Tethyan changes shaped aquatic diversification

The Tethys Ocean existed between the continents of Gondwana and Laurasia from the Triassic to the Pliocene. Analyses of multiple biogeographic and phylogenetic histories reveal that the subsequent breakup of the Tethys greatly influenced the distributions of many species. The ancestral Tethyan realm broke into five biogeographic provinces, including the present‐day East Pacific, West Atlantic, East Atlantic, Mediterranean Sea, and Indo‐West Pacific. Palaeogeographic maps illustrate the Mesozoic Atlantic opening, the Cenozoic closure of the Tethys, the Messinian Salinity Crisis, the mid‐Miocene closure of the Central American Seaway, and Quaternary geological changes. Further, we consider Cenozoic sea‐level changes and the formation of freshwater habitats. These reconstructions allow assessment of patterns of aquatic diversification for marine and freshwater animals, and comparison of vicariance and dispersal processes. Estimated divergence times indicate that fragmentation of the Tethys was responsible for the vicariant speciation of aquatic animals because these dates are consistent with associated tectonic events. The opening of the Atlantic Ocean during the Cretaceous is responsible for the earliest isolation between the West and East Atlantic. The mid‐Miocene closure of the Tethys, which blocked global equatorial currents, appears to have isolated the Atlantic/Mediterranean Sea and Indo‐West Pacific. Finally, formation of the Isthmus of Panama isolated East Pacific and West Atlantic marine organisms. Dispersals related to the Messinian Salinity Crisis and Quaternary sea‐level changes influenced population structuring. Tethyan changes affected marine habitats, created new freshwater habitats, inland caves and ancient lakes along the Alps and Himalayas, and influenced anchialine caves at the edge of the ancient sea. The extensive new habitats provided opportunities for colonisation and rapid diversification. Future work should focus on testing the biological impact of the series of Tethyan changes.     

Origins of the Greenland shark (Somniosus microcephalus): Impacts of ice‐olation and introgression

Herein, we use genetic data from 277 sleeper sharks to perform coalescent‐based modeling to test the hypothesis of early Quaternary emergence of the Greenland shark (Somniosus microcephalus) from ancestral sleeper sharks in the Canadian Arctic‐Subarctic region. Our results show that morphologically cryptic somniosids S. microcephalus and Somniosus pacificus can be genetically distinguished using combined mitochondrial and nuclear DNA markers. Our data confirm the presence of genetically admixed individuals in the Canadian Arctic and sub‐Arctic, and temperate Eastern Atlantic regions, suggesting introgressive hybridization upon secondary contact following the initial species divergence. Conservative substitution rates fitted to an Isolation with Migration (IM) model indicate a likely species divergence time of 2.34 Ma, using the mitochondrial sequence DNA, which in conjunction with the geographic distribution of admixtures and Pacific signatures likely indicates speciation associated with processes other than the closing of the Isthmus of Panama. This time span coincides with further planetary cooling in the early Quaternary period followed by the onset of oscillating glacial‐interglacial cycles. We propose that the initial S. microcephalus–S. pacificus split, and subsequent hybridization events, were likely associated with the onset of Pleistocene glacial oscillations, whereby fluctuating sea levels constrained connectivity among Arctic oceanic basins, Arctic marginal seas, and the North Atlantic Ocean. Our data demonstrates support for the evolutionary consequences of oscillatory vicariance via transient oceanic isolation with subsequent secondary contact associated with fluctuating sea levels throughout the Quaternary period—which may serve as a model for the origins of Arctic marine fauna on a broad taxonomic scale.     

PHYLOGEOGRAPHY OF THE TRUMPETFISHES (AULOSTOMUS): RING SPECIES COMPLEX ON A GLOBAL SCALE

Abstract.— The distribution of circumtropical marine species is limited by continental boundaries, cold temperate conditions, and oceanic expanses, but some of these barriers are permeable over evolutionary time scales. Sister taxa that evolved in separate ocean basins can come back into contact, and the consequences of this renewed sympatry may be a key to understanding evolutionary processes in marine organisms. The circumtropical trumpetfishes (Aulostomus) include a West Atlantic species (A. maculatus), an Indian‐Pacific species (A. chinensis), and an East Atlantic species (A. strigosus) that may be the product of a recent invasion from the Indian Ocean. To resolve patterns of divergence and speciation, we surveyed 480 bp of mitochondrial DNA cytochrome b in 196 individuals from 16 locations. Based on a conventional molecular clock of 2% sequence divergence per million years, the deepest partitions in a neighbor‐joining tree (d= 0.063‐0.082) are consistent with separation of West Atlantic and Indian‐Pacific species by the Isthmus of Panama, 3–4 million years ago. By the same criteria, trumpetfish in the East Atlantic were isolated from the Indian Ocean about 2.5 million years ago (d= 0.044‐0.054), coincident with the advent of glacial cycles and cold‐water upwelling around South Africa. Continental barriers between tropical oceans have only rarely been surmounted by trumpetfishes, but oceanic barriers do not appear to be substantial, as indicated by weak population partitioning (ø_ST= 0.093) in A. chinensis across the Indian and Pacific Oceans. Finally, morphological and mitochondrial DNA data indicate hybridization of A. strigosus and A. maculatus in Brazil. After 3–4 million years and a globe‐spanning series of vicariant and dispersal events, trumpetfish lineages have come back into contact in the southwest Atlantic and appear to be merging. This ring species phenomenon may occur in a broad array of marine organisms, with clear implications for the production and maintenance of biodiversity in marine ecosystems.     

Phylogeny and historical biogeography of the cave-adapted shrimp genus Typhlatya (Atyidae) in the Caribbean Sea and western Atlantic

Aim To infer phylogenetic relationships among five species of the cave‐adapted shrimp genus Typhlatya in order to test competing hypotheses of dispersal and colonization of the disjunct cave localities occupied by these five species. Location Typhlatya species are found in caves and anchialine ponds across the northern margin of the Caribbean Sea, along the Mediterranean and Adriatic coasts and on oceanic islands in the Atlantic and eastern Pacific oceans. This study focuses on five species, one from Bermuda, one from the Caicos Islands and three from the Yucatan Peninsula of Mexico. Methods Partial sequences (c. 1400 bp) from the mitochondrial cytochrome b, 16S rDNA and COI genes were obtained from representative samples of the five species. Phylogenetic inference was carried out with maximum parsimony and maximum likelihood analyses. Parsimony networks were constructed for the Bermudian species Typhlatya iliffei and one Yucatan species Typhlatya mitchelli, to determine the degree of connectivity among populations inhabiting different cave systems. Results All three land masses were recovered as monophyletic. The two insular marine species from Bermuda and the Caicos Islands formed a clade, while the three continental freshwater species from the Yucatan Peninsula formed another. Within both Bermuda and the Yucatan, shared haplotypes were found in different cave systems, suggesting recent or ongoing gene flow among populations in both locales. Main conclusions The two insular marine Typhlatya species originated from an ancestral marine population, possibly already cave‐adapted, that is suggested to have colonized the Caicos Islands and subsequently dispersed to Bermuda via the Gulf Stream. Divergence estimates suggest that colonization occurred before the formation of present‐day anchialine cave habitat, which did not form on either island until the late Pliocene to early Pleistocene. Divergence estimates also indicate that the Yucatan freshwater species split before the formation of freshwater cave habitat in the Yucatan. These species could have inhabited crevicular marine habitats before the late Pliocene/early Pleistocene in the Yucatan or elsewhere in the Caribbean, and subsequently migrated to freshwater caves once they formed.     

Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae

Marine–freshwater and freshwater–marine transitions have been key events in the evolution of life, and most major groups of organisms have independently undergone such events at least once in their history. Here, we first compile an inventory of bidirectional freshwater and marine transitions in multicellular photosynthetic eukaryotes. While green and red algae have mastered multiple transitions in both directions, brown algae have colonized freshwater on a maximum of six known occasions, and angiosperms have made the transition to marine environments only two or three times. Next, we review the early evolutionary events leading to the colonization of current habitats. It is commonly assumed that the conquest of land proceeded in a sequence from marine to freshwater habitats. However, recent evidence suggests that early photosynthetic eukaryotes may have arisen in subaerial or freshwater environments and only later colonized marine environments as hypersaline oceans were diluted to the contemporary level. Although this hypothesis remains speculative, it is important to keep these alternative scenarios in mind when interpreting the current habitat distribution of plants and algae. Finally, we discuss the roles of structural and functional adaptations of the cell wall, reactive oxygen species scavengers, osmoregulation, and reproduction. These are central for acclimatization to freshwater or to marine environments. We observe that successful transitions appear to have occurred more frequently in morphologically simple forms and conclude that, in addition to physiological studies of euryhaline species, comparative studies of closely related species fully adapted to one or the other environment are necessary to better understand the adaptive processes.     

Contrasting demographic history and gene flow patterns of two mangrove species on either side of the Central American Isthmus

Comparative phylogeography offers a unique opportunity to understand the interplay between past environmental events and life‐history traits on diversification of unrelated but co‐distributed species. Here, we examined the effects of the quaternary climate fluctuations and palaeomarine currents and present‐day marine currents on the extant patterns of genetic diversity in the two most conspicuous mangrove species of the Neotropics. The black (Avicennia germinans, Avicenniaceae) and the red (Rhizophora mangle, Rhizophoraceae) mangroves have similar geographic ranges but are very distantly related and show striking differences on their life‐history traits. We sampled 18 Atlantic and 26 Pacific locations for A. germinans (N = 292) and R. mangle (N = 422). We performed coalescence simulations using microsatellite diversity to test for evidence of population change associated with quaternary climate fluctuations. In addition, we examined whether patterns of genetic variation were consistent with the directions of major marine (historical and present day) currents in the region. Our demographic analysis was grounded within a phylogeographic framework provided by the sequence analysis of two chloroplasts and one flanking microsatellite region in a subsample of individuals. The two mangrove species shared similar biogeographic histories including: (1) strong genetic breaks between Atlantic and Pacific ocean basins associated with the final closure of the Central American Isthmus (CAI), (2) evidence for simultaneous population declines between the mid‐Pleistocene and early Holocene, (3) asymmetric historical migration with higher gene flow from the Atlantic to the Pacific oceans following the direction of the palaeomarine current, and (4) contemporary gene flow between West Africa and South America following the major Atlantic Ocean currents. Despite the remarkable differences in life‐history traits of mangrove species, which should have had a strong influence on seed dispersal capability and, thus, population connectivity, we found that vicariant events, climate fluctuations and marine currents have shaped the distribution of genetic diversity in strikingly similar ways.     

DO FRESHWATER FISHES DIVERSIFY FASTER THAN MARINE FISHES? A TEST USING STATE‐DEPENDENT DIVERSIFICATION ANALYSES AND MOLECULAR PHYLOGENETICS OF NEW WORLD SILVERSIDES (ATHERINOPSIDAE)

Freshwater habitats make up only ～0.01% of available aquatic habitat and yet harbor 40% of all fish species, whereas marine habitats comprise >99% of available aquatic habitat and have only 60% of fish species. One possible explanation for this pattern is that diversification rates are higher in freshwater habitats than in marine habitats. We investigated diversification in marine and freshwater lineages in the New World silverside fish clade Menidiinae (Teleostei, Atherinopsidae). Using a time‐calibrated phylogeny and a state‐dependent speciation–extinction framework, we determined the frequency and timing of habitat transitions in Menidiinae and tested for differences in diversification parameters between marine and freshwater lineages. We found that Menidiinae is an ancestrally marine lineage that independently colonized freshwater habitats four times followed by three reversals to the marine environment. Our state‐dependent diversification analyses showed that freshwater lineages have higher speciation and extinction rates than marine lineages. Net diversification rates were higher (but not significant) in freshwater than marine environments. The marine lineage‐through time (LTT) plot shows constant accumulation, suggesting that ecological limits to clade growth have not slowed diversification in marine lineages. Freshwater lineages exhibited an upturn near the recent in their LTT plot, which is consistent with our estimates of high background extinction rates. All sequence data are currently being archived on Genbank and phylogenetic trees archived on Treebase.     

Species Diversity,Phylogeny and Large Scale Biogeographic Patterns of the Genus Padina (Phaeophyceae,Dictyotales)

The brown algal genus Padina (Dictyotales, Phaeophyceae) is distributed worldwide in tropical and temperate seas. Global species diversity and distribution ranges, however, remain largely unknown. Species‐level diversity was reassessed using DNA‐based, algorithmic species delineation techniques based on cox3 and rbcL sequence data from 221 specimens collected worldwide. This resulted in estimates ranging from 39 to 61 putative species (ESUs), depending on the technique as well as the locus. We discuss the merits, potential pitfalls, and evolutionary and biogeographic significance of algorithmic species delineation. We unveil patterns whereby ESUs are in all but one case restricted to either the Atlantic or Indo‐Pacific Ocean. Within ocean basins we find evidence for the vast majority of ESUs to be confined to a single marine realm. Exceptions, whereby ESUs span up to three realms, are located in the Indo‐Pacific Ocean. Patterns of range‐restricted species likely arise by repeated founder events and subsequent peripatric speciation, hypothesized to dominate speciation mechanisms for coastal marine organisms in the Indo‐Pacific. Using a three‐gene (cox3, psaA and rbcL), relaxed molecular clock phylogenetic analysis we estimated divergence times, providing a historical framework to interpret biogeographic patterns.     

Tracking the history of an invasion: the freshwater croakers (Teleostei: Sciaenidae) in South America

In this study, the competing hypotheses of single vs. double colonisation events for freshwater Pachyurinae (Sciaenidae) in South America is tested and the historical biogeography of the expansion of this clade within the continent is reconstructed based on phylogenetic analysis. Parsimony and Bayesian inference (BI) for 19 marine and freshwater species assigned to Sciaenidae, Haemulidae and Polypteridae were determined based on partial sequences of the mitochondrial 16S and cytochrome b genes and fragments of the nuclear Tmo‐4C4 and rhodopsin genes. A parsimonious ancestral character reconstruction of euryhalinity was performed on a clade of families of closely related fishes to evaluate the role of ecological fitting in the colonisation of freshwater by a marine sciaenid. The parsimony and BI phylogenetic hypotheses for the concatenated sequences supported the monophyly of the freshwater Sciaenidae. Divergence of the two freshwater clades of Sciaenidae, Pachyurinae and Plagioscion, occurred within the Amazon Basin. Within Pachyurinae, two clades were recovered: one composed of species from the Amazon and the Paraná Basin and a second with representatives from the São Francisco and south‐eastern Atlantic basins. The results were compatible with the hypothesis of a single colonisation event of South American freshwater habitats by a marine lineage. The hypothesis of gradual adaptation to freshwater was rejected in favour of the hypothesis of ecological fitting. Sciaenidae, or a subordinate lineage within the family, is ancestrally capable of withstanding exposure to low‐salinity habitats, which putatively facilitated the colonisation of freshwater habitats. The subsequent diversification and expansion of Pachyurinae across South America followed this colonisation and replicated the general pattern of the area relationships of South American river basins for several other fish groups.     

Patterns of genetic diversity of the cryptogenic red alga Polysiphonia morrowii (Ceramiales,Rhodophyta) suggest multiple origins of the Atlantic populations

The red alga Polysiphonia morrowii, native to the North Pacific (Northeast Asia), has recently been reported worldwide. To determine the origin of the French and Argentine populations of this introduced species, we compared samples from these two areas with samples collected in Korea and at Hakodate, Japan, the type locality of the species. Combined analyses of chloroplastic (rbcL) and mitochondrial (cox1) DNA revealed that the French and Argentine populations are closely related and differ substantially from the Korean and Japanese populations. The genetic structure of P. morrowii populations from South Atlantic and North Atlantic, which showed high haplotype diversity compared with populations from the North Pacific, suggested the occurrence of multiple introduction events from areas outside of the so‐called native regions. Although similar, the French and Argentine populations are not genetically identical. Thus, the genetic structure of these two introduced areas may have been modified by cryptic and recurrent introduction events directly from Asia or from other introduced areas that act as introduction relays. In addition, the large number of private cytoplasmic types identified in the two introduced regions strongly suggests that local populations of P. morrowii existed before the recent detection of these invasions. Our results suggest that the most likely scenario is that the source population(s) of the French and Argentine populations was not located only in the North Pacific and/or that P. morrowii is a cryptogenic species.     

Phylogenetic relationships of tropical western Atlantic snappers in subfamily Lutjaninae (Lutjanidae: Perciformes) inferred from mitochondrial DNA sequences

Phylogenetic relationships among 20 nominal species of tropical lutjanine snappers (Lutjanidae) (12 from the western Atlantic, one from the eastern Pacific, and seven from the Indo‐Pacific) were inferred based on 2206 bp (712 variable, 614 parsimony informative) from three protein‐coding mitochondrial genes. Also included in the analysis were DNA sequences from two individuals, identified initially as Lutjanus apodus, which were sampled off the coast of Bahia State in Brazil (western Atlantic), and from three individuals labelled as ‘red snapper’ in the fish market in Puerto Armuelles, Panama (eastern Pacific). Bayesian posterior probabilities and maximum‐likelihood bootstrap percentages strongly supported monophyly of all lutjanines sampled and the hypothesis that western Atlantic lutjanines are derived from an Indo‐Pacific lutjanine lineage. The phylogenetic hypothesis also indicated that oceans where lutjanines are distributed (western Atlantic, eastern Pacific, and Indo‐Pacific) are not reciprocally monophyletic for the species distributed within them. There were three strongly supported clades that included all western Atlantic lutjanines: one included six species of Lutjanus from the western Atlantic, two species of Lutjanus from the eastern Pacific, and the monotypic genera Rhomboplites and Ocyurus (western Atlantic); one that included three, probably four, species of Lutjanus in the western Atlantic; and one that included Lutjanus cyanopterus (western Atlantic), an unknown species of Lutjanus from the eastern Pacific, and three species of Lutjanus from the Indo‐Pacific. Molecular‐clock calibrations supported an early Miocene diversification of an Indo‐Pacific lutjanine lineage that dispersed into the western Atlantic via the Panamanian Gateway. Divergent evolution among these lutjanines appears to have occurred both by vicariant and ecological speciation: the former following significant geographic or geological events, including both shoaling and closure of the Panamanian Gateway and tectonic upheavals, whereas the latter occurred via phenotypic diversification inferred to indicate adaptation to life in different habitats. Taxonomic revision of western Atlantic lutjanines appears warranted in that monotypic Ocyurus and Rhomboplites should be subsumed within the genus Lutjanus. Finally, it appears that retail mislabelling of ‘red snapper’ in commercial markets extends beyond the USA. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 915–929.     

The relative importance of geography and ecology in species diversification: evidence from a tropical marine intertidal snail (Nerita)

Aim In this study, I examined the relative contributions of geography and ecology to species diversification within the genus Nerita, a prominent clade of marine snails that is widely distributed across the tropics and intertidal habitats. Specifically, I tested whether geographical patterns of speciation correspond primarily to allopatric or sympatric models, and whether habitat transitions have played a major role in species diversification. Location Indo‐West Pacific, eastern Pacific, Atlantic, tropical marine intertidal. Methods I used a previously reconstructed molecular phylogeny of Nerita as a framework to assess the relative importance of geographical and ecological factors in species diversification. To evaluate whether recently diverged clades exhibit patterns consistent with allopatric or sympatric speciation, I mapped the geo‐graphical distribution of each species onto the species‐level phylogeny, and examined the relationship between range overlap and time since divergence using age–range correlation analyses. To determine the relative contribution of habitat transitions to divergence, I traced shifts in intertidal substrate affinity and vertical zonation across the phylogeny using parsimony, and implemented randomization tests to evaluate the resulting patterns of ecological change. Results Within the majority of Nerita clades examined, age–range correlation analysis yielded a low intercept and a positive slope, similar to that expected under allopatric speciation. Approximately 75% of sister species pairs have maintained allopatric distributions; whereas more distantly related sister taxa often exhibited complete or nearly complete geographical overlap. In contrast, only 19% of sister species occupy distinct habitats. For both substrate and zonation, habitat transitions failed to concentrate towards either the tips or the root of the phylogeny. Instead, habitat shifts have occurred throughout the history of Nerita, with a general transition from the lower and mid‐littoral towards the upper and supra‐littoral zones, and multiple independent shifts from hard (rock) to softer substrates (mangrove, mud and sand). Main conclusions Both geography and ecology appear to have influenced diversification in Nerita, but to different extents. Geography seems to play a principal role, with allopatric speciation driving the majority of Nerita divergences. Habitat transitions appear insignificant in shaping the early and recent history of speciation, and promoting successive diversification in Nerita; however, shifts may have been important for respective divergences (i.e. those that correspond to the transitions) and enhancing diversity throughout the clade.     

Distinct patterns of hybridization across a suture zone in a coral reef fish (Dascyllus trimaculatus)

Hybrid zones are natural laboratories for investigating the dynamics of gene flow, reproductive isolation, and speciation. A predominant marine hybrid (or suture) zone encompasses Christmas Island (CHR) and Cocos (Keeling) Islands (CKE), where 15 different instances of interbreeding between closely related species from Indian and Pacific Oceans have been documented. Here, we report a case of hybridization between genetically differentiated Pacific and Indian Ocean lineages of the three‐spot dascyllus, Dascyllus trimaculatus (Rüppell, 1829). Field observations indicate there are subtle color differences between Pacific and Indian Ocean lineages. Most importantly, population densities of color morphs and genetic analyses (mitochondrial DNA and SNPs obtained via RADSeq) suggest that the pattern of hybridization within the suture zone is not homogeneous. At CHR, both color morphs were present, mitochondrial haplotypes of both lineages were observed, and SNP analyses revealed both pure and hybrid genotypes. Meanwhile, in CKE, the Indian Ocean color morphs were prevalent, only Indian Ocean mitochondrial haplotypes were observed, and SNP analysis showed hybrid individuals with a large proportion (~80%) of their genotypes assigning to the Indian Ocean lineage. We conclude that CHR populations are currently receiving an influx of individuals from both ocean basins, with a greater influence from the Pacific Ocean. In contrast, geographically isolated CKE populations appear to be self‐recruiting and with more influx of individuals from the Indian Ocean. Our research highlights how patterns of hybridization can be different at scales of hundreds of kilometers, due to geographic isolation and the history of interbreeding between lineages.     

A molecular evaluation of the Liagoraceae sensu lato (Nemaliales,Rhodophyta) in Bermuda including Liagora nesophila sp. nov. and Yamadaella grassyi sp. nov.

We have undertaken a comprehensive, molecular‐assisted alpha‐taxonomic examination of the rhodophyte family Liagoraceae sensu lato, a group that has not previously been targeted for molecular studies in the western Atlantic. Sequence data from three molecular markers indicate that in Bermuda alone there are 10 species in nine different genera. These include the addition of three genera to the flora — Hommersandiophycus, Trichogloeopsis, and Yamadaella. Liagora pectinata, a species with a type locality in Bermuda, is phylogenetically allied with Indo‐Pacific species of Hommersandiophycus, and the species historically reported as L. ceranoides for the islands is morphologically and genetically distinct from that taxon, and is herein described as L. nesophila sp. nov. Molecular sequence data have also uncovered the Indo‐Pacific L. mannarensis in Bermuda, a long‐distance new western Atlantic record. DNA sequences of Trichogloeopsis pedicellata from the type locality (Bahamas) match with local specimens demonstrating its presence in Bermuda. We described Yamadaella grassyi sp. nov. from Bermuda, a species phylogenetically and morphologically distinct from the generitype, Y. caenomyce of the Indo‐Pacific. Our data also indicated a single species each of Ganonema, Gloiocallis, Helminthocladia, Titanophycus, and Trichogloea in the flora.     

Hybridization at an ecotone: ecological and genetic barriers between three Iberian vipers

The formation of stable genetic boundaries between emerging species is often diagnosed by reduced hybrid fitness relative to parental taxa. This reduced fitness can arise from endogenous and/or exogenous barriers to gene flow. Although detecting exogenous barriers in nature is difficult, we can estimate the role of ecological divergence in driving species boundaries by integrating molecular and ecological niche modelling tools. Here, we focus on a three‐way secondary contact zone between three viper species (Vipera aspis, V. latastei and V. seoanei) to test for the contribution of ecological divergence to the development of reproductive barriers at several species traits (morphology, nuclear DNA and mitochondrial DNA). Both the nuclear and mitochondrial data show that all taxa are genetically distinct and that the sister species V. aspis and V. latastei hybridize frequently and backcross over several generations. We find that the three taxa have diverged ecologically and meet at a hybrid zone coincident with a steep ecotone between the Atlantic and Mediterranean biogeographical provinces. Integrating landscape and genetic approaches, we show that hybridization is spatially restricted to habitats that are suboptimal for parental taxa. Together, these results suggest that niche separation and adaptation to an ecological gradient confer an important barrier to gene flow among taxa that have not achieved complete reproductive isolation.     

Does your lip stick? Evolutionary aspects of the mouth morphology of the Indo‐Pacific clinging goby of the Sicyopterus genus (Teleostei: Gobioidei: Sicydiinae) based on mitogenome phylogeny

Sicydiinae gobies have an amphidromous life cycle. Adults grow, feed, and reproduce in rivers, while larvae have a marine dispersal phase. Larvae recruit back to rivers and settle in upstream habitats. Within the Sicydiinae subfamily, the Sicyopterus genus, one of the most diverse (24 species), is distributed in the tropical islands of the Indo‐Pacific. One of the characters used to determine Sicyopterus species is the upper lip morphology, which can be either smooth, crenulated, or with papillae, and with (2 or 3) or without clefts. The mouth is used as a secondary locomotor organ along with the pelvic sucker. It is thus strongly related to the climbing ability of species and is of major importance for the upstream migration and the colonization of insular freshwater systems. The mouth also has an important role in the feeding mechanism of these herbivorous species. In this paper, we have established a molecular phylogeny of the genus based on the 13 mitochondrial protein‐coding genes to discuss the relationship between 18 Sicyopterus species. There is a well‐supported dichotomy in the molecular phylogeny of the Sicyopterus genus and this separation into two clades is also morphologically visible, with the distinction of species with three clefts and species with 0 or 2 clefts on the upper lip. The mouth morphology can thus be separated with regard to the molecular phylogeny obtained. The evolution of the mouth morphology is discussed in terms of the adaptation of the Sicyopterus genus to settlement and life in tropical insular river systems.     

A molecular analysis of relationships and biogeography within a species complex of Holarctic fish (genus Osmerus)

Episodes of trans-Arctic faunal exchange and isolation between the north Pacific and Atlantic ocean basins have been implicated as important historic geological events contributing to extant patterns of genetic diversity and structure in Holarctic faunas. We made a further test of the significance of such biogeographic events by examining mitochondrial DNA (mtDNA) restriction fragment length and cytochrome b sequence polymorphism among north Pacific and Arctic, north-western Atlantic (north-eastern North American), and north-eastern Atlantic (European) regional forms of the boreal smelt, genus Osmerus. Our analyses also assessed whether the regional forms within this ‘species complex’: (i) represent a single widely distributed and polytypic species, or is composed of three geographically distinct species, and (ii) resulted from a single split from north Pacific ancestral Osmerus or two independent Pacific-Atlantic divergences. MtDNA sequence divergence estimates among forms ranged from 5.6–8.9% and from 6.1–8.5% based on restriction fragment and 300 base pairs of cytochrome b sequencing, respectively. Divergence within forms averaged less than 0.5% for fragment analysis and no differences were detected from sequence analysis. Provisional dating of lineage separations in Osmerus based on our sequence divergence estimates suggested a mid-Pliocene to early Pleistocene time frame for diversification among the forms. These estimated lineage separation dates support the idea that geological events in ‘Beringia’ and the surrounding trans-Arctic area (e.g. opening of the Bering Seaway, Pleistocene glacial advances), occurring over a similar time frame, have influenced radiation in Osmerus. Phenetic and parsimony analyses of the sequence divergence estimates and of sequence polymorphisms suggested that the north Pacific/Arctic form and the northwestern Atlantic form shared a common ancestor more recently than either has with the north-eastern Atlantic form, thus supporting the hypothesis that the species complex has arisen from two independent Pacific-Atlantic divergences probably beginning during the mid-Pliocene.     

Tethyan vicariance, relictualism and speciation: evidence from a global molecular phylogeny of the opisthobranch genus Bulla

Aim Our aims were: (1) to reconstruct a molecular phylogeny of the cephalaspidean opisthobranch genus Bulla, an inhabitant of shallow sedimentary environments; (2) to test if divergence times are consistent with Miocene and later vicariance among the four tropical marine biogeographical provinces; (3) to examine the phylogenetic status of possible Tethyan relict species; and (4) to infer the timing and causes of speciation events. Location Tropical and warm‐temperate regions of the Atlantic, Indo‐West Pacific, Australasia and eastern Pacific. Methods Ten of the 12 nominal species of Bulla were sampled, in a total sample of 65 individuals, together with cephalaspidean outgroups. Phylogenetic relationships were inferred by Bayesian analysis of partial sequences of the mitochondrial cytochrome c oxidase I (COI) and 16S rRNA and nuclear 28S rRNA genes. Divergence times and rates of evolution were estimated using uncorrelated relaxed‐clock Bayesian methods with fossil calibrations (based on literature review and examination of fossil specimens), implemented in beast . The geographical pattern of speciation was assessed by estimating the degree of overlap between sister lineages. Results Four clades were supported: Indo‐West Pacific (four species), Australasia (one species), Atlantic plus eastern Pacific (three species) and Atlantic (two species), with estimated mean ages of 35–46 Ma. Nominal species were monophyletic, but deep divergences were found within one Indo‐West Pacific and one West Atlantic species. Species‐level divergences occurred in the Miocene or earlier. The age of a sister relationship across the Isthmus of Panama was estimated at 7.9–32.1 Ma, and the divergence of a pair of sister species on either side of the Atlantic Ocean occurred 20.4–27.2 Ma. Main conclusions Fossils suggest that Bulla originated in the Tethys realm during the Middle Eocene. Average ages of the four main clades fall in the Eocene, and far pre‐date the 18–19 Ma closure of the Tethys Seaway. This discrepancy could indicate earlier vicariant events, selective extinction or errors of calibration. Similarly, the transisthmian divergence estimate far pre‐dates the uplift of the Panamanian Isthmus at about 3 Ma. Speciation events occurred in the Miocene, consistent with tectonic events in the central Indo‐West Pacific, isolation of the Arabian Sea by upwelling and westward trans‐Atlantic dispersal. Differences in habitat between sister species suggest that ecological speciation may also have played a role. The basal position of the Australasian species supports its interpretation as a Tethyan relict.