Chemical differentiation of Bolivian Cedrela species as a tool to trace illegal timber trade

Combating illegal timber trade requires the ability to identify species and verify geographic origin of timber. Forensic techniques that independently verify the declared species and geographic origin are needed, as current legality procedures are based on certificates and documents that can be falsified. Timber from the genus Cedrela is among the most economically valued tropical timbers worldwide. Three Cedrela species are included in the Appendix III of CITES: C. fissilis, C. odorata and C. angustifolia (listed as C. lilloi). Cedrela timber is currently traded with false origin declarations and under a different species name, but tools to verify this are lacking. We used Direct Analysis in Real Time Time-of-Flight Mass Spectrometry (DART-TOFMS) to chemically identify Cedrela species and sites of origin. Heartwood samples from six Cedrela species (the three CITES-listed species plus C. balansae, C. montana and C. saltensis) were collected at 11 sites throughout Bolivia. Mass spectra detected by DART-TOFMS comprised 1062 compounds; their relative intensities were analysed using Principal Component Analyses, Kernel Discriminant Analysis (KDA) and Random Forest analyses to check discrimination potential among species and sites. Species were identified with a mean discrimination error of 15–19 per cent, with substantial variation in discrimination accuracy among species. The lowest error was observed in C. fissilis (mean = 4.4 per cent). Site discrimination error was considerably higher: 43–54 per cent for C. fissilis and 42–48 per cent for C. odorata. These results provide good prospects to differentiate C. fissilis from other species, but at present there is no scope to do so for other tested species. Thus, discrimination is highly species specific. Our findings for tests of geographic origin suggest no potential to discriminate at the studied scale and for the studied species. Cross-checking results from different methods (KDA and Random Forest) reduced discrimination errors. In all, the DART-TOFMS technique allows independent verification of claimed identity of certain Cedrela species in timber trade

Digest : Following clines along an Amazonian hybrid zone*

The shape and position of clines can provide crucial insights into the evolutionary forces at work in hybrid zones. In this issue, Del-Rio and colleagues applied cline theory to a hybrid zone between two antbird species in Amazonia. A narrow and displaced mitochondrial cline suggests that the selected genetic marker failed to track the northward movement of this hybrid zone, possibly due to reduced fitness of female hybrids

Multispecies hybridization in birds

Hybridization is not always limited to two species; often multiple species are interbreeding. In birds, there are numerous examples of species that hybridize with multiple other species. The advent of genomic data provides the opportunity to investigate the ecological and evolutionary consequences of multispecies hybridization. The interactions between several hybridizing species can be depicted as a network in which the interacting species are connected by edges. Such hybrid networks can be used to identify ‘hub-species’ that interbreed with multiple other species. Avian examples of such ‘hub-species’ are Common Pheasant (Phasianus colchicus), Mallard (Anas platyrhynchos) and European Herring Gull (Larus argentatus). These networks might lead to the formulation of hypotheses, such as which connections are most likely conducive to interspecific gene flow (i.e. introgression). Hybridization does not necessarily result in introgression. Numerous statistical tests are available to infer interspecific gene flow from genetic data and the majority of these tests can be applied in a multispecies setting. Specifically, model-based approaches and phylogenetic networks are promising in the detection and characterization of multispecies introgression. It remains to be determined how common multispecies introgression in birds is and how often this process fuels adaptive changes. Moreover, the impact of multispecies hybridization on the build-up of reproductive isolation and the architecture of genomic landscapes remains elusive. For example, introgression between certain species might contribute to increased divergence and reproductive isolation between those species and other related species. In the end, a multispecies perspective on hybridization in combination with network approaches will lead to important insights into the history of life on this planet

Ghost Introgression : Spooky Gene Flow in the Distant Past

Evolution is a continuous trial and error process in which most lineages go extinct without leaving fossil remains. Many of these lineages would be closely related and occasionally hybridized with lineages that gave rise to extant species. Hence, it is likely that one can find genetic signatures of these ancient introgression events in present-day genomes, so-called ghost introgression. The increasing availability of high-quality genome assemblies for non-model organisms and the development of more sophisticated methods for detecting introgression will undoubtedly reveal more cases of ghost introgression, indicating that the Tree of Life is even more reticulated than assumed. The presence of ghost introgression has important consequences for the study of numerous evolutionary processes, including adaptation, speciation, and macroevolutionary patterns. In addition, detailed studies of introgressed regions could provide insights into the morphology of the extinct lineage, providing an unexpected link between genomics and the fossil record. Hence, new methods that take into account ghost introgression will need to be developed

Avian Introgression Patterns are Consistent With Haldane’s Rule

According to Haldane’s Rule, the heterogametic sex will show the greatest fitness reduction in a hybrid cross. In birds, where sex is determined by a ZW system, female hybrids are expected to experience lower fitness compared to male hybrids. This pattern has indeed been observed in several bird groups, but it is unknown whether the generality of Haldane’s Rule also extends to the molecular level. First, given the lower fitness of female hybrids, we can expect maternally inherited loci (i.e., mitochondrial and W-linked loci) to show lower introgression rates than biparentally inherited loci (i.e., autosomal loci) in females. Second, the faster evolution of Z-linked loci compared to autosomal loci and the hemizygosity of the Z-chromosome in females might speed up the accumulation of incompatible alleles on this sex chromosome, resulting in lower introgression rates for Z-linked loci than for autosomal loci. I tested these expectations by conducting a literature review which focused on studies that directly quantified introgression rates for autosomal, sex-linked, and mitochondrial loci. Although most studies reported introgression rates in line with Haldane’s Rule, it remains important to validate these genetic patterns with estimates of hybrid fitness and supporting field observations to rule out alternative explanations. Genomic data provide exciting opportunities to obtain a more fine-grained picture of introgression rates across the genome, which can consequently be linked to ecological and behavioral observations, potentially leading to novel insights into the genetic mechanisms underpinning Haldane’s Rule

Digest: Recent movement of a flicker hybrid zone

What biotic and abiotic factors drive hybrid zone movement? By resampling a classic hybrid zone between two subspecies of woodpeckers, Aguillon and Rohwer documented a westward shift that may be attributed to changes in land use or climate. These findings highlight the increasing influence of anthropogenic activities on hybridization events

Digest : Avian genomes are permeable to introgression for a few million years

How wide is the time window for introgression after divergence? Pulido-Santacruz et al. addressed this question by studying the evolutionary history of the bird genus Dendrocincla. They found five introgression events that occurred between a few hundred thousand and around 2.5 million years after divergence. The introgressed genomic proportion declined exponentially with the age of the hybridizing taxa. Hence, this study suggests that species boundaries are permeable up to 2.5 million years after divergence

Multispecies hybridization in birds

Hybridization is not always limited to two species; often multiple species are interbreeding. In birds, there are numerous examples of species that hybridize with multiple other species. The advent of genomic data provides the opportunity to investigate the ecological and evolutionary consequences of multispecies hybridization. The interactions between several hybridizing species can be depicted as a network in which the interacting species are connected by edges. Such hybrid networks can be used to identify hub-species' that interbreed with multiple other species. Avian examples of such hub-species' are Common Pheasant (Phasianus colchicus), Mallard (Anas platyrhynchos) and European Herring Gull (Larus argentatus). These networks might lead to the formulation of hypotheses, such as which connections are most likely conducive to interspecific gene flow (i.e. introgression). Hybridization does not necessarily result in introgression. Numerous statistical tests are available to infer interspecific gene flow from genetic data and the majority of these tests can be applied in a multispecies setting. Specifically, model-based approaches and phylogenetic networks are promising in the detection and characterization of multispecies introgression. It remains to be determined how common multispecies introgression in birds is and how often this process fuels adaptive changes. Moreover, the impact of multispecies hybridization on the build-up of reproductive isolation and the architecture of genomic landscapes remains elusive. For example, introgression between certain species might contribute to increased divergence and reproductive isolation between those species and other related species. In the end, a multispecies perspective on hybridization in combination with network approaches will lead to important insights into the history of life on this planet