Endemic diversification of the monophyletic cottoid fish species flock in Lake Baikal explored with mtDNA sequencing

In the ancient Lake Baikal in East Siberia, cottoid fishes have diversified into an endemic flock of 33 species. From an ancestral shallow-water, benthic life-style, Baikalian cottoids have shifted to deep-water life in environments even below 1500 m, and also colonized the pelagic habitat. We examined phylogenetic relationships among 22 Baikalian and 10 extra-Baikalian cottoid taxa using a total of 2822 bp of mitochondrial DNA sequence, from complete sequences of ATPase 8 and 6 and cytochrome b genes and the control region. Unlike in earlier studies, we found strong support for a monophyly of the whole endemic Baikalian cottoid diversity. The Baikalian clade, currently assigned to three families and 12 genera, appears to be nested within the Holarctic freshwater genus Cottus. In the molecular phylogeny, all but one of the current Baikalian genera formed well-supported monophyletic groups. However, the topology was inconsistent with the present morphology-based familial subdivision; particularly in positioning the genus Batrachocottus of Cottidae within Abyssocottidae. The branching order of the Baikalian genera could not be resolved completely, however; short basal branches indicate rapid diversification early in the history of the species flock. Using synonymous divergence rates from other fish species for calibration, the diversification of the Baikalian cottoids seems to have started in the Pliocene or early Pleistocene.     

Gill structure and morphometry of its respiratory components in the Baikalian golomyankas (Comephoridae; Cottoidei; Pisces)

All four pairs of gill arches in Comephorus baicalensis and C. dybowskii (Comephoridae) are well developed. In contrast to other Baikalian cottoids (Cottidae, Abyssocottidae), the slit behind the fourth arch is well preserved and two hemibranchs are present on this arch in the golomyankas. These anatomical details are considered to contradict the opinion that Comephoridae may be derived from other Baikalian Cottoidei. The arches in golomyankas are elongated and equipped with very strong spinous gill-rakers. The external sides of the upper and lower jaws are covered with numerous spinelike denticles, the apices of which are bent toward the mouth cavity. The denticles and the spinous gill-rakers are considered to be a specific adaptation in sluggish golomyankas for precise grasping of zooplankton of different sizes, as well as fish larvae. Respiratory components of the gill apparatus in the golomyankas are reduced considerably because of short gill filaments and the sparce distribution of small respiratory lamella on them. The allometry for the relationships between gill respiratory surface area (GRSA) and body mass (Y = aW^b) in both golomyankas is biphasic: in juvenile immature C. baicalensis Yj = 199.013 W^1.057 and in mature animals Ym = 322.354 W^0.731. In C. dybowskii the values for immature and mature animals are Yj = 95.736 W^1.190 and Ym = 199.609W^0.707, respectively. In the sluggish bathypelagic golomyankas (Comephorus) the GRSA per body weight unit is 5–6 times smaller than that found in more active secondarily pelagic Baikalian sculpins (Cottocomephorus) examined previously. © 1996 Wiley-Liss, Inc.     

Nuclear markers reveal unexpected genetic variation and a Congolese-Nilotic origin of the Lake Victoria cichlid species flock

Phylogenetic analyses based on mitochondrial (mt) DNA have indicated that the cichlid species flock of the Lake Victoria region is derived from a single ancestral species found in East African rivers, closely related to the ancestor of the Lake Malawi cichlid species flock. The Lake Victoria flock contains ten times less mtDNA variation than the Lake Malawi radiation, consistent with current estimates of the ages of the lakes. We present results of a phylogenetic investigation using nuclear (amplified fragment length polymorphism) markers and a wider coverage of riverine haplochromines. We demonstrate that the Lake Victoria-Edward flock is derived from the morphologically and ecologically diverse cichlid genus Thoracochromis from the Congo and Nile, rather than from the phenotypically conservative East African Astatotilapia. This implies that the ability to express much of the morphological diversity found in the species flock may by far pre-date the origin of the flock. Our data indicate that the nuclear diversity of the Lake Victoria-Edward species flock is similar to that of the Lake Malawi flock, indicating that the genetic diversity is considerably older than the 15 000 years that have passed since the lake began to refill. Most of this variation is manifested in trans-species polymorphisms, indicating very recent cladogenesis from a genetically very diverse founder stock. Our data do not confirm strict monophyly of either of the species flocks, but raise the possibility that these flocks have arisen from hybrid swarms.     

Boomeranging around Australia: Historical biogeography and population genomics of the anti‐equatorial fish Microcanthus strigatus (Teleostei: Microcanthidae)

The geographic distributions of marine fishes have been shaped by ancient vicariance and ongoing dispersal events. Some species exhibit anti‐equatorial distributions, inhabiting temperate regions on both sides of the tropics while being absent from equatorial latitudes. The perciform fish Microcanthus strigatus (the stripey) exhibits such a distribution with disjunct populations occurring in East Asia, Hawaii, Western Australia, and the southwest Pacific. Here, we examine the historical biogeography and evolutionary history of M. strigatus, based on more than 80 specimens sampled from the four major populations. We analysed 36 morphological characters, three mitochondrial markers, and two sets of 7,120 and 12,771 single‐nucleotide polymorphisms from the nuclear genome. Our results suggest that M. strigatus represents a cryptic species complex comprising at least two genetically distinct populations worthy of species‐level recognition, with one population exhibiting strong genetic structuring but with intermittent, historical gene flow. We provide evidence for a southwest Pacific origin for the ancestral Microcanthus and explain how past connectivity between these regions might have given rise to the relationships observed in present‐day marine fauna. Our ancestral range reconstructions and molecular‐clock analyses support a southwest Pacific centre of origin for Microcanthus, with subsequent colonization of Western Australia through the Bass Strait followed by transequatorial dispersals to the Northern Hemisphere during the Pleistocene. Our results detail an anti‐tropical dispersal pattern that is highly unusual and previously undocumented, thereby emphasizing the importance of integrative systematics in the evaluation of widespread species.     

Tracing ancestry with chromosomal sequences

Most of the large Drosophila species of Hawaii are single-island endemics. Chromosomal sequences show that species at the new end of the archipelago have been derived stepwise from ancestral populations on older islands. The oldest high island has an endemic species with sequences that match some in the Nearctic-Palearctic robusta species group. Colonization from a continent by long-distance dispersal seems a likely origin for the Hawaiian drosophilids. Telmatogeton, a worldwide genus of marine midges, has five Hawaiian species inhabiting freshwater streams. Chromosomal sequences of a marine species in Hawaiian waters match the freshwater forms, indicating colonization from the ocean.     

Role of thyroid hormone in the ontogeny and morphological diversification of <Emphasis Type="Italic">Barbus intermedius sensu</Emphasis> Banister, 1973 of Lake Tana in Ethiopia

The role of thyroid hormone in the ontogeny of the Large African barb Barbus intermedius sensu Banister, 1973 (Teleostei; Cyprinidae)—the presumed ancestral species for the species flock of the large African Barbs of Lake Tana (Ethiopia)—has been studied. It has been shown that the thyroid hormone influences the timing and rate of many morphogenetic processes and a change in its level causes heterochronies, affecting the definitive morphology of fish. Thus, it can be assumed that even a slight change in activity of the thyroid axis during ontogeny can be the cause of wide morphological variability in B. intermedius and may underlie explosive diversification of the species flock of Large African Barbs of Lake Tana.     

Genetic differentiation,polyploidization and hybridization in northern EuropeanDactylorhiza (Orchidaceae): Evidence from allozyme markers

Taxa endemic to North-western Europe are rare, but the orchid genusDactylorhiza contains several species restricted to this area. Evidence from morphological and cytological studies have indicated that some species may have arisen recently and may be of hybrid origin. In the present report, I use allozymes to characterize the genomes in various species ofDactylorhiza and evaluate the possibilities for rapid evolutionary change in the genus. Allotetraploid species have evolved repeatedly from two principal diploid ancestral lineages. These lineages include extant diploid and autotetraploid species, from which allotetraploid derivatives may still arise. It is suggested that allotetraploidization dominates over introgression as speciation mechanism in the genus. The more common and widespread allotetraploid species could be characterized by their allozyme characters over considerable distances, indicating that each of them may have a unique origin and that they have spread from their ancestral populations to the present distribution areas. However, it is also possible that some allotetraploid species contain local populations that have been independently derived from the ancestral lineages.     

Mitogenomic Evidence for an Indo-West Pacific Origin of the Clupeoidei (Teleostei: Clupeiformes)

The clupeoid fishes are distributed worldwide, with marine, freshwater and euryhaline species living in either tropical or temperate environments. Regional endemism is important at the species and genus levels, and the highest species diversity is found in the tropical marine Indo-West Pacific region. The clupeoid distribution follows two general pattern of species richness, the longitudinal and latitudinal gradients. To test historical hypotheses explaining the formation of these two gradients, we have examined the early biogeography of the Clupeoidei in reconstructing the evolution of their habitat preferences along with their ancestral range distributions on a time-calibrated mitogenomic phylogeny. The phylogenetic results support the distinction of nine main lineages within the Clupeoidei, five of them new. We infer several independent transitions from a marine to freshwater environment and from a tropical to temperate environment that occurred after the initial diversification period of the Clupeoidei. These results combined with our ancestral range reconstruction hypothesis suggest that the probable region of origin and diversification of the Clupeoidei during the Cretaceous period was the tropical marine precursor to the present Indo-West Pacific region. Thus, our study favors the hypotheses of “Region of origin” and “Tropical conservatism” to explain the origins of the longitudinal and latitudinal gradients of clupeoid species richness, respectively. Additional geological and paleontological evidence further define the tropical marine paleo-region of origin as the eastern Tethys Sea region. The Cretaceous fossil record of the Clupeoidei is partially incongruent with the results here as it contains taxa found outside this region. We discuss three possible causes of conflict between our biogeographical hypothesis and the distributions of the Cretaceous clupeoid fossils: regional extinction, incomplete taxonomic sampling and incorrect timescale estimation.     

ORIGIN AND RECENT ENDEMIC DIVERGENCE OF A CASPIAN MYSIS SPECIES FLOCK WITH AFFINITIES TO THE “GLACIAL RELICT” CRUSTACEANS IN BOREAL LAKES

Aspects of the evolution of intralacustrine species flocks and of the origin of the Arctic or “glacial-relict” zoogeographical element in Eurasian inland waters were elucidated in an allozyme study of the crustacean genus Mysis. This element, of supposedly northern marine ancestry, is represented by vicarious taxa in the deeper parts of the Caspian Sea (an enclosed ancient basin) and in young boreal lakes. The three endemic Caspian Mysis species studied are very close genetically (Nei's D = 0.06), which suggests a recent intrabasin radiation and rapid morphological divergence. This is in contrast to the pattern in postglacial Holarctic boreal lakes, where the Mysis relicta group is represented by a set of morphologically uniform but probably much older sibling species (D = 0.3–0.6). The results provide a parallel to those on the recent diversification of some fish species flocks in ancient freshwater lakes. The situation is, however, unusual in that the Caspian sympatric Mysis flock is pelagic, and conditions promoting speciation through allopatric isolation or spatial segregation by trophic substrate specialization seem implausible. The monophyletic Caspian Mysis clade shows a relatively strong divergence from both the northern lacustrine and the Arctic marine congeners (D = 0.6–1.0); the phylogenetic branching order of these three zoogeographical groups is not conclusively resolved. The results contradict the prevailing hypothesis of a recent Pleistocene origin of the Caspian Arctic element by invasion from Eastern European continental proglacial lakes that drained south to the Caspian basin during the glacial maxima and served as refugia for the boreal lacustrine taxa.     

Reticulate sympatric speciation in Cameroonian crater lake cichlids

BACKGROUND: Traditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically. RESULTS: We reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni. CONCLUSIONS: Our findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.     

Historical biogeography of the widespread macroalga Sargassum (Fucales,Phaeophyceae)

Sargassum is a cosmopolitan brown algal genus spanning the three ocean basins of the Atlantic, Pacific and Indian Oceans, inhabiting temperate, subtropical and tropical habitats. Sargassum has been postulated to have originated in the Oligocene epoch approximately 30 mya according to a broad phylogenetic analysis of brown macroalgae, but its diversification to become one of the most widespread and speciose macroalgal genera remains unclear. Here, we present a Bayesian molecular clock study, which analyzed data from the order Fucales of the brown algal crown radiation (BACR) group to reconstruct a time-calibrated phylogeny of the Sargassum clade. Our phylogeny included a total of 120 taxa with 99 Sargassum species sampled for three molecular markers – ITS-2, cox3 and rbcLS – calibrated with an unambiguous Sargassaceae fossil from between the lower and middle Miocene. The analysis revealed a much later origin of Sargassum than expected at about 6.7 mya, with the genus diversifying since approximately 4.3 mya. Current geographic distributions of Sargassum species were then analyzed in conjunction with the time-calibrated phylogeny using the dispersal-extinction-cladogenesis (DEC) model to estimate ancestral ranges of clades in the genus. Results strongly support origination of Sargassum in the Central Indo-Pacific (CIP) region with subsequent independent dispersal events into other marine realms. The longer history of diversification in the ancestral CIP range could explain the much greater diversity there relative to other marine areas today. Analyses of these dynamic processes, when fine-tuned to a higher spatial resolution, enable the identification of evolutionary hotspots and provide insights into long-term dispersal patterns.     

Evolutionary Relationships in the Sand-Dwelling Cichlid Lineage of Lake Tanganyika Suggest Multiple Colonization of Rocky Habitats and Convergent Origin of Biparental Mouthbrooding

The cichlid species flock of Lake Tanganyika is comprised of seven seeding lineages that evolved in step with changes of the lake environment. One seeding lineage diversified into at least six lineages within a short period of time. Our study focuses on the diversification of one of these lineages, the Ectodini, comprising highly specialized, sand- and rock-dwelling species. They display two distinct breeding styles: maternal and biparental mouthbrooding. By analyzing three mtDNA gene segments in 30 species representing all 13 described genera, we show that the Ectodini rapidly diversified into four clades at the onset of their radiation. The monotypic genus Grammatotria is likely to represent the most ancestral split, followed by the almost contemporary origin of three additional clades, the first comprising the benthic genus Callochromis, the second comprising the benthic genera Asprotilapia, Xenotilapia, Enantiopus, and Microdontochromis, and the third comprising the semi-pelagic genera Ophthalmotilapia, Cardiopharynx, Cyathopharynx, Ectodus, Aulonocranus, Lestradea, and Cunningtonia. Our study confirms the benthic and sand-dwelling life-style as ancestral. Rocky habitats were colonized independently in the Xenotilapia- and Ophthalmotilapia-clade. The Xenotilapia-clade comprises both maternal and biparental mouthbrooders. Their mode of breeding appears to be highly plastic: biparental mouthbrooding either evolved once in the common ancestor of the clade, to be reverted at least three times, or evolved at least five times independently from a maternally mouthbrooding ancestor. Furthermore, the genera Xenotilapia, Microdontochromis, Lestradea, and Ophthalmotilapia appeared paraphyletic in our analyses, suggesting the need of taxonomic revision.     

Ancestry to an endemic radiation in Lake Tanganyika? Evolution of the viviparous gastropod Potadomoides Leloup, 1953 in the Congo River system (Caenogastropoda,Cerithioidea, Paludomidae)

Providing another spectacular model for understanding speciation and radiation, the origin of the gastropod species flock in Lake Tanganyika (with an estimated age of approximately 12 Myr) remained enigmatic to date. Although, for a long time, an in situ radiation was assumed, Lake Tanganyika could have functioned as a reservoir for ancient African lineages, implying that the now lacustrine taxa originiated elsewhere. However, the fluviatile gastropod fauna of adjacent river systems in Central and East Africa is only poorly known. Here, we provide conchological, anatomical, phylogenetical, and biogeographical data on the fluviatile genus Potadomoides Leloup, 1953, which was hitherto regarded as ancestral to the entire Tanganyika gastropod radiation. The type species Potadomoides pelseneeri is restricted to the delta region of the Malagarasi River east of Lake Tanganyika, whereas three congeneric species (Potadomoides bequaerti, Potadomoides hirta, and Potadomoides schoutedeni) inhabit the Congo River with its tributaries Lualaba and Luvua, west of the Tanganyikan Rift. We describe and document, with scanning electron microscopy, the ontogenetic development of embryos of this uterine brooder as well as the detailed reproductive anatomy. Phylogenetic analysis of 44 morphological characters (including adult and embryonic shell, operculum, radula, reproductive tract) for 15 paludomid taxa could not support monophyly of the Tanganyika species flock. Instead, we found two major lineages that colonized Lake Tanganyika independently, one comprising the Nassopsinae Kesteven, 1903 (= Lavigeriinae Thiele, 1925) with the riverine Potadomoides plus the lacustrine Lavigeria and Vinundu, the second comprising the riverine Cleopatra together with the rest of the lacustrine species (except for Tiphobia horei). The analysis identifies Potadomoides as paraphyletic, with the uterine brooder P. pelseneeri being the sister taxon to the uterine brooder Lavigeria plus the oviparous Vinundu, but not to the entire Tanganyika species flock. We reconstruct the independent evolution of an fluviolacustrine taxon Nassopsinae for which we evaluate the synapomorphic characters, in particular those of reproductive biology, and discuss systematic and evolutionary implications of repeated origin of (ovo‐)viviparity in these limnic Cerithioidea. Finally, we outline a hypothesis on the evolutionary history of Potadomoides in the context of the gastropod radiation in Lake Tanganyika. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92 , 367–401.     

Molecular analyses of ostracod flocks from Lake Baikal and Lake Tanganyika

Ancient lakes are excellent laboratories for evolutionary research, where species can be studied in the cradle where they originated. In this article, we investigate two endemic ostracod species flocks from the two oldest lakes in the world, Lake Baikal (LB) (ca. 28 myr) and Lake Tanganyika (LT) (ca. 12 myr), with DNA sequence data. Nuclear ITS1 failed to resolve the phylogeny of both flocks. Whilst most phylogenetic relationships of the Tanganyika flock are resolved with mitochondrial COI, the Baikalian tree contains multifurications of up to seven different clades. The Tanganyikan Cyprideis flock shows higher genetic variability, which matches its higher morphological variability. A significant deviation from a constant divergence rate through time indicates that the Cytherissa flock most likely experienced explosive speciation events during its earlier history. Comparative analyses of substitution rates furthermore revealed that they are not clock-wise for COI. Ancestral Cytherissa probably radiated in LB 5–8 myr ago, around the time when the cold, oxygenated abyss was formed in LB. The Tanganyikan Cyprideis flock is almost twice as old as the Baikalian Cytherissa flock, and possibly older than LT itself, ca. 15 myr. The Cyprideis flock has survived drastic lake level changes and resulting salinity crises during its entire history.     

Demography and speciation history of the homoploid hybrid pine Pinus densata on the Tibetan Plateau

Pinus densata is an ecologically successful homoploid hybrid that inhabits vast areas of heterogeneous terrain on the south‐eastern Tibetan Plateau as a result of multiple waves of colonization. Its region of origin, route of colonization onto the plateau and the directions of introgression with its parental species have previously been defined, but little is known about the isolation and divergence history of its populations. In this study, we surveyed nucleotide polymorphism over eight nuclear loci in 19 representative populations of P. densata and its parental species. Using this information and coalescence simulations, we assessed the historical changes in its population size, gene flow and divergence in time and space. The results indicate a late Miocene origin for P. densata associated with the recent uplift of south‐eastern Tibet. The subsequent differentiation between geographical regions of this species began in the late Pliocene and was induced by regional topographical changes and Pleistocene glaciations. The ancestral P. densata population had a large effective population size but the central and western populations were established by limited founders, suggesting that there were severe bottlenecks during the westward migration out of the ancestral hybrid zone. After separating from their ancestral populations, population expansion occurred in all geographical regions especially in the western range. Gene flow in P. densata was restricted to geographically neighbouring populations, resulting in significant differentiation between regional groups. The new information on the divergence and demographic history of P. densata reported herein enhances our understanding of its speciation process on the Tibetan Plateau.     

The role of the thyroid gland hormone in ontogenesis and morphological diversification of Barbus intermedius sensu of Lake Tana in Ethiopia

The role of thyroid gland hormone in the ontogenesis of the Large African Barb Barbus intermedius sensu Banister, 1973 (Teleostei; Cyprinidae)--the supposed ancestral species which produced the species flock of the large African Barbs of the Lake Tana (Ethiopia)--has been studied. It has been shown that the thyroid hormone influences the period of many morphological processes and a change in its level causes heterochrony, reflecting on the definitive morphology of fish. Thus, it can be assumed that even a slight change in activity of the thyroid axis in the process of ontogenesis can be the cause of wide variability of the morphological features of B. intermedius and is at the basis for the fast diversification of the species flock of Big African Barbs of Lake Tana.     

A tale of time and depth: intralacustrine radiation in endemic Gammarus species flock from the ancient Lake Ohrid

Tentatively dated, the Plio‐/Pleistocene origin of the ancient Lake Ohrid on the Balkan Peninsula makes it the oldest ancient lake in Europe. Given the surface area of the lake and the adjusted endemicity rate, it may be also defined as the most diverse of all the ancient lakes in the world. From all the animal groups endemic to this lake, gammarids are amongst the most scarcely known in terms of their diversity and phylogenetic relationships. Partial DNA sequences of two mitochondrial genes, cytochrome oxidase subunit I (cox1) and 16S ribosomal RNA (16S rRNA) of eight known endemic Gammarus species from the Lake Ohrid valley were analysed. Phylogenetic analyses showed that endemic Gammarus species comprise an ancient species flock, with Gammarus sketi from the feeder springs being their sister taxon outside the lake. Amongst the species inhabiting the lake, Gammarus solidus and Gammarus salemaai are morphologically and molecularly well defined. By contrast, Gammarus ochridensis, Gammarus parechiniformis, Gammarus lychnidensis, and Gammarus stankokaramani revealed high discrepancy between morphological and genetic data. None of these morphospecies form a monophyletic clade and a significant degree of apparent gene flow occurs between them. This could be caused by incomplete lineage sorting and/or hybridization events. Two novel mtDNA lineages were found within the lake, possibly constituting two new species (Gammarus sp. 1 and Gammarus sp. 2). Molecular clock analysis showed that the split between G. sketi and the Gammarus species flock from the lake occurred approximately 5–7 Mya, whereas within the flock there were at least two intralacustrine radiations: one estimated at 2–3 Mya and the second at less than 1 Mya. The first one could be associated with the origin of the lake and the second with the lake water‐level fluctuations during Pleistocene. © 2013 The Linnean Society of London     

Aerial release of CO2 and respiratory exchange ratio in intertidal fishes out of water

Synopsis Many fishes in the marine intertidal zone have the ability to exchange respiratory gases in air during brief periods out of water. Previous studies on marine amphibious fishes such as mudskippers and rockskippers have shown that they release CO₂ in air at a rate consistent with its release in aquatic respiration. Respiratory Exchange Ratios (RER, CO₂ released per O₂ consumed) are between 0.7 and 1.0, similar to Respiratory Quotients (CO₂ produced metabolically per O₂ consumed). However, previous studies of temperate intertidal fishes emerged only at low tides have been less consistent, with two species reported similar in ability to the amphibious skipper fishes, and two others reported with much lower RERs in air. This study examines 12 species of fishes, from six families (Batrachoididae, Cottidae, Gobiesocidae, Kyphosidae, Pholididae, and Stichaeidae), present in the intertidal zone of California. All 12 species exchanged O₂ and CO₂ in air. Like the amphibious skipper fishes of marine tropical waters, many intertidal fishes of the temperate zone release CO₂ during low tide emergence in quantities sufficient to match its metabolic production. These results indicate that CO₂ release is not hampered by the change in respiratory medium from water to air.     

Speciation of endemic Lake Tana barbs (Cyprinidae, Ethiopia) driven by trophic resource partitioning; a molecular and ecomorphological approach

A unique species flock of large barbs (Barbus spp.) from Lake Tana is presented, from the level of fish stocks to molecules. Evidence is given for the species status of 14 morphotypes of large barbs. They distinctly differ in: (1) head and body morphometrics, (2) food preferences, (3) distribution patterns, (4) maximal body size, (5) spawning area and period, and (6) molecular genetic characters. Most types show early morphological divergence at small size. Major Histocompatibility Complex (MHC) genes, encoding cell membrane proteins involved in defence against pathogens, were found to be diagnostic for the species' genetic identity. A strong selective pressure on particular amino acid positions in the MHC protein sequence most probably arose in response to different pathogen loads from the newly invaded ecological niches after formation of Lake Tana, ca. two million years ago.Arguments for a sympatric origin of this species flock are discussed. An evolutionary scenario suggests a riverine ancestral Barbus intermedius invading Lake Tana after its formation by volcanic blocking of the Blue Nile river and its isolation by waterfalls. Specialisation for particular food types and disruptive selection on many feeding structures are hypothesized as the evolutionary drive in speciation. The causal relationship between the diversity in feeding structures and food types is explained from experiments and models. As an example, the potential food niche of three barb species is predicted from parameter values measured from a large set of feeding structures and tested against the actual gut contents. The co-occurrence of eight piscivorous barb species is unique for cyprinid fish, which lack oral jaw teeth and a stomach. The significance of this aquatic ecosystem as a multidisciplinary evolutionary laboratory and the need for a wise balance between exploitation and conservation is stressed.     

Phylogenetic Relationships Among East African Haplochromine Fish as Revealed by Short Interspersed Elements (SINEs)

Genomic DNA libraries were prepared from two endemic species of Lake Victoria haplochromine (cichlid) fish and used to isolate and characterize a set of short interspersed elements (SINEs). The distribution and sequences of the SINEs were used to infer phylogenetic relationships among East African haplochromines. The SINE-based classification divides the fish into four groups, which, in order of their divergence from a stem lineage, are the endemic Lake Tanganyika flock (group 1); fish of the nonendemic, monotypic, widely distributed genus Astatoreochromis (group 2); the endemic Lake Malawi flock (group 3); and group 4, which contains fish from widely dispersed East African localities including Lakes Victoria, Edward, George, Albert, and Rukwa, as well as many rivers. The group 4 haplochromines are characterized by a subset of polymorphic SINEs, each of which is present in some individuals and absent in others of the same population at a given locality, the same morphologically defined species, and the same mtDNA-defined haplogroup. SINE-defined group 4 contains six of the seven previously described mtDNA haplogroups. One of the polymorphic SINEs appears to be fixed in the endemic Lake Victoria flock; four others display the presence-or-absence polymorphism within the species of this flock. These findings have implications for the origin of Lake Victoria cichlids and for their founding population sizes.