Genome size as a determinant of growth and life-history traits in crustaceans

Genome size varies tremendously both within and among taxa, and strong correlations between genome size and various physiological and ecological attributes suggest that genome size is a key trait of organisms, yet the causalities remains vague. In the present study, we tested how genome size is related to key physiological and ecological properties in five large orders of crustaceans: Decapoda, Cladocera, Amphipoda, Calanoida, and Cyclopoida. These span a wide range in sizes, habitats and life-history traits. To some extent, genome size reflected phylogenetic footprints but, generally, a very wide range in genome size was found within all orders. Genome size was positively correlated with body size in Amphipoda, Cladocera, and Copepoda, but not for Decapoda in general. This could indicate that the evolution of body size occurs mainly by changing cell size for the three first orders, whereas it is more attributed to cell numbers for Decapoda. Cladocera, with direct development and a high growth rate, have minute genomes compared to copepods that possess a more complex life history, whereas, within Decapoda and Amphipoda, developmental complexity is not related to genome size. The present study suggests that, within the crustaceans, selection for a wide variety of life-history strategies has led to widely different genome sizes.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 393–399.     

Evolutionary and ecological implications of genome size in the North American endemic sagebrushes and allies (Artemisia, Asteraceae)

The genome size of 51 populations of 20 species of the North American endemic sagebrushes (subgenus Tridentatae ), related species, and some hybrid taxa were assessed by flow cytometry, and were analysed in a phylogenetic framework. Results were similar for most Tridentatae species, with the exception of three taxonomically conflictive species:  Artemisia bigelovii Gray,  Artemisia pygmaea Gray, and  Artemisia rigida Gray. Genome size homogeneity (together with the high morphological, chemical, and karyological affinities, as well as low DNA sequence divergence) could support a recent diversification process in this geographically restricted group, thought to be built upon a reticulate evolutionary framework. The Tridentatae and the other North American endemic Artemisia show a significantly higher genome size compared with the other subgenera. Our comparative analyses including genome size results, together with different kinds of ecological and morphological traits, suggest an evolutionary change in lifestyle strategy linked to genome expansion, in which junk or selfish DNA accumulation might be involved. Conversely, weed or invasive behaviour in Artemisia is coupled with lower genome sizes. Data for both homoploid and polyploid hybrids were also assessed. Genome sizes are close to the expected mean of parental species for homoploid hybrids, but are lower than expected in the allopolyploids, a phenomenon previously documented to be related with polyploidy.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 631–649.     

Genetic differentiation on multiple spatial scales in an ecotype-forming marine snail with limited dispersal: Littorina saxatilis

The population genetic structure of marine species lacking free-swimming larvae is expected to be strongly affected by random genetic drift among populations, resulting in genetic isolation by geographical distance. At the same time, ecological separation over microhabitats followed by direct selection on those parts of the genome that affect adaptation might also be strong. Here, we address the question of how the relative importance of stochastic vs. selective structuring forces varies at different geographical scales. We use microsatellite DNA and allozyme data from samples of the marine rocky shore snail Littorina saxatilis over distance scales ranging from metres to 1000 km, and we show that genetic drift is the most important structuring evolutionary force at distances > 1 km. On smaller geographical scales (< 1 km), divergent selection between contrasting habitats affects population genetic structure by impeding gene flow over microhabitat borders (microsatellite structure), or by directly favouring specific alleles of selected loci (allozyme structure). The results suggest that evolutionary drivers of population genetic structure cannot a priori be assumed to be equally important at different geographical scales. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 31–40.     

The evolutionary pathway of light emission in myodocopid Ostracoda

The evolutionary history of bioluminescence and iridescence in myodocopid ostracods was estimated by phylogenetic analysis of mitochondrial 16S ribosomal RNA sequences. The inferred phylogeny of the myodocopids suggests that the common ancestor of Myodocopida evaluated in this study exhibits iridescence. This type of light emission was once lost and recaptured independently in the descendant lineages. Bioluminescent species also evolved from non-luminous ancestral species. In the suborder Myodocopina, all the bioluminescent species form a monophyletic group, suggesting that bioluminescence evolved only once. Structural differences between two bioluminescent groups in the order Myodocopida suggests independent origins for bioluminescence.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 449–455.     

Genome size evolution and polyploidy in the Daphnia pulex complex (Cladocera: Daphniidae)

Genome size was estimated in 49 clones of the Daphnia pulex complex from temperate and subarctic locations using flow cytometry and microsatellite DNA analyses. Significant genome size differences were found in diploid species belonging to the two genetically distinct groups (the pulicaria and the tenebrosa groups), with clones from the tenebrosa group having genome sizes 22% larger than those in the pulicaria group. Combined flow cytometry and microsatellite DNA analyses revealed that nearly all polyploid clones in the D. pulex complex are triploid and not tetraploid, as was previously suggested. Sequencing analyses of the ND5 gene to position clones in their respective clades within the D. pulex complex have uncovered three triploid clones of Daphnia middendorffiana with a D. pulex maternal parent. This result was unexpected because Daphnia pulicaria has always been identified as the maternal parent of these hybrid polyploid clones. Triploid clones likely owe their origins to interactions between sexual and asexual populations. Further interactions in the tenebrosa group have generated tetraploid clones but these events have been rare.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 68–79.     

Historical diversification of floodplain forest specialist species in the Amazon: a case study with two species of the avian genus Xiphorhynchus (Aves: Dendrocolaptidae)

Phylogeographical and population genetics methods are used to reconstruct the diversification history of two species of the genus Xiphorhynchus (Aves: Dendrocolaptidae) associated with seasonally flooded forest types in Amazonia. Sequences of the mitochondrial gene cytochrome b were assessed for 21 and 30 individuals, belonging to eight and ten populations, of Xiphorhynchus kienerii and Xiphorhynchus obsoletus , respectively. Uncorrected genetic distances among unique haplotypes recovered ranged only from 0.01% to 0.4% for both species. Over 90% of the genetic variation detected in both species was partitioned within populations, and therefore was not structured geographically. Mismatch distributions and values of Tajima's D -tests indicate that both X. kienerii and X. obsoletus have had small evolutionary effective population sizes, but experienced a recent demographic expansion. These demographic expansions are tentatively dated as occurring over the last 18 000 years BP, a time frame which coincides with the establishment of the early and mid-Holocene age floodplain forest in most of central and eastern Amazonia, following a period of increased river stages throughout the basin. Based on phylogenetic, phylogeographical, and populations genetics data obtained for X. kienerii and X. obsoletus , an evolutionary scenario is proposed to account for the historical diversification of floodplain specialist species in Amazonia.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 383–395.     

An extraordinary shift in life habit within a genus of cyclopid copepods in Lake Tanganyika

We here describe a new species of cyclopid copepod, Eucyclops bathanalicola sp. nov. , parasitic on a gastropod endemic to Lake Tanganyika, Bathanalia straeleni (Cerithioidea, Paludomidae). E. bathanalicola is distinguished by the possession of praecoxal claws on the maxillules, by the modified maxillae which lack any trace of an endopod on the powerful distal claw, and by the reduction of the maxillipeds to minute unarmed lobes. In the character states exhibited by the female body, antennules and swimming legs 1–5, the new species closely resembles a typical free living Eucyclops . The impact of the adoption of parasitism as a life habit is expressed primarily in the modification of the postmandibular mouthparts. As a member of the Cyclopidae, this species represents a unique foray into a parasitic lifestyle from an otherwise free-living group of copepods inhabiting Lake Tanganyika. This is the first record of a parasitic copepod on a mollusc host within this ancient lake and only the second family of freshwater gastropods reported to host copepods.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 146 , 275–285. No claim to original US government works.     

Genetic parsimony: a factor in the evolution of complexity, order and emergence

Two conjectures, drawn from Gregory Chaitin's Algorithmic Information Theory, are examined with respect to the relationship between an algorithm and its product; in particular his finding that, where an algorithm is minimal, its length provides a measure of the complexity of the product. Algorithmic complexity is considered from the perspective of the relationship between genotype and phenotype, which Chaitin suggests is analogous to other algorithm-product systems. The first conjecture is that the genome is a minimal set of algorithms for the phenotype. Evidence is presented for a factor, here termed 'genetic parsimony', which is thought to have helped minimize the growth of genome size during evolution. Species that depend on rapid replication, such as prokaryotes which are generally r -selected are more likely to have small genomes, while the K -strategists accumulate introns and have large genomes. The second conjecture is that genome size could provide a measure of organism complexity. A surrogate index for coding DNA is in agreement with an established phenotypic index (number of cell types), in exhibiting an evolutionary trend of increasing organism complexity over time. Evidence for genetic parsimony indicates that simplicity in coding has been selected, and is responsible for phenotypic order. It is proposed that order evolved because order in the phenotype can be encoded more economically than disorder. Thus order arises due to selection for genetic parsimony, as does the evolution of other 'emergent' properties.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 295–308.     

Relations between microhabitat use and limb shape in phrynosomatid lizards

With the exception of the well-documented case for anoline lizards, recent studies have found few evolutionary relationships between morphology and habitat use in lizards despite clear-cut biomechanical predictions. One of the factors typically hampering these analyses is the clustering of habitat use within evolutionary lineages. In the present study, body shape was quantified for male and female lizards of 30 species of phrynosomatid lizards. This group was selected as little clustering of ecological variables seemed to be present. The results of traditional analyses indicate that evolutionary correlates of habitat use were prominent in the hindlimbs of both sexes. Species living in open habitats are characterized by longer femurs, and longer hindlimbs relative to the forelimb. Moreover, males from ground-dwelling species utilizing open habitats have longer toes on the hind foot than males from climbing species. Phylogenetic analyses indicated strong evolutionary associations between habitat use and the relative length of front and hindlimbs, with species from open terrestrial habitats having significantly shorter frontlimbs relative to their hindlimb than rock or tree climbing species. Evolutionary associations between morphology and habitat use were generally stronger for male lizards, indicating a potentially important contribution of sexual selection to the evolution of differences in limb proportions.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 149–163.     

Convergent body flattening in a clade of tropical rock-using lizards (Scincidae: Lygosominae)

The repeated occurrence of similar morphologies in organisms from similar habitats provides good evidence of convergent selection, and convergent patterns of evolutionary change. In lizards, a flattened morphology has often been noted; however, whether this trait is convergent in specific habitats has never been tested using phylogenetic methods. The present study examined patterns of morphological convergence in 18 species of tropical Lygosomine skinks from three broad habitat categories (generalist, leaf litter-dwelling, and rock-using species). In general, although there where relatively few morphological differences of species from different habitats, phylogenetic analyses revealed that rock-using species have consistently and repeatedly evolved a dorsoventrally flattened head and body. The adaptive basis of this flattened morphology is consistent with both biomechanical predictions of performance (e.g. climbing locomotion) and ecology (e.g. use of rock crevices, camouflage) of species that occupy rocky habitats.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 399–411.     

Spartina anglica C. E. Hubbard: a natural model system for analysing early evolutionary changes that affect allopolyploid genomes

Spartina anglica arose during the end of the 19th century in England by hybridization between the indigenous Spartina maritima and the introduced East American Spartina alterniflora and following genome duplication of the hybrid ( S.  ×  townsendii ). This system allows investigations of the early evolutionary changes that accompany stabilization of a new allopolyploid species in natural populations. Various molecular data indicate that S. anglica has resulted from a unique parental genotype. This young species contains two distinctly divergent homoeologous genomes that have not undergone extensive change since their reunion. No burst of retroelements has been encountered in the F₁ hybrid or in the allopolyploid, suggesting a 'structural genomic stasis' rather than 'rapid genomic changes'. However, modifications of the methylation patterns in the genomes of S.  ×  townsendii and S. anglica indicate that in this system, epigenetic changes have followed both hybridization and polyploidization.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 475–484.     

A new diploid species of Hieracium sect. Pannosa (Asteraceae) from Bulgaria

Hieracium petrovae Vladimirov & Szeląg sp. nov. , a new diploid (2 n  = 18) species in H. sect. Pannosa Zahn, is described and illustrated from the Rhodope Mountains, South Bulgaria, and compared with related taxa. It grows in relict habitats in crevices of limestone rock together with many Balkan endemics. H. petrovae is morphologically similar to taxa from the H. pannosum , H. pilosissimum and H. heldreichii groups (collective species sensu Zahn) to some of which it is a presumed ancestral species.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 261–265.     

The enigma of the landlocked Baikal and Caspian seals addressed through phylogeny of phocine mitochondrial sequences

The endemic seals of Lake Baikal ( Phoca sibirica ) and of the Caspian Sea ( Phoca caspica ) inhabit ancient continental basins that have remained isolated from primary marine seal habitats for millions of years. The species have been united with the Arctic ringed seal, Phoca hispida , into (sub)genus Pusa , but the age and route of invasions to/from the continental basins remain controversial. A phylogenetic analysis of nine northern phocines based on three mitochondrial genes (Cyt b , COI, COII, total 3369 bp) provided no support for the monophyly of the Pusa group. The three species are involved in an apparent polytomy with the boreal harbour seal, Phoca vitulina , and grey seal, Halichoerus grypus . From the average estimated interspecies divergence (4.1%), the radiation of this group plausibly took place in the Late Pliocene 2–3 Mya. This dating does not fit the prevailing hypotheses on the origin of the landlocked taxa in association with Middle Pleistocene glacial events, or of the Caspian seal as a direct descendant of Miocene fossil phocines of the continental Paratethyan basin. The current phocine diversity more likely results from marine radiations, and the continental seals invaded their basins through Plio-Pleistocene (marine) connections from the north. The palaeohydrography that would have enabled the invasions at that time still remains an enigma.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 61–72.     

The house mouse: a model and motor for evolutionary understanding

Commensal house mice have spread from their probable origin in north India, differentiating into a number of forms described variously as species, semispecies or subspecies. The different taxa can breed together and exchange genes but they retain their distinctiveness (although Mus ( musculus ) molossinus of Japan seems to be the result of a complete fusion between M.  ( m .)  musculus and M.  ( m .)  castaneus ). The most widespread form is M.  ( m. )  domesticus , which has successfully colonized every continent as a commensal, albeit with varying contributions from other Mus genomes. It has also been domesticated as the laboratory mouse. This means that the same genome is exposed to a wide variety of environments and gives tremendous opportunities for exploring the operation of different evolutionary mechanisms. Mice have accompanied evolutionary understanding from early Darwinian days – confirming Mendelian ratios and showing they applied in mammals, providing data on rates of evolution, and representing examples of dominance modification, differential survival, competition and other indicators of a struggle for existence. However, they have an unfulfilled potential to drive as well as to illuminate evolutionary theory – by revealing more about, for example, the interactions between gene flow and social determinants, constraints on introgression and physiological adjustments. This potential can be explored through our ever-deepening knowledge of the genome and molecular mechanisms, and by the application of new techniques, but its most effective agent will always be the cross-disciplinary synergy of visionary scientists like Julian Huxley, Charles Elton and Louis Thaler.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 335–347.     

Mitochondrial DNA phylogeny of the woodpecker genus Veniliornis (Picidae, Picinae) and related genera implies convergent evolution of plumage patterns

The woodpecker genus Veniliornis comprises 12 species, all restricted to the New World tropics. The seemingly distantly related genus Picoides is broadly distributed in Eurasia and North America with two putative species, P. lignarius and P. mixtus , occurring in South America. The two genera are clearly distinct with respect to general plumage colouration and patterning as well as habitat utilization and thus traditionally have been placed in different tribes. Phylogenetic analyses of mtDNA sequences from the COI and cyt b genes indicated that both genera are reciprocally paraphyletic. The two South American species of Picoides belong to a clade comprising most species of Veniliornis , but V. fumigatus of Central and north-western South America belongs to a clade comprising species of Picoides . The mtDNA tree also indicated that Veniliornis is not closely related to the genus Piculus, as is implicit in current classifications. Misclassifications involving Veniliornis at both the generic and tribal levels appear to result from convergent evolution of plumage traits in specific forest types. We infer that the common ancestor of Veniliornis entered South America approximately at the time the Isthmus of Panama was formed, and diversification within South America was rapid.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 611–624.     

Historical biogeography of Hexabathynella , a cosmopolitan genus of groundwater Syncarida (Crustacea, Bathynellacea, Parabathynellidae)

Hexabathynella is the only cosmopolitan genus of the order Bathynellacea (Crustacea). The known species number 18, found in Europe (9), Africa (1), South America (2), North America (3) and Australia and New Zealand (3). Phylogenetic analysis suggests that the least derived species are those from South America and the most derived those from the Iberian Peninsula, North America and Australia. The five species with the most plesiomorphic characters occur in salt or brackish water, which supports a marine origin for the genus. Phylogenetic and biogeographical analyses suggest that the distribution of the genus can be explained by dispersion and a double vicariant biogeographical model based on plate tectonics and the evolution of the Tethys during the Mesozoic and Cenozoic.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 78 , 457–466.     

Cytological evidence of natural hybridization in Brachiaria brizantha Stapf (Gramineae)

Two accessions of Brachiaria brizantha under cytological analysis showed 2 n  = 5x = 45 chromosomes. Pentaploidy probably resulted from natural hybridization between two species that were not closely related: an apomictic tetraploid male (2 n  = 4x = 36), and a sexual diploid female (2 n  = 2x = 18). The lack of affinity between genomes was clearly indicated by asynchrony during meiosis. The haploid genome ( n  = 9) showed unique behaviour, remaining univalent during prophase I and metaphase I, and undergoing sister-chromatid segregation and lagging at anaphase I. The laggard genome did not always reach the poles in time to be included in the telophase nucleus. However, when the inclusion was effective, this genome was distributed peripherally, changing the otherwise spherical nucleus shape. In the second division, the haploid genome behaved similarly, but as there was sister-chromatid segregation during the first division, the chromatids were slow to reach the poles, forming several micronuclei at telophase II. The two accessions were characterized as allo-autopentaploids, with the tetraploid genome (2 n  = 4x = 36) designated as B (from B. brizantha ) and the haploid genome as X, representing a species with a distinct genome having little affinity with the B genome. Thus, the hybrids' genome composition is represented by BBBBX. By comparing their meiotic behaviour with that observed in synthetic hybrids between B. brizantha and B. ruziziensis analysed previously, B. ruziziensis is the putative diploid sexual parent species in these pentaploid accessions.  © 2006 The Linnean Society of London , Botanical Journal of the Linnean Society , 2006, 150 , 441–446.     

Risk of predation as a promoting factor of species divergence in threespine sticklebacks (Gasterosteus aculeatus L.)

Icelandic freshwater systems are geologically young and contain only six species of freshwater fish. As these species colonized Icelandic fresh waters they were presented with a diversity of unique, uncontested habitats and food resources, promoting the evolution of new behaviour strategies crucial to the formation of new morphs and speciation. To determine the likelihood that predation threat could affect the antipredator behaviour and possibly the sympatric divergence of prey populations, we analysed antipredator behaviour of seven groups of Icelandic threespine sticklebacks ( Gasterosteus aculeatus ): two marine groups, one group from a lake without piscine predators, and two polymorphic lake populations, each with two groups occupying unique habitats. Shoaling cohesion, school formation and duration, and vigilance in predator inspection/avoidance behaviour varied greatly among groups. The differences appeared to be related to the risk of predation as well as to opportunities and constraints set by the different habitats. Antipredator behaviour was especially pronounced and differed extensively in two polymorphic forms from the lake Thingvallavatn, where predation risk is very high. By keeping the two morphs separate in their respective habitats, high predation risk may be a contributing factor in promoting the habitat-specific divergence of G. aculeatus seen in the lake. This suggests that in situations where refuge habitats are spatially separated, the risk of predation may contribute to the evolution of separate sympatric forms of small fish such as G. aculeatus .  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 189–203.