Cohesion, Gene flow, and the Nature of Species

A far-reaching and influential view in evolutionary biology claims that species are cohesive units held together by gene flow. Biologists have recognized empirical problems facing this view; after sharpening the expression of the view, we present novel conceptual problems for it. At the heart of these problems is a distinction between two importantly different concepts of cohesion, what we call integrative and response cohesion. Acknowledging the distinction problematizes both the explanandum of species cohesion and the explanans of gene flow that are central to the view we discuss. We conclude by tracing four broader implications for the study and conceptualization of species

The bracteatus pineapple genome and domestication of clonally propagated crops

Domestication of clonally propagated crops such as pineapple from South America was hypothesized to be a 'one-step operation'. We sequenced the genome of Ananas comosus var. bracteatus CB5 and assembled 513 Mb into 25 chromosomes with 29,412 genes. Comparison of the genomes of CB5, F153 and MD2 elucidated the genomic basis of fiber production, color formation, sugar accumulation and fruit maturation. We also resequenced 89 Ananas genomes. Cultivars 'Smooth Cayenne' and 'Queen' exhibited ancient and recent admixture, while 'Singapore Spanish' supported a one-step operation of domestication. We identified 25 selective sweeps, including a strong sweep containing a pair of tandemly duplicated bromelain inhibitors. Four candidate genes for self-incompatibility were linked in F153, but were not functional in self-compatible CB5. Our findings support the coexistence of sexual recombination and a one-step operation in the domestication of clonally propagated crops. This work guides the exploration of sexual and asexual domestication trajectories in other clonally propagated crops

Natural Kinds in Evolution and Systematics: Metaphysical and Epistemological Considerations

Despite the traditional focus on metaphysical issues in discussions of natural kinds in biology, epistemological considerations are at least as important. By revisiting the debate as to whether taxa are kinds or individuals, I argue that both accounts are metaphysically compatible but one or the other approach can be pragmatically preferable depending on the epistemic context. Recent objections against construing species as homeostatic property cluster kinds are also addressed. The second part of the paper broadens the perspective by considering homologues as another example of natural kinds, comparing them with analogues as functionally defined kinds. Given that there are various types of natural kinds, I discuss the different theoretical purposes served by diverse kind concepts, suggesting that there is no clear-cut distinction between natural kinds and other kinds, such as functional kinds. Rather than attempting to offer a unique metaphysical account of ‘natural’ kind, a more fruitful approach consists in the epistemological study of how different natural kind concepts are employed in scientific reasoning

Trans-Arctic asymmetries, melting pots and weak species cohesion in the low-dispersal amphiboreal seaweed Fucus distichus

Amphiboreal taxa are often composed of vicariant phylogroups and species complexes whose divergence and phylogeographic affinities reflect a shared history of chronic isolation and episodic trans-Arctic dispersal. Ecological filters and shifting selective pressures may also promote selective sweeps, niche shifts and ecological speciation during colonization, but these are seldom considered at biogeographical scales. Here we integrate genetic data and Ecologic Niche Models (ENMs) to investigate the historical biogeography and cohesion of the polymorphic rockweed Fucus distichus throughout its immense amphiboreal range, focusing on trans-Arctic asymmetries, glacial/interglacial dynamics, and integrity of sympatric eco-morphotypes. Populations were sampled throughout the Pacific and the Atlantic, from southern rear-edges to the high-Arctic. They were genotyped for seven microsatellites and an mtDNA spacer, and genetic diversity and structure were assessed from global to local scales. ENMs were used to compare niche divergence and magnitude of post-glacial range shifts in Pacific versus Atlantic sub-ranges. Haplotypic and genotypic data revealed distinct and seemingly isolated Pacific vs Arctic/Atlantic gene-pools, with finer-scale regional sub-structuring pervasive in the Pacific. MtDNA diversity was highly structured and overwhelmingly concentrated in the Pacific. Regionally, Alaska showed the highest intra-population diversity but the lowest levels of endemism. Some sympatric/parapatric ecotypes exhibited distinct genotypic/ haplotypic compositions. Strikingly, niche models revealed higher Pacific tolerance to maximum temperatures and predicted a much more consolidated presence in the NE Atlantic. Glacial and modern ranges overlapped extensively in the Pacific, whereas the modern Atlantic range was largely glaciated or emerged during the Last Glacial Maximum. Higher genetic and ecogeographic diversity supports a primary Pacific diversification and secondary Atlantic colonization, also likely reflecting the much larger and more stable climatic refugia in the Pacific. The relic distribution and reduced ecological/morphological plasticity in the NE Atlantic are hypothesized to reflect functional trans-Arctic bottlenecks, recent colonization or competition with congeners. Within the Pacific, Alaska showed signatures of a post-glacial melting pot of eastern and southern populations. Genetic/ecotypic variation was generally not sufficiently discontinuous or consistent to justify recognizing multiple taxonomic entities, but support a separate species in the eastern Pacific, at the southern rear-edge. We predict that layered patterns of phylogeographic structure, incipient speciation and niche differences might be common among widespread low-dispersal amphiboreal taxa

The role of genome and gene regulatory network canalization in the evolution of multi-trait polymorphisms and sympatric speciation

<p>Abstract</p> <p>Background</p> <p>Sexual reproduction has classically been considered as a barrier to the buildup of discrete phenotypic differentiation. This notion has been confirmed by models of sympatric speciation in which a fixed genetic architecture and a linear genotype phenotype mapping were assumed. In this paper we study the influence of a flexible genetic architecture and non-linear genotype phenotype map on differentiation under sexual reproduction.</p> <p>We use an individual based model in which organisms have a genome containing genes and transcription factor binding sites. Mutations involve single genes or binding sites or stretches of genome. The genome codes for a regulatory network that determines the gene expression pattern and hence the phenotype of the organism, resulting in a non-linear genotype phenotype map. The organisms compete in a multi-niche environment, imposing selection for phenotypic differentiation.</p> <p>Results</p> <p>We find as a generic outcome the evolution of discrete clusters of organisms adapted to different niches, despite random mating. Organisms from different clusters are distinct on the genotypic, the network and the phenotypic level. However, the genome and network differences are constrained to a subset of the genome locations, a process we call genotypic canalization. We demonstrate how this canalization leads to an increased robustness to recombination and increasing hybrid fitness. Finally, in case of assortative mating, we explain how this canalization increases the effectiveness of assortativeness.</p> <p>Conclusion</p> <p>We conclude that in case of a flexible genetic architecture and a non-linear genotype phenotype mapping, sexual reproduction does not constrain phenotypic differentiation, but instead constrains the genotypic differences underlying it. We hypothesize that, as genotypic canalization enables differentiation despite random mating and increases the effectiveness of assortative mating, sympatric speciation is more likely than is commonly suggested.</p

Endosymbiotic bacteria nodulating a new endemic lupine Lupinus mariae-josephi from alkaline soils in Eastern Spain represent a new lineage within the Bradyrhizobium genus

Lupinus mariae-josephi is a recently described endemic Lupinus species from a small area in Eastern Spain where it thrives in soils with active lime and high pH. The L. mariae-josephi root symbionts were shown to be very slow-growing bacteria with different phenotypic and symbiotic characteristics from those of Bradyrhizobium strains nodulating other Lupinus. Their phylogenetic status was examined by multilocus sequence analyses of four housekeeping genes (16S rRNA, glnII, recA, and atpD) and showed the existence of a distinct evolutionary lineage for L. mariae-josephi that also included Bradyrhizobium jicamae. Within this lineage, the tested isolates clustered in three different sub-groups that might correspond to novel sister Bradyrhizobium species. These core gene analyses consistently showed that all the endosymbiotic bacteria isolated from other Lupinus species of the Iberian Peninsula were related to strains of the B. canariense or B. japonicum lineages and were separate from the L. mariae-josephi isolates. Phylogenetic analysis based on nodC symbiotic gene sequences showed that L. mariae-josephi bacteria also constituted a new symbiotic lineage distant from those previously defined in the genus Bradyrhizobium. In contrast, the nodC genes of isolates from other Lupinus spp. from the Iberian Peninsula were again clearly related to the B. canariense and B. japonicum bv. genistearum lineages. Speciation of L. mariae-josephi bradyrhizobia may result from the colonization of a singular habitat by their unique legume host

Species divergence and maintenance of species cohesion of three closely related Primula species in the Qinghai-Tibet Plateau

Understanding the relative roles of geography and ecology in driving speciation, population divergence and maintenance of species cohesion is of great interest to molecular ecology. Closely related species that are parapatricly distributed in mountainous areas provide an ideal model to evaluate these key issues, especially when genomic data are analyzed within a spatially and ecologically explicit context. Here we used three closely related species of Primula that occur in the Himalayas, the Hengduan Mountains and Northeast Qinghai-Tibet Plateau (QTP) to examine the effects of geography and ecology on interspecific divergence and maintenance of species cohesion. We used genomic data for 770 samples of the three species using restriction site-associated DNA (RAD) sequencing and combined approximate Bayesian computation (ABC) modeling, Bayesian generalized linear mixed modeling (GLMM) and species distribution modeling (SDM). The three species are clearly delimited by the RADseq data. Further ABC modeling indicates that the three species originated in the Himalayas and diverged from each other following the uplifts of the Hengduan Mountains and the Northern QTP during the Pliocene. After a long period of divergence, the three species came into secondary contact triggered by past climatic changes but with no significant introgression. The three species display complex and different drivers of genomic variation, which provides further insights into the effects of geographical and ecological factors on maintaining species cohesion. Our findings highlight the significance of combining the use of population genomics with environmental data when evaluating the effects of geography and ecology on interspecific divergence and maintenance of closely related specie

Average nucleotide identity of genome sequences supports the description of Rhizobium lentis sp. nov., Rhizobium bangladeshense sp. nov. and Rhizobium binae sp. nov. from lentil (Lens culinaris) nodules

Rhizobial strains isolated from effective root nodules of field-grown lentil (Lens culinaris) from different parts of Bangladesh were previously analysed using sequences of the 16S rRNA gene, three housekeeping genes (recA, atpD and glnll) and three nodulation genes (nodA, nodC and nodD), DNA fingerprinting and phenotypic characterization. Analysis of housekeeping gene sequences and DNA fingerprints indicated that the strains belonged to three novel clades in the genus Rhizobium. In present study, a representative strain from each clade was further characterized by determination of cellular fatty acid compositions, carbon substrate utilization patterns and DNA DNA hybridization and average nucleotide identity (ANI) analyses from whole-genome sequences. DNA DNA hybridization showed 50-62 % relatedness to their closest relatives (the type strains of Rhizobium etli and Rhizobium phaseoh) and 50-60 % relatedness to each other. These results were further supported by ANI values, based on genome sequencing, which were 87-92 % with their close relatives and 88-89 % with each other. On the basis of these results, three novel species, Rhizobium lentis sp. nov. (type strain BLR27(T)=LMG 28441(T)=DSM 29286(T)), Rhizobium bangladeshense sp. nov. (type strain BLR175(T)=LMG 28442(T)=DSM 29287(T)) and Rhizobium binae sp. nov. (type strain BLR195(T)=LMG 28443(T)=DSM 29288(T)), are proposed. These species share common nodulation genes (nodA, nodC and nodD) that are similar to those of the symbiovar viciae