroyo et al (2002) gave further insights into styledimorphic Narcissus papyraceus, a relatively basal species in the phylogeny of the genus (Santos-Gally et al 2012) that presents LSI (Simón-Porcar et al 2015) Populations of this species show a bimodal but frequently overlapping distribution of style lengths in dimorphic populations, with the least frequent position located exactly at the level of the lower anther whorl (see an example population, data from other populations not shown). The disruption between long and short styles seems to be consistently minor in populations of the Moroccan Narcissus brousonetti, another basal species in the phylogeny of the genus (Santos-Gally et al 2012) which holds LSI and whose classification as style-dimorphic was subjected to controversy (Santos-Gally et al 2015) In this species the lower anther whorl is presented in a relaxed manner very close to the upper whorl, above the floral tube, where this is wide. The distribution of style morphs in the genus and beyond would result from the interplay between selection at the individual and species level (see Raduski et al 2011 for an interesting discussion on this topic)
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