Interspecific mating in the Piriqueta caroliniana (Turneraceae) complex: effects of pollen load size and composition

Two taxa of Piriqueta (P. caroliniana and P. viridis) form a broad hybrid zone that extends over much of the central Florida peninsula. We used genetic markers to examine the strength of the isolation barriers between these taxa and the patterns of mating at the initial stages of hybridization. Regression models were employed to analyze the effects of pollen load size and the proportions of intra- and interspecific pollen on the frequency of first-generation (F1) hybrid formation. Overall, the postpollination mating barriers between these two taxa were relatively weak. However, there were significant effects of pollen load composition and size on the patterns of hybridization in both taxa with frequency-dependent responses to composition in both taxa. The frequency of F1 hybrid formation was generally lower than expected based on the frequency of each pollen type on the stigma for P. caroliniana recipients. The lower frequencies of F1 seeds in this taxon were apparently due to a greater competitive ability for intraspecific pollen, since hybrid seed formation decreased with increasing pollen load size. Pollen from P. caroliniana donors was also competitively superior on P. viridis recipients, leading to higher than expected frequencies of hybrid seed formation. Pollen from P. caroliniana did suffer higher rates of pollen-tube attrition than intraspecific pollen on P. viridis recipients, so the frequency of hybrid seed formation would be lower when pellen load sizes were small. In general, reproductive isolation mechanisms were stronger in P. caroliniana, suggesting that introgression should occur into P. viridis when these taxa come into close contact. Comparison of these expected patterns of mating to the distribution of hybrid genotypes in Florida provides insights into the relative roles of mating and selection in the evolution of hybrid populations of Piriqueta.     

Comparative phylogeography of the Veronica alpina complex in Europe and North America

The Veronica alpina complex comprises eight species of alpine habitats over a wide range of mountain systems in the Northern Hemisphere. The occurrence of sympatric species in the European and North American mountain systems allowed us not only to investigate the effect of the ice ages on intraspecific phylogeographical patterns and genetic diversity in different continents of the Northern Hemisphere, but also to compare these patterns in closely related species. Plastid DNA trnL-F sequences and AFLP (amplified fragment length polymorphism) fingerprints were used to infer the phylogenetic history of the group and phylogeographical patterns within species. Hybrid origin of tetraploid eastern North American V. wormskjoldii from western North American V. nutans (= V. wormskjoldii s.l.) and Eurasian V. alpina is suggested. A number of phylogeographical groups have been found both in V. alpina from Europe and in V. nutans from western North America. Phylogeographical substructuring in the Alps is inferred for V. alpina but not for V. bellidioides, which is moreover characterized by an overall very low genetic diversity. Western North American V. cusickii is much more genetically diverse than its sympatric relative, V. nutans, an effect that is likely due to differences in the breeding system. Populations of V. nutans are differentiated into three groups, those from the Cascades and from the southern and the northern Rocky Mountains. Genetic diversity seems to be higher in the North American V. nutans than in the morphologically and ecologically similar European V. alpina. A possible scenario to explain this pattern is suggested.     

A case of recent long-distance dispersal in the Piriqueta caroliniana complex

We investigated restriction fragment length polymorphism (RFLP) variation in the chloroplast DNA regions trnS and trnC in 53 populations of the herbaceous perennial Piriqueta caroliniana complex, encompassing its known North American range. The goals of this study were to expand the geographic range of a previous study and to determine the origin of an apparently anomalous population, which possessed a phenotype that was incongruous with other populations in the region. The phylogeography of these populations was investigated with nested clade analysis of a one-step network phylogeny that was based on the RFLP variation of the chloroplast genome. We found evidence of restricted gene flow and past fragmentation, which is consistent with a previous study on the phylogeography of this species complex. The morphological and genetic profile of the one anomalous population indicates that it recently originated from south Florida. Given the urban location of this geographically disjunct population, it is probable that this is an example of a recent human-mediated long-distance dispersal event.     

Patterns in the geographical range sizes of ectotherms in North America

The distributions of homeothermic mammals and birds in continental North America show a distinct pattern in the configuration of their geographical ranges. Smaller ranges tend to be elongated north-south while larger ranges tend to be elongated east-west. To examine the generality of this pattern in ectotherms, we analyzed the distribution on continental North America of 139 species of mosquitoes, 164 amphibians, and 221 reptiles. Unlike birds and mammals, small ranges of ectotherms were not elongated north-south and the small ranges of snakes were elongated east-west. The distribution of ectotherms with small ranges does not appear to be affected by the major topographic features of North America which tend to run north-south. Like birds and mammals, large ranges of mosquitoes and reptiles but not amphibians are elongated east-west. The east-west orientation of mosquitoes with large ranges is not attributable to the three largest genera in North America taken singly, Aedes, Culex, or Anopheles, but appears only when all genera are pooled. The east-west orientation of reptiles with large ranges is attributable to turtles and snakes but not lizards. Climatic zones may thus affect the distribution of mosquitoes, turtles, and snakes with large ranges but are not the major determinants of range dimensions among ectotherms in general. Received: 1 September 1997 / Accepted: 8 February 1998     

Genetic consequences of glacial survival: the late Quaternary history of balsam poplar (Populus balsamifera L.) in North America

Aim Beringia, the unglaciated region encompassing the former Bering land bridge, as well as the land between the Lena and Mackenzie rivers, is recognized as an important refugium for arctic plants during the last ice age. Compelling palaeobotanical evidence also supports the presence of small populations of boreal trees within Beringia during the Last Glacial Maximum. The occurrence of balsam poplar (Populus balsamifera) in Beringia provides a unique opportunity to assess the implications of persistence in a refugium on present‐day genetic diversity for this boreal tree species. Location North America. Methods We sequenced three variable non‐coding regions of the chloroplast genome (cpDNA) from 40 widely distributed populations of balsam poplar across its North American range. We assessed patterns of genetic diversity, geographic structure and historical demography between glaciated and unglaciated regions of the balsam poplar’s range. We also utilized a coalescent model to test for divergence between regions. Results Levels of genetic diversity were consistently greater for populations at the southern margin (θ_W = 0.00122) than in the central (θ_W = 0.00086) or northern (θ_W = 0.00034) regions of the current distribution of balsam poplar, and diversity decreased with increasing latitude (R² = 0.49, P < 0.01). We detected low, but significant, structure (F_CT = 0.05, P = 0.05), among regions of P. balsamifera’s distribution. The cpDNA genealogy was shallow, however, showing an absence of highly differentiated chloroplast haplotypes. Coalescent analyses supported a model of divergence between the southern ice margin and the northern unglaciated region of balsam poplar’s distribution, but analyses of other regional comparisons did not converge. Main conclusions The palaeobotanical record supports the presence of a Beringian refugium for balsam poplar, but we were unable to definitively identify the presence of known refugial populations based on genetic data alone. Balsam poplar populations from Beringia are not a significant reservoir of cpDNA diversity today. Unique alleles that may have been present in the small, isolated populations that survived within Beringia were probably lost through genetic drift or swamped by post‐glacial, northward migration from populations south of the ice sheets.     

Nuclear vs. plastid data: complex Pleistocene history of a circumpolar key species

To fully understand the contemporary genetic structure of plants, both nuclear and plastid markers are needed. Three chloroplast DNA (cpDNA) lineages, which probably diverged before the major Pleistocene glaciations, have been identified in the circumpolar/circumboreal Vaccinium uliginosum. Here we investigate its nuclear DNA variation using nuclear ribosomal internal transcribed spacer (ITS) sequences, DNA ploidy level measurements and amplified fragment length polymorphisms (AFLPs). We also extend the cpDNA dataset. Two ITS lineages, corresponding to diploids and tetraploids, respectively, were identified. However, both main sequence types apparently occurred in most individual plants but showed ploidy-biased homogenization and possibly reflect paralogy predating the origin of V. uliginosum. The ploidy levels were largely consistent with the cpDNA lineages, suggesting that the initial cpDNA divergence followed early polyploidizations. Five main AFLP groups were identified, consistent with recent glacial refugia in Beringia, western Siberia, the southern European mountains and areas south/east of the Scandinavian and Laurentide ice sheets. Except from the southern European mountains, there has been extensive expansion from all refugia, resulting in several contact zones. Surprisingly, the presumably older ploidy and cpDNA patterns were partly inconsistent with the main AFLP groups and more consistent with AFLP subgroups. A likely major driver causing the inconsistencies is recent nuclear gene flow via unreduced pollen from diploids to tetraploids. This may prevent cytoplasmic introgression and result in overlayed patterns formed by processes dominating at different time scales. The data also suggest more recent polyploidizations, as well as several chloroplast capture events, further complicating this scenario. This study highlights the importance of combining different marker systems to unravel intraspecific histories.     

Molecular insights into the colonization and chromosomal diversification of Madeiran house mice

The colonization history of Madeiran house mice was investigated by analysing the complete mitochondrial (mt) D-loop sequences of 156 mice from the island of Madeira and mainland Portugal, extending on previous studies. The numbers of mtDNA haplotypes from Madeira and mainland Portugal were substantially increased (17 and 14 new haplotypes respectively), and phylogenetic analysis confirmed the previously reported link between the Madeiran archipelago and northern Europe. Sequence analysis revealed the presence of four mtDNA lineages in mainland Portugal, of which one was particularly common and widespread (termed the 'Portugal Main Clade'). There was no support for population bottlenecks during the formation of the six Robertsonian chromosome races on the island of Madeira, and D-loop sequence variation was not found to be structured according to karyotype. The colonization time of the Madeiran archipelago by Mus musculus domesticus was approached using two molecular dating methods (mismatch distribution and Bayesian skyline plot). Time estimates based on D-loop sequence variation at mainland sites (including previously published data from France and Turkey) were evaluated in the context of the zooarchaeological record of M. m. domesticus. A range of values for mutation rate (μ) and number of mouse generations per year was considered in these analyses because of the uncertainty surrounding these two parameters. The colonization of Portugal and Madeira by house mice is discussed in the context of the best-supported parameter values. In keeping with recent studies, our results suggest that mutation rate estimates based on interspecific divergence lead to gross overestimates concerning the timing of recent within-species events.     

The role of disjunction and postglacial population expansion on phylogeographical history and genetic diversity of the circumboreal plant Chamaedaphne calyculata

In the last decade a number of studies has illustrated quite different phylogeographical patterns amongst plants with a northern present‐day geographical distribution, spanning the entire circumboreal region and/or circumarctic region and southern mountains. These works, employing several marker systems, have brought to light the complex evolutionary histories of this group. Here I focus on one circumboreal plant species, Chamaedaphne calyculata (leatherleaf), to unravel its phylogeographical history and patterns of genetic diversity across its geographical range. A survey of 29 populations with combined analyses of chloroplast DNA (cpDNA), internal transcribed spacer (ITS) and AFLP markers revealed structuring into two groups: Eurasian/north‐western North American, and north‐eastern North American. The present geographical distribution of C. calyculata has resulted from colonization from two putative refugial areas: east Beringia and south‐eastern North America. The variation of chloroplast DNA (cpDNA) and ITS sequences strongly indicated that the evolutionary histories of the Eurasian/north‐western North American and the north‐eastern North American populations were independent of each other because of a geographical disjunction in the distribution area and ice‐sheet history between north‐eastern and north‐western North America. Mismatch analysis using ITS confirmed that the present‐day population structure is the result of rapid expansion, probably since the last glacial maximum. The AFLP data revealed low genetic diversity of C. calyculata (P = 19.5%, H = 0.085) over the whole geographical range, and there was no evidence of loss of genetic diversity within populations in the continuous range, either at the margins or in formerly glaciated and nonglaciated regions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 761–775.     

Chloroplast DNA intraspecific phylogeography of plants from the Pacific Northwest of North America

Molecular studies of plants from the Pacific Northwest of North America suggest a recurrent pattern of genetic differentiation and geographic structuring. In each of five angiosperms and one fern species representing diverse life histories, cpDNA data indicate two clades of populations that are geographically structured. A northern group comprises populations from Alaska to central or southern Oregon, whereas populations from central Oregon southward to northern California form a southern group. In several of these species, a few populations having southern genotypes may have survived in glacial refugia further north in the Olympic Peninsula, Queen Charlotte Islands, and Prince of Wales Island. Allozyme data reveal a similar pattern of differentiation in several other plants from the Pacific Northwest. North-south partitioning of genotypes has also been reported for several animal species from this region. On a broader geographic scale, northsouth partitioning of genotypes has also been observed in other plants from western North America having a variety of geographic distributions. Some species also display a reduction of genetic variability in the northern portion of their range compared to the south. The data suggest strongly that past glaciation profoundly influenced the genetic architecture of the flora and fauna of the Pacific Northwest. Two alternative hypotheses are advanced to explain the geographic structuring of genotypes. First, past glaciation may have created discontinuities in the geographic distributions of plant species, with populations surviving in several well-isolated northern and southern refugia. Following glaciation, migration of genetically differentiated, once-isolated populations resulted in the formation of a continuous geographic distribution with a major genetic discontinuity. Alternatively, plants survived and subsequently migrated northward from a southern refugium, and a genotype became fixed in one or a few populations at the leading edge of recolonization. Subsequent long-distance dispersal from this leading edge resulted in a relatively uniform northern genotype that differs from the southern genotype(s). Whatever the underlying mechanism, Pleistocence glaciation may have molded the intraspecific genetic architecture of both plants and animals from the Pacific Northwest in a geographically similar manner. Future studies should seek to obtain a comprehensive phylogeography for regions that includes a diversity of both plants and animals.Dedicated to emer. Univ.-Prof. DrFriedrich Ehrendorfer on the occasion of his 70th birthday     

Evidence for multiple sources of invasion and intraspecific hybridization in Brachypodium sylvaticum (Hudson) Beauv. in North America

We compared the levels and distribution of genetic diversity in Eurasian and North American populations of Brachypodium sylvaticum (Huds.) Beauv. (false brome), a newly invasive perennial bunchgrass in western North America. Our goals were to identify source regions for invasive populations, determine the number of independent invasion events, and assess the possibility that postinvasion bottlenecks and hybridization have affected patterns of genetic diversity in the invaded range. We tested the hypothesis that this Eurasian grass was accidentally introduced into two areas in Oregon and one site in California by examining nuclear microsatellites and chloroplast haplotype variation in 23 introduced and 25 native populations. In the invaded range, there was significantly lower allelic richness (R(S)), observed heterozygosity (H(O)) and within-population gene diversity (H(S)), although a formal test failed to detect a significant genetic bottleneck. Most of the genetic variation existed among populations in the native range but within populations in the invaded range. All of the allelic variation in the invaded range could be explained based on alleles found in western European populations. The distribution of identified genetic clusters in the North American populations and the unique alleles associated with them is consistent with two historical introductions in Oregon and a separate introduction to California. Further analyses of population structure indicate that intraspecific hybridization among genotypes from geographically distinct regions of western Europe occurred following colonization in Oregon. The California populations, however, are more likely to be derived from one or perhaps several genetically similar regions in the native range. The emergence and spread of novel recombinant genotypes may be facilitating the rapid spread of this invasive species in Oregon.     

The history and geography of diversification within the butterfly genus Lycaeides in North America

The Lycaeides butterfly species complex in North America consists of two nominal, morphologically defined species. These butterflies are ecologically diverse and appear to be distributed as a geographically complex mosaic of locally differentiated populations that may be undergoing adaptive radiation. We asked whether patterns of molecular genetic variation within the species complex are congruent with currently recognized morphological species and whether the distribution of molecular variation is consistent with the hypothesis that Pleistocene climate changes contributed to the process of differentiation within the genus. Variation in the form of the genitalia from 726 males from 59 populations clearly distinguishes both species with only six populations containing morphologically intermediate or ambiguous individuals. However, partitioning of molecular variance in a 236 bp section of the mitochondrial AT-rich region from 628 individuals (57 populations) surveyed using single strand conformation polymorphism analysis (SSCP) indicates that only 26% of the total genetic variation is distributed along nominal species boundaries as defined by morphology. Instead, three phylogeographical groups were detected, represented by three major haplotype clades, which account for 90% of the total genetic variance. Pleistocene glaciations appear to have fostered divergence during glacial maxima, while post-glacial range expansions created opportunities for gene exchange and reticulation along suture zones between geographical groups. Data presented here allow us to make inferences about the history of the species complex. However, evidence of ancestral polymorphism and reticulation limit our ability to define species boundaries based on mitochondrial DNA sequence variation.     

Bryophyte disjunctions in the Northern Hemisphere: Europe and North America*

Two major disjunctions are recognized between the bryofloras of Europe and North America: Amphi-Atlantic and Western Europe—Western North America. Each of these has specifically segregated floras related to climatic regimes and correlated with different historical development. An interpretation of the disjunctions is based on reconstruction of past continental interconnections coincident with reconstructed climates and an understanding of the biology of the bryophytes.     

Recent and rapid diversification of the small carpenter bees in eastern North America

Bees fulfil a critical ecological role as pollinators, significantly contributing to the reproductive success of myriad angiosperm species. Although increasingly appreciated for their agricultural contributions, relatively little is yet known about the natural history of the vast majority of the more than 20 000 wild bee species worldwide. The small carpenter bee genus Ceratina occurs globally, and is represented in North America by its most recently diverged subgenera, Ceratinula and Zadontomerus. Recent genetic analysis of eastern Ceratina (Zadontomerus) supports the existence of five closely related, yet genetically distinct species living in sympatry. This phylogeographical study employs molecular barcoding of the most comprehensive specimen collection yet assembled to confirm the identities of these recently diverged eastern North American Ceratina (Zadontomerus) species. Delineation of extant population structure, evolutionary history and known range of this emerging model native pollinator are greatly improved by this study. We consider ecological and behavioural factors potentially contributing to the maintenance of genetic identity among these sympatric species.     

Holarctic phylogeography of an asexual species complex – II. Allozymic variation and clonal structure in Arctic Daphnia

Previous mitochondrial (mt)DNA sequencing and restriction fragment length polymorphism (RFLP) studies have shown that the Holarctic Daphnia pulex complex is divisible into two major groups (pulicaria and tenebrosa) that exhibit distinct phylogeographic patterns. Here we examine allozymic variation at six polymorphic enzyme loci to ascertain clonal structure and clonal distribution patterns within each group. Specimens were collected from a total of 850 populations encompassing the Arctic. A significant negative relationship (Mantel test) between similarity of regional clonal arrays and geographical distance was observed. A small fraction of clones in each group was widespread (in the order of 1000s of kilometres). However, most clones were restricted to single regions, and were often found only in a single population. These data indicate that the population genetic structure is highly fragmented in this complex, but the potential for long-distance passive dispersal exists. Further, ‘hot spots’ of high clonal richness and diversity were found in each group, which is concordant with earlier work. In addition, ≈ 20% of pulicaria group clones possess nuclear genes from tenebrosa, while approximately 10% of tenebrosa group clones harbour pulicaria nuclear genes. These data indicate nuclear introgression between the two groups, which was found to be prominent in a broad zone of secondary contact encompassing parts of northwestern Russia, northern Fennoscandia, Svalbard, and extending into the high eastern Canadian Arctic.     

A hierarchical classification of freshwater mussel diversity in North America

Aim North America harbours the most diverse freshwater mussel fauna on Earth. This fauna has high endemism at the continental scale and within individual river systems. Previous faunal classifications for North America were based on intuitive, subjective assessments of species distributions, primarily the occurrence of endemic species, and do not portray continent‐wide patterns of faunal similarity. The aim of this study is to provide an analytical portrayal of patterns of mussel diversity in a hierarchical framework that informs the biogeographical history of the fauna. Location The study considered the mussel fauna of North America from the Rio Grande system northwards. Methods Patterns of mussel faunal similarity in 126 river systems or lake watersheds across North America were examined. The dataset was developed from the literature and consisted of recent species presence/absence (282 species) in each drainage unit; subspecies were not included. Patterns of mussel diversity were examined with hierarchical cluster analysis, based on a pairwise distance matrix between all drainage units. Results Cluster analysis revealed 17 faunal provinces within four major faunal regions: Mississippian, Atlantic, Eastern Gulf and Pacific. The Mississippian Region dominates the North American fauna with 11 provinces, including five not recognized by previous classifications: Mississippi Embayment, Upper Mississippi, Great Plains, Ohioan and Pontchartrain–Pearl–Pascagoula. Within the Eastern Gulf Region (containing three provinces), the Escambia–Choctawhatchee Province is distinctive from the Apalachicolan Province, under which it was previously subsumed. Patterns of diversity in the Atlantic Region (two provinces) and Pacific Region (one province) were similar to previous classifications. Main conclusions The classification proposed in this study largely corroborates earlier schemes based on the occurrence of endemic species but identifies additional heterogeneity that reflects unique assemblages of widely distributed species. The study proposes a hierarchical structure that illustrates relationships among these provinces. Although some provinces in the Mississippian Region have high endemism, all Mississippian provinces share a group of widely distributed species. The Atlantic and Eastern Gulf regions have distinctive, endemic faunas suggesting limited past connectivity with the Mississippian Region. The Pacific Region is the most distinct fauna in North America and bears close affinity to the Eurasian mussel fauna.     

Identification of glacial refugia in south-eastern North America by phylogeographical analyses of a forest understorey plant, Trillium cuneatum

Aim We examine several hypotheses emerging from biogeographical and fossil records regarding glacial refugia of a southern thermophilic plant species. Specifically, we investigated the glacial history and post‐glacial colonization of a forest understorey species, Trillium cuneatum. We focused on the following questions: (1) Did T. cuneatum survive the Last Glacial Maximum (LGM) in multiple refugia, and (if so) where were they located, and is the modern genetic structure congruent with the fossil record‐based reconstruction of refugia for mesic deciduous forests? (2) What are the post‐glacial colonization patterns in the present geographical range? Location South‐eastern North America. Methods We sampled 45 populations of T. cuneatum throughout its current range. We conducted phylogeographical analyses based on maternally inherited chloroplast DNA (cpDNA haplotypes) and used TCS software to reconstruct intraspecific phylogeny. Results We detected six cpDNA haplotypes, geographically highly structured into non‐overlapping areas. With one exception, none of the populations had mixed haplotype composition. TCS analysis resulted in two intraspecific cpDNA lineages, with one clade subdivided further by shallower diversification. Main conclusions Our investigation revealed that T. cuneatum survived the LGM in multiple refugia, belonging to two (western, eastern) genealogical lineages geographically structured across south‐eastern North America. The western clade is confined to the south‐western corner of T. cuneatum’s modern range along the Lower Mississippi Valley, where fossil records document a major refugium of mesic deciduous forest. For the eastern clade, modern patterns of cpDNA haplotype distribution suggest cryptic vicariance, in the form of forest contractions and subsequent expansions associated with Pleistocene glacial cycles, rather than simple southern survival and subsequent northward colonization. The north–south partitioning of cpDNA haplotypes was unexpected, suggesting that populations of this rather southern thermophilic species may have survived in more northern locations than initially expected based on LGM climate reconstruction, and that the Appalachian Mountains functioned as a barrier to the dispersal of propagules originating in more southern refugia. Furthermore, our results reveal south‐west to north‐east directionality in historical migration through the Valley and Ridge region of north‐west Georgia.     

Glacial-induced altitudinal migrations in Armeria (Plumbaginaceae) inferred from patterns of chloroplast DNA haplotype sharing

In contrast to northern European areas where large-scale migrations occurred to recolonize territories after glacial periods, species in southern regions survived and diverged without large geographical displacements. As a result of the importance of orography in much of the southern areas, such displacements must have involved populations ascending or descending mountains. The present study provides support for glacial-induced altitudinal migrations from chloroplast phylogeographic patterns in Armeria (Plumbaginaceae) in southeast Spain. One hundred and five sequences of the trnL-F spacer were obtained from seven species. Fifteen different haplotypes were recognized, their genealogy was inferred, and associations with geography were explored using nested clade analysis. Seven instances were detected in which the same haplotype is shared by two or three species within a particular massif. In all the cases, at least one of the species involved displayed different haplotypes in other areas; in most, the haplotype shared is predominant either in one of the species involved or in the massif. These patterns of haplotype sharing strongly suggest horizontal transfer between species. In one of the massifs (Sierra Nevada) the three species involved in haplotype sharing (A. splendens, A. filicaulis ssp. nevadensis, A. villosa ssp. bernisii) occur at markedly different altitudinal belts. It is argued that altitudinal migrations within the contraction-expansion model provide the best explanation for the current pattern, and that at least in one case it resulted in the formation of a new hybrid taxon, A. filicaulis ssp. nevadensis.