Restricted pollen flow of Dieffenbachia seguine populations in fragmented and continuous tropical forest

Frugivore-Mediated Selection in A Habitat Transformation Scenario

Plant-animal interactions are strong drivers of phenotypic evolution. However, the extent to which anthropogenic habitat transformation creates new selective scenarios for plant-animal interactions is a little explored subject. We examined the effects of native forest replacement by exotic Eucalyptus trees on the frugivore-mediated phenotypic selection coefficients imposed by the relict marsupial Dromiciops gliroides upon traits involved in frugivore attraction and germination success of the mistletoe Tristerix corymbosus (Loranthaceae). We found significant gradients for seed weight and sugar content along the native - transformed habitat gradient. While selection for larger seed weight was more relevant in native habitats, fruits with intermediate sugar content were promoted in transformed habitats. The spatial habitat structure and microclimate features such as the degree of sunlight received influenced the natural selection processes, as they correlated with the phenotypic traits analysed. The response of this plant-frugivore interaction to human disturbance seemed to be context-dependent, in which extremely transformed habitats would offer new opportunities for natural selection on dispersal-related traits. Even in recent transformation events like this, human disturbance acts as a strong contemporary evolution driver.

Life history and past demography maintain genetic structure, outcrossing rate, contemporary pollen gene flow of an understory herb in a highly fragmented rainforest

INTRODUCTION

Theory predicts that habitat fragmentation, by reducing population size and increasing isolation among remnant populations, can alter their genetic diversity and structure. A cascade of effects is expected: genetic drift and inbreeding after a population bottleneck, changes in biotic interactions that may affect, as in the case of plants, pollen dynamics, mating system, reproductive success. The detection of the effects of contemporary habitat fragmentation on the genetic structure of populations are conditioned by the magnitude of change, given the few number of generations since the onset of fragmentation, especially for long-lived organisms. However, the present-day genetic structure of populations may bear the signature of past demography events. Here, we examine the effects of rainforest fragmentation on the genetic diversity, population structure, mating system (outcrossing rate), indirect gene flow and contemporary pollen dynamics in the understory herb Aphelandra aurantiaca. Also, we assessed its present-day genetic structure under different past demographic scenarios.

METHODS

Twelve populations of A. aurantiaca were sampled in large (4), medium (3), and small (5) forest fragments in the lowland tropical rainforest at Los Tuxtlas region. Variation at 11 microsatellite loci was assessed in 28-30 reproductive plants per population. In two medium- and two large-size fragments we estimated the density of reproductive plants, and the mating system by analyzing the progeny of different mother plants per population.

RESULTS

Despite prevailing habitat fragmentation, populations of A. aurantiaca possess high genetic variation (He  = 0.61), weak genetic structure (Rst  = 0.037), and slight inbreeding in small fragments. Effective population sizes (Ne ) were large, but slightly lower in small fragments. Migrants derive mostly from large and medium size fragments. Gene dispersal is highly restricted but long distance gene dispersal events were detected. Aphelandra aurantiaca shows a mixed mating system (tm  = 0.81) and the outcrossing rate have not been affected by habitat fragmentation. A strong pollen pool structure was detected due to few effective pollen donors (Nep ) and low distance pollen movement, pointing that most plants received pollen from close neighbors. Past demographic fluctuations may have affected the present population genetic structure as Bayesian coalescent analysis revealed the signature of past population expansion, possibly during warmer conditions after the last glacial maximum.

DISCUSSION

Habitat fragmentation has not increased genetic differentiation or reduced genetic diversity of A. aurantiaca despite dozens of generations since the onset of fragmentation in the region of Los Tuxtlas. Instead, past population expansion is compatible with the lack of observed genetic structure. The predicted negative effects of rainforest fragmentation on genetic diversity and population structure of A. aurantiaca seem to have been buffered owing to its large effective populations and long-distance dispersal events. In particular, its mixed-mating system, mostly of outcrossing, suggests high efficiency of pollinators promoting connectivity and reducing inbreeding. However, some results point that the effects of fragmentation are underway, as two small fragments showed higher membership probabilities to their population of origin, suggesting genetic isolation. Our findings underscore the importance of fragment size to maintain genetic connectivity across the landscape.

Paternity analysis reveals significant isolation and near neighbor pollen dispersal in small Cariniana legalis Mart. Kuntze populations in the Brazilian Atlantic Forest

Throughout the world, large trees are increasingly rare. Cariniana legalis is the tallest tree species of the Brazilian Atlantic Forest, reaching up to 60 m in height. Due to extensive deforestation of the Atlantic Forest, remnant C. legalis populations are small and spatially isolated, requiring the development of strategies for their conservation. For in situ and ex situ genetic conservation to be effective, it is important to understand the levels and patterns of spatial genetic structure (SGS), and gene flow. We investigated SGS and pollen flow in three small, physically isolated C. legalis stands using microsatellite loci. We measured, mapped, and sampled all C. legalis trees in the three stands: 65 trees from Ibicatu population, 22 trees from MGI, and 4 trees from MGII. We also collected and genotyped 600 seeds from Ibicatu, 250 seeds from MGI, and 200 seeds from MGII. Significant SGS was detected in Ibicatu up to 150 m, but substantial levels of external pollen flow were also detected in Ibicatu (8%), although not in MGI (0.4%) or MGII (0%). Selfing was highest in MGII (18%), the smallest group of trees, compared to MGI (6.4%) and Ibicatu (6%). In MGI and MGII, there was a strong pattern of mating among near‐neighbors. Seed collection strategies for breeding, in situ and ex situ conservation and ecological restoration, must ensure collection from seed trees located at distances greater than 350 m and from several forest fragments.

Interactions between insect herbivores and plant mating systems

Self-pollination is common in plants, and limited seed and pollen dispersal can create localized inbreeding even within outcrossing plants. Consequently, insects regularly encounter inbred plants in nature. Because inbreeding results in elevated homozygosity, greater expression of recessive alleles, and subsequent phenotypic changes in inbred plants, inbreeding may alter plant-insect interactions. Recent research has found that plant inbreeding alters resistance and tolerance to herbivores, alters the attraction and susceptibility of plants to insects that vector plant pathogens, and alters visitation rates of insect pollinators. These results suggest that interactions with insects can increase or decrease inbreeding depression (the loss of fitness due to self-fertilization) and subsequently alter the evolution of selfing within plant populations. Future work needs to focus on the mechanisms underlying genetic variation in the effects of inbreeding on plant-insect interactions and the consequences of altered plant-insect interactions on the evolution of plant defense and plant mating systems.

Effects of forest fragmentation on assemblages of pollinators and floral visitors to male- and female-phase inflorescences of Astrocaryum mexicanum (Arecaceae) in a Mexican rain forest

We examined the consequences of habitat fragmentation on the assemblage of floral visitors and pollinators to male- and female-phase inflorescences of the understorey dominant palm Astrocaryum mexicanum at the Los Tuxtlas tropical rain forest. In six forest fragments ranging from 2 to 700 ha, we collected all floral visitors, pollinators and non-pollinators, to male- and female-phase inflorescences at the time of their greatest activity. We used multivariate and mixed-effects models to explore differences in guild composition between sexual phases of inflorescences and the effects of forest fragment size on several metrics of the assemblages of floral visitors. We detected 228 786 floral visitors, grouped into 57 species, across the six forest fragments. On average, abundance and species richness of floral visitors to female-phase inflorescences were higher than to male-phase ones. Forest fragmentation had no effect on species richness but negatively affected Shannon's diversity index. Overall, the most abundant species of floral visitors were predominantly found in inflorescences of plants from the large fragments. In contrast, most of the less common species were more abundant in the smallest fragments. The abundance of pollinators (those found on inflorescences of both phases and dusted with pollen that was carried to flower stigmas), and the ratio of pollinators to other floral visitors, increased with fragment size in both sexual phases of the inflorescences but these effects were significantly stronger on male-phase inflorescences than on female-phase inflorescences. These results show that tropical forest fragmentation correlates with changes in the composition of flower visitors to a dominant palm, with a reduction in the abundance of pollinators, but that such changes co-vary with the sexual phase of the plants.