An experimental evaluation of self-interference in Narcissus assoanus: functional and evolutionary implications

Abstract

Floral traits that reduce self‐pollination in hermaphroditic plants have usually been interpreted as mechanisms that limit the genetic consequences of self‐fertilization. However, the avoidance of sexual conflict between female and male function (self‐interference) may also represent an important selection pressure for the evolution of floral traits, particularly in self‐incompatible species. Here, we use experimental manipulations to investigate self‐interference in Narcissus assoanus, a self‐incompatible species with a stigma‐height dimorphism in which the degree of spatial separation between sex organs (herkogamy) differs strikingly between the long‐ and short‐styled morphs (hereafter L‐ and S‐morphs). We predicted that weak herkogamy in the L‐morph would cause greater self‐pollination and hence self‐interference. Experimental self‐pollination reduced seed set when it occurred prior to, or simultaneously with, cross‐pollination in the L‐morph, but only if it occurred prior to cross‐pollination in the S‐morph. In the field, autonomous self‐pollination was greater in the L‐morph than the S‐morph, but we found no evidence that self‐interference reduced maternal or paternal fitness in either morph. One‐day‐old flowers of the L‐morph have reduced stigma receptivity and hence exhibit protandry, whereas stigma receptivity and anther dehiscence are concurrent in the S‐morph. This suggests that the two style morphs have alternative strategies for reducing self‐interference: dichogamy in the L‐morph and herkogamy in the S‐morph. These results provide insight into the mechanisms that reduce sexual conflict in hermaphrodite plants and are of significance for understanding the evolution and maintenance of sexual polymorphisms.

Darwin's legacy: the forms, function and sexual diversity of flowers

“…Studies of the influence of stigma-height dimorphism on pollen export and receipt in both heterostylous and nonheterostylous groups would be valuable to determine the extent of inter-morph mating. Stigma-height dimorphism is especially well represented in Narcissus, where it occurs in approximately 12 species and has been studied intensively in at least four (Dulberger 1964;Arroyo & Dafni 1995;Baker et al 2000a,b;Arroyo et al 2002;Cesaro et al 2004). The inheritance of style length is the same as that in most distylous species (Lewis & Jones 1992) with the L-morph of genotype ss and the S-morph Ss (Dulberger 1967).…”

The evolution of polymorphic sexual systems in daffodils (Narcissus)

“…Furthermore, in Narcissus species with stylar polymorphism, most of which possess late-acting self-incompatibility, self-pollination prior to cross-pollination reduces seed set in comparison to flowers that receive only cross pollen (e.g. N. assoanus, Cesaro et al, 2004;N. papyraceus, Arroyo et al, 2002; N. triandrus, Barrett et al, 1997), probably by interfering with signalling between ovules and cross-pollen tubes (ovule discounting: Barrett et al, 1996;Sage et al, 1999).…”

Spatial ecology of mating success in a sexually polymorphic plant

“…Population surveys of N. assoanus have revealed wide variation in the relative frequencies of the L-and S-morphs (Baker et al 2000a), which is thought to reflect asymmetries in the mating patterns of the two morphs Baker et al 2000b;Cesaro et al 2004). Specifically, the S-morph is likely to be less proficient at intra-morph mating than the L-morph because the large spatial separation of stigmas and anthers (herkogamy) in the S-morph reduces the precision of intra-morph pollen transfer.…”