Patterns of variation in distylous traits and reproductive consequences in Erythroxylum species and populations

Features of floral odor and nectar in the distylous Luculia pinceana (Rubiaceae) promote compatible pollination by hawkmoths

It is hypothesized that in heterostylous plant species, standardization of signals of floral attraction between different morphs is advantageous, encouraging flower visitors to switch between morphs. It remains unclear whether signals of floral attraction (floral odor and properties of nectar) are similar between morphs in distylous species pollinated by hawkmoths, and how these relate to hawkmoth behavior. We observed the behavior of visitors to distylous Luculia pinceana (Rubiaceae), collected and analyzed floral odor, and examined properties of nectar (volume, sugar concentration, and composition) of long‐styled and short‐styled morphs during the day and night. Pollinator responses to the floral scent were tested with a Y‐tube olfactometer. We conducted diurnal and nocturnal pollination treatments and six other pollination treatments to test the importance of nocturnal pollinators and to examine the self‐incompatibility system. A species of hawkmoth, Cechenena lineosa, was the effective pollinator. The floral odor was rich in methyl benzoate, and sucrose was dominant in the nectar. There were no significant differences between the two morphs in the methyl benzoate content or the properties of nectar. Flowers released more methyl benzoate and secreted larger volumes of nectar with lower sugar concentration at night than during the day. The hawkmoth had a significant preference for methyl benzoate. Luculia pinceana was partially self‐incompatible and relied on nocturnal pollinators for reproductive success. This study verifies that floral attraction signals are consistent between different morphs in this distylous species, promoting compatible pollination, and the features and the diel pattern of these signals between day and night are adapted to hawkmoth behavior.

Direct evidence supporting Darwin's hypothesis of cross-pollination promoted by sex organ reciprocity

The floral phenotype plays a main role in the attraction and fit of pollinators. Both perianth traits and the positioning of sex organs can be subjected to natural selection and determine nonrandom mating patterns in populations. In stylar-polymorphic species, the Darwinian hypothesis predicts increased mating success between individuals with sex organs at equivalent heights (i.e. with higher reciprocity). We used paternity analyses in experimental populations of a stylar-dimorphic species. By comparing the observed mating patterns with those expected under random mating, we tested the effects of sex organ reciprocity and perianth traits on mating success. We also analysed phenotypic selection on perianth traits through female and male functions. The (dis)similarity of parental perianth traits had no direct effects on the mating patterns. Sex organ reciprocity had a positive effect on mating success. Narrow floral tubes increased this effect in upper sex organs. Perianth traits showed little signs of phenotypic selection. Female and absolute fitness measures resulted in different patterns of phenotypic selection. We provide precise empirical evidence of the Darwinian hypothesis about the functioning of stylar polymorphisms, demonstrating that mating patterns are determined by sex organ reciprocity and only those perianth traits which are critical to pollinator fit.

Variation in pollen sterility and gender specialization: an investigation with distylous species of Erythroxylum (Erythroxylaceae)

Differences in pollen sterility between morphs in distylous populations may represent the first step in the evolution of gender specialization. Theoretically, for partially or completely male sterile individuals to persist they must have some fitness advantage that compensates for the loss of male function. Gender specialization is considered a widespread process in Erythroxylum, but male sterility and the resource reallocation to female function have been investigated in few species and populations. In 18 populations of four distylous species of Erythroxylum, we quantified the levels of male sterility, estimated through pollen sterility, in short- and long-styled flowers to test if sterility is morph-biased. In one population per species, we also described the frequency of floral visitors, the production of flowers and fruits, and the quality of fruits and seeds of short- and long-styled plants to evaluate the expression of trade-offs in allocation to male and female function. In some populations of E. campestre and E. deciduum, short-styled flowers possessed higher levels of pollen sterility than long-styled flowers. Although most flowers of E. suberosum and E. tortuosum also expressed pollen sterility, the frequency of sterility was similar between morphs in all populations. Differences in reproductive output between morphs occurred only in populations of species with morph-biased sterility, but none of the variations reflect resource allocation to female fitness of short-styled plants. Differences in the level of sterility between morphs indicates the potential for gender specialization in populations of E. campestre and E. deciduum, despite the apparent lack of a trade-off in allocation.