Euglossine bees mediate only limited long-distance gene flow in a tropical vine

Evolutionary convergence on hummingbird pollination in Neotropical Costus provides insight into the causes of pollinator shifts

The evolution of hummingbird pollination is common across angiosperms throughout the Americas, presenting an opportunity to examine convergence in both traits and environments to better understand how complex phenotypes arise. Here we examine independent shifts from bee to hummingbird pollination in the Neotropical spiral gingers (Costus) and address common explanations for the prevalence of transitions from bee to hummingbird pollination. We use floral traits of species with observed pollinators to predict pollinators of unobserved species and reconstruct ancestral pollination states on a well-resolved phylogeny. We examine whether independent transitions evolve towards the same phenotypic optimum and whether shifts to hummingbird pollination correlate with elevation or climate. Traits predicting hummingbird pollination include small flower size, brightly colored floral bracts and the absence of nectar guides. We find many shifts to hummingbird pollination and no reversals, a single shared phenotypic optimum across hummingbird flowers, and no association between pollination and elevation or climate. Evolutionary shifts to hummingbird pollination in Costus are highly convergent and directional, involve a surprising set of traits when compared with other plants with analogous transitions and refute the generality of several common explanations for the prevalence of transitions from bee to hummingbird pollination.

Population structure in Neotropical plants: Integrating pollination biology, topography and climatic niches

Animal pollinators mediate gene flow among plant populations, but in contrast to well-studied topographic and (Pleistocene) environmental isolating barriers, their impact on population genetic differentiation remains largely unexplored. Comparing how these multifarious factors drive microevolutionary histories is, however, crucial for better resolving macroevolutionary patterns of plant diversification. Here we combined genomic analyses with landscape genetics and niche modelling across six related Neotropical plant species (424 individuals across 33 localities) differing in pollination strategy to test the hypothesis that highly mobile (vertebrate) pollinators more effectively link isolated localities than less mobile (bee) pollinators. We found consistently higher genetic differentiation (FST ) among localities of bee- than vertebrate-pollinated species with increasing geographical distance, topographic barriers and historical climatic instability. High admixture among montane populations further suggested relative climatic stability of Neotropical montane forests during the Pleistocene. Overall, our results indicate that pollinators may differentially impact the potential for allopatric speciation, thereby critically influencing diversification histories at macroevolutionary scales.

Using ecological context to interpret spatiotemporal variation in natural selection

Spatiotemporal variation in natural selection is expected, but difficult to estimate. Pollinator-mediated selection on floral traits provides a good system for understanding and linking variation in selection to differences in ecological context. We studied pollinator-mediated selection in five populations of Dalechampia scandens (Euphorbiaceae) in Costa Rica and Mexico. Using a nonlinear path-analytical approach, we assessed several functional components of selection, and linked variation in pollinator-mediated selection across time and space to variation in pollinator assemblages. After correcting for estimation error, we detected moderate variation in net selection on two out of four blossom traits. Both the opportunity for selection and the mean strength of selection decreased with increasing reliability of cross-pollination. Selection for pollinator attraction was consistently positive and stronger on advertisement than reward traits. Selection on traits affecting pollen transfer from the pollinator to the stigmas was strong only when cross-pollination was unreliable and there was a mismatch between pollinator and blossom size. These results illustrate how consideration of trait function and ecological context can facilitate both the detection and the causal understanding of spatiotemporal variation in natural selection.

Mixed mating system and variable mating patterns in tropical woody bamboos

BACKGROUND

So far, little is known in detail about mating systems of woody bamboos. Paternity analysis of offspring improved our understanding of these systems, and contributed to their germplasm conservation and genetic improvement.

RESULTS

In this study, a paternity analysis of offspring from two consecutive mass or sporadically flowering events of Dendrocalamus membranaceus and D. sinicus were conducted to determine their mating system and pollen dispersal using the program COLONY based on simple sequence repeat (SSR) markers. Two sporadically flowering populations of D. sinicus (C1, C2) obtained relatively high paternity assignments rates (69.0-71.4%). Meanwhile, among three populations of D. membranaceus, the sporadically flowering population A also had much higher paternity assignments rates (56.4%) than mass flowering populations B1(28.6%) and B2 (42.5%). Both D. membranaceus and D. sinicus had mixed mating systems while their mating patterns were variable depending on pollination conditions. The maximum pollen dispersal distances were 90 m and 4378 m for D. membranaceus and D. sinicus populations, respectively, and the mating distances of these two species focused on ranges of ca. 0-50 m and 0-1500 m, respectively.

CONCLUSIONS

These results revealed for the first time variable mating patterns in woody bamboos. This suggests half-sib seeds from the same bamboo clump may have different male parents and it is crucial to clarify genetic origin in woody bamboos' breeding programs. The results also indicate the importance of pollinators in the mating systems of tropical woody bamboos.

Hybridization and geographic distribution shapes the spatial genetic structure of two co-occurring orchid species

Multiple ecological and life-history traits shape the fine-scale spatial genetic structure (FSGS) of a given population. The occurrence in core versus peripheral populations, levels of outcrossing, pollen and seed dispersal, and hybridization are important biological properties that influence the kinship of individuals within populations. We examined spatial genetic structure within 15 populations of Epidendrum fulgens and E. puniceoluteum distributed along a linear gradient of Brazilian coastal vegetation, including both allopatric and sympatric populations where the two orchid species hybridize. We analyzed 581 mapped specimens using nine simple sequence repeat loci, aiming to investigate how geographic distribution and hybridization shape within-population FSGS. A significant increase in FSGS was found towards peripheral populations, compared to core populations. Analysis of short-distance and long-distance components of FSGS identified biparental inbreeding and higher levels of FSGS at peripheral populations, when compared to core populations. In contrast, the relatively high density of reproductive adults in core populations potentially leads to highly overlapping seed and pollen movement, decreasing FSGS. Hybridization was an important factor shaping within-population spatial genetic structure at sympatric sites, decreasing the FSGS observed in parental species. Our results indicate that different ecological forces act in concert to create a gradient of FSGS along species distribution ranges, shaped by extensive levels of intraspecific and interspecific gene exchange.

Genetic diversity and genetic structure of Decalobanthus boisianus in Hainan Island, China

Decalobanthus boisianus is a native plant of Hainan Island, China, which has caused considerable damage to tropical forest ecosystems in recent decades. Understanding the genetic diversity and structure of this species can facilitate uncovering the molecular mechanism of its invasive ability. Here, we collected 77 individuals of D. boisianus spanning 8 distribution areas with a gradient of human disturbance intensity (i.e., low, moderate, and high disturbance intensity groups) to assess patterns of genetic diversity and structure using inter simple sequence repeat (ISSR) markers. We found that a total of 220 loci were scored with 13 primers using ISSR methods, and that 198 loci were polymorphic. The genetic diversity of D. boisianus among these eight forests decreased with increasing human disturbance intensity. Over 70% of the total genetic variation was present within populations, while less than 30% of variation was found among populations. There was a high gene flow (1.27) among them due to a lack of effective geographic barriers. The mean Nei's genetic distance of D. boisianus populations was found to be relatively small (i.e., 0.07), and the average genetic similarity of the eight populations was high (i.e., 0.93). Our findings indicate that the genetic diversity of D. boisianus correlated to human disturbance density, and that D. boisianus populations in Hainan Island have frequent gene exchange. We suggest that reduce deforestation to decrease human disturbance may be a good way to prevent the invasion of D. boisianus.

No evidence that seed predators constrain pollinator-mediated trait evolution in a tropical vine

PREMISE OF THE STUDY

Turnover in biotic communities across heterogeneous landscapes is expected to lead to variation in interactions among plants, their mutualists, and their antagonists. Across a fragmented landscape in northern Costa Rica, populations of the euphorb vine Dalechampia scandens vary widely in mating systems and associated blossom traits. Previous work suggested that populations are well adapted to the local reliability of pollination by apid and megachilid bees. We tested whether variation in the intensity of predispersal seed predation by seed weevils in the genus Nanobaris also contributes to the observed variation in blossom traits.

METHODS

We studied spatiotemporal variation in the relationships between floral advertisement and the probability of seed predation within three focal populations. Then we assessed among-population covariation of predation rate, pollination reliability, mating system, and blossom traits across 20 populations.

KEY RESULTS

The probability of seed predation was largely unrelated to variation in floral advertisement both within focal populations and among the larger sample of populations. The rate of seed predation was only weakly associated with the rate of cross-pollination (allogamy) in each population but tended to be proportionally greater in populations experiencing less reliable pollination.

CONCLUSIONS

These results suggest that geographic variation in the intensity of antagonistic interactions have had only minor modifying effects on the evolutionary trajectories of floral advertisement in plant populations in this system. Thus, pollinator-driven floral trait evolution in D. scandens in the study area appears not to be influenced by conflicting seed-predator-mediated selection.

Intersexual conflict over seed size is stronger in more outcrossed populations of a mixed-mating plant

In polyandrous species, fathers benefit from attracting greater maternal investment toward their offspring at the expense of the offspring of other males, while mothers should usually allocate resources equally among offspring. This conflict can lead to an evolutionary arms race between the sexes, manifested through antagonistic genes whose expression in offspring depends upon the parent of origin. The arms race may involve an increase in the strength of maternally versus paternally derived alleles engaged in a "tug of war" over maternal provisioning or repeated "recognition-avoidance" coevolution where growth-enhancing paternally derived alleles evolve to escape recognition by maternal genes targeted to suppress their effect. Here, we develop predictions to distinguish between these two mechanisms when considering crosses among populations that have reached different equilibria in this intersexual arms race. We test these predictions using crosses within and among populations of Dalechampia scandens (Euphorbiaceae) that presumably have experienced different intensities of intersexual conflict, as inferred from their historical differences in mating system. In crosses where the paternal population was more outcrossed than the maternal population, hybrid seeds were larger than those normally produced in the maternal population, whereas when the maternal population was more outcrossed, hybrid seeds were smaller than normal. These results confirm the importance of mating systems in determining the intensity of intersexual conflict over maternal investment and provide strong support for a tug-of-war mechanism operating in this conflict. They also yield clear predictions for the fitness consequences of gene flow among populations with different mating histories.