Transalpine colonisation and partial phylogeographic erosion by dispersal in the common vole (Microtus arvalis)

The colonisation history and genetic structure of the common vole ( Microtus arvalis ) was investigated in the region of the Alps by analysing the mitochondrial cytochrome b gene (mtDNA) and 19 microsatellite loci (nucDNA) for 137 voles from 52 localities. mtDNA data provided a much refined distribution of three highly divergent evolutionary lineages in the region compared to previous studies. Although high mountain ranges are widely accepted to be barriers for colonisation processes for many organisms and especially small terrestrial mammals, our phylogeographic analyses showed clear evidence of four transalpine colonisation events by the common vole. Individual-based phylogenetic analyses of nucDNA and two alternative Bayesian-clustering approaches revealed a deep genetic structure analogous to mtDNA. Incongruence between nucDNA and mtDNA at the individual level was restricted to the regions of contact between the lineages. mtDNA patterns and strong female philopatry in M. arvalis suggest that the crossings of the Alps occurred during the colonisation of the region when it was free from ice after the last glaciation. nucDNA patterns suggest that some of the transalpine elements of this phylogeographic pattern were subsequently eroded by male-biased gene flow. We conclude that the combination of phylogeography and landscape genetics at the individual level can provide very detailed insights into colonisation events and may even allow differentiation between historical and more recent processes.     

Intraspecific phylogeography in the sedge frog Litoria fallax (Hylidae) indicates pre-Pleistocene vicariance of an open forest species from eastern Australia

The eastern sedge frog Litoria fallax (Anura: Hylidae) is common throughout the open forests and coastal wetlands along the eastern coast of Australia. Its range spans four biogeographical zones from northern Queensland to central New South Wales. Phylogenetic analysis of mitochondrial DNA (mtDNA) haplotypes of 87 L. fallax individuals from 22 populations identified two major mtDNA lineages, differing by 11-12% sequence divergence. The two clades of haplotypes were separated by the McPherson Range, indicating that this mesic upland area has acted as a major long-term barrier to gene flow for this open forest species. Slight isolation by distance was observed within both the northern and southern lineages but was insufficient to explain the large sequence divergence between lineages. Within the northern lineage, additional phylogeographical structure was observed across the relatively dry Burdekin Gap which separates Atherton populations from all populations in the central and eastern Queensland biogeographical zones. There was less phylogeographical structure in the southern lineage suggesting historical gene flow across the drier portions of the Great Dividing Range. These data, together with recent observations of deep phylogeographical divergences in rainforest-restricted Litoria suggest that the east coast hylids of Australia represent an old (Tertiary) radiation. Individual species of Litoria have been strongly affected by climatic and ecological barriers to gene flow during the Quaternary.     

MITOCHONDRIAL DNA VARIATION IN THE FIELD VOLE (MICROTUS AGRESTIS): REGIONAL POPULATION STRUCTURE AND COLONIZATION HISTORY

Restriction fragment length polymorphism analysis of mitochondrial DNA (mtDNA) was used to examine the genetic structure among field voles (Microtus agrestis) from southern and central Sweden. A total of 57 haplotypes was identified in 158 voles from 60 localities. Overall mtDNA diversity was high, but both haplotype and nucleotide diversity exhibited pronounced geographic heterogeneity. Phylogenetic analyses revealed a shallow tree with seven primary mtDNA lineages separated by sequence divergences ranging from 0.6% to 1.0%. The geographic structure of mtDNA diversity and lineage distribution was complex but strongly structured and deviated significantly from an equilibrium situation. The extensive mtDNA diversity observed and the recent biogeographic history of the region suggests that the shallow mtDNA structure in the field vole cannot be explained solely by stochastic lineage sorting in situ or isolation by distance. Instead, the data suggest that the genetic imprints of historical demographic conditions and vicariant geographic events have been preserved and to a large extent determine the contemporary geographic distribution of mtDNA variation. A plausible historical scenario involves differentiation of mtDNA lineages in local populations in glacial refugia, a moving postglacial population structure, and bottlenecks and expansions of mtDNA lineages during the postglacial recolonization of Sweden. By combining the mtDNA data with an analysis of Y-chromosome variation, a specific population unit was identified in southwestern Sweden. This population, defined by a unique mtDNA lineage and fixation of a Y-chromosome variant, probably originated in a population bottleneck in southern Sweden about 12,000 to 13,000 calendar years ago.     

Biogeographic history of an Australian freshwater shrimp, Paratya australiensis (Atyidae): the role life history transition in phylogeographic diversification

The widespread distribution of the freshwater shrimp Paratya australiensis in eastern Australia suggests that populations of this species have been connected in the past. Amphidromy is ancestral in these shrimps, although many extant populations are known to be restricted to freshwater habitats. In this study, we used a fragment of the cytochrome c oxidase I mitochondrial DNA (mtDNA) gene to examine diversity within P. australiensis and to assess the relative importance of amphidromy in its evolutionary history. We hypothesized that if transitions from an amphidromous to a freshwater life history were important, then we would find a number of divergent lineages restricted to single or groups of nearby drainages. Alternatively, if amphidromy was maintained within the species historically, we expected to find lineages distributed over many drainages. We assumed that the only way for divergence to occur within amphidromous lineages was if dispersal was limited to between nearby estuaries, which, during arid periods in the earth's history, became isolated from one another. We found nine highly divergent mtDNA lineages, estimated to have diverged from one another in the late Miocene/early Pliocene, when the climate was more arid than at present. Despite this, the geographic distribution of lineages and haplotypes within lineages did not support the notion of a stepping-stone model of dispersal between estuaries. We conclude that the extensive divergence has most likely arisen through a number of independent amphidromy-freshwater life history transitions, rather than via historical isolation of amphidromy populations. We also found evidence for extensive movement between coastal and inland drainages, supporting the notion that secondary contact between lineages may have occurred as a result of drainage rearrangements. Finally, our data indicate that P. australiensis is likely a complex of cryptic species, some of which are widely distributed, and others geographically restricted.     

Molecular Phylogeography and Population Genetic Structure of an Endangered Species Pachyhynobius shangchengensis (hynobiid Salamander) in a Fragmented Habitat of Southeastern China

The salamander Pachyhynobius shangchengensis (Hynobiidae) is a vulnerable species restricted to a patchy distribution associated with small mountain streams surrounded by forested slopes in the Mount Dabieshan region in southeastern China. However, molecular phylogeography and population genetic structure of P. shangchengensis remain poorly investigated. In this study, we explored the genetic structure and phylogeography of P. shangchengensis based on partial sequences of the mitochondrial DNA (mtDNA) cytochrome b and cytochrome c oxidase subunit I genes. Fifty-one haplotypes and four clades were found among 93 samples. Phylogenetic analyses revealed four deeply divergent and reciprocally monophyletic mtDNA lineages that approximately correspond to four geographic regions separated by complicated topography and long distances. The distinct geographic distributions of all lineages and the estimated divergence time suggest spatial and temporal separation coinciding with climatic changes during the Pleistocene. Analysis of molecular variance indicated that most of the observed genetic variation occurred among the four groups, implying long-term interruption of gene flow, and the possible separation of P. shangchengensis into four management units for conservation.     

THE ORIGIN OF WEST EUROPEAN SUBSPECIES OF HONEYBEES (APIS MELLIFERA): NEW INSIGHTS FROM MICROSATELLITE AND MITOCHONDRIAL DATA

Apis mellifera is composed of three evolutionary branches including mainly African (branch A), western and northern European (branch M), and southeastern European (branch C) populations. The existence of morphological clines extending from the equator to the Polar Circle through Morocco and Spain raised the hypothesis that the branch M originated in Africa. Mitochondrial DNA analysis revealed that branches A and M were characterized by highly diverged lineages implying very remote links between both branches. It also revealed that mtDNA haplotypes from lineages A coexisted with haplotypes M in the Iberian Peninsula and formed a south-north frequency cline, suggesting that this area could be a secondary contact zone between the two branches. By analyzing 11 populations sampled along a France-Spain/Portugal-Morocco-Guinea transect at 8 microsatellite loci and the DraI RFLP of the COI-COII mtDNA marker, we show that Iberian populations do not present any trace of “africanization” and are very similar to French populations when considering microsatellite markers. Therefore, the Iberian Peninsula is not a transition area. The higher haplotype A variability observed in Spanish and Portuguese samples compared to that found in Africa is explained by a higher mutation rate and multiple and recent introductions. Selection appears to be the best explanation to the morphological and allozymic clines and to the diffusion and maintenance of African haplotypes in Spain and Portugal.     

The utility of AFLPs for supporting mitochondrial DNA phylogeographical analyses in the Taiwanese bamboo viper, Trimeresurus stejnegeri

An amplified fragment length polymorphism (AFLP) assay was performed on individuals representing discrete haplotypes from two genetically distinct mtDNA lineages of the bamboo viper, Trimeresurus stejnegeri (Schmidt), within Taiwan. AFLP (525 polymorphic markers from five primer pairs) and mtDNA genetic distances were highly correlated and an analysis of molecular variance, and a Bayesian approach similarly partitioned estimates of genetic similarity according to the mtDNA phylogeographical pattern. These results are discussed in relation to biogeographical hypotheses, comparative rates of mtDNA molecular evolution, and in the identification of evolutionary significant units of Taiwanese T. stejnegeri. In spite of the high degree of congruence between the genetic datasets, the AFLP phylogenetic analysis did not support the mtDNA tree, suggesting that no contemporary barriers to gene flow exist between individuals from the two mtDNA lineages.     

PERCHED AT THE MITO‐NUCLEAR CROSSROADS: DIVERGENT MITOCHONDRIAL LINEAGES CORRELATE WITH ENVIRONMENT IN THE FACE OF ONGOING NUCLEAR GENE FLOW IN AN AUSTRALIAN BIRD

Relationships among multilocus genetic variation, geography, and environment can reveal how evolutionary processes affect genomes. We examined the evolution of an Australian bird, the eastern yellow robin Eopsaltria australis, using mitochondrial (mtDNA) and nuclear (nDNA) genetic markers, and bioclimatic variables. In southeastern Australia, two divergent mtDNA lineages occur east and west of the Great Dividing Range, perpendicular to latitudinal nDNA structure. We evaluated alternative scenarios to explain this striking discordance in landscape genetic patterning. Stochastic mtDNA lineage sorting can be rejected because the mtDNA lineages are essentially distinct geographically for > 1500 km. Vicariance is unlikely: the Great Dividing Range is neither a current barrier nor was it at the Last Glacial Maximum according to species distribution modeling; nuclear gene flow inferred from coalescent analysis affirms this. Female philopatry contradicts known female‐biased dispersal. Contrasting mtDNA and nDNA demographies indicate their evolutionary histories are decoupled. Distance‐based redundancy analysis, in which environmental temperatures explain mtDNA variance above that explained by geographic position and isolation‐by‐distance, favors a nonneutral explanation for mitochondrial phylogeographic patterning. Thus, observed mito‐nuclear discordance accords with environmental selection on a female‐linked trait, such as mtDNA, mtDNA–nDNA interactions or genes on W‐chromosome, driving mitochondrial divergence in the presence of nuclear gene flow.     

DISCORDANT DISTRIBUTION OF POPULATIONS AND GENETIC VARIATION IN A SEA STAR WITH HIGH DISPERSAL POTENTIAL

Patiria miniata, a broadcast‐spawning sea star species with high dispersal potential, has a geographic range in the intertidal zone of the northeast Pacific Ocean from Alaska to California that is characterized by a large range gap in Washington and Oregon. We analyzed spatial genetic variation across the P. miniata range using multilocus sequence data (mtDNA, nuclear introns) and multilocus genotype data (microsatellites). We found a strong phylogeographic break at Queen Charlotte Sound in British Columbia that was not in the location predicted by the geographical distribution of the populations. However, this population genetic discontinuity does correspond to previously described phylogeographic breaks in other species. Northern populations from Alaska and Haida Gwaii were strongly differentiated from all southern populations from Vancouver Island and California. Populations from Vancouver Island and California were undifferentiated with evidence of high gene flow or very recent separation across the range disjunction between them. The surprising and discordant spatial distribution of populations and alleles suggests that historical vicariance (possibly caused by glaciations) and contemporary dispersal barriers (possibly caused by oceanographic conditions) both shape population genetic structure in this species.     

Composition of native and introduced mtDNA lineages in <Emphasis Type="Italic">Coregonus</Emphasis> sp. in two Austrian lakes: evidence for spatio-temporal segregation of larvae?

We hypothesized that there is spatio-temporal genetic (mtDNA) structure of native and introduced larval whitefish (Coregonus sp.) in two Austrian lakes (Traunsee and Hallstättersee). Larval whitefish were sampled from 12 sites in each lake and screened for variation in the mtDNA NADH-1 gene. Based on the sequencing of adult fish together with existing GenBank sequences, an RFLP protocol was developed to assign haplotypes from larval samples into one of two divergent lineages. All but one site (pelagic) in Traunsee contained both haplotypes, thus there was no support for spatial segregation of mtDNA groups in that lake. However, weekly sampling from December to May in Traunsee revealed a temporal pattern, with the native haplotypes dominating in December and January before the appearance of the introduced Baltic clade. In Hallstättersee, only three of the 12 sites sampled revealed haplotypes from the introduced clade and thus spatial segregation seems operative on that lake. Our results imply that differences in the spawning ecology of the two groups maintain sufficient reproductive isolation to be reflected in distinct larval occurrence in space and time highly consistent with genetic differences on the mtDNA level. If the two lineages were highly introgressed, we would expect to find little or no correspondence between spatio-temporal patterns in larval distribution and the differentiated mtDNA lineages.     

Mitochondrial DNA variability in viviparous and ovoviviparous populations of the urodele Salamandra salamandra

Mitochondrial DNA analysis of 13 populations of S. salamandra along a transect across the North of the Iberian Peninsula showed values of divergence between haplotypes ranging from d = 0.41% to 5.91%. Phenetic and cladistic analysis grouped the isofemale lineages into two main clusters with contrasting phylogeographic patterns. The first group encompasses populations located at each extreme of the Iberian Peninsula. Despite the large geographic distance separating these populations, they exhibit only a minor degree of divergence among haplotypes. In contrast, much higher diversification, in both number of distinct haplotypes, and overall genetic divergence, was observed in the second group of phylogenetically related populations. Surprisingly, this process of radiation and divergence in mtDNA haplotypes occurred in populations in close geographic proximity. All populations sampled in this group are located within a 300 km range, in the central part of our transect across the Northern edge of the Peninsula. Most populations in the central range of our transect exhibit viviparous reproduction — which is derived and highly unusual among urodeles. The genetic distances measured among Asturian (central portion of our transect), viviparous populations are higher than the distances measured between the two main taxonomic clusters. A viviparous population showing an unusual level of mtDNA heterogenetiy is reported and the potential implications of this focus of localized variability are discussed. The dynamics of isofemale lineages among the two reproductive modes was further explored in combination with the previous allozyme data. Several nuclear markers suggest that major mtDNA divergences could be explained by long-term extrinsic barriers to gene flow. Isofemale lineages indicate a narrow secondary contact zone among populations with different reproductive patterns. The existence of viviparous and ovovivparous populations sharing a common haplotype suggests that reproductive transition in S. salamandra could have arisen in absence of genetic mtDNA differentiation. We finally outline a genetic model system where the acquisition of water independence from a primitively aquatic dependent amphibian life cycle can be analyzed from a microevolutionary perspective.     

Lineage Divergence and Historical Gene Flow in the Chinese Horseshoe Bat (Rhinolophus sinicus)

Closely related taxa living in sympatry provide good opportunities to investigate the origin of barriers to gene flow as well as the extent of reproductive isolation. The only two recognized subspecies of the Chinese rufous horseshoe bat Rhinolophus sinicus are characterized by unusual relative distributions in which R. s. septentrionalis is restricted to a small area within the much wider range of its sister taxon R. s. sinicus. To determine the history of lineage divergence and gene flow between these taxa, we applied phylogenetic, demographic and coalescent analyses to multi-locus datasets. MtDNA gene genealogies and microsatellite-based clustering together revealed three divergent lineages of sinicus, corresponding to Central China, East China and the offshore Hainan Island. However, the central lineage of sinicus showed a closer relationship with septentrionalis than with other lineages of R. s. sinicus, in contrary to morphological data. Paraphyly of sinicus could result from either past asymmetric mtDNA introgression between these two taxa, or could suggest septentrionalis evolved in situ from its more widespread sister subspecies. To test between these hypotheses, we applied coalescent-based phylogenetic reconstruction and Approximate Bayesian Computation (ABC). We found that septentrionalis is likely to be the ancestral taxon and therefore a recent origin of this subspecies can be ruled out. On the other hand, we found a clear signature of asymmetric mtDNA gene flow from septentrionalis into central populations of sinicus yet no nuclear gene flow, thus strongly pointing to historical mtDNA introgression. We suggest that the observed deeply divergent lineages within R. sinicus probably evolved in isolation in separate Pleistocene refugia, although their close phylogeographic correspondence with distinct eco-environmental zones suggests that divergent selection might also have promoted broad patterns of population genetic structure.     

The divergence of two independent lineages of an endemic Chinese gecko,Gekko swinhonis,launched by the Qinling orogenic belt

The genetic structure and demographic history of an endemic Chinese gecko, Gekko swinhonis, were investigated by analysing the mitochondrial cytochrome b gene and 10 microsatellite loci for samples collected from 27 localities. Mitochondrial DNA data provided a detailed distribution of two highly divergent evolutionary lineages, between which the average pairwise distance achieved was 0.14. The geographic division of the two lineages coincided with a plate boundary consisting of the Qinling and Taihang Mts, suggesting a historical vicariant pattern. The orogeny of the Qinling Mts, a dispersal and major climatic barrier of the region, may have launched the independent lineage divergence. Both lineages have experienced recent expansion, and the current sympatric localities comprised the region of contact between the lineages. Individual‐based phylogenetic analyses of nucDNA and Bayesian‐clustering approaches revealed a deep genetic structure analogous to mtDNA. Incongruence between nucDNA and mtDNA at the individual level at localities outside of the contact region can be explained by the different inheritance patterns and male‐biased dispersal in this species. High genetic divergence, long‐term isolation and ecological adaptation, as well as the morphological differences, suggest the presence of a cryptic species.     

THE COMBINED EFFECTS OF RIVERS AND REFUGIA GENERATE EXTREME CRYPTIC FRAGMENTATION WITHIN THE COMMON GROUND SKINK (SCINCELLA LATERALIS)

Rivers can act as both islands of mesic refugia for terrestrial organisms during times of aridification and barriers to gene flow, though evidence for long-term isolation by rivers is mixed. Understanding the extent to which riverine barrier effects can be heightened for populations trapped in mesic refugia can help explain maintenance and generation of diversity in the face of Pleistocene climate change. Herein, we implement phylogenetic and population genetic approaches to investigate the phylogeographic structure and history of the ground skink, Scincella lateralis , using mtDNA and eight nuclear loci. We then test several predictions of a river–refugia model of diversification. We recover 14 well-resolved mtDNA lineages distributed east–west along the Gulf Coast with a subset of lineages extending northward. In contrast, ncDNA exhibits limited phylogenetic structure or congruence among loci. However, multilocus population structure is broadly congruent with mtDNA patterns and suggests that deep coalescence rather than differential gene flow is responsible for mtDNA–ncDNA discordance. The observed patterns suggest that most lineages originated from population vicariance due to riverine barriers strengthened during the Plio–Pleistocene by a climate-induced coastal distribution. Diversification due to rivers is likely a special case, contingent upon other environmental or biological factors that reinforce riverine barrier effects.     

Genetic differentiation among North Atlantic killer whale populations

Population genetic structure of North Atlantic killer whale samples was resolved from differences in allele frequencies of 17 microsatellite loci, mtDNA control region haplotype frequencies and for a subset of samples, using complete mitogenome sequences. Three significantly differentiated populations were identified. Differentiation based on microsatellite allele frequencies was greater between the two allopatric populations than between the two pairs of partially sympatric populations. Spatial clustering of individuals within each of these populations overlaps with the distribution of particular prey resources: herring, mackerel and tuna, which each population has been seen predating. Phylogenetic analyses using complete mitogenomes suggested two populations could have resulted from single founding events and subsequent matrilineal expansion. The third population, which was sampled at lower latitudes and lower density, consisted of maternal lineages from three highly divergent clades. Pairwise population differentiation was greater for estimates based on mtDNA control region haplotype frequencies than for estimates based on microsatellite allele frequencies, and there were no mitogenome haplotypes shared among populations. This suggests low or no female migration and that gene flow was primarily male mediated when populations spatially and temporally overlap. These results demonstrate that genetic differentiation can arise through resource specialization in the absence of physical barriers to gene flow.     

Genetic diversity of North American populations of Cristatella mucedo, inferred from microsatellite and mitochondrial DNA

Research over the past 20 years has shown, with the help of molecular markers, that the population genetics and distribution patterns of freshwater invertebrates in North America are often more complex than was previously believed. Here we extend this research to an, as yet, unstudied but widespread and common group, the freshwater bryozoans. Colonies of the bryozoan Cristatella mucedo were collected from a number of lakes across central North America, and were characterized genetically by analysis of microsatellite loci and mitochondrial DNA (mtDNA) cytochrome b sequences. The microsatellites illustrate a pattern of generally diverse and highly differentiated populations that contain little evidence of recent gene flow. The mtDNA sequences yielded highly variable levels of divergence, ranging from 0.0 to 8.8% within populations, and 0.0 to 9.8% among populations. The multiple divergent mtDNA lineages within populations provide evidence for repeated colonization events. The lack of clustering of haplotypes by site suggests that there has been widespread dispersal of multiple genetic lineages since the last ice age. While some of the haplotype lineages may have evolved in disjunct glacial refugia, the maximum levels of divergence predate the time since the last glacial-interglacial cycles. It is likely that multiple factors including vicariance events, patterns of dispersal, localized extinction, and an unusual life history, explain the unique phylogeographic patterns evident today in populations of C. mucedo.     

Mitochondrial DNA phylogeography of the three-spined stickleback (Gasterosteus aculeatus) in Europe-evidence for multiple glacial refugia

An analysis of mitochondrial DNA sequence variation in 172 three-spined sticklebacks (Gasterosteus aculeatus) sampled across the European distribution range revealed three major evolutionary lineages occupying relatively large and separate geographic areas. The trans-Atlantic lineage comprised of populations spanning from the East Coast of USA to the continental Europe and was basal group to the other European lineages in the phylogeny. The European lineage included populations located in the Western and Eastern Europe, British Isles, Scandinavia as well as some parts of the Mediterranean region. The third lineage was specific to the Black Sea drainages. The within lineage structure was characterized by significant excess of low frequency haplotypes and star-like mtDNA genealogies, which suggest a recent population expansions to the formerly glaciated marine and freshwater environments. A coalescent-based method dated the splits between the major lineages to have occurred during the Saalian and Weichselian glaciations in the late Pleistocene, depending on the molecular clock calibration. The coalescent simulations further indicate high degree of genetic diversity within the lineages and a substantial increase in the genetic diversity in the European lineage relative to the ancestral level. In addition to the three major lineages, the freshwater populations in R. Neretva and L. Skadar in the Adriatic Sea coast region harboured unique and highly divergent haplotypes suggesting long independent histories of these populations. Evidence from mtDNA analyses suggests that these populations deserve a status of an evolutionary significant unit.     

Current geography masks dynamic history of gene flow during speciation in northern Australian birds

Genome divergence is greatly influenced by gene flow during early stages of speciation. As populations differentiate, geographic barriers can constrain gene flow and so affect the dynamics of divergence and speciation. Current geography, specifically disjunction and continuity of ranges, is often used to predict the historical gene flow during the divergence process. We test this prediction in eight meliphagoid bird species complexes codistributed in four regions. These regions are separated by known biogeographical barriers across northern Australia and Papua New Guinea. We find that bird populations currently separated by terrestrial habitat barriers within Australia and marine barriers between Australia and Papua New Guinea have a range of divergence levels and probability of gene flow not associated with current range connectivity. Instead, geographic distance and historical range connectivity better predict divergence and probability of gene flow. In this dynamic environmental context, we also find support for a nonlinear decrease of the probability of gene flow during the divergence process. The probability of gene flow initially decreases gradually after a certain level of divergence is reached. Its decrease then accelerates until the probability is close to zero. This implies that although geographic connectivity may have more of an effect early in speciation, other factors associated with higher divergence may play a more important role in influencing gene flow midway through and later in speciation. Current geographic connectivity may then mislead inferences regarding potential for gene flow during speciation under a complex and dynamic history of geographic and reproductive isolation.     

Genetic variation in natural populations of the aphid Rhopalosiphum padi as revealed by maternally inherited markers

A survey on 148 clones of the aphid Rhopalosiphum padi from 11 widespread localities has been carried out to study the genetic structure of populations of this species as revealed by mitochondrial DNA restriction site and length polymorphisms as well as by restriction site analysis of a maternally inherited plasmid carried by the aphid eubacterial endosymbiont Buchnera aphidicola. Our results support the existence in the area under study of two main aphid maternal lineages strikingly coincidental with the two main reproductive categories displayed by this species. Those aphid clones possessing an incomplete life cycle that lacks the sexual phase (anholocyclic or androcyclic clones) show mitochondrial DNA (mtDNA) haplotype I and plasmid haplotype I, whereas those clones displaying the complete life cycle (holocyclic clones) posses some other distinct mtDNA haplotypes closely related to each other and plasmid haplotype II. While restriction-site analysis of maternally inherited markers points to a relatively ancient origin of anholocycly/androcycly (between 460 000 and 1 400 000 years) followed by interrupted gene flow with respect to the ancestral holocyclic population, mtDNA size variation also suggests that historical stochastic processes have a different effect on the evolution of both main aphid lineages. Evidence of occasional nuclear gene flow between lineages and its consequences on the correspondence between maternally inherited haplotypes and life cycle are also presented and discussed.     

GENE GENEALOGY AND DIFFERENTIATION AMONG ARBOREAL SPINY RATS (RODENTIA: ECHIMYIDAE) OF THE AMAZON BASIN: A TEST OF THE RIVERINE BARRIER HYPOTHESIS

Sequence variation in the mitochondrial cytochrome b gene was examined in the arboreal spiny rat, Mesomys hispidus, collected at 15 sites along the Rio Juruá in western Amazonia, Brazil, to determine the importance of riverine barriers in the diversification of this taxon. Twenty individual haplotypes were uncovered, most of which were unique to single localities but some of which were shared among adjacent sites either along or across the river. Genealogical analyses suggest that gene flow is limited and, in combination with the unique distribution of most haplotypes, suggest that populations of this species are strongly substructured along the river. Thus, most sharing of haplotypes between adjacent localities is probably caused by historical association rather than to ongoing gene flow. Two haplotype clades were uncovered, but these correspond to headwaters versus mouth areas, not to opposite sides of the river, as would be expected by the Riverine Barrier Hypothesis. Moreover, haplotype sharing across the river was greater at its mouth than in the headwaters, a pattern opposite that expected if the river were a substantive barrier. Broader scale phylogeographic patterns of this species show that both clades have relationships to areas well outside the Rio Juruá basin. This suggests that the basin represents a relatively recent point of invasion between two more broadly distributed and differentiated geographic units of the species.