THE EVOLUTIONARY RADIATION OF DIVERSE OSMOTOLERANT PHYSIOLOGIES IN KILLIFISH (FUNDULUS SP.)

Fish have radiated to exploit diverse habitats, but little is known about the evolutionary lability and directionality of associated physiological specialization. Killifish of the genus Fundulus present a compelling system to explore the evolution of osmotic tolerance because closely related species have evolved to occupy most osmotic niches, and physiological osmotic tolerance data are available for most species. This study seeks to determine the number of times, and the rate at which, alternate osmotic tolerance physiologies have evolved, and to determine the directionality of physiological transitions, by mapping comparative physiology data to a molecular phylogeny for the genus. Character mapping and phylogeographic inference indicate that freshwater tolerance is derived, can evolve rapidly, has evolved several times within the genus, and that variation in osmotic tolerance contributes to defining species distributions. The derivation of alternate physiologies within Fundulus appears associated with contraction of physiological plasticity rather than shifts in tolerance ranges, and the degree of contraction is surprisingly similar across convergent physiological types. The rate of physiological transition is relatively high within Fundulus compared to other taxa, but directionality from high salt tolerance to intolerance appears to be the rule. Together, these comparative physiology and phylogenetic data yield insight into the patterns of evolution of ecological specialization.     

A molecular phylogeny of the genus Alloteropsis (Panicoideae, Poaceae) suggests an evolutionary reversion from C4 to C3 photosynthesis

Background and Aims: The grass Alloteropsis semialata is the only plant species withboth C₃ and C₄ subspecies. It therefore offers excellent potentialas a model system for investigating the genetics, physiologyand ecological significance of the C₄ photosynthetic pathway.Here, a molecular phylogeny of the genus Alloteropsis is constructedto: (a) confirm the close relationship between the C₃ and C₄subspecies of A. semialata; and (b) infer evolutionary relationshipsbetween species within the Alloteropsis genus. Methods: The chloroplast gene ndhF was sequenced from 12 individuals,representing both subspecies of A. semialata and all four ofthe other species in the genus. ndhF sequences were added tothose previously sequenced from the Panicoideae, and used toconstruct a phylogenetic tree. Key Results: The phylogeny confirms that the two subspecies of A. semialataare among the most recently diverging lineages of C₃ and C₄taxa currently recognized within the Panicoideae. Furthermore,the position of the C₃ subspecies of A. semialata within theAlloteropsis genus is consistent with the hypothesis that itsphysiology represents a reversion from C₄ photosynthesis. Thedata point to a similar evolutionary event in the Panicum stenodes–P.caricoides–P. mertensii clade. The Alloteropsis genusis monophyletic and occurs in a clade with remarkable diversityof photosynthetic biochemistry and leaf anatomy. Conclusions: These results confirm the utility of A. semialata as a modelsystem for investigating C₃ and C₄ physiology, and provide moleculardata that are consistent with reversions from C₄ to C₃ photosynthesisin two separate clades. It is suggested that further phylogeneticand functional investigations of the Alloteropsis genus andclosely related taxa are likely to shed new light on the mechanismsand intermediate stages underlying photosynthetic pathway evolution.     

The Morphology and Evolution of the Ear in Actinopterygian Fishes

SYNOPSIS. In this paper we consider various aspects of the anatomyand ultrastructure of the actinopterygian ear and make a numberof suggestions on the possible adaptive significance of thestructural specializations. The focus of the arguments is basedupon the substantial inter-specific variation in teleost auditorysystems as measured anatomically, behaviorally, and physiologically.It is potentially of considerable significance that the majorpoints of inter-specific variation in the teleost ear are associatedwith the gross morphology and ultrastructure of the otolithicorgan most often implicated in sound detection, the sacculus.Analysis of patterns of sacculus ultrastructure has led to theconclusion that there are, in effect, only about five differentsaccular ultrastructural patterns but that these patterns arebroadly found throughout the teleost fishes. Based upon patternsof inter-specific variation in the sacculus and in other aspectsof the ear and more peripheral auditory structures (e.g., swimbladder),it is argued that adaptations encountered in the teleost auditorysystem cannot be used as reliable taxonomic indicators amongfishes. Rather, it is proposed that the teleost auditory systemis quite maleable in the evolutionary sense, and that interspecificsimilarities in many features of the auditory system reflectconvergent evoluuon, rather than phylogenetic affinities. Theactual selective pressures operating in the evoluuon of thefish auditory system are still essentially unknown. In addition,we cannot be certain that similar ear patterns in differentspecies reflect convergent evolution (or common ancestry), orthat conversely, different ear patterns among species reflectdifferences in auditory function.     

A Study of the Evolutionary Relationships of Fundulus Topminnows (Teleostei: Fundulidae)

A hypothesis of the evolutionary relationships of thirty-fourspecies of Fundulus and their closer relatives is presented.The study is restricted to morphological characters which couldbe determined to be primitive or derived using the methods ofphylogenetic systematics. Following Parenti, a family Fundulidaeis recognized. It is composed of four genera of North Americankillifishes. (Cyprinodontidae as usually constitutedis polyphyletic.)No convincing characters demonstrate that Fundulus is a naturalgroup (a monophyletic group). However, all species of Fundulusshare four derived characters with Lucania. The sister group(closest genealogical relative) of Fundulus plus Lucania isa group composed of the genera Adinia and Leptolucania. Fourmonophyletic groups of Fundulus can be recognized based on sharedderived characters: (1) subgenus Fundulus (seven species excludingthree species placed in Fontinus); (2) subgenus Fonlinus (sixspecies); (3) subgenus Xensima (five species); and (4) subgenusZygonectes (thirteen species). Three species, F. (Plancterus)zebrinus, F. lima, and F. parvipinnis are of uncertain affinitieswithin the Lucania-Fundulus clade     

Competition and Coexistence Among Four Estuarine Species of Fundulus

Four sympatric species of Fundulus (F. heteroclitus, F. majalis,F. diaphanus, and F. luciae) are distributed along tidal heightand salinity gradients such that F. heteroclitus co-occurs witheach species, while the remaining species rarely occur together.Feeding habitats of all the species are similar, and food hasbeen shown to limit population size of F. heteroclitus. Thispaper examines whether competition is an important structuringforce within this guild by addressing two questions: 1) is thespatial separation exhibited by three of the species due tophysiological barriers or due to competitive exlusion? and 2)when F. heteroclitus occurs with other Fundulus species doescompetition for resources take place? Laboratory studies indicate that all four species are tolerantof a wide range of environmental conditions; available evidencesuggests that physiological barriers are an unlikely explanationfor spatial segregation among some members of this guild. Competitiveexclusion seems a likely alternate explanation, but is supportedonly by studies of diet overlap. Similarly, only inference fromdiet overlap is available to answer the second question. A fieldexperiment is presented here in which F. heteroclitus and F.majalis were placed in enclosures separately and together. Competitionbetween these species appears to be important, and similar fieldexperiments are recommended to investigate competitive interactionsamong other species pairs within the genus     

Genetic relationships and variation in reproductive strategies in four closely related bromeliads adapted to neotropical 'inselbergs': Alcantarea glaziouana, A. regina, A. geniculata and A. imperialis (Bromeliaceae)

Background and Aims: Bromeliads (Bromeliaceae) adapted to rock outcrops or ‘inselbergs’in neotropical rain forests have been identified as suitableplant models for studying population divergence and speciationduring continental plant radiations. Little is known about geneticrelationships and variation in reproductive strategies withinand among inselberg-adapted species, yet knowledge of theseparameters is important for understanding divergence processesand for conservation planning. Methods: Nuclear microsatellites were used to assess the role of clonalreproduction, estimate genetic diversity and explore geneticrelationships and variation in reproductive strategies for atotal of 15 populations of four closely related Alcantarea inselbergspecies in south-eastern Brazil: A. glaziouana, A. regina, A.geniculata and A. imperialis. Key Results: Clonal propagation is frequent in coastal populations of A.glaziouana and A. regina, but absent in the high-altitude speciesA. geniculata and A. imperialis. Considerable variation in clonaldiversity, gene diversity (H_e), allelic richness, and Wright'sinbreeding coefficient (F_IS) exists within and between speciesof Alcantarea. A Bayesian analysis of coastal inselberg speciesindicated pronounced genetic structure. A neighbor-joining analysisgrouped populations of each species together with moderate bootstrapsupport, except for the high altitude species A. imperialis. Conclusions: The coastal inselberg species A. glaziouana and A. regina tendto propagate asexually via vegetative clonal growth, and bothreproductive strategies and breeding systems vary greatly betweenpopulations and species of Alcantarea. The microsatellite dataindicate a history of hybridization and reticulation involvingthe high-altitude species A. geniculata and A. imperialis inareas of co-occurrence. The results highlight the need to understandsimilarities and differences in reproductive strategies bothwithin and between related species for conservation planningand as a basis for understanding evolutionary processes in tropicalradiations.     

Extensive evolutionary rate variation in floral color determining genes in the genus Ipomoea

Flower color is an important adaptive trait in many plant species because it determines reproductive success through differential attractiveness to insect pollinators. The genus Ipomoea is a pan tropically distributed plant genus characterized by showy flowers that often differ in color among closely related species. Flower color is determined primarily by products of the anthocyanin biosynthesis pathway. To determine whether flower color evolution among members of the genus is driven by adaptive molecular evolution of the anthocyanin pathway genes, we analyzed data from 10 genes sequenced from 19 species of Ipomoea . Six protein-coding genes from the anthocyanin pathway were drawn for evolutionary analysis in addition to three genes from the unrelated leucine biosynthesis pathway and one MADS box regulatory gene for comparison. The analyses provided: (i) no convincing evidence for positive selection on anthocyanin pathway structural genes, or on the other sampled genes, despite shifts in flower color among species included in the sample; (ii) pathway position correlated weakly with estimates of the intensity of evolutionary constraint on the anthocyanin pathway enzyme coding genes; and (iii) there was substantial gene-specific heterogeneity in the rates of synonymous site evolution. Synonymous rate heterogeneity does not appear to be accounted for by codon bias or local contextual or compositional sequence differences, leading us to implicate heterogeneous rates of mutation among genes as the most probable cause of synonymous rate heterogeneity.     

Evolution of the leaf economics spectrum in herbs: Evidence from environmental divergences in leaf physiology across Helianthus (Asteraceae)

The leaf economics spectrum (LES) describes a major axis of plant functional trait variation worldwide, defining suites of leaf traits aligned with resource‐acquisitive to resource‐conservative ecological strategies. The LES has been interpreted to arise from leaf‐level trade‐offs among ecophysiological traits common to all plants. However, it has been suggested that the defining leaf‐level trade‐offs of the LES may not hold within specific functional groups (e.g., herbs) nor within many groups of closely related species, which challenges the usefulness of the LES paradigm across evolutionary scales. Here, we examine the evolution of the LES across 28 species of the diverse herbaceous genus Helianthus (the sunflowers), which occupies a wide range of habitats and climate variation across North America. Using a phylogenetic comparative approach, we find repeated evolution of more resource‐acquisitive LES strategies in cooler, drier, and more fertile environments. We also find macroevolutionary correlations among LES traits that recapitulate aspects of the global LES, but with one major difference: leaf mass per area is uncorrelated with leaf lifespan. This indicates that whole‐plant processes likely drive variation in leaf lifespan across Helianthus, rather than leaf‐level trade‐offs. These results suggest that LES patterns do not reflect universal physiological trade‐offs at small evolutionary scales.     

A holistic approach to taxonomic evaluation of two closely related endangered freshwater mussel species, the oyster mussel Epioblasma capsaeformis and tan riffleshell Epioblasma florentina walkeri (Bivalvia: Unionidae)

Species in the genus Epioblasma have specialized life historyrequirements and represent the most endangered genus of freshwatermussels (Unionidae) in the world. A genetic characterizationof extant populations of the oyster mussel E. capsaeformis andtan riffleshell E. florentina walkeri sensu late was conductedto assess taxonomic validity and to resolve conservation issuesfor recovery planning. These mussel species exhibit pronouncedphenotypic variation, but were difficult to characterize phylogeneticallyusing DNA sequences. Monophyletic lineages, congruent with phenotypicvariation among species, were obtained only after extensiveanalysis of combined mitochondrial (1396 bp of 16S, cytochrome-b,and ND1) and nuclear (515 bp of ITS-1) DNA sequences. Incontrast, analysis of variation at 10 hypervariable DNA microsatelliteloci showed moderately to highly diverged populations basedon F_ST and R_ST values, which ranged from 0.12 to 0.39 and 0.15to 0.71, respectively. Quantitative variation between specieswas observed in fish-host specificity, with transformation successof glochidia of E. capsaeformis significantly greater (P<0.05)on greenside darter Etheostoma blennioides, and that of E. f.walkeri significantly greater (P<0.05) on fantail darterEtheostoma flabellare. Lengths of glochidia differed significantly(P<0.001) among species and populations, with mean sizesranging from 241 to 272 µm. The texture and colourof the mantle-pad of E. capsaeformis sensu stricto is smoothand bluish-white, whereas that of E. f. walkeri is pustuledand brown, with tan mottling. Based on extensive molecular,morphological and life history data, the population of E. capsaeformisfrom the Duck River, Tennessee, USA is proposed as a separatespecies, and the population of E. f. walkeri from Indian Creek,upper Clinch River, Virginia, USA is proposed as a distinctsubspecies. (Received 23 February 2005; accepted 16 January 2006)     

Clonal behaviour in closely related plants

The comparison of published data on clonal morphology of closely related species and/or data on comparative morphology of larger groups of related species showed that most of the variation observed is more related to history than to putative selection pressures within present plant communities. Historical factors include both phylogeny and climatic conditions experienced in the past. Inheritance of clonal growth traits is variable even within one genus and is hard to predict namely in polyploid groups. Community parameters and architectural traits may be closely related in the case of hybrid speciation when the divergence of newly formed taxa may be very rapid and easily influenced by short term community parameters; however there are very few well documented examples.     

Allozyme Variation in Turkmenian Populations of Wild Barley, Hordeum spontaneum Koch.

The extent and structure of genetic variation in 720 individualsrepresenting 36 populations of wild barley, Hordeum spontaneum,from Central Asia (Turkmenistan) were determined using starchgel electrophoresis of eight water soluble leaf proteins encodedby 13 loci. The populations were grouped into seven ecogeographicregions. The study found: (a) a similar amount of within populationgenetic diversity (H_e = 0.106), but lower total genetic diversity(H_T = 0.166) to that reported for Middle East populations ofH. spontaneum; (b) of the total genetic diversity, 61% was attributableto variation within populations, 27% between populations ofa region, and 12% among regions; (c) of the 42 alleles found,11 were ubiquitous, 22 were widespread and common, three localand common and seven local and rare; (d) there was a poor correlationbetween population genetic and geographic distances; and (e)the frequencies of only a few alleles correlated significantlywith climatic or geographic parameters. The extent and structureof genetic variation of Turkmenian populations, which representthe Central Asian part of the species' range, were significantlydifferent in some important aspects from Middle Eastern andeastern Mediterranean populations. The mosaic pattern of geneticvariation found in wild barley in the Middle East is less pronouncedin populations from Central Asia where there is less geneticdifferentiation among populations and regions, and more ubiquitousor common and fewer localized alleles. Copyright 2001 Annalsof Botany Company Allozyme, Central Asia, genetic diversity, Hordeum spontaneum, wild barley     

The causes of variation in tree seedling traits: the roles of environmental selection versus chance

A key aspect of biodiversity is the great quantitative variation in functional traits observed among species. One perspective asserts that trait values should converge on a single optimum value in a particular selective environment, and consequently trait variation would reflect differences in selective environment, and evolutionary outcomes would be predictable. An alternative perspective asserts that there are likely multiple alternative optima within a particular selective environment, and consequently different lineages would evolve toward different optima due to chance. Because there is evidence for both of these perspectives, there is a long-standing controversy over the relative importance of convergence due to environmental selection versus divergence due to chance in shaping trait variation. Here, I use a model of tree seedling growth and survival to distinguish trait variation associated with multiple alternative optima from variation associated with environmental differences. I show that variation in whole plant traits is best explained by environmental differences, whereas in organ level traits variation is more affected by alternative optima. Consequently, I predict that in nature variation in organ level traits is most closely related to phylogeny, whereas variation in whole plant traits is most closely related to ecology.     

Is geographic variation within species related to macroevolutionary patterns between species?

The relationship between microevolution and macroevolution is a central topic in evolutionary biology. An aspect of this relationship that remains very poorly studied in modern evolutionary biology is the relationship between within‐species geographic variation and among‐species patterns of trait variation. Here, we tested the relationship between climate and morphology among and within species in the salamander genus Plethodon. We focus on a discrete colour polymorphism (presence and absence of a red dorsal stripe) that appears to be related to climatic distributions in a common, wide‐ranging species (Plethodon cinereus). We find that this trait has been variable among (and possibly within) species for >40 million years. Furthermore, we find a strong relationship among species between climatic variation and within‐species morph frequencies. These between‐species patterns are similar (but not identical) to those in the broadly distributed Plethodon cinereus. Surprisingly, there are no significant climate–morphology relationships within most other polymorphic species, despite the strong between‐species patterns. Overall, our study provides an initial exploration of how within‐species geographic variation and large‐scale macroevolutionary patterns of trait variation may be related.     

Evolutionary rate variation at multiple levels of biological organization in plant mitochondrial DNA

We examined patterns of mitochondrial polymorphism and divergence in the angiosperm genus Silene and found substantial variation in evolutionary rates among species and among lineages within species. Moreover, we found corresponding differences in the amount of polymorphism within species. We argue that, along with our earlier findings of rate variation among genes, these patterns of rate heterogeneity at multiple phylogenetic scales are most likely explained by differences in underlying mutation rates. In contrast, no rate variation was detected in nuclear or chloroplast loci. We conclude that mutation rate heterogeneity is a characteristic of plant mitochondrial sequence evolution at multiple biological scales and may be a crucial determinant of how much polymorphism is maintained within species. These dramatic patterns of variation raise intriguing questions about the mechanisms driving and maintaining mutation rate heterogeneity in plant mitochondrial genomes. Additionally, they should alter our interpretation of many common phylogenetic and population genetic analyses.     

Molecular phylogeny reveals the repeated evolution of complex male genital traits in the New Zealand moth genus Izatha (Lepidoptera: Xyloryctidae)

Male genitalia are among the most rapidly evolving and divergent morphological structures and sexual selection is known to drive this phenomenon in many taxa. Because of their diversity, even within a single genus, genital characters are frequently used to infer relationships among closely‐related species. Moths within the genus Izatha (Xyloryctidae) are ideal candidates for investigating the phylogenetic patterns of genital evolution as they display great variation in male genital structure and complexity. We determined the evolutionary relationships among 31 species of Izatha by constructing a molecular phylogeny of the genus based on the mitochondrial cytochrome oxidase subunit I gene and the isocitrate dehydrogenase and carbamoylphosphate synthase domain protein nuclear genes. This allowed estimations of ancestral male genital character states and patterns of male genital diversification using maximum‐likelihood models. The genus is divided into two well‐supported clades and two poorly supported clades at the root of the phylogeny with incomplete phylogenetic resolution within two species groups, likely due to rapid speciation. Izatha display a number of apomorphic phallic traits including cornuti (sclerotized spines) which are either discharged into the female during copulation (deciduous cornuti) or fixed to the male phallus (compound and fish‐hook cornuti). Within the genus, there is a reduction of secondary genital characters – the uncus and gnathos – but an elaboration of another grasping structure, the juxta; the potential origin and functionality of these male genital traits are discussed. Overall, some male genital characters provided a good indication of species relationships; however, several parts of the complex male genitalia of Izatha show evidence of homoplasy and convergence highlighting the problems of using these traits in determining species relationships. Additionally, this convergence has highlighted that complex genital structures may evolve repeatedly and independently within a lineage.     

Cis-regulatory evolution of chalcone-synthase expression in the genus Arabidopsis

The contribution of cis-regulation to adaptive evolutionary change is believed to be essential, yet little is known about the evolutionary rules that govern regulatory sequences. Here, we characterize the short-term evolutionary dynamics of a cis-regulatory region within and among two closely related species, A. lyrata and A. halleri, and compare our findings to A. thaliana. We focused on the cis-regulatory region of chalcone synthase (CHS), a key enzyme involved in the synthesis of plant secondary metabolites. We observed patterns of nucleotide diversity that differ among species but do not depart from neutral expectations. Using intra- and interspecific F1 progeny, we have evaluated functional cis-regulatory variation in response to light and herbivory, environmental cues, which are known to induce CHS expression. We find that substantial cis-regulatory variation segregates within and among populations as well as between species, some of which results from interspecific genetic introgression. We further demonstrate that, in A. thaliana, CHS cis-regulation in response to herbivory is greater than in A. lyrata or A. halleri. Our work indicates that the evolutionary dynamics of a cis-regulatory region is characterized by pervasive functional variation, achieved mostly by modification of response modules to one but not all environmental cues. Our study did not detect the footprint of selection on this variation.     

Comparison of Gastrulation in Frogs and Fish

SYNOPSIS. Comparative embryological studies of frogs and fishprovide valuable information about the mechanisms and evolutionof vertebrate development. First, by mapping developmental datafrom a range of species onto a cladogram, one can distinguishgeneral features of a ground plan from variation within it.Two studies illustrate this: comparison of gastrulation mechanismsin sturgeon and Xenopus, and morphogenesis of the dorsal mesodermin five species of anurans. Second, phylogenetic analysis ofdevelopmental data makes it possible to identify radical departuresfrom the ground plan among related groups. Teleost gastrulationis a highly derived process that appears to have little in commonwith the ancestral version. However, teleost gastrulation mayhave evolved as a result of two specific developmental changes:loss of bottle cells in the surface layer, and changes in theyolk. The phylogenetic distribution of developmental charactersforms the basis for mechanistic hypotheses about the originsof major evolutionary changes in development     

Variation in Concentration of Ribulose-1, 5-bisphosphate Carboxylase in Leaves of Barley, Wheat and Hybrids of Crested Wheatgrasses

Amounts of the enzyme ribulose-1,5-bisphosphate carboxylasewere estimated in seedling leaves of barley (Hordewn vulgareL.) and flag leaves of wheat (Triticum aestitum L.) by radialimmuno diffusion. A fourfold variation among barley varietiesfor amount of RuBPCase at the seedling stage was observed (c.3.5–15mg g^–1 fr. wt). Range in variation for amountof flag leaf RuBPCase among wheat varieties was 6-09-9.39 mgRuBPCase g^–1 fr. wt. F₁ hybrids from interspecific andintergeneric crosses of crested wheatgrasses (Agropyron andElymus spp.) and their amphidiploid analogues were comparedfor amount of RuBPCase in the most recent fully expanded leavesharvested before seed set. Amount of enzyme varied from 3.4to 77.6 mg g^–1 fr. wt among the hybrids. No effect chromosomenumber on enzyme concentration was observed among 13 hybridsand their amphidiploid counterparts. Key words: RuBPCase, wheatgrasses