When looks can kill: the evolution of sexually dimorphic floral display and the extinction of dioecious plants

Dioecious plants (with separate male and female individuals) more often have drab, inconspicuous flowers than related bisexual plants. Models indicate, however, that similar conditions favour the evolution of showy floral displays in dioecious and bisexual plants. One difference, however, is that dioecious plants may evolve floral displays that are sexually dimorphic. We show that males are more likely to evolve showy flowers than females in animal-pollinated plants, especially when pollinators are abundant. We demonstrate that this dimorphism places showy dioecious plants at a much higher risk of extinction during years of low pollinator abundance because pollinators may fail to visit female flowers. The higher extinction risk of showy dioecious plants provides an explanation for the fact that dioecious plants that do persist tend to have inconspicuous flowers and are more often wind pollinated. It may also help explain why dioecious plants are less species-rich than related bisexual plants.     

Y Fuse? Sex Chromosome Fusions in Fishes and Reptiles

Chromosomal fusion plays a recurring role in the evolution of adaptations and reproductive isolation among species, yet little is known of the evolutionary drivers of chromosomal fusions. Because sex chromosomes (X and Y in male heterogametic systems, Z and W in female heterogametic systems) differ in their selective, mutational, and demographic environments, those differences provide a unique opportunity to dissect the evolutionary forces that drive chromosomal fusions. We estimate the rate at which fusions between sex chromosomes and autosomes become established across the phylogenies of both fishes and squamate reptiles. Both the incidence among extant species and the establishment rate of Y-autosome fusions is much higher than for X-autosome, Z-autosome, or W-autosome fusions. Using population genetic models, we show that this pattern cannot be reconciled with many standard explanations for the spread of fusions. In particular, direct selection acting on fusions or sexually antagonistic selection cannot, on their own, account for the predominance of Y-autosome fusions. The most plausible explanation for the observed data seems to be (a) that fusions are slightly deleterious, and (b) that the mutation rate is male-biased or the reproductive sex ratio is female-biased. We identify other combinations of evolutionary forces that might in principle account for the data although they appear less likely. Our results shed light on the processes that drive structural changes throughout the genome.     

Animal dispersal dynamics promoting dioecy over hermaphroditism

Because of the separation of sexual function to male and female individuals, dioecious species have fewer pollen and seed bearers and thus experience disadvantages due to increased aggregation of reproductive function. Because of this disadvantage, models predict that dioecious females must have substantially more than twice the fecundity of hermaphrodites, yet empirical data suggest that female fecundity advantages are commonly much lower. Here, we incorporate animal foraging dynamics--and the heightened dispersal of seeds that may accompany increases in fecundity of dioecious females--into a spatially explicit mathematical model. We focus on the competition for germination sites with varying seed production, seed dispersal ability, and mortality, and we find that preferential foraging on dioecious females reduces the stringent fecundity requirements of dioecy to values in accordance with empirical estimates. This finding contributes to our understanding of the correlation between dioecy and fleshy fruits and highlights the importance of mutualist dispersers to dioecious species.     

Frozen F1's amidst a masterpiece of nature: new insights into the rare hybrid origin of gynogenesis in the Amazon molly (Poecilia formosa)

All‐female ‘species’ of fish have been shown to be great models in ecological and evolutionary studies because of the insights they can provide into the origin and evolution of asexuality, the ecology of hybrids, associations between genotype and environment, and the maintenance of sex. Gynogenetic organisms that evolved from sexual ancestors, and combine the disadvantageous traits from sexuality and asexuality, have long baffled evolutionary biologists trying to understand their origin and persistence with their sympatric sexual counterparts. In this issue, a new study using an integrated molecular phylogenetic and classical genetic approach has uncovered compelling evidence regarding the obscure asexual origin of the Amazon molly, Poecilia formosa. By performing an extensive phylogeographic analysis, Stöck et al. (2010) provide evidence that the Amazon molly arose only once within its history, with monophyly being strongly supported by mitochondrial DNA and microsatellite analyses. This result, combined with an elaborate failed attempt to resynthesize the lineage, suggests that vertebrate gynogens such as the Amazon molly are not rare because they are at a disadvantage to their sexual counterparts, but because the genomic conditions under which they arise are rare. Organisms that apparently combine the disadvantages of both sexuality and asexuality remain difficult to understand from both an ecological and an evolutionary perspective, and Stöck et al. (2010) highlight several outstanding important questions. Nonetheless, given that we now have a better knowledge of the origin and history of this unique ‘species’, this should allow researchers to better understand how these frozen F1’s can persist amidst the masterpiece of nature.     

Extinction Risk Escalates in the Tropics

The latitudinal biodiversity gradient remains one of the most widely recognized yet puzzling patterns in nature [1]. Presently, the high level of extinction of tropical species, referred to as the “tropical biodiversity crisis”, has the potential to erode this pattern. While the connection between species richness, extinction, and speciation has long intrigued biologists [2], [3], these interactions have experienced increased poignancy due to their relevancy to where we should concentrate our conservation efforts. Natural extinction is a phenomenon thought to have its own latitudinal gradient, with lower extinction rates in the tropics being reported in beetles, birds, mammals, and bivalves [4]–[7]. Processes that have buffered ecosystems from high extinction rates in the past may also buffer ecosystems against disturbance of anthropogenic origin. While potential parallels between historical and present-day extinction patterns have been acknowledged, they remain only superficially explored and plant extinction patterns have been particularly neglected. Studies on the disappearances of animal species have reached conflicting conclusions, with the rate of extinction appearing either higher [8] or lower [9] in species richness hotspots. Our global study of extinction risk in vascular plants finds disproportionately higher extinction risk in tropical countries, even when indicators of human pressure (GDP, population density, forest cover change) are taken into account. Our results are at odds with the notion that the tropics represent a museum of plant biodiversity (places of historically lowered extinction) and we discuss mechanisms that may reconcile this apparent contradiction.     

Ecology in the age of DNA barcoding: the resource,the promise and the challenges ahead

Ten years after DNA barcoding was initially suggested as a tool to identify species, millions of barcode sequences from more than 1100 species are available in public databases. While several studies have reviewed the methods and potential applications of DNA barcoding, most have focused on species identification and discovery, and relatively few have addressed applications of DNA barcoding data to ecology. These data, and the associated information on the evolutionary histories of taxa that they can provide, offer great opportunities for ecologists to investigate questions that were previously difficult or impossible to address. We present an overview of potential uses of DNA barcoding relevant in the age of ecoinformatics, including applications in community ecology, species invasion, macroevolution, trait evolution, food webs and trophic interactions, metacommunities, and spatial ecology. We also outline some of the challenges and potential advances in DNA barcoding that lie ahead.     

Genetic population structure in prickly sculpin (Cottus asper) reflects isolation‐by‐environment between two life‐history ecotypes

Life‐history transitions have evolved repeatedly in numerous taxa, although the ecological and evolutionary conditions favouring such transitions in the presence of gene flow remain poorly understood. The present study aimed to disentangle the effects of isolation‐by‐distance and isolation‐by‐environment on genetic differentiation between two sympatric life‐history ecotypes. Using 14 microsatellite loci, we first characterized amphidromous and freshwater groups of Cottus asper in a high gene flow setting in the Lower Fraser River system (south‐western British Columbia, Canada) to test for the effects of habitat and geographical distance on the distribution of life‐history ecotypes. Within the main river channel, no genetic differentiation was found, whereas tributaries even close to the estuary were genetically differentiated. Partial mantel tests confirmed that genetic differentiation between river tributaries and the main channel was independent from geographical distance, with distance‐scaled migration rates indicating reduced gene flow from the main channel into the tributaries. Our results suggest that isolation‐by‐environment can play an important role for the early stage of life‐history transitions, and may promote differentiation among life‐history ecotypes despite the presence of gene flow. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 943–957.     

Traits and phylogenetic history contribute to network structure across Canadian plant–pollinator communities

Interaction webs, or networks, define how the members of two or more trophic levels interact. However, the traits that mediate network structure have not been widely investigated. Generally, the mechanism that determines plant-pollinator partnerships is thought to involve the matching of a suite of species traits (such as abundance, phenology, morphology) between trophic levels. These traits are often unknown or hard to measure, but may reflect phylogenetic history. We asked whether morphological traits or phylogenetic history were more important in mediating network structure in mutualistic plant-pollinator interaction networks from Western Canada. At the plant species level, sexual system, growth form, and flower symmetry were the most important traits. For example species with radially symmetrical flowers had more connections within their modules (a subset of species that interact more among one another than outside of the module) than species with bilaterally symmetrical flowers. At the pollinator species level, social species had more connections within and among modules. In addition, larger pollinators tended to be more specialized. As traits mediate interactions and have a phylogenetic signal, we found that phylogenetically close species tend to interact with a similar set of species. At the network level, patterns were weak, but we found increasing functional trait and phylogenetic diversity of plants associated with increased weighted nestedness. These results provide evidence that both specific traits and phylogenetic history can contribute to the nature of mutualistic interactions within networks, but they explain less variation between networks.