Stephen R. Downie | University of Illinois at Urbana-Champaign (original) (raw)

Papers by Stephen R. Downie

Tracing the invasion history of a parasitoid wasp in North America

A note on the taxonomic status of Euphrasia randii (Scrophulariaceae)

Canadian Journal of Botany, 1990

ABSTRACT A reexamination of the phenetic relationships among the previously recognized infraspeci... more ABSTRACT A reexamination of the phenetic relationships among the previously recognized infraspecific taxa of Euphrasia randii using principal component analyses and four clustering methods indicates that the recognition of these infraspecific taxa is unwarranted. This complex exists as one morphologically variable taxon, E. randii.

Morphological, cytological, and flavonoid variability of the Arnica angustifolia aggregate (Asteraceae)

Canadian Journal of Botany, 1988

ABSTRACT The Arnica angustifolia aggregate is a circumpolar taxon previously consisting of a numb... more ABSTRACT The Arnica angustifolia aggregate is a circumpolar taxon previously consisting of a number of geographically distinct infra-specific taxa. Cluster and principal component analyses of 99 populations revealed the aggregate to be best represented by two subspecies: A. angustifolia subsp. angustifolia (a combination of the previously recognized subspecies angustifolia, attenuata, sornborgeri, intermedia, iljinii, and alpina and A. plantaginea) and A. angustifolia subsp. tomentosa. Arnica angustifolia subsp. angustifolia is polymorphic and varied in ploidy level and foliar flavonoid chemistry. No significant differences were found among the means of 14 characters in plants collected from six broadly delimited geographic areas. Four cytotypes (2n = 38, 57, 76, and 95) and seven flavonoid glycosides (five flavonols and two flavones) were discerned. The aggregate is predominantly agamospermous with amphimictic phases in unglaciated Alaska and west central Yukon. Disjunct distributions are probably the result of survival in refugia during the late Wisconsinan.

A systematic study of Peucedanum nebrodense

Rhodora, 2005

Five infraspecific taxa have been recognized in Cymopterus glomeratus (¼ C. acaulis): vars. glome... more Five infraspecific taxa have been recognized in Cymopterus glomeratus (¼ C. acaulis): vars. glomeratus (¼ acaulis), fendleri, greeleyorum, higginsii, and parvus. The results of previous phylogenetic analyses of DNA sequence and morphological data have supported the close association of these five varieties, although the relationships among them could not be discerned. The recognition of infraspecific taxa within C. glomeratus is controversial. Multivariate analysis of variance and principal component analysis of 288 specimens representing the morphological variability and geographic distribution of this species complex were conducted to test the validity of these infraspecific taxa. Results show that most of the characters previously used to recognize these varieties are highly variable within the taxa. Although analysis of variance demonstrated some statistical differences among the varieties, patterns were not consistent. No clearly separated clusters are revealed in the principal component analysis and all five varieties were intermixed on the plots of the principal components. On the basis of the results of both phylogenetic and multivariate analyses, we propose that plants in this species complex be recognized as one species, C. glomeratus, with no varieties. The nomenclature and typification of this species are presented.

Le statut des Euphrasia borealis, E. nemorosa et E. stricta dans l'est de l'Amérique du Nord : une analyse numérique

Canadian Journal of Botany, Feb 1, 2011

ABSTRACT The taxonomic status of Euphrasia borealis, E. nemorosa, and E. stricta in eastern North... more ABSTRACT The taxonomic status of Euphrasia borealis, E. nemorosa, and E. stricta in eastern North America has been the subject of considerable controversy. A sample of 184 operational taxonomic units were scored for 58 morphological characters, and the resulting data matrix was subjected to cluster and principal component analyses. Two groups were recognized on the basis of bract and upper cauline leaf characters and designated as E. nemorosa and E. stricta. These two groups were then subjected to several discriminant analyses (stepwise, linear, and canonical) using quantitative data. The results do not support the recognition of E. borealis and E. nemorosa as separate taxa. These plants should be treated as a single species, for which the correct name is E. nemorosa. Euphrasia stricta is, however, morphologically distinct from E. nemorosa. Some geographic differentiation was observed between plants of E. nemorosa from Newfoundland and those from Quebec, but not enough to warrant the recognition of infraspecific taxa. No evidence of hybridization between E. nemorosa and E. stricta was found. Key words: Euphrasia borealis, Euphrasia nemorosa, Euphrasia stricta, numerical analysis, North America, hybridization.

Taxon, 2010

Phylogenetic analyses of the cpDNA trnQ-trnK 5′ exon region for 27 genera and 42 species of Sanic... more Phylogenetic analyses of the cpDNA trnQ-trnK 5′ exon region for 27 genera and 42 species of Saniculoideae and early diverging lineages of Apioideae were carried out to assess or confirm the tribal placements of the following anomalous genera: Annesorhiza, and Phlyctidocarpa. To accommodate these unique early diverging members of the Apiaceae and to reflect their relationships, a new tribal classification system has become necessary. Many of the early diverging genera (herein referred to as the protoapioids) can readily be distinguished from the euapioids (the remaining apioids) by the presence of scattered druse crystals in the mesocarp. The major morphological discontinuity within the family, however, lies between the combined protoapioids and euapioids (representing an expanded Apioideae s.l., including the Saniculoideae) and the subfamilies Azorelloideae and Mackinlayoideae. The broadened subfamily Apioideae is diagnostically different from the other subfamilies in the absence of rhomboidal crystals, the presence of druse crystals scattered throughout the mesocarp (subsequently lost in the euapioids), and the non-woody endocarp. No such diagnostic characters are available to support the traditional or recently expanded concept of Saniculoideae. The broadened concept of Apioideae is also supported by the sporadic presence of true wings. This character can be variously interpreted from a phylogenetic point of view, but nevertheless has considerable diagnostic value. A new tribal classification system for the protoapioids is proposed on the basis of molecular, morphological and anatomical evidence. This new system is intended to be practical and non-hierarchical to allow for future realignments amongst the tribes, as more evidence becomes available. It makes provision for hitherto poorly known African taxa and comprises the following eight tribes, five of which are newly described: Annesorhizeae, Choritaenieae, Heteromorpheae, Lichtensteinieae, Marlothielleae, Phlyctidocarpeae, Saniculeae and Steganotaenieae.

Taxonomy of Arnica (Asteraceae) subgenus Arctica

Rhodora, 1988

Plant Systematics and Evolution, Oct 9, 2009

The genus Billburttia is described to include two new species endemic to Madagascar, B. capensoid... more The genus Billburttia is described to include two new species endemic to Madagascar, B. capensoides and B. vaginoides. Both species were tentatively placed within the problematic genus Peucedanum L. (as Peucedanum sp. A and Peucedanum sp. B) based on their dorsally compressed fruits lacking prominent dorsal ribs and with winged marginal ribs. Recently, however, the African members of Peucedanum have been shown to be only distantly related to the type of the genus and have therefore been segregated into six African endemic genera. While the Malagasy species appear superficially similar to members from one of these segregate genera, viz. Notobubon, they differ markedly in their fruit anatomical characters, notably a narrower commissure, six commissural vittae, vascular tissue in the tip of the ribs and sphaerocrystals distributed in and around the epidermis. The latter two characters apparently represent apomorphies for Billburttia. The non-peucedanoid affinity of the genus, as suggested by the fruit anatomical data, was confirmed using ITS and rps16 intron sequences. Both parsimony and Bayesian analyses of these data place Billburttia within the tribe Apieae of subfamily Apioideae and not closely related to either Peucedanum (Selineae) or the African peucedanoid genera (Lefebvrea clade of Tordylieae).

Plant Syst Evol, 2000

Evolutionary relationships among members of Apiaceae (Umbelliferae) tribe Caucalideae Spreng. and... more Evolutionary relationships among members of Apiaceae (Umbelliferae) tribe Caucalideae Spreng. and related taxa were inferred from maximum parsimony analyses of chloroplast DNA restriction sites and rpsl6 intron sequences and the results compared to an existing phylogeny for the group based on nuclear ribosomal DNA internal transcribed spacer sequences. While these three data sets were not similar in size or composition, the relationships among the shared taxa, with few exceptions, were concordant. Three major lineages are recognized, coinciding with the previously delimited Scandiceae subtribes Daucinae Dumort.

Rhodora, 2005

Biological Approaches and Evolutionary Trends in Plants, 1990

The demise of Peucedanum (Apiaceae) in Africa

Previous mapping studies have revealed that the frequency and large size of inverted repeat junct... more Previous mapping studies have revealed that the frequency and large size of inverted repeat junction shifts in Apiaceae plastomes are unusual among angiosperms. To further examine plastome structural organization and inverted repeat evolution in the Apiales (Apiaceae + Araliaceae), we have determined the complete plastid genome sequences of five taxa, namely Anthriscus cerefolium (154,719 base pairs), Crithmum maritimum (158,355 base pairs), Hydrocotyle verticillata (153,207 base pairs), Petroselinum crispum (152,890 base pairs), and Tiedemannia filiformis subsp. greenmanii (154,737 base pairs), and compared the results obtained to previously published plastomes of Daucus carota subsp. sativus and Panax schin-seng. We also compared the five Apiaceae plastomes to identify highly variable noncoding loci for future molecular evolutionary and systematic studies at low taxonomic levels. With the exceptions of Crithmum and Petroselinum, which each demonstrate a~1.5 kilobase shift of its LSC-IR B junction (J LB ), all plastomes are typical of most other non-monocot angiosperm plastid DNAs in their structural organization, gene arrangement, and gene content. Crithmum and Petroselinum also incorporate novel DNA in the LSC region adjacent to the LSC-IR A junction (J LA ). These insertions (of 1,463 and 345 base pairs, respectively) show no sequence similarity to any other region of their plastid genomes, and BLAST searches of the Petroselinum insert resulted in multiple hits to angiosperm mitochondrial genome sequences, indicative of a mitochondrial to plastid transfer of DNA. A comparison of pairwise sequence divergence values and numbers of variable and parsimony-informative alignment positions (among other sequence characteristics) across all introns and intergenic spacers >150 base pairs in the five Apiaceae plastomes revealed that the rpl32-trnL, trnE-trnT, ndhF-rpl32, 5 0 rps16-trnQ, and trnT-psbD intergenic spacers are among the most fast-evolving loci, with the trnD-trnY-trnE-trnT combined region presenting the greatest number of potentially informative characters overall. These regions are therefore likely to be the best choices for molecular evolutionary and systematic studies at low taxonomic levels. Repeat analysis revealed direct and inverted dispersed repeats of 30 base pairs or more that may be useful in population-level studies. These structural and sequence analyses contribute to a better understanding of plastid genome evolution in the Apiales and provide valuable new information on the phylogenetic utility of plastid noncoding loci, enabling further molecular evolutionary and phylogenetic studies on this economically, ecologically, and medicinally important group of flowering plants.

Systematic Botany, 2014

The genera Lomatium and Cymopterus, along with many others, form a group that has been referred t... more The genera Lomatium and Cymopterus, along with many others, form a group that has been referred to previously as the perennial endemic Apiaceae subfamily Apioideae of western North America. This group of ecologically important and widespread species has been the target of numerous systematic studies, but the evolutionary relationships among these species remain elusive. Here we show that this confusion is due to high levels of morphological parallelism and homoplasy in the characters that have traditionally been used to define them, a result that is concordant with previous studies of the group. We explore patterns of evolution in traditionally important morphological characters using Bayesian stochastic character mapping on a phylogeny constructed from novel nrDNA and cpDNA sequence data for 96 specific and infraspecific taxa of the estimated 200 species in the group. We consider the implications of these results for taxonomic classification, the evolution of morphologies, and the utility of these morphologies to delimit small and large clades. Lomatium concinnum is newly combined as Cymopterus glomeratus var. concinnus and the new combination Cymopterus glomeratus var. greeleyorus is made.

Multiple Independent Losses of the rpoC1 Intron in Angiosperm Chloroplast DNA's

Systematic Botany, 1996

Systematic Botany, 2008

Oxypolis and Ptilimnium are two endemic genera of North America within tribe Oenantheae (Apiaceae... more Oxypolis and Ptilimnium are two endemic genera of North America within tribe Oenantheae (Apiaceae subfamily Apioideae). Both genera are small, with 12 species currently recognized. Some members of each genus share an unusual leaf morphology. Rather than having the pinnately compound leaves that generally characterize apioid umbellifers, they have highly reduced, linear, terete, hollow, septate appendages known as rachis leaves. It has long been questioned whether the species with rachis leaves should be placed in separate genera. In this study, we use data from the internal transcribed spacer region of nuclear ribosomal DNA to explore relationships within and between these genera. A total of 147 internal transcribed spacer sequences were obtained from multiple accessions of all species of Oxypolis and Ptilimnium and for several other genera from tribe Oenantheae. These included Lilaeopsis, which also has rachis leaves, and Cynosciadium and Limnosciadium, which have rachis-like leaves. These sequence data were analyzed using maximum parsimony, maximum likelihood, and Bayesian methods. The results from each of these analyses were congruent and suggest that neither Oxypolis nor Ptilimnium as presently defined is monophyletic. Oxypolis and Ptilimnium are each separated into two clades according to leaf morphology. The rachis-leaved Oxypolis species are provisionally recognized as the genus Tiedemannia and the rachis-leaved Ptilimnium species are provisionally recognized as the genus Harperella, pending further investigation. The relationships among these four clades and the genera Cynosciadium, Daucosma, and Limnosciadium are not clear. Nevertheless, it appears that the rachis-leaf habit has evolved multiple times in the tribe. Geographic structure is apparent in the phylogenetic trees and, pending further study, may suggest new taxa; in addition, the presence of O. occidentalis on the Queen Charlotte Islands, well-separated from other populations of this species, suggests that it may have survived the last glaciation in a refugium in this area.

South African Journal of Botany, 2009

The Cape endemic genus Capnophyllum Gaertn. is revised. As a result of valuable recent collection... more The Cape endemic genus Capnophyllum Gaertn. is revised. As a result of valuable recent collections and extensive fieldwork, this hitherto neglected genus was found to comprise four annual species, two of which are newly described, namely C. lutzeyeri Magee and B.-E.van Wyk, and C. macrocarpum Magee and B.-E.van Wyk. The four species are distinguished from one another by their fruit morphology (relative length of the styles, the shape and position of the stylopodium, fruit size, surface sculpturing, and the presence or absence of a sterile apical portion) and fruit anatomy (marginal wings slightly or prominently involute and secondary ribs present or absent). A comprehensive key to the species, their complete nomenclature and typification, together with complete descriptions and known geographical distributions for all the species are presented and illustrated.

Tracing the invasion history of a parasitoid wasp in North America

A note on the taxonomic status of Euphrasia randii (Scrophulariaceae)

Canadian Journal of Botany, 1990

Morphological, cytological, and flavonoid variability of the Arnica angustifolia aggregate (Asteraceae)

Canadian Journal of Botany, 1988

A systematic study of Peucedanum nebrodense

Rhodora, 2005

Le statut des Euphrasia borealis, E. nemorosa et E. stricta dans l'est de l'Amérique du Nord : une analyse numérique

Canadian Journal of Botany, Feb 1, 2011

Taxon, 2010

Taxonomy of Arnica (Asteraceae) subgenus Arctica

Rhodora, 1988

Plant Systematics and Evolution, Oct 9, 2009

Plant Syst Evol, 2000

Rhodora, 2005

Biological Approaches and Evolutionary Trends in Plants, 1990

The demise of Peucedanum (Apiaceae) in Africa

Systematic Botany, 2014

Multiple Independent Losses of the rpoC1 Intron in Angiosperm Chloroplast DNA's

Systematic Botany, 1996

Systematic Botany, 2008

South African Journal of Botany, 2009

Generic delimitations within the Sium alliance (Apiaceae tribe Oenantheae) inferred from cpDNA rps16-5'trnK(UUU) and nrDNA ITS sequences

Taxon, Jul 31, 2009

Dispersal patterns in space and time: a case study of Apiaceae subfamily Apioideae

Journal of Biogeography, 2013

ABSTRACT Aim: To analyse spatial and temporal patterns of dispersal events in the euapioids (Apia... more ABSTRACT Aim: To analyse spatial and temporal patterns of dispersal events in the euapioids (Apiaceae subfamily Apioideae). Location: Worldwide, with an emphasis on the Northern Hemisphere. Methods: A phylogeny of euapioids was inferred from 1194 nuclear ribosomal DNA internal transcribed spacer (nrDNA ITS) sequences using Bayesian methods. The reconstruction of ancestral areas was performed simultaneously with phylogenetic inference using a Markov discrete phylogeographical model with Bayesian stochastic search variable selection (BSSVS). Routes with significant non-zero migration rates were identified using Bayes factors. For each significant track and each tree, the distribution of dispersals in time was scored and the asymmetry of dispersals was evaluated. Results: The root of the euapioid umbellifers was reconstructed at c. 44.51 Ma (95% highest posterior density interval: 39.11—51.55 Ma). The Eastern Asiatic Region was reconstructed as the most probable ancestral area for the root of the tree. Seventeen migration routes have significant non-zero migration rates. Five of these tracks represent long-distance transoceanic routes: (1) western Eurasia—North America, (2) Eastern Asiatic Region—North America, (3) Australia—Neotropical Kingdom, (4) Australia—Neantarctic, and (5) Neotropical Kingdom—sub-Saharan Africa. Most dispersal events occurred among the areas that comprise the major diversification centres of apioid umbellifers: western Eurasia and the Mediterranean, Irano-Turanian and Eastern Asiatic regions. The floristic exchange among these regions was more or less symmetrical in all directions and continuous in time. The exchange between North America and the Eastern Asiatic Region was asymmetrical; the asymmetry of dispersals between western Eurasia and North America was less pronounced and not statistically significant. Floristic exchange was significantly asymmetrical for the Nearctic—Neotropical Kingdom, Nearctic—Neantarctic, western Eurasia—sub-Saharan Africa, and western Eurasia—Siberia migration tracks. Main conclusions: The observed dispersal pattern — intense and symmetrical within the same climatic zone versus scarce and often unidirectional between climatic zones or along long-distance transoceanic tracks — suggests that both the availability of suitable habitats and geographical barriers have played crucial roles in determining the present distribution of euapioid umbellifers.

Recurrent short-distance dispersal explains wide distributions of hydrophytic umbellifers (Apiaceae tribe Oenantheae)

Journal of Biogeography, 2014

ABSTRACT AimLong-distance dispersal (LDD) by migratory birds is often invoked to explain the broa... more ABSTRACT AimLong-distance dispersal (LDD) by migratory birds is often invoked to explain the broad and disjunct distributions of many aquatic plants. Such distributions may also be achieved by recurrent short-distance dispersal (SDD) to adjacent areas and extinctions in connecting areas. To test the relative importance of LDD and SDD in shaping the distributions of hydrophytes, we examined different dispersal models for a clade of hydrophytic umbellifers (Apiaceae tribe Oenantheae).LocationWorldwide, with emphasis on Eurasian–North American disjunctions.MethodsA dated phylogeny of the group was obtained with Bayesian methods using nrDNA internal transcribed spacer (ITS) and cpDNA rps16–trnK sequences from 100 species and infraspecific taxa of Oenantheae and two outgroup species. Ancestral habits were inferred using maximum likelihood (R package ape). Six connectivity models were compared using a maximum-likelihood-based method (Lagrange), four with symmetrical dispersal rate matrices and two assuming asymmetrical exchanges between Eurasia and North America, with or without spatial and temporal constraints on LDD.ResultsThe age of the crown node of Oenantheae was estimated at 26.3 Ma, and western Europe was reconstructed as its most likely ancestral area. The divergence between Peucedanum sandwicense, an endemic of Hawaii, and its sister Oenanthe occurred 17.2 Ma, pre-dating the emergence of the Hawaiian Islands. Throughout the phylogeny, the hydrophytic habit (including helophytes and amphiphytes) dominated. Of the six connectivity models considered, the model restricting all intracontinental and transoceanic LDDs and assuming an almost unidirectional dispersal from Eurasia to North America received the highest likelihood score. This model was also characterized by the highest dispersal rate. A stratified model assuming a higher probability for dispersals between Eurasia and North America when these continents were connected with land bridges received a lower likelihood score.Main conclusionsThe results suggest that the broad and often disjunct distribution of Oenantheae hydrophytes is mostly achieved through an increased dispersal rate and recurrent SDD rather than frequent LDD. Our data confirm the asymmetry of the floristic exchange between Eurasia and North America and do not support the increase of this exchange when the continents were connected with land bridges.

The Utility of Morphological Characters for Inferring Phylogeny in Scandiceae Subtribe Scandicinae (Apiaceae)

Annals of the Missouri Botanical Garden, 2001

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