ABSTRACT
One of the most common evolutionary transitions in angiosperms is the reproductive change from outcrossing to selfing, commonly associated with changes in floral biology and genetic diversity. Here, we aim to test whether self-compatibility leads to a reduction of floral traits and genetic diversity. For this, we experimentally estimate levels of self-compatibility, measure three floral traits and estimate four genetic diversity parameters using nine microsatellites in nine Calceolaria species. Our analysis indicated that four of the study species were self-incompatible. In addition, we found that self-compatible species did not show a reduction in floral traits size, but rather displayed larger corolla and elaiophore areas. Our analyses of genetic diversity identified larger allele number and observed heterozygosity in selfers than in outcrossers, but did not find larger inbreeding in the self-compatible species. Even though our results contradict our expectations, in the case of Calceolaria, their high dependence on only two genera of oil-bees puts the genus in a vulnerable reproductive position, probably facilitating the evolution of reproductive assurance mechanisms in the absence of pollinators. As a result, plants maintain their pollinator attraction traits while evolving the ability to self, possibly in a delayed way.
ABSTRACT
AbstractPitcairnioideae is the second most diverse subfamily of bromeliads (Bromeliaceae), a group exclusive to tropical regions of the New World. Pitcairnioid bromeliads have floral traits assumed to promote outcrossing through biotic pollination systems; however, the reproductive biology of most of the species of this group has not been documented. Pitcairnia heterophylla is an epiphytic (seldom saxicolous) bromeliad occurring from Southern Mexico, into the Northern Andes. We studied the pollination and breeding system of P. heterophylla in an epiphytic population at a mountain forest in Costa Rica from January to April 2013. We performed hand pollination experiments (agamospermy, autonomous self-pollination, hand self-pollination and hand cross-pollination) on 89 flowers from 23 individuals (3-6 flowers per individual) in 2013 flowering season. Nectar production was measured on 18 unvisited flowers of six individuals with a hand-held refractometer. Simultaneously, floral visitors were recorded on eight individuals with trail cameras for a total of 918 hours (115 ± 52 hours per individual, mean ± SE). Under natural conditions, seed set (540.4 ± 55.2) was similar to manually selfed flowers (516.3 ± 41.5) and autonomously selfed flowers (521.1 ± 29.0), but lower to manually outcrossed flowers (670.2 ± 31.3). The flowers of P. heterophylla are self-compatible, capable of autonomous pollination, and non-agamospermous. Intrafloral self-pollination is facilitated by adichogamy and lack of floral herkogamy. The scentless red flowers of P. heterophylla with tubular corollas and nectar production suggested ornithophilic pollination which was confirmed by video recording of 46 hummingbird visits. The most common floral visitor was the short-billed hummingbird Lampornis calolaemus which accounted for 78 % of the visits. However, the visitation rate during the flowering season was low (0.6 visits per day per plant). Selfing in P. heterophylla might be explained as a mechanism of reproductive assurance and to reduce interspecific pollen flow with taxonomically unrelated plants.
ResumenLa subfamilia Pitcairnioideae es la segunda más diversa de las bromelias (Bromeliaceae), un grupo exclusivo de la región Neotropical. Las bromelias pitcairnioideas poseen rasgos florales que se asume promueven el exo-cruzamiento a través de sistemas de polinización biótica; sin embargo, la biología reproductiva de la mayoría de especies de este grupo no se ha documentado. Pitcairnia heterophylla es una bromelia epífita (raramente saxícola) que se encuentra en el sur de México, América Central y el norte de los Andes. Se estudió la polinización y el sistema de apareamiento de una población P. heterophylla en un bosque montano en Costa Rica entre enero y abril 2013. Se realizaron cuatro experimentos de polinización (agamospermia, auto-polinización espontánea, auto-polinización manual, exo-polinización manual) en 89 flores de 23 individuos (3-6 flores por individuo). Se cuantificó la producción de néctar de 18 flores en seis individuos con un refractómetro. Simultáneamente, se registraron los visitantes florales de ocho individuos en el campo con ayuda de cámaras trampa por un total de 918 horas (115 ± 52 horas por individuo, promedio ± ES). Bajo condiciones naturales la producción de semillas fue similar (540.4 ± 55.2) a la producción de las flores auto-polinizadas manualmente (516.3 ± 41.5) y las flores auto-polinizadas espontáneamente (521.1 ± 29.0), pero fue menor a las flores exo-cruzadas (670.2 ± 31.3). Las flores de P. heterophylla son auto-compatibles, con alta capacidad de polinización espontánea y sin capacidad de agamospermia. La auto-polinización intra-floral es facilitada por la ausencia de dicogamia y hercogamia floral. Las flores rojas, sin aroma, con corola tubular y con producción de néctar sugirieron polinización por ornitofilia, lo que se confirmó con el registro de 46 visitas por colibríes (Apodiformes: Trochilidae). El visitante floral más común fue Lampornis calolaemus (78 % de las visitas). A pesar de esto, la tasa de visitas durante el periodo de estudio fue baja (0.6 visitas por día por planta). La capacidad de autofecundación de P. heterophylla puede ser explicada como un mecanismo de aseguramiento reproductivo, así como para reducir el flujo de polen inter-específico de plantas taxonómicamente no relacionadas.
ABSTRACT
BACKGROUND AND AIMS: The underlying evolutionary processes of pollinator-driven floral diversification are still poorly understood. According to the Grant-Stebbins model speciation begins with adaptive local differentiation in the response to spatial heterogeneity in pollinators. Although this crucial process links the micro- and macroevolution of floral adaptation, it has received little attention. In this study geographical phenotypic variation was investigated in Patagonian Calceolaria polyrhiza and its pollinators, two oil-collecting bee species that differ in body size and geographical distribution. METHODS: Patterns of phenotypic variation were examined together with their relationships with pollinators and abiotic factors. Six floral and seven vegetative traits were measured in 45 populations distributed across the entire species range. Climatic and edaphic parameters were determined for 25 selected sites, 2-16 bees per site of the most frequent pollinator species were captured, and a critical flower-bee mechanical fitting trait involved in effective pollination was measured. Geographical patterns of phenotypic and environmental variation were examined using uni- and multivariate analyses. Decoupled geographical variation between corolla area and floral traits related to the mechanical fit of pollinators was explored using a Mantel test. KEY RESULTS: The body length of pollinators and the floral traits related to mechanical fit were strongly correlated with each other. Geographical variation of the mechanical-fit-related traits was decoupled from variation in corolla size; the latter had a geographical pattern consistent with that of the vegetative traits and was mainly affected by climatic gradients. CONCLUSIONS: The results are consistent with pollinators playing a key role in shaping floral phenotype at a geographical scale and promoting the differentiation of two floral ecotypes. The relationship between the critical floral-fit-related trait and bee length remained significant even in models that included various environmental variables and an allometric predictor (corolla area). The abiotic environment also has an important role, mainly affecting floral size. Decoupled geographical variation between floral mechanical-fit-related traits and floral size would represent a strategy to maintain plant-pollinator phenotypic matching in this environmentally heterogeneous area.