RESUMO

Different populations of plant species can adapt to their local pollinators and diverge in floral traits accordingly. Floral traits are subject to pollinator-driven natural selection to enhance plant reproductive success. Studies on temperate plant systems have shown pollinator-driven selection results in floral trait variation along elevational gradients, but studies in tropical systems are lacking. We analyzed floral traits and pollinator assemblages in the Neotropical bee-pollinated taxon Costus guanaiensis var. tarmicus across four sites along a steep elevational gradient in Peru. We found variations in floral traits of size, color, and reward, and in the pollinator assemblage along the elevational gradient. We examined our results considering two hypotheses, (1) local adaptation to different bee assemblages, and (2) the early stages of an evolutionary shift to a new pollinator functional group (hummingbirds). We found some evidence consistent with the adaptation of C. guanaiensis var. tarmicus to the local bee fauna along the studied elevational gradient. Corolla width across sites was associated with bee thorax width of the local most frequent pollinator. However, we could not rule out the possibility of the beginning of a bee-to-hummingbird pollination shift in the highest-studied site. Our study is one of the few geographic-scale analyses of floral trait and pollinator assemblage variation in tropical plant species. Our results broaden our understanding of plant-pollinator interactions beyond temperate systems by showing substantial intraspecific divergence in both floral traits and pollinator assemblages across geographic space in a tropical plant species.

Diferentes poblaciones de una especie de planta pueden adaptarse a sus polinizadores locales y, en consecuencia, divergir en los rasgos florales. Los rasgos florales están sujetos a la selección natural impulsada por los polinizadores para mejorar el éxito reproductivo de la planta. Los estudios en sistemas de plantas de zonas templadas han demostrado que la selección impulsada por los polinizadores da como resultado una variación de los rasgos florales a lo largo de gradientes de altitud, pero faltan estudios en sistemas de plantas de zonas tropicales. Analizamos las características florales y los ensamblajes de polinizadores en el taxón Neotropical polinizado por abejas Costus guanaiensis var. tarmicus en cuatro sitios a lo largo de un empinado gradiente altitudinal en Perú. Encontramos variación en los rasgos florales de tamaño, color y recompensa, y en los ensamblajes de polinizadores a lo largo del gradiente altitudinal. Examinamos nuestros resultados considerando dos hipótesis, (1) la adaptación local a diferentes ensamblajes de abejas polinizadoras y (2) las primeras etapas de un desplazamiento evolutivo a un nuevo grupo funcional polinizador (colibríes). Encontramos evidencia que respalda la adaptación de C. guanaiensis var. tarmicus a la fauna de abejas local a lo largo del gradiente altitudinal estudiado. El ancho de la corola de la flor a lo largo de los sitios de estudio se asoció con el ancho del tórax de la especie de abeja local más frecuentemente registrada en las flores. Sin embargo, no pudimos descartar la posibilidad del comienzo de un desplazamiento de polinización de abeja a colibrí en el sitio más alto estudiado. Nuestro estudio es uno de los pocos análisis a escala geográfica de variación de rasgos florales y ensamblaje de polinizadores en una especie tropical. Nuestros resultados amplían la comprensión de las interacciones planta­polinizador más allá de los sistemas templados al mostrar una sustancial divergencia intraespecífica tanto en los rasgos florales como en los ensamblajes de polinizadores en una especie tropical.

RESUMO

Speciation involves the evolution of traits and genetic differences that contribute to reproductive isolation and the cessation of gene flow, and studying closely related species and divergent populations gives insight into how these phenomena proceed. Here, we document patterns of gene flow within and between two members of a rapid Neotropical species radiation, Costus pulverulentus and Costus scaber (Costaceae). These species co-occur in the tropical rainforest and share pollinators, but are reproductively isolated by a series of prezygotic barriers, some of which show evidence of reinforcement at sympatric sites. Here, we genotype microsatellite markers in plants from eight sites that span the geographical range of both species, including four sympatric sites. We also genotype putative hybrids found at two sympatric sites. We find high levels of genetic isolation among populations within each species and low but detectable levels of introgression between species at sympatric sites. Putative hybrids identified by morphology are consistent with F1 or more advanced hybrids. Our results highlight the effectiveness of prezygotic isolating mechanisms at maintaining species boundaries in young radiations and provide empirical data on levels of gene flow consistent with reinforcement.

Assuntos

RESUMO

Plant-pollinator interactions are great model systems to investigate mutualistic relationships. We compared pollinator effectiveness between facultative and obligate nectar-feeding bats to determine how foraging specialization influences mutualistic interactions in a bat-adapted cactus. We predicted that a specialized nectarivorous bat would deliver more pollen than an opportunistic nectar-feeding bat because of specialized adaptations to nectar feeding that indicate close association with their food plants. Counter to our predictions, the opportunistic Antrozous pallidus delivered significantly more pollen grains per visit than the specialized Leptonycteris yerbabuenae. Higher pollinator effectiveness, based on visitation rates and pollen deposition levels, varied between species by site, and although A. pallidus visits flowers much less frequently than L. yerbabuenae over all sites, it is likely an effective and reliable pollinator of Pachycereus pringlei in Baja, Mexico. Our results suggest that morphological adaptations and dietary specialization on nectar do not necessarily confer advantages for pollination over less specialized plant visitors and highlight the reciprocally exploitative nature of mutualisms.

Assuntos

RESUMO

Empirical estimates of the relative importance of different barriers to gene flow between recently diverged species are important for understanding processes of speciation. I investigated the factors contributing to reproductive isolation between Costus pulverulentus and C. scaber (Costaceae), two closely related hummingbird-pollinated understory Neotropical herbs. I studied broad-scale geographic isolation, microhabitat isolation, flowering phenology, overlap in pollinator assemblages, floral constancy by pollinators, mechanical floral isolation, pollen-pistil interactions, seed set in interspecific crosses, and postzygotic isolation (hybrid seed germination, greenhouse survival to flowering, and pollen fertility). Aside from substantial geographic isolation, I found evidence for several factors contributing to reproductive isolation in the sympatric portion of their geographic ranges, but the identity and relative strength of these factors varied depending on the direction of potential gene flow. For C. pulverulentus as the maternal parent, mechanical floral isolation was the most important factor, acting as a complete block to interspecific pollen deposition. For C. scaber as the maternal parent, microhabitat isolation, pollinator assemblage, mechanical floral isolation, and postpollination pollen-pistil incompatibility were important. Overall, prezygotic barriers were found to be strong, resulting in 100% reproductive isolation for C. pulverulentus as the maternal parent and 99.0% reproductive isolation for C. scaber as the maternal parent. Some postzygotic isolation also was identified in the F1 generation, increasing total isolation for C. scaber to 99.4%. The results suggest that ecological factors, including habitat use and plant-pollinator interactions, contributed to speciation in this system and evolved before extensive intrinsic postzygotic isolation.

Assuntos