Bruce A. Ford

The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms.

A population genetic approach was used to explore the evolutionary biology of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. (Viscaceae). Arceuthobium americanum infects three principal hosts and has the most extensive geographical range of any North American dwarf mistletoe. Based on the lack of apparent morphological and phenological differences between populations of A. americanum, past researchers have found no evidence for recognizing infraspecific taxa. In this study, molecular analysis using amplified fragment length polymorphism (AFLP) analysis indicated that A. americanum is divided into three distinct genetic races, each associated with a different host taxon in regions of allopatry: (i) Pinus banksiana in western Canada; (ii) Pinus contorta var. murrayana in the Sierra Nevada and Cascade Mountain ranges in the western US; and (iii) Pinus contorta var. latifolia in the western US and Canada. These observations suggest that host identity, geographical isolation and environmental factors have contributed to race formation in A. americanum. The lack of fine‐scale patterning within each of the A. americanum races is attributed to random dispersal of seeds over long distances by animal vectors. Historical factors such as glaciations and founder events have also influenced structuring and genetic diversity in A. americanum populations. Given sufficient time, it is possible that these races will become reproductively isolated and undergo speciation.

Allozyme Diversity and Genetic Relationships among North American Members of the Short-Beaked Taxa of Carex sect. Vesicariae (Cyperaceae)

Morphological studies of the short-beaked members of Carex sect. Vesicariae have indicated that three species can be recognized: C. saxatilis (incl. C. miliaris and C. physocarpa), C. membranacea, and C. rotundata. To assess genetic relationships among species and gene diversity within species, 44 populations from throughout North America were surveyed electrophoretically for 12 enzymes encoded by 17 putative loci. Populations of the same species exhibited high genetic identities, with average values exceeding 0.950. Interspecific genetic identities ranged from 0.636 (between C. saxatilis and C. rotundata) to 0.880 (C. membranacea and C. rotundata). Alleles for the isozymes AAT-1, AAT-2, 6-PGD, PGI-2, PGM-2, and SKDH were useful in distinguishing C. saxatilis from C. membranacea and C. rotundata. Carex membranacea and C. rotundata were best distinguished by alleles of 6-PGD. There was little allozyme differentiation within the C. saxatilis complex; however, a weak east/west cline was evident. Within populations, observed heterozygosity closely matched expected levels of heterozygosity: F values averaged 0.039, indicating outcrossing is prevalent. Genetic variation was found to be largely apportioned within populations as Neis gene diversity statistic (GST) was close to 0.19 in each species. Past electrophoretic studies of Carex have disclosed low levels of intrapopulation genetic variation and high levels of interpopulation differentiation indicating high levels of inbreeding. Large population sizes, dichogamy, a rhizomatous habit, and self-incompatibility are discussed as possible mechanisms for maintaining genetic diversity within populations of the short-beaked species.

Phylogeny and Evolution in Cariceae (Cyperaceae): Current Knowledge and Future Directions

The goal of this study was to review the impact of DNA sequence analyses on our understanding of Cariceae phylogeny, classification and evolution. To explore character evolution, 105 taxa from four different studies were included in an nrDNA ITS + ETS 1f analysis of all recognized genera (Carex, Cymophyllus, Kobresia, Schoenoxiphium, Uncinia) and Carex subgenera (Carex, Psyllophora, Vignea, Vigneastra). As in previous analyses, four major Cariceae clades were recovered: (1) a “Core Carex Clade” (subg. Carex, Vigneastra, Psyllophora p.p); (2) A “Vignea Clade” (subg. Vignea, Psyllophora p.p.); (3) a “Schoenoxiphium Clade” (Schoenoxiphium, subg. Psyllophora p.p.), and (4) a “Core Unispicate Clade” (Uncinia, Kobresia, subg. Psyllophora p.p.). All studies provide strong support (86–100% BS) for the Core Carex and Vignea Clades, but only weak to moderate support (<50%–78% BS) for the Core Unispicate and Schoenoxiphium Clades. The relationships of these groups are unresolved. Studies suggest that Carex is either paraphyletic with respect to all Cariceae genera or to all genera except Schoenoxiphium. Kobresia is a grade, but Uncinia and possibly Schoenoxiphium are monophyletic. The monotypic Cymophyllus is indistinct from Carex subg. Psyllophora species. Character analyses indicate that inflorescence proliferation and reduction have occurred in all major clades, and that the Cariceae’s unisexual flowers have evolved from perfect flowers. The ancestor to Cariceae possessed a multispicate inflorescence with cladoprophylls and female spikelets with tristigmatic gynoecia and closed utricles. This morphology is most similar to extant Carex subg. Carex species, which contradicts the nearly unanimous assumption that the highly compound inflorescences of Schoenoxiphium are primitive. Since taxonomic sampling and statistical support for phylogenies have generally been poor, we advocate the temporary maintenance of the four traditional Carex subgenera with androgynous unispicate species placed within subg. Psyllophora and dioecious and gynaecandrous unispicate species distributed amongst subgenera Carex and Vignea. A collective effort focused on developing new nuclear markers, on increasing taxonomic and geographic sampling, and on studying development within the context of phylogeny, is needed to develop a phylogenetic classification of Cariceae.

Allozyme variation and genetic relationships among species in the Carex willdenowii complex (Cyperaceae).

A taxonomic study by Naczi, Reznicek, and Ford (American Journal of Botany, 85, 434–447, 1998) has determined that three species (Carex willdenowii, C. basiantha, and C. superata) can be recognized within the C. willdenowii complex. To determine the amount of genetic divergence within and between these species, allozyme analyses were conducted on 14 populations distributed from Pennsylvania to eastern Texas. Seventeen loci were surveyed, 13 of which were polymorphic, with all populations being polymorphic at one or more loci. Interspecific genetic identities ranged from 0.560 (C. willdenowii and C. basiantha) to 0.807 (C. basiantha and C. superata). Alleles for the isozymes Aat‐1, Dia‐1, Idh‐2, Mdh‐2, Per‐1, Pgm‐1, and Pgm‐2 served to distinguish C. willdenowii from C. basiantha and C. superata. Carex basiantha and C. superata were recognized by alleles of Mdh‐2, Pgm‐1, and Tpi‐2. The genetic identities of populations within species were high and exceeded 0.957. A caespitose growth habit and perigynia in close proximity to the staminate flowers suggest adaptations for selfing and therefore low levels of heterozygosity. Paradoxically, the values for expected heterozygosities (H exp) were always lower than those obtained by direct count (H obs): F values were highly negative, indicating heterozygous excess. Disassortative mating and selection are discussed as possible mechanisms for maintaining heterozygous excess within populations.

Morphological, geographical, and ecological differentiation in the Carex willdenowii complex (Cyperaceae).

Field studies as well as principal components analysis and analyses of variance of specimen measurements revealed morphologic variation within Carex willdenowii correlated with differences in geographical distribution and habitat characteristics. C. willdenowii is actually a complex of three species, C. basiantha Steudel, C. willdenowii Willdenow, and C. superata Naczi, Reznicek, & B.A. Ford, sp. nov. Carex basiantha is a calciphile of moist forests in the southern United States. It has relatively long culms and terminal spikes with long staminate portions. Carex willdenowii is a calcifuge of dry forests in the northeastern United States and immediately adjacent Canada. It has relatively long culms and terminal spikes with short staminate portions. Carex superata is a facultative calciphile of moist to dry forests of the southeastern United States. It has relatively short culms and terminal spikes with long staminate portions. Synonymies, typifications, descriptions, and citations of representative specimens are provided for each species, along with an identification key. The recognition of previously unsuspected diversity in a species from a region with a relatively well-known flora suggests the systematics of other common, widespread, and morphologically divergent species should be investigated.

Phylogeny of Carex subg. Vignea (Cyperaceae) Based on Non-coding nrDNA Sequence Data

Abstract Carex subg. Vignea is characterized by sessile bisexual spikes, distigmatic flowers, and the lack of cladoprophylls. Phylogenies reconstructed using nrDNA internal and external transcribed spacer (ITS and ETS 1f) sequences for 100 vignean taxa support this subgenus as monophyletic. The atypical C. gibba is sister to all remaining taxa. Many clades in the remainder of the subgenus do not correspond to easily defined morphological groups, with species representative of several disparate sections frequently contained within a single clade. Many traditionally recognized sections are not supported, although others such as sects. Ovales, Stellulatae, and Glareosae are monophyletic. Tree topologies indicate that gynaecandry has evolved multiple times in subg. Vignea. Species of uncertain subgeneric affinity are variously placed in our analysis. Carex fecunda, previously linked to subg. Vignea, is positioned within the outgroup composed of species traditionally placed in subg. Carex and Vigneastra. While species with highly compound inflorescences are often regarded as ancestral in Carex, our results indicate that this inflorescence type has evolved multiple times and is derived within subg. Vignea.

Comparative population structure and genetic diversity of Arceuthobium americanum (Viscaceae) and its Pinus host species: insight into host–parasite evolution in parasitic angiosperms

In a recent study we revealed that the parasitic angiosperm Arceuthobium americanum is comprised of three distinct genetic races, each associated with a different host in regions of allopatry. In order to assess the role of host identity and geographical isolation on race formation in A. americanum, we compared the genetic population structure of this parasite with that of its three principal hosts, Pinus banksiana, Pinus contorta var. latifolia and Pinus contorta var. murrayana. Despite the fact that A. americanum was divided into three genetic races, hosts were divided into only two genetic groups: (i) Pinus banksiana and hybrids, and (ii) P. contorta var. latifolia and var. murrayana. These findings suggest that factors such as geographical isolation and adaptation to different environmental conditions are important for race formation in the absence of host‐driven selection pressures. To assess factors impacting population structure at the fine‐scale, genetic and geographical distance matrices of host and parasite were compared within A. americanum races. The lack of a relationship between genetic and geographical distance matrices suggests that isolation‐by‐distance plays a negligible role at this level. The effect of geographical isolation may have been diminished because of the influence of factors such as random seed dispersal by animal vectors or adaptation to nongeographically patterned environmental conditions. Host–parasite interactions might also have impacted the fine‐scale structure of A. americanum because the parasite and host were found to have similar patterns of gene flow.

Megaphylogenetic Specimen-Level Approaches to the Carex (Cyperaceae) Phylogeny Using ITS, ETS, and matK Sequences: Implications for Classification

Abstract We present the first large-scale phylogenetic hypothesis for the genus Carex based on 996 of the 1983 accepted species (50.23%). We used a supermatrix approach using three DNA regions: ETS, ITS and matK. Every concatenated sequence was derived from a single specimen. The topology of our phylogenetic reconstruction largely agreed with previous studies. We also gained new insights into the early divergence structure of the two largest clades, core Carex and Vignea clades, challenging some previous evolutionary hypotheses about inflorescence structure. Most sections were recovered as non-monophyletic. Homoplasy of characters traditionally selected as relevant for classification, historical misunderstanding of how morphology varies across Carex, and regional rather than global views of Carex diversity seem to be the main reasons for the high levels of polyphyly and paraphyly in the current infrageneric classification.

The impact of species-specific traits and phylogenetic relatedness on allozyme diversity inCarex sect.Phyllostachys (Cyperaceae)

Allozyme variation was examined inCarex sect.Phyllostachys (Cyperaceae) to study the effects of species-specific traits and phylogenetic relatedness on genetic structure. In contrast to the findings of similar studies, genetic variability in thePhyllostachys is poorly correlated with geographic range and putative differences in breeding systems (as inferred from morphology). This suggests that other patterns of evolution, colonization, and gene flow characterize the species found in this section. Fixation indices are negative for all populations suggesting that mechanisms such as disassortative mating and selection are maintaining heterozygous excess within populations. Closely related taxa often exhibit different genetic variability statistics. In some instances, however, clades (e.g.C. jamesii andC. juniperorum) display very similar levels of genetic variability despite marked differences in species-specific traits. Recent speciation coupled with the ability to maintain historical levels of variation within populations may be factors accounting for this phenomenon. Contrary to similar studies, species restricted to known glacial refugia have lower genetic diversity than those species that underwent mass migrations in response to deglaciation. Narrowly endemic species were found to partition their genetic diversity within, as opposed to between populations. The opposite trend was evident in wider ranging congeners.

The Taxonomy of the Circumpolar Short-Beaked Taxa of Carex sect. Vesicariae (Cyperaceae)

Within the short-beaked taxa of Carex sect. Vesicariae, C. membranacea and C. rotundata were found to form a close species pair with anatomical data providing the most convincing evidence for the separation of these two species. Carex membranacea has V-shaped leaves, a median vascular bundle keel, a prominent bulliform cell layer, and triangular stems with a discontinuous layer of sclerenchyma. In contrast, C. rotundata has crescentiform leaves, which lack a median vascular keel and bulliform cell layer, and circular stems with an almost continuous layer of sclerenchyma adjacent to the epidermis. Numerical analyses of macromorphological characters indicate that, within the C. saxatilis complex, plants from western North America (C. physocarpa) tend to be robust individuals with long peduncles on the pistillate spikes, wide leaves, and large perigynia. These characters, and others, decrease in size eastward across the North American continent with intermediate plants usually referred to as C. saxatilis and small plants referred to as C. miliaris. This east/west cline is confounded by large amounts of variation within small geographic areas and phenotypic plasticity. Eurasian material is less variable than that from North America and is included within the range of variation found in North America. It is concluded that a single distigmatic species, C. saxatilis, should be recognized. Anatomically, C. saxatilis is indistinguishable from C. membranacea. Evidence is presented to suggest that this similarity is the result of homoplasy or stasis in anatomical characters rather than an indication of a close evolutionary relationship.