FORAGER BEHAVIOR, POLLEN DISPERSAL, AND INBREEDING IN TWO SPECIES OF HUMMINGBIRD‐POLLINATED PLANTS

Pollinator type strongly impacts gene flow within and among plant populations for six Neotropical species

Animal pollinators directly affect plant gene flow by transferring pollen grains between individuals. Pollinators with restricted mobility are predicted to limit gene flow within and among populations, whereas pollinators that fly longer distances are likely to promote genetic cohesion. These predictions, however, remain poorly tested. We examined population genetic structure and fine-scale spatial genetic structure (FSGS) in six perennial understory angiosperms in Andean cloud forests of northwestern Ecuador. Species belong to three families (Gesneriaceae, Melastomataceae, and Rubiaceae), and within each family we paired one insect-pollinated with one hummingbird-pollinated species, predicting that insect-pollinated species have greater population differentiation (as quantified with the F_ST statistic) and stronger FSGS (as quantified with the S_P statistic) than hummingbird-pollinated species. We confirmed putative pollinators through a literature review and fieldwork, and inferred population genetic parameters with a genome-wide genotyping approach. In two of the three species pairs, insect-pollinated species had much greater (>2-fold) population-level genetic differentiation and correspondingly steeper declines in fine-scale genetic relatedness. In the Gesneriaceae pair, however, F_ST and S_P values were similar between species and to those of the other hummingbird-pollinated plants. In this pair, the insect pollinators are euglossine bees (as opposed to small bees and flies in the other pairs), which are thought to forage over large areas, and therefore may provide similar levels of gene flow as hummingbirds. Overall, our results shed light on how different animal pollination modes influence the spatial scale of plant gene flow, suggesting that small insects strongly decrease genetic cohesion.

The evolution, ecology, and conservation of hummingbirds and their interactions with flowering plants

The ecological co-dependency between plants and hummingbirds is a classic example of a mutualistic interaction: hummingbirds rely on floral nectar to fuel their rapid metabolisms, and more than 7000 plant species rely on hummingbirds for pollination. However, threats to hummingbirds are mounting, with 10% of 366 species considered globally threatened and 60% in decline. Despite the important ecological implications of these population declines, no recent review has examined plant-hummingbird interactions in the wider context of their evolution, ecology, and conservation. To provide this overview, we (i) assess the extent to which plants and hummingbirds have coevolved over millions of years, (ii) examine the mechanisms underlying plant-hummingbird interaction frequencies and hummingbird specialization, (iii) explore the factors driving the decline of hummingbird populations, and (iv) map out directions for future research and conservation. We find that, despite close associations between plants and hummingbirds, acquiring evidence for coevolution (versus one-sided adaptation) is difficult because data on fitness outcomes for both partners are required. Thus, linking plant-hummingbird interactions to plant reproduction is not only a major avenue for future coevolutionary work, but also for studies of interaction networks, which rarely incorporate pollinator effectiveness. Nevertheless, over the past decade, a growing body of literature on plant-hummingbird networks suggests that hummingbirds form relationships with plants primarily based on overlapping phenologies and trait-matching between bill length and flower length. On the other hand, species-level specialization appears to depend primarily on local community context, such as hummingbird abundance and nectar availability. Finally, although hummingbirds are commonly viewed as resilient opportunists that thrive in brushy habitats, we find that range size and forest dependency are key predictors of hummingbird extinction risk. A critical direction for future research is to examine how potential stressors - such as habitat loss and fragmentation, climate change, and introduction of non-native plants - may interact to affect hummingbirds and the plants they pollinate.

From pollen dispersal to plant diversification: genetic consequences of pollination mode

Pollinators influence patterns of plant speciation, and one intuitive hypothesis is that pollinators affect rates of plant diversification through their effects on pollen dispersal. By specifying mating events and pollen flow across the landscape, distinct types of pollinators may cause different opportunities for allopatric speciation. This pollen dispersal-dependent speciation hypothesis predicts that pollination mode has effects on the spatial context of mating events that scale up to impact population structure and rates of species formation. Here I consider recent comparative studies, including genetic analyses of plant mating events, population structure and comparative phylogenetic analyses, to examine evidence for this model. These studies suggest that highly mobile pollinators conduct greater gene flow within and among populations, compared to less mobile pollinators. These differences influence patterns of population structure across the landscape. However, the effects of pollination mode on speciation rates is less predictable. In some contexts, the predicted effects of pollen dispersal are outweighed by other factors that govern speciation rates. A multiscale approach to examine effects of pollination mode on plant mating system, population structure and rates of diversification is key to determining the role of pollen dispersal on plant speciation for model clades.

VARIATION IN THE FLOWERING PHENOLOGY OF AN EPIPHYTIC BROMELIAD ALONG AN ELEVATIONAL GRADIENT

Elevational patterns in flowering phenology have been reported for trees, shrubs and herbs. However, for vascular epiphytes that rely on atmospheric sources for humidity and nutrients, and depend on phorophyte microhabitat, elevational patterns of variation are unknown. In this study, we described the flowering phenology of Tillandsia carlos-hankii, an epiphytic bromeliad, along an elevational gradient in Capulálpam de Méndez, Oaxaca. We analyzed the onset, seasonality and duration of flowering along and within different elevation zones, and we evaluated the effect of phorophyte features (tree height, DBH and canopy diameter) on flowering start date and duration. From June 2016 to May 2017, we periodically recorded phenological data from six populations along three elevation zones (“low” 2151 to 2283 m. a. s. l., “medium”: 2284 to 2416 m. a. s. l. and “high” 2417 to 2548 m. a. s. l.), monitoring two population per zone. Start of flowering occurred between December and January, beginning six to 16 days earlier at low elevations than in the other zones, although this difference was not statistically significant. We observed marked flowering seasonality at all the elevation zones, with differences between zones (W≥18.49, p<0.0001) and between the populations at medium and high elevations (W≥8.57, p≤0.05). Flowering duration spanned fromDecember to May. Phorophyte features were not related to the onset or duration of flowering (t≤-1.47, p≥0.14, in all cases). Our results suggest that vascular epiphytes follow the same elevational patterns in phenology as other life forms, and that populations in the same elevation range can vary. The causes of such intra-elevational variation merit further investigation.

DIRECT AND INDIRECT ESTIMATES OF NEIGHBORHOOD AND EFFECTIVE POPULATION SIZE IN A TROPICAL PALM, ASTROCARYUM MEXICANUM

To estimate the relative importance of genetic drift, the effective population size ∗∗∗(N_e ) can be used. Here we present estimates of the effective population size and related measures in Astrocaryum mexicanum, a tropical palm from Los Tuxtlas rain forest, Veracruz, Mexico. Seed and pollen dispersal were measured. Seeds are primarily dispersed by gravity and secondarily dispersed by small mammals. Mean primary and secondary dispersal distances for seeds were found to be small (0.78 m and 2.35 m, respectively). A. mexicanum is beetle pollinated and pollen movements were measured by different methods: a) using fluorescent dyes, b) as the minimum distance between active female and male inflorescences, and c) using rare allozyme alleles as genetic markers. All three estimates of pollen dispersal were similar, with a mean of approximately 20 m. Using the seed and pollen dispersal data, the genetic neighborhood area (A) was estimated to be 2,551 m² . To obtain the effective population size, three different overlapping generation methods were used to estimate an effective density with demographic data from six permanent plots. The effective density ranged from 0.040 to 0.351 individuals per m² . The product of effective density and neighborhood area yields a direct estimate of the neighborhood effective population size (N_b ). N_b ranged from 102 to 895 individuals. Indirect estimates of population size and migration rate (Nm) were obtained using F_st for five different allozymic loci for both adults and seeds. We obtained a range of Nm from 1.2 to 19.7 in adults and a range of Nm from 4.0 to 82.6 for seeds. We discuss possible causes of the smaller indirect estimates of Nm relative to the direct and compare our estimates with values from other plant populations. Gene dispersal distances, neighborhood size, and effective population size in A. mexicanum are relatively high, suggesting that natural selection, rather than genetic drift, may play a dominant role in patterning the genetic variation in this tropical palm.

Floral display and mating patterns within populations of the neotropical epiphytic orchid,Laeliarubescens (Orchidaceae)

Pollinator behavior plays a central role in determining patterns of pollen-mediated gene movement in zoophilous angiosperms. A species' floral display can strongly influence the behavior of its pollinators and thereby affect its evolutionary pathway. We used paternity analysis to directly measure and describe mating patterns within 15 populations of the epiphytic orchid, Laelia rubescens, in Costa Rican dry forest. Strict correlated mating by orchids allows inference of the precise multilocus diploid genotype of the pollen parents. Our data show that mean effective population sizes were small (11.2 in 1999 and 11.8 in 2000) relative to the number of flowering genets (63 and 56, respectively). Fewer genets were reproductively successful as females than males. The relationship between reproductive success (RS) and floral display within three cluster size classes was consistent between years, with large (>30 inflorescences) and small (≤10 inflorescences) clusters often having significantly lower RS than expected, while the RS of medium-sized clusters (11-30 inflorescences) often significantly exceeded expectations. Paternity analysis allowed us to take advantage of the pollination biology of L. rubescens to provide unusually detailed insights into mating patterns, pollen-mediated gene movement and RS for populations of this epiphytic orchid, an herbaceous perennial, distributed in three-dimensional space.

Interannual genetic heterogeneity of pollen pools accepted by Quercus salicina individuals

Since flowering often varies among years in wind-pollinated woody species, the genetic composition of pollen pools accepted by seed parents can differ between years. The interannual heterogeneity of pollen flow may be important for maintaining genetic diversity within populations because it can increase genetic variation within populations and the effective sizes of the populations. In this study we examined heterogeneity, using paternity analysis and analysis of molecular variance, in the genetic composition of pollen pools among different reproductive years for six Quercus salicina seed parents in an 11.56-ha plot in a temperate old-growth evergreen broadleaved forest. The genotypes at seven microsatellite loci were determined for 111 adult trees and 777 offspring of the six seed parents in 2-5 reproductive years. Genetic differentiation of pollen pools among different reproductive years for each seed parent was significant over all seed parents and for each of four seed parents that were analysed for more than 2 years, but not for either of the other two seed parents (analysed for 2 years). For both the pollen pools originating from inside the plot and those originating from outside it, genetic differentiation among different reproductive years for each seed parent was significant over all seed parents. However, among-year genetic differentiation in the pollen pools originating from within the plot was detected for all four of the seed parents that were analysed for more than 2 years, but for only one of the four in the pools originating from outside the plot. Genetic diversity (estimated as allelic richness and gene diversity) was higher for pollen pools over all reproductive years than for pollen pools in single years. These results indicate that the year-to-year genetic variation of pollen pools increases genetic diversity in offspring and is strongly affected by the variation in pollen parents within the plot because of their high pollination contributions. The high year-to-year variation in pollen parents within the plot and overall supports the hypothesis that the offspring produced across years represent a larger genetic neighbourhood.

Outcrossing and paternity analysis of Pinus densiflora (Japanese red pine) by microsatellite polymorphism

This study employed microsatellite loci to analyse outcrossing rate and pollen dispersal in Japanese red pine (Pinus densiflora) in an isolated stand. The average offspring outcrossing rate for 29 cones was 0.955. Significant differences in outcrossing rates between offspring groups on individual branches that extended in different directions at different heights were not detected. Male parents of 874 offspring collected from the maternal tree were assessed by exclusion using polymorphisms at three microsatellite loci. Paternity analysis indicated that at least 31% of the offspring were fertilized by pollen from trees outside the stand. The average distance of pollen migration within the study stand was 68 m, with a maximum value of 325 m. There was excess mating with nearby P. densiflora trees, of which only a few were predominant pollen donors. In addition, a weakly directional bias in P. densiflora pollination was also detected in the study stand, suggesting that female strobili on a branch of the maternal tree were more easily fertilized by pollen from trees in that direction.

Characterization of highly variable (GA/CT) n microsatellites in the bur oak, Quercus macrocarpa

The objective of this study was to ascertain the usefulness of polymerase chain reaction (PCR)-based microsatellite analysis for studying pollination and parentage in a wind-pollinated temperate tree. A small insert genomic library of the bur oak (Quercus macrocarpa) was constructed and screened for the presence of (CA/GT) n and (GA/CT) n repeats. The proportion of positive clones yielded estimates of 3×10(5) such dinucleotide repeats per genome, roughly comparable to abundances reported in other eukaryotic genomes. Thirteen positive clones were sequenced. In contrast to mammalian genomes, the (GA/CT) n motif was more abundant than the (CA/GT) n motif in these clones. The (GA/CT) n repeats also showed longer average repeat length (mean n=16.2 versus 7.3), suggesting that they are better candidates for yielding polymorphic genetic markers in oak genomes. Indeed, a survey of adult bur oaks and offspring in a small stand in northern Illinois at 3 of these (GA/CT) n microsatellite loci revealed Mendelian inheritance and extremely high levels of polymorphism, with the number of alleles at each locus ranging from 11-20 and heterozygosity ranging from 0.66 to 0.75. These results, indicating that (GA/CT) n microsatellites are both abundant and highly polymorphic in the bur oak genome, suggest that such genetic markers have tremendous potential for applications for studies of parentage, pollination and dispersal in temperate trees.