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Cronk, Quentin; Boehm, Mannfred M. A.; Cronk, Quentin C. B. — 2022-07-07 <jats:p>The extinction of species before they are discovered and named (dark extinction, DE) is widely inferred as a significant part of species loss in the ‘pre-taxonomic’ period (approx. 1500–1800 CE) and, to some extent, in the ‘taxonomic period’ (approx. 1800–present) as well. The discovery of oceanic islands and other pristine habitats by European navigators and the consequent introduction of destructive mammals, such as rats and goats, started a process of anthropogenic extinction. Much ecosystem change happened before systematic scientific recording, so has led to DE. Statistical methods are available to robustly estimate DE in the ‘taxonomic period’. For the ‘pre-taxonomic period’, simple extrapolation can be used. The application of these techniques to world birds, for example, suggests that approximately 56 DEs occurred in the ‘taxonomic period’ (1800–present) and approximately 180 in the ‘pre-taxonomic period’ (1500–1800). Targeting collection activities in extinction hotspots, to make sure organisms are represented in collections before their extinction, is one way of reducing the number of extinct species without a physical record (providing that collection efforts do not themselves contribute to species extinction).</jats:p>

Geraldes, Armando; DiFazio, Steve P.; Slavov, Gancho T.; Ranjan, Priya; Muchero, Wellington; Hannemann, Jan; Gunter, Lee E.; Wymore, Ann M.; Grassa, Christopher J.; Farzaneh, Nima; Porth, Ilga; Mckown, Athena D.; Skyba, Oleksandr; Li, Eryang; Fujita, Miki; Klápště, Jaroslav; Martin, Joel; Schackwitz, Wendy; Pennacchio, Christa; Rokhsar, Daniel; Friedmann, Michael C.; Wasteneys, Geoffrey O.; Guy, Robert D.; El-Kassaby, Yousry A.; Mansfield, Shawn D.; Cronk, Quentin C. B.; Ehlting, Juergen; Douglas, Carl J.; Tuskan, Gerald A. — 2013-01-16 Genetic mapping of quantitative traits requires genotypic data for large numbers of markers in many individuals. For such studies, the use of large single nucleotide polymorphism (SNP) genotyping arrays still offers the most cost-effective solution. Herein we report on the design and performance of a SNP genotyping array for Populus trichocarpa (black cottonwood). This genotyping array was designed with SNPs pre-ascertained in 34 wild accessions covering most of the species latitudinal range. We adopted a candidate gene approach to the array design that resulted in the selection of 34 131 SNPs, the majority of which are located in, or within 2 kb of, 3543 candidate genes. A subset of the SNPs on the array (539) was selected based on patterns of variation among the SNP discovery accessions. We show that more than 95% of the loci produce high quality genotypes and that the genotyping error rate for these is likely below 2%. We demonstrate that even among small numbers of samples (n = 10) from local populations over 84% of loci are polymorphic. We also tested the applicability of the array to other species in the genus and found that the number of polymorphic loci decreases rapidly with genetic distance, with the largest numbers detected in other species in section Tacamahaca. Finally, we provide evidence for the utility of the array to address evolutionary questions such as intraspecific studies of genetic differentiation, species assignment and the detection of natural hybrids.

Geraldes, Armando; Hefer, Charles A.; Capron, Arnaud; Kolosova, Natalia; Martinez-Nuñez, Felix; Soolanayakanahally, Raju Y.; Stanton, Brian; Guy, Robert D.; Mansfield, Shawn D.; Douglas, Carl J.; Cronk, Quentin C. B. — 2015-02-26 All species of the genus Populus (poplar, aspen) are dioecious, suggesting an ancient origin of this trait. Despite some empirical counter examples, theory suggests that nonrecombining sex-linked regions should quickly spread, eventually becoming heteromorphic chromosomes. In contrast, we show using whole-genome scans that the sex-associated region in Populus trichocarpa is small and much younger than the age of the genus. This indicates that sex determination is highly labile in poplar, consistent with recent evidence of ‘turnover’ of sex-determination regions in animals. We performed whole-genome resequencing of 52 P. trichocarpa (black cottonwood) and 34 Populus balsamifera (balsam poplar) individuals of known sex. Genomewide association studies in these unstructured populations identified 650 SNPs significantly associated with sex. We estimate the size of the sex-linked region to be ~100 kbp. All SNPs significantly associated with sex were in strong linkage disequilibrium despite the fact that they were mapped to six different chromosomes (plus 3 unmapped scaffolds) in version 2.2 of the reference genome. We show that this is likely due to genome misassembly. The segregation pattern of sex-associated SNPs revealed this to be an XY sex-determining system. Estimated divergence times of X and Y haplotype sequences (6–7 Ma) are much more recent than the divergence of P. trichocarpa (poplar) and Populus tremuloides (aspen). Consistent with this, in P. tremuloides, we found no XY haplotype divergence within the P. trichocarpa sex-determining region. These two species therefore have a different genomic architecture of sex, suggestive of at least one turnover event in the recent past.

Suarez-Gonzalez, Adriana; Hefer, Charles A.; Lexer, Christian; Cronk, Quentin C.B.; Douglas, Carl J.; Cronk, Quentin C. B. — 2018-02-26 Introgression can introduce novel genetic variation at a faster rate than mutation alone, and result in adaptive introgression when adaptive alleles are maintained in the recipient genome over time by natural selection. A previous study from our group demonstrated adaptive introgression from Populus balsamifera into P. trichocarpa in a target genomic region. Here we expanded our local ancestry analysis to the whole genome of both parents to provide a comprehensive, unbiased view of introgression patterns and to identify additional candidate regions for adaptive introgression genome-wide. Populus trichocarpa is a large, fast-growing tree of mild coastal regions of the Pacific northwest, whereas P. balsamifera is a smaller stature tree of continental and boreal regions with intense winter-cold. The species are parapatric with extensive hybridization. Here, using local ancestry analysis, we detected asymmetric patterns of introgression across the whole genome of these two species of poplar trees adapted to contrasting environments, with stronger introgression from P. balsamifera to P. trichocarpa than vice versa. Admixed P. trichocarpa individuals showed more genomic regions with unusually high levels of introgression (19 regions) compared with admixed P. balsamifera (9 regions) but also the largest introgressed peak (1.02 Mb). Our analysis also revealed numerous candidate regions for adaptive introgression with strong signals of selection, notably related to disease resistance, and enriched for genes that may play crucial roles for survival and adaptation. Furthermore, we revealed overrepresentation of subtelomeric regions in P. balsamifera introgression into P. trichocarpa and possible protection of the sex-determining regions from interspecific gene flow.

Geraldes, Armando J. M.; Farzaneh, Nima; Grassa, Christopher J.; McKown, Athena D.; Guy, Robert D.; Mansfied, Shawn D.; Douglas, Carl J.; Cronk, Quentin C. B.; Geraldes, Armando; Mansfield, Shawn D. — 2014-07-10 Populus trichocarpa is an ecologically important tree across western North America. We used a large population sample of 498 accessions over a wide geographical area genotyped with a 34K Populus SNP array to quantify geographical patterns of genetic variation in this species (landscape genomics). We present evidence that three processes contribute to the observed patterns: (1) introgression from the sister species P. balsamifera (2) isolation-by-distance and (3) natural selection. Introgression was detected only at the margins of the species’ distribution. Isolation-by-distance was significant across the sampled area as a whole, but no evidence of restricted gene flow was detected in a core of drainages from southern British Columbia. We identified a large number of FST outliers. GO analyses revealed that FST outliers are overrepresented in genes involved in circadian rhythm and response to red/far-red light when the entire dataset is considered, while in southern British Columbia heat response genes are overrepresented. We also identified strong correlations between geoclimate variables and allele frequencies at FST outlier loci that provide clues regarding the selective pressures acting at these loci.

Dempewolf, Hannes; Tesfaye, Misteru; Teshome, Abel; Bjorkman, Anne; Andrew, Rose L.; Scascitelli, Moira; Black, Scott; Bekele, Endashaw; Engels, Johannes M. M.; Cronk, Quentin C. B.; Rieseberg, Loren H.; Bjorkman, Anne D. — 2015-02-26 Noug (Guizotia abyssinica) is a semi-domesticated oil-seed crop, which is primarily cultivated in Ethiopia. Unlike its closest crop relative, sunflower, noug has small seeds, small flowering heads, many branches, many flowering heads, indeterminate flowering, and it shatters in the field. Here we conducted common garden studies and microsatellite analyses of genetic variation to test whether high levels of crop-wild gene flow and/or unfavorable phenotypic correlations have hindered noug domestication. With the exception of one population, analyses of microsatellite variation failed to detect substantial recent admixture between noug and its wild progenitor. Likewise, only very weak correlations were found between seed mass and the number or size of flowering heads. Thus, noug's ‘atypical’ domestication syndrome does not seem to be a consequence of recent introgression or unfavorable phenotypic correlations. Nonetheless, our data do reveal evidence of local adaptation of noug cultivars to different precipitation regimes, as well as high levels of phenotypic plasticity, which may permit reasonable yields under diverse environmental conditions. Why noug has not been fully domesticated remains a mystery, but perhaps early farmers selected for resilience to episodic drought or untended environments rather than larger seeds. Domestication may also have been slowed by noug's outcrossing mating system.

Percy, Diana M.; Page, Roderic D. M.; Cronk, Quentin C. B.; Cronk, Quentin C.B.; Page, Roderic D.M. — 2018-07-18 An increasing number of plant-insect studies using phylogenetic analysis suggest that cospeciation events are rare in plant–insect systems. Instead, nonrandom patterns of phylogenetic congruence are produced by phylogenetically conserved host switching (to related plants) or tracking of particular resources or traits (e.g., chemical). The dominance of host switching in many phytophagous insect groups may make the detection of genuine cospeciation events difficult. One important test of putative cospeciation events is to verify whether reciprocal speciation is temporally plausible. We explored techniques for double-dating of both plant and insect phylogenies. We use dated molecular phylogenies of a psyllid (Hemiptera)–Genisteae (Fabaceae) system, a predominantly monophagous insect–plant association widespread on the Atlantic Macaronesian islands. Phylogenetic reconciliation analysis suggests high levels of parallel cladogenesis between legumes and psyllids. However, dating using molecular clocks calibrated on known geological ages of the Macaronesian islands revealed that the legume and psyllid radiations were not contemporaneous but sequential. Whereas the main plant radiation occurred some 8 million years ago, the insect radiation occurred about 3 million years ago. We estimated that >60% of the psyllid speciation has resulted from host switching between related hosts. The only evidence for true cospeciation is in the much more recent and localized radiation of genistoid legumes in the Canary Islands, where the psyllid and legume radiations have been partially contemporaneous. The identification of specific cospeciation events over this time period, however, is hindered by the phylogenetic uncertainty in both legume and psyllid phylogenies due to the apparent rapidity of the species radiations.

Huang, Daisie I.; Hefer, Charles A.; Kolosova, Natalia; Douglas, Carl J.; Cronk, Quentin C. B. — 2015-05-18 As molecular phylogenetic analyses incorporate ever-greater numbers of loci, cases of cytonuclear discordance – the phenomenon in which nuclear gene trees deviate significantly from organellar gene trees – are being reported more frequently. Plant examples of topological discordance, caused by recent hybridization between extant species, are well known. However, examples of branch-length discordance are less reported in plants relative to animals. We use a combination of de novo assembly and reference-based mapping using short-read shotgun sequences to construct a robust phylogeny of the plastome for multiple individuals of all the common Populus species in North America. We demonstrate a case of strikingly high plastome divergence, in contrast to little nuclear genome divergence, in two closely related balsam poplars, Populus balsamifera and Populus trichocarpa (Populus balsamifera ssp. trichocarpa). Previous studies with nuclear loci indicate that the two species (or subspecies) diverged since the late Pleistocene, whereas their plastomes indicate deep divergence, dating to at least the Pliocene (6–7 Myr ago). Our finding is in marked contrast to the estimated Pleistocene divergence of the nuclear genomes, previously calculated at 75 000 yr ago, suggesting plastid capture from a ‘ghost lineage’ of a now-extinct North American poplar.