The role of short-tongued insects in floral variation across the range of a style-dimorphic plant

Abstract

Background and aims: Heterostyly and related style polymorphisms are suitable model systems to evaluate the importance of functional pollinators in the maintenance of population variability. In Narcissus papyraceus different functional pollinators, incompatibility system and flower morphology have been proposed to influence the maintenance of polymorphism through their effect on disassortative mating. Here a test is done to find out if the visitation rate of long- versus short-tongued pollinators correlates with the morph ratio and if the latter is related to other flower traits of the species across its main geographic range.

Methods: Floral traits from 34 populations in the south-west of the Iberian Peninsula and in north-west Africa were measured, perianth variation was described and a comparison was made of allometric relationships between sex organs and floral tube. Correlations between pollinator guilds, stigma-anther separation of reciprocal morphs (our proxy for disassortative mating) and morph-ratio variation were analysed. Finally, the incompatibility system of the species in the northern and southern borders of its distribution are described.

Key results: Flowers from southern populations were significantly larger than flowers from centre and northern populations. The abundance of short-styled plants decreased gradually with increasing distance from the core region (the Strait of Gibraltar), with these disappearing only in the northern range. Although there was a significant difference in stigma-anther separation among populations, morph ratio was not associated with reciprocity or floral tube length. Long-style morph frequency increased with short-tongued pollinator visitation rate. Populations from both edges of the distribution range were self-incompatible and within- and between-morph compatible.

Conclusions: The style morph ratio changed gradually, whereas perianth trait variation showed abrupt changes with two morphotypes across the range. The positive relationship between the visitation rate of short-tongued pollinators and the decrease of the short-style morph supports our initial hypothesis. The results highlight the importance of different pollinators in determining the presence of style polymorphism.

Fig. 1.

Distribution and morph-ratio variation of 34 populations of Narcissus papyraceus in the Guadalquivir river basin, Strait of Gibraltar and Sebou river basin. Each pie chart represents the morph ratio in a population (black = L-morph; white = S-morph). Moran's I correlogram of morph-ratio variation is shown in the inset graph (significant and non-significant autocorrelation coefficients as indicated, calculated from their moments; see PASSaGE v. 1·1 manual; Rosenberg, 2004).

Fig. 2.

Principal component analysis of five morphological traits from the Guadalquivir river basin, Strait of Gibraltar and Sebou river basin populations of Narcissus papyraceus. The first two axes account for 78 % and 18 % of the total variation, respectively. Monomorphic, anisoplethic and isoplethic populations are as indicated in the key. The Guadalquivir river basin and Strait of Gibraltar populations are depicted by black symbols, the Sebou river basin populations by grey symbols. Each population is represented with standard error of PCA1 and PCA2 factor loadings. Loadings for all the variables of the PCA are given in the top right corner. Arrows indicate populations from an intermediate geographic location between the Strait of Gibraltar and the Sebou river basin.

Fig. 3.

Diurnal and nocturnal, and short- and long-tongued pollinator visitation rates (total visits h⁻¹) in nine populations of Narcissus papyraceus. Data from two monomorphic, three anisoplethic and four isoplethic populations (population numbers 3, 6, 7, 11, 13, 17, 19 and 21; see Table 1 for details). Diurnal short-tongued pollinators of the three types of population are shown separately (grey); long-tongued pollinators are divided into diurnal and nocturnal pollinators, as indicated.

Fig. 4.

Relationship between short-tongued pollinator visitation rates (total visits h⁻¹) and the proportion of L-morph plants in nine populations of Narcissus papyraceus.

Fig. 5.

Seed-set (seed to ovule number) of L- and S-morphs in two anisoplethic population of Narcissus papyraceus from the Sebou river basin after hand pollination experiments (see Table 1 for location of populations). Values indicated are the means ± s.e. of seed production following intramorph (L × L, S × S), intermorph (L× S, S× L), self- pollination (L× , S×) and prior self-pollination plus cross-pollination (L× +a, S× +a).