Phylogeny of South American Bufo (Anura: Bufonidae) inferred from combined evidence

Despite the considerable research that has focused on the evolutionary relationships and biogeography of the genus Bufo, an evolutionary synthesis of the entire group has not yet emerged. In the present study, almost 4 kb of DNA sequence data from mitochondrial (12S, tRNA^Val, and 16S) and nuclear (POMC; Rag-1) genes, and 83 characters from morphology were analysed to infer a phylogeny of South American toads. Phylogenies were reconstructed with parsimony and maximum likelihood and Bayesian model-based methods. The results of the analysis of morphological data support the hypothesis that within Bufo , some skull characters (e.g. frontoparietal width), correlated with the amount of cranial ossification, are prone to homoplasy. Unique and unreversed morphological synapomorphies are presented that can be used to diagnose recognized species groups of South American toads. The results of all phylogenetic analyses support the monophyly of most species groups of South American Bufo . In most DNA-only and combined analyses, the South American (minus the B. guttatus and part of the ' B. spinulosus ' groups), North American, Central American, and African lineages form generally well-supported clades: ((((((((South America) (North America + Central America)) Eurasia) Africa) Eurasia) South America) West Indies) South America). This result confirms and extends prior studies recovering South American Bufo as polyphyletic. The biogeographical results indicate that: (1) The origin of Bufo predates the fragmentation of Gondwana; (2) Central and North American species compose the sister group to a large, 'derived' clade of South American Bufo ; and (3) Eurasian species form the sister group to the New World clade.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 146 , 407–452.     

Phylogeography of the bushmaster (Lachesis tnuta: Viperidae): implications for neotropical biogeography, systematics, and conservation

We used mitochondrial gene sequences to reconstruct phylogenetic relationships among subspecies of the bushmaster, Lachesis muta. These large vipers are widely distributed in lowland tropical forests in Central and South America, where three of four allopatric subspecies are separated by montane barriers. Our phylogeny indicates that the four subspecies belong to two clades, the Central American and South American lineages. We use published molecular studies of other taxa to estimate a 'reptilian mtDNA rate' and thus temporal boundaries for major lineage divergences in Lachesis. We estimate that the Central and South American forms diverged 18-6 Mya, perhaps due to the uplifting of the Andes, whereas the two Central American subspecies may have diverged 11-4 Mya with the uprising of the Cordillera de Talamanca that separates them today. South American bushmasters from the Amazon Basin and the Atlantic Forest are not strongly differentiated, perhaps due to episodic gene flow during the Pleistocene, when suitable habitat for this species was at times more continuous. Our results agree with previous evidence that genetic divergence among some neotropical vertebrates pre-dated Pleistocene forest fragmentation cycles and the appearance of the Panamanian Isthmus. Based on morphological, behavioral, and molecular evidence, we recognize three species of Lachesis. In addition to L. muta, the widespread South American form, the Central American forms are treated as distinct species (L. meknocephak and L. stenophrys), each deserving of special conservation status due to restricted distribution and habitat destruction.     

Molecular phylogeny and historical biogeography of the Holarctic wetland leaf beetle of the genus Plateumaris

Leaf beetles of the genus Plateumaris inhabit wetlands across the temperate zone of the Holarctic region. To explore the phylogeographic relationships among North American, East Asian, and European members of this genus and the origin of the species endemic to Japan, we studied the molecular phylogeny of 20 of the 27 species in this genus using partial sequences of mitochondrial cytochrome oxidase subunit I (COI) and the 16S and nuclear 28S rRNA genes. The molecular phylogeny revealed that three species endemic to Europe are monophyletic and sister to the remaining 11 North American and six Asian species. Within the latter clade, North American and Asian species did not show reciprocal monophyly. Dispersal-vicariance analysis and divergence time estimation revealed that the European and North America-Asian lineages diverged during the Eocene. Moreover, subsequent differentiation occurred repeatedly between North American and Asian species, which was facilitated by three dispersal events from North America to Asia and one in the opposite direction during the late Eocene through the late Miocene. Two Japanese endemics originated from different divergence events; one differentiated from the mainland lineage after differentiation from the North American lineage, whereas the other showed a deep coalescence from the North American lineage with no present-day sister species on the East Asian mainland. This study of extant insects provides molecular phylogenetic evidence for ancient vicariance between Europe and East Asia-North America, and for more recent (but pre-Pleistocene) faunal exchanges between East Asia and North America.     

American Indian prehistory as written in the mitochondrial DNA: a review.

Native Americans have been divided into three linguistic groups: the reasonably well-defined Eskaleut and Nadene of northern North America and the highly heterogeneous Amerind of North, Central, and South America. The heterogeneity of the Amerinds has been proposed to be the result of either multiple independent migrations or a single ancient migration with extensive in situ radiation. To investigate the origin and interrelationship of the American Indians, we examined the mitochondrial DNA (mtDNA) variation in 87 Amerinds (Pima, Maya, and Ticuna of North, Central, and South America, respectively), 80 Nadene (Dogrib and Tlingit of northwest North America and Navajo of the southwest North America), and 153 Asians from 7 diverse populations. American Indian mtDNAs were found to be directly descended from five founding Asian mtDNAs and to cluster into four lineages, each characterized by a different rare Asian mtDNA marker. Lineage A is defined by a HaeIII site gain at np 663, lineage B by a 9-bp deletion between the COII and tRNA(Lys) genes, lineage C by a HincII site loss at np 13259, and lineage D by an AluI site loss at np 5176. The North, Central, and South America Amerinds were found to harbor all four lineages, demonstrating that the Amerinds originated from a common ancestral genetic stock. The genetic variation of three of the four Amerind lineages (A, C, and D) was similar with a mean value of 0.084%, whereas the sequence variation in the fourth lineage (B) was much lower, raising the possibility of an independent arrival. By contrast, the Nadene mtDNAs were predominantly from lineage A, with 27% of them having a Nadene-specific RsaI site loss at np 16329. The accumulated Nadene variation was only 0.021%. These results demonstrate that the Amerind mtDNAs arose from one or maybe two Asian migrations that were distinct from the migration of the Nadene and that the Amerind populations are about four times older than the Nadene.     

Mitochondrial DNA phylogeny of North American field crickets: perspectives on the evolution of life cycles,songs, and habitat associations

North American field crickets (genus Gryllus) exhibit a diversity of life cycles, habitat associations, and calling songs. However, patterns of evolution for these ecological and behavioral traits remain uncertain in the absence of a robust phylogenetic framework. Analyses of morphological variation have provided few clues about species relationships in the genus Gryllus. Here we use comparisons of mitochondrial DNA restriction site maps for 29 individuals representing 11 species (including potential outgroups) to examine relationships among eastern North American field crickets. Initially chosen as likely outgroup taxa, the two European species of Gryllus do not obviously fall outside of an exclusively North American clade and (based on amount of sequence divergence) appear to have diverged from North American lineages at about the same time that major North American lineages diverged from each other. The egg-overwintering crickets comprise a strongly supported monophyletic group, but relationships among these three closely related species cannot be resolved. The mtDNA data are consistent with a single origin of egg diapause and do not support a model of recent life cycle divergence and allochronic speciation for Gryllus pennsylvanicus and G. veletis. The two crickets are not sister species, despite remarkable similarity in morphology, habitat, and calling song. This conclusion is consistent with published data on allozyme variation in North American field crickets. The habitat associations of eastern North American field crickets have been labile, but calling songs sometimes have remained virtually unchanged across multiple speciation events.     

Possible migration routes into South America deduced from mitochondrial DNA studies in Colombian Amerindian populations

Mitochondrial DNA haplotype studies have been useful in unraveling the origins of Native Americans. Such studies are based on restriction site and intergenic deletion/insertion polymorphisms, which define four main haplotype groups common to Asian and American populations. Several studies have characterized these lineages in North, Central, and South American Amerindian, as well as Na Dene and Aleutian populations. Siberian, Central Asian, and Southeast Asian populations have also been analyzed, in the hope of fully depicting the route(s) of migration between Asia and America. Colombia, a key route of migration between North and South America, has until now not been studied. To resolve the current lack of information about Colombian Amerindian populations, we have investigated the presence of the founder haplogroups in 25 different ethnic groups from all over the country. The present research is part of an interdisciplinary program, Expedición Humana, fostered by the Universidad Javeriana and Dr. J. E. Bernal V. The results show the presence of the four founder A-D Amerindian lineages, with varied distributions in the different populations, as well as the presence of other haplotypes in frequencies ranging from 3% to 26%. These include some unique or private polymorphisms, and also indicate the probable presence of other Asian and a few non-Amerindian lineages. A spatial structure is apparent for haplogroups A and D, and to a lesser extent for haplogroup C. While haplogroup A and D frequencies in Colombian populations from the northwestern side of the Andes resemble those seen in Central American Amerindians more than those seen in South American populations, their frequencies on the southeastern side more closely resemble the bulk of South American frequencies so far reported, raising the question as to whether they reflect more than one migration route into South America. High frequencies of the B lineage are also characteristic of some populations. Our observations may be explained by historical events during the pre-Columbian dispersion of the first settlers and, later, by disruptions caused by the European colonization.     

Phylogenetic relationships of Iberian cyprinids: systematic and biogeographical implications.

The phylogenetic relationships among all Iberian endemic cyprinids were inferred using the complete nucleotide sequence of the cytochrome b gene. The inferred molecular phylogeny included representatives from Central European, Asian and North African species, and is highly congruent with previous phylogenies based on osteological characters. Iberian cyprinids were grouped into only five, very speciose lineages (with the exception of the monotypic Anaecypris): Barbus, Luciobarbus, Chondrostoma, Leuciscus and Anaecypris. The existence of such a relatively small number of Iberian cyprinid lineages can be explained by the historical isolation of the Iberian Peninsula. North African and Asian barbels are the sister group of Iberian Luciobarbus, supporting a south-eastern route of colonization of the Iberian Peninsula for this subgenus. With leuciscins, Anaecypris hispanica was considered a relict species as it could not be related to any other Iberian cyprinid. The phylogenetic relationships among the main lineages of Iberian cyprinids based on cytochrome b sequence data supported the traditional division of the Cyprinidae into two subfamilies: Cyprininae and Leuciscinae.     

Origin and evolution of North American polyploid Silene (Caryophyllaceae)

Nuclear DNA sequences from introns of the low-copy nuclear gene family encoding the second largest subunit of RNA polymerases and the ribosomal internal transcribed spacer (ITS) regions, combined with the psbE-petL spacer and the rps16 intron from the chloroplast genome were used to infer origins and phylogenetic relationships of North American polyploid Silene species and their closest relatives. Although the vast majority of North American Silene species are polyploid, which contrasts to the diploid condition dominating in other parts of the world, the phylogenetic analyses rejected a single origin of the North American polyploids. One lineage consists of tetraploid Silene menziesii and its diploid allies. A second lineage, Physolychnis s.l., consists of Arctic, European, Asian, and South American taxa in addition to the majority of the North American polyploids. The hexaploid S. hookeri is derived from an allopolyploidization between these two lineages. The tetraploid S. nivea does not belong to any of these lineages, but is closely related to the European diploid S. baccifera. The poor resolution within Physolychnis s.l. may be attributed to rapid radiation, recombination among homoeologues, homoplasy, or any combination of these factors. No extant diploid donors could be identified in Physolychnis s.l.     

Mitochondrial genetic variation in European, African and Indian cattle populations

Mitochondrial DNA from representative animals of 13 different cattle breeds was assayed for restriction fragment length polymorphisms (RFLP) to determine phylogenetic relationships and levels of variation among breeds; 16 different mitotypes were found, described by 20 polymorphisms. Within these 16 mitotypes two major lineages were apparent: an Afro-European and an Asian type. These were found to differ at over 2.3% of sites surveyed. None of the mitotypes found in the Asian lineage was detectable in the Afro-European lineage and vice versa. Within each of the major mitotypes there were no further significant differences within or among breeds. Using rates of mitochondrial evolution estimated from other species, the two lineages were estimated to have diverged between 575000 and 1150000 years ago; well outside the 10000 years bp timeframe postulated by a single domestication hypothesis. The results presented are concordant with those generated in other studies and provide strong evidence for an independent domestication of Asian Bos indicus. Furthermore, the grouping of all African indicine populations within the clade containing all Bos taurus lineages points to the hybrid origins of the humped cattle of that continent.     

Parallel evolution of site‐specific changes in divergent caribou lineages

The parallel evolution of phenotypes or traits within or between species provides important insight into the basic mechanisms of evolution. Genetic and genomic advances have allowed investigations into the genetic underpinnings of parallel evolution and the independent evolution of similar traits in sympatric species. Parallel evolution may best be exemplified among species where multiple genetic lineages, descended from a common ancestor, colonized analogous environmental niches, and converged on a genotypic or phenotypic trait. Modern North American caribou (Rangifer tarandus) originated from three ancestral sources separated during the Last Glacial Maximum (LGM): the Beringian–Eurasian lineage (BEL), the North American lineage (NAL), and the High Arctic lineage (HAL). Historical introgression between the NAL and the BEL has been found throughout Ontario and eastern Manitoba. In this study, we first characterized the functional differentiation in the cytochrome‐b (cytB) gene by identifying nonsynonymous changes. Second, the caribou lineages were used as a direct means to assess site‐specific parallel changes among lineages. There was greater functional diversity within the NAL despite the BEL having greater neutral diversity. The patterns of amino acid substitutions occurring within different lineages supported the parallel evolution of cytB amino acid substitutions suggesting different selective pressures among lineages. This study highlights the independent evolution of identical amino acid substitutions within a wide‐ranging mammal species that have diversified from different ancestral haplogroups and where ecological niches can invoke parallel evolution.     

Phylogenetic relationships among North American Toads, genus Bufo

Relationships among 14 species of North American Bufo were studied using the quantitative immunological technique of micro-complement fixation. Separate antisera to serum albumins of five North American Bufo and one Central American Bufo were used as probes of phylogenetic relationships. A cladogram for these species constructed with the albumin data was compatible with phylogenetic interpretations based on morphological analyses and hybridization studies. Using albumin as a molecular dating device radiations within the B. americanus, B. cognatus and B. boreas species groups were each estimated to be Pleistocene events. Separation of North and Central American lineages is estimated to have occurred as early as the upper Oligocene.     

Analysis of DNA sequences of six chloroplast and nuclear genes suggests incongruence, introgression, and incomplete lineage sorting in the evolution of Lespedeza (Fabaceae)

The genus Lespedeza (Fabaceae) consists of 40 species disjunctively distributed in East Asia and eastern North America. Phylogenetic relationships of all Lespedeza species and closely related genera were reconstructed using maximum parsimony, maximum likelihood, and Bayesian analyses of sequence data from five chloroplast (rpl16, rpl32-trnL, rps16-trnQ, trnL-F, and trnK/matK) and one nuclear (ITS) DNA regions. All analyses yielded consistent relationships among major lineages. Our results suggested that Campylotropis, Kummerowia, and Lespedeza are monophyletic, respectively. Lespedeza is resolved as sister to Kummerowia and these two together are further sister to Campylotropis. Neither of the two subgenera, subgen. Lespedeza and subgen. Macrolespedeza, in Lespedeza based on morphological characters, is recovered as monophyletic. Within Lespedeza, the North American clade is retrieved as sister to the Asian clade. The nuclear and chloroplast markers showed incongruent phylogenetic signals at shallow-level phylogeny, which may point to either introgression or incomplete lineage sorting in Lespedeza. The divergence times within Lespedeza and among related genera were estimated using Bayesian approach with BEAST. It is assumed that following the divergence between Kummerowia and Lespedeza in Asia in the late Miocene, the ancestor of Lespedeza diverged into the North American and the Asian lineages. The North American ancestor quickly migrated to North America through the Bering land bridge in the late Miocene. The North American and Asian lineages started to diversify almost simultaneously in the late Miocene but resulted in biased numbers of species in two continents.     

Pan-African Genetic Structure in the African Buffalo (Syncerus caffer): Investigating Intraspecific Divergence

The African buffalo (Syncerus caffer) exhibits extreme morphological variability, which has led to controversies about the validity and taxonomic status of the various recognized subspecies. The present study aims to clarify these by inferring the pan-African spatial distribution of genetic diversity, using a comprehensive set of mitochondrial D-loop sequences from across the entire range of the species. All analyses converged on the existence of two distinct lineages, corresponding to a group encompassing West and Central African populations and a group encompassing East and Southern African populations. The former is currently assigned to two to three subspecies (S. c. nanus, S. c. brachyceros, S. c. aequinoctialis) and the latter to a separate subspecies (S. c. caffer). Forty-two per cent of the total amount of genetic diversity is explained by the between-lineage component, with one to seventeen female migrants per generation inferred as consistent with the isolation-with-migration model. The two lineages diverged between 145 000 to 449 000 years ago, with strong indications for a population expansion in both lineages, as revealed by coalescent-based analyses, summary statistics and a star-like topology of the haplotype network for the S. c. caffer lineage. A Bayesian analysis identified the most probable historical migration routes, with the Cape buffalo undertaking successive colonization events from Eastern toward Southern Africa. Furthermore, our analyses indicate that, in the West-Central African lineage, the forest ecophenotype may be a derived form of the savanna ecophenotype and not vice versa, as has previously been proposed. The African buffalo most likely expanded and diverged in the late to middle Pleistocene from an ancestral population located around the current-day Central African Republic, adapting morphologically to colonize new habitats, hence developing the variety of ecophenotypes observed today.     

Molecular Systematics and Evolution of Arabidopsis and Arabis

Abstract: We provide a phylogenetic analysis of the genera Arabidopsis and Arabis based on nuclear ribosomal DNA sequences. We show that traditional taxonomical concepts within tribe Ara-bideae, which includes these genera, are highly artificial. Arabis and Arabidopsis are paraphyletic and consist of several different independent lineages. The genus Capsella, originally placed in tribe Lepideae, is related to North American Arabis and the Arabidopsis thaliana lineage. Other genera, including East Asian Yin-shania, North American Halimolobus, cosmopolitan Barbarea and Cardamine, and European Aubrieta are positioned among different Arabis lineages. One Arabis species, Arabis pauciflora, is only distantly related to tribe Arabideae. Base chromosome number reduction from n = 8 to n = 5 to 7 occurred several times, suggesting that lower base chromosome numbers than n = 8 are derived in tribe Arabideae. Current knowledge on the evolution and systematics of the genera Arabis and Arabidopsis and relationships within the mustard family are summarized and discussed in the light of convergent evolution and transfer of knowledge from Arabidopsis thaliana as a molecular model plant to other species of the Cruciferae.     

The Great American Interchange in birds: a phylogenetic perspective with the genus Trogon

The 'Great American Interchange' (GAI) is recognized as having had a dramatic effect on biodiversity throughout the Neotropics. However, investigation of patterns in Neotropical avian biodiversity has generally been focused on South American taxa in the Amazon Basin, leaving the contribution of Central American taxa under-studied. More rigorous studies of lineages distributed across the entire Neotropics are needed to uncover phylogeographical patterns throughout the area, offering insights into mechanisms that contribute to overall Neotropical biodiversity. Here we use mitochondrial DNA sequence data and intensive geographical sampling from the widespread Neotropical avian genus Trogon to investigate the role of the GAI in shaping its phylogeographical history. Our results show that genetic diversity in Trogon exceeds the perceived biodiversity, and that the GAI resulted in lineage diversification within the genus. Despite greater diversity in South America, a Central American centre of origin with multiple and independent dispersals into South America is indicated. These dispersals were followed by the evolution of divergent lineages associated with the Andes Mountains and other South American geographical features. According to our phylogenetic reconstructions, several species, which were originally defined by morphological characters, are nonmonophyletic. In sum, our results elucidate the evolutionary history of Trogon , reveal patterns obscured by extant biodiversity, and serve as a biogeographical model to consider in future studies.     

Phylogeny and Phytogeography of Eupatorium (Eupatorieae, Asteraceae): Insights from Sequence Data of the nrDNA ITS Regions and cpDNA RFLP

Eupatorium were examined by sequencing the internal transcribed spacers (ITS) of nuclear ribosomal DNA and restriction site analysis of chloroplast DNA. Molecular data provided strong evidence that (1) this genus originated in North America, (2) the genus diverged into three morphological species groups, Eutrochium, Traganthes and Uncasia in North America, and (3) one of the North American Uncasia lineages migrated into temperate Europe and eastern Asia over the Bering land bridge. The estimated divergence times support a late Miocene to early Pliocene migration from North America to Eurasia via the Bering land bridge. A European species was sister to all of the eastern Asian species examined. The disjunct distribution pattern of the genus Eupatorium is incongruent with the classical Arcto-Tertiary geoflora concept. Received 13 September 1999/ Accepted in revised form 4 January 2000     

Cladistic biogeography of Gleditsia (Leguminosae) based on ndhF and rpl16 chloroplast gene sequences

We used cladistic analysis of chloroplast gene sequences (ndhF and rpl16) to test biogeographic hypotheses in the woody genus Gleditsia. Previous morphological comparisons suggested the presence of two eastern Asian-eastern North American species pairs among the 13 known species, as well as other intra- and inter-continental disjunctions. Results from phylogenetic analyses, interpreted in light of the amount of sequence divergence observed, led to the following conclusions. First, there is a fundamental division of the genus into three clades, only one of which contains both Asian and North American species. Second, the widespread and polymorphic Asian species, G. japonica, is sister to the two North American species, G. triacanthos and G. aquatica, which themselves are closely related inter se, but are both polymorphic and paraphyletic. Third, the lone South American Gleditsia species, G. amorphoides, forms a clade with two eastern Asian species. Gleditsia thus appears to have only one Asian-North American disjunction and no intercontinental species pairs. Low sequence divergence between G. amorphoides and its closest Asian relatives implicates long-distance dispersal in the origin of this unusual disjunction. Sequence divergence between Asian and North American Gleditsia is much lower than between Asian and North American species of its closest relative, Gymnocladus. Estimates of Asian-North American divergence times for Gymnocladus are in general accordance with fossil data, but estimates for Gleditsia suggest recent divergences that conflict with ages of known North American Gleditsia fossils.     

Phylogenetic analysis of the perissodactylan family Tapiridae using mitochondrial cytochromec oxidase (COII) sequences

The four extant members of the family Tapiridae have a disjunct, relictual distribution, with three species being Neotropical (Tapirus bairdii, T. terrestris, andT. pinchaque) and one found in Southeast Asia (T. indicus). Little recent work on tapir systematics have appeared, and no molecular studies of this group have been published. A phylogenetic analysis was undertaken using sequences of the mitochondrial cytochromec oxidase subunit II gene (COII) from representatives of the four species of tapirs, as well as a representative outgroup,Equus caballus. Analyses of the COII sequences indicate a close relationship between the two South American species of tapirs,T. terrestris andT. pinchaque, and estimates of divergence dates using rates of COII evolution are compatible with migration of a single tapir lineage into South America following the emergence of the isthmus of Panama, about 3 million years bp. Various methods of analysis, including maximum parsimony, maximum likelihood, and neighbor-joining, provided poorer resolution of other tapir relationship. The COII data suggest that three distinct tapir mitochondrial lineages, a South American (represented byT. terrestris andT. pinchaque), a Central American (represented byT. bairdii), and an Asian (represented byT. indicus) diverged relatively rapidly, 20–30 million years bp. Another goal of this study was to calibrate the rate of COII evolution in a eutherian mammal group which has a good fossil record, such as perissodactyls, to estimate accurately the rate of COII evolution in a nonprimate mammalian group. The rate of COII evolution in equids and tapirs has been relatively constant and, using corrected distances, calibrated to be approximately 0.22% lineage/million years. This rate is three-to fourfold lower than that of hominoid primates.     

Molecular evolution of rDNA external transcribed spacer and phylogeny of sect. Petota (genus Solanum)

The 5(') external transcribed spacer (ETS) region of ribosomal DNA of 30 species of Solanum sect. Petota and the European Solanum dulcamara were compared. Two structural elements can be distinguished in the ETS: (i). a variable region (VR), demonstrating significant structural rearrangements and (ii). a conservative region (CR), evolving mainly by base substitutions. In VR, a conservative element (CE) with similarity to the ETS of distantly related Nicotiana is present. The ancestral organization of ETS (variant A) was found for non-tuber-bearing species of ser. Etuberosa, tuber-bearing wild potatoes of Central American ser. Bulbocastana, Pinnatisecta, and Polyadenia and S. dulcamara. Duplication of CE took place in the ETS of species from ser. Commersoniana and Circaeifolia (variant B). South American diploids and Mexican polyploids from superser. Rotata also possess two CE, and additionally two duplications around CE1 are present in VR (variant C). Three major lineages could be distinguished: non-tuber-bearing species of ser. Etuberosa, tuber-bearing Central American diploids and all South American species radiated from a common ancestor at early stages of evolution, indicating a South American origin of the tuber-bearing species. Later, Central and South American diploids evolved further as independent lineages. South American species form a monophyletic group composed of series with both stellata and rotata flower morphology. Solanum commersonii represents a sister taxon for all rotata species, whereas ser. Circaeifolia diverged earlier. Two main groups, C1 and C2, may be distinguished for species possessing ETS variant C. C1 contains ser. Megistacroloba, Conicibaccata, Maglia, and Acaulia, whereas all diploids of ser. Tuberosa are combined into C2. A closer relationship of Solanum chacoense (ser. Yungasensa) to the C2 group was found. The origin of polyploid species Solanum maglia, Solanum acaule, Solanum tuberosum, Solanum iopetalum, and Solanum demissum is discussed.     

Speciation and hybridization among Houstonia (Rubiaceae) species: the influence of polyploidy on reticulate evolution

Chloroplast and nuclear DNA sequence variation among populations and species was used to examine the phylogenetic history and hybridization of the North American Houstonia lineage. The ancestral species in the lineage do not show evidence of hybridization; however, the more recently derived species in eastern North America contain a wide degree of morphological and genetic variation both within and among species. Furthermore, there is a clear association between hybridization and polyploidy in the Houstonia lineage, with all potential hybrids occurring among species that contain polyploid populations. This suggests that polyploidy may break down species barriers and allow hybridization among lineages. These results indirectly support speciation models that involve genetic incompatibilities among species arising from gene silencing or genomic reorganization.