Putting the biological species concept to the test: using mating networks to delimit species

The oak syngameon: more than the sum of its parts

One of Anthropocene's most daunting challenges for conservation biology is habitat extinction, caused by rapid global change. Tree diversity has persisted through previous episodes of rapid change, even global extinctions. Given the pace of current change, our management of extant diversity needs to facilitate and even enhance the natural ability of trees to adapt and diversify. Numerous processes contribute to this evolutionary flexibility, including introgression, a widespread yet under-studied process. Reproductive networks, in which species remain distinct despite interspecific gene flow, are called syngameons, a concept largely inspired from work focusing on Quercus. Delineating and analyzing such species groups, empirically and theoretically, will provide insights into the nonadditive effects on evolution of numerous partially interfertile species exchanging genetic material episodically under changing environmental conditions. To conserve tree diversity, crossing experiments designed with an empirical and theoretical understanding of the constituent syngameon should be set up to assist diversification and adaptation in the Anthropocene. Our increasingly detailed knowledge of the oak genome and of oak interspecific and intraspecific phenotypic variation will improve our ability to sustain the diversity of this tree through an unpredictable and unprecedented future.

Oak genome reveals facets of long lifespan

Oaks are an important part of our natural and cultural heritage. Not only are they ubiquitous in our most common landscapes¹ but they have also supplied human societies with invaluable services, including food and shelter, since prehistoric times². With 450 species spread throughout Asia, Europe and America³, oaks constitute a critical global renewable resource. The longevity of oaks (several hundred years) probably underlies their emblematic cultural and historical importance. Such long-lived sessile organisms must persist in the face of a wide range of abiotic and biotic threats over their lifespans. We investigated the genomic features associated with such a long lifespan by sequencing, assembling and annotating the oak genome. We then used the growing number of whole-genome sequences for plants (including tree and herbaceous species) to investigate the parallel evolution of genomic characteristics potentially underpinning tree longevity. A further consequence of the long lifespan of trees is their accumulation of somatic mutations during mitotic divisions of stem cells present in the shoot apical meristems. Empirical⁴ and modelling⁵ approaches have shown that intra-organismal genetic heterogeneity can be selected for⁶ and provides direct fitness benefits in the arms race with short-lived pests and pathogens through a patchwork of intra-organismal phenotypes⁷. However, there is no clear proof that large-statured trees consist of a genetic mosaic of clonally distinct cell lineages within and between branches. Through this case study of oak, we demonstrate the accumulation and transmission of somatic mutations and the expansion of disease-resistance gene families in trees.

Rapid song divergence leads to discordance between genetic distance and phenotypic characters important in reproductive isolation

The criteria for species delimitation in birds have long been debated, and several recent studies have proposed new methods for such delimitation. On one side, there is a large consensus of investigators who believe that the only evidence that can be used to delimit species is molecular phylogenetics, and with increasing numbers of markers to gain better support, whereas on the other, there are investigators adopting alternative approaches based largely on phenotypic differences, including in morphology and communication signals. Yet, these methods have little to say about rapid differentiation in specific traits shown to be important in reproductive isolation. Here, we examine variation in phenotypic (morphology, plumage, and song) and genotypic (mitochondrial and nuclear DNA) traits among populations of yellow-rumped tinkerbird Pogoniulus bilineatus in East Africa. Strikingly, song divergence between the P. b. fischeri subspecies from Kenya and Zanzibar and P. b. bilineatus from Tanzania is discordant with genetic distance, having occurred over a short time frame, and playback experiments show that adjacent populations of P. b. bilineatus and P. b. fischeri do not recognize one another's songs. While such rapid divergence might suggest a founder effect following invasion of Zanzibar, molecular evidence suggests otherwise, with insular P. b. fischeri nested within mainland P. b. fischeri. Populations from the Eastern Arc Mountains are genetically more distant, yet share the same song with P. b. bilineatus from Coastal Tanzania and Southern Africa, suggesting they would interbreed. We believe investigators ought to examine potentially rapid divergence in traits important in species recognition and sexual selection when delimiting species, rather than relying entirely on arbitrary quantitative characters or molecular markers.