Morpho-karyological Types of Dugesia (Turbellaria, Tricladida) in Israel and Their Distribution Patterns

Populations of Dugesia in Israel have been classified on morphological and karyological criteria into 5 types. Two of these are morphologically similar to the Dugesia lugubris group (n = 4) but differ karyologically ( Types 1 and 2 : n=8), and three types are morphologically similar to the Dugesia gonocephala group ( n = 8) but two differ karyologically (Types 4 and 5: n = 9). The three ' gonocephala ' types are distributed in three distinct areas of Israel with little overlap, while the two ' lugubris ' types are found in springs of one of these areas, The Rift Valley, where special localised conditions prevail. Possible relationships between Types 3, 4 and 5 are discussed with particular reference to their karyotypes and the presence of supernumerary chromosomes in Type 5.     

The karyotypes of Dugesia species from Spain (Turbellaria,Tricladida)

Some species of Planarians, new to Spain, are recorded. Dugesia polychroa, D. sicula, D. iberica and D. gonocephala s. 1. have been investigated karyologically. The former possesses a diploid complement characteristic of the biotype A (2n = 8); the second is diploid with 2n = 18; diploidy and triploidy were found in sexual populations of D. iberica with n = 8. Triploidy occurred in all the asexual strains of the D. gonocephala group with a basic number of either 8 or 9. In this latter case B-chromosomes were occasionally found.     

Karyology and karyotype analysis of diploid freshwater planarian populations of the Dugesia gonocephala group (Platyhelminthes,Tricladida) found in Sardinia

Karyology and karyotype analysis were carried out on freshwater planarian populations of the Dugesia gonocephala group. The strains studied were all diploid with chromosomic number 2n = 16; n = 8. They came from 12 sites mainly localized on the west of the island of Sardinia. Three karyotypes indicated with the letters A, B and C were found in which eight homomorphic pairs of chromosomes were easily identified. In karyotype A all chromosomes are metacentric. Ten populations of the twelve examined showed this karyotype which appears to be the most common. In karyotype B the seventh pair of chromosomes is submetacentric. This karyotype is quite common having been previously found in another eight Sardinian localities. Karyotype C differs from the others in having submetacentric third and seventh pairs of the chromosome complement. It was found in only one locality. The differences observed between these three karyotypes could be interpreted either as sign of differentiation at species level, or as an intraspecific variation due to chromosome mutations (pericentric inversions). This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of dopaminergic agents on monoamine levels and motor behaviour in planaria

1. Dopamine, serotonin and, in lesser amounts, norepinephrine were detected in Dugesia gonocephala using electrochemical detection coupled with liquid chromatography (LCED). 2. Treatment with L-dopa induced hyperkinesias, and a rise in dopamine, serotonin and norepinephrine content, whereas reserpine reduced motor activity and the concentrations of all three monoamines. 3. Haloperidol reduced motor activity and dopamine and norepinephrine levels. 4. Apomorphine induced hyperkinesias without altering monoamine levels.     

Chromosome bands in freshwater triclads

Four species of Triclads belonging to three families, Dugesia polychroa and Dugesia mediterranea (Dugesiidae), Planaria torva (Planariidae) and Dendrocoelum lacteum (Dendrocoelidae) were studied for chromosome banding: C-banding for all the species, ASG banding for Dugesia polychroa and G-banding for Dugesia polychroa and Dugesia mediterranea. C-banding results for Dugesia lugubris-polychroa group suggest a high level of heterochromatin evolution in the group. Small bands strictly limited to the centromeric region were seen in Dugusia lugubris and this pattern is similar to the patterns of some species belonging to other families, notably Planaria torva and Dendrocoelum lacteum. This could be considered a primitive character. In contrast numerous heterochromatin bands which are useful to characterize the different karyotypes were seen in Dugesia polychroa and Dugesia mediterranea. Our data suggest that heterochromatin is important in freshwater triclad speciation.     

Polymorphism and divergence of multilocus DNA markers in sibling species Chironomus riparius Meigen and Chironomus piger Strenzke (Diptera, Chironomidae)

Intra- and interspecific variation and divergence of multilocus markers for genomic DNA of the sibling species from the thimmi group, Chironomus riparius and C. piger, were studied by PCR with arbitrary primers (RAPD). A high level of RAPD polymorphism was determined in both laboratory and natural populations of these species. The genetic distances were estimated between the C. riparius populations and between the sibling species C. riparius and C. piger. The genetic distance between C. riparius and C. piger was 4 to 5 times higher than that between the C. riparius populations. A comparison of the variation and divergence for the RAPD markers with those for other genomic markers--enzyme-coding genes and chromosomes (linked gene groups)--showed that different components of the genome differed in their contribution to the genome divergence.     

Mixed signals from hybrid genomes

Admixture results from interbreeding between individuals from different populations or species that were previously genetically isolated from each other (Fig.  1 ). Identifying admixture events in the genome is not always a straightforward task, because the genetic signature left behind fades with time as recombination events fragment the genomic segments introduced during the interbreeding event. Additionally, when the genetic architecture of populations or species that admix is not very different (e.g. they coalesce to a common ancestor recently), admixture signatures may be difficult to detect. Ignoring the effects of admixture can, however, pose severe problems for population genetic analyses that rely on the distribution of polymorphic markers across the genome. In this issue of Molecular Ecology, Bosse et al. ( 2014 ) analyse genomic data from modern pigs to understand hybridization processes that occurred between domestic pigs from European and Asiatic origin, and between pigs and wild boars. Their results are interesting regarding the fine‐scale distribution of admixture across the pig genome, and the way in which this admixture biases estimates of the effective population size in European domestic pigs. The implications of these results are significant, as they serve as a cautionary note on genomic analyses that depend on the distribution of polymorphic variants in potentially admixed populations.     

Recurrent selection explains parallel evolution of genomic regions of high relative but low absolute differentiation in a ring species

Recent technological developments allow investigation of the repeatability of evolution at the genomic level. Such investigation is particularly powerful when applied to a ring species, in which spatial variation represents changes during the evolution of two species from one. We examined genomic variation among three subspecies of the greenish warbler ring species, using genotypes at 13 013 950 nucleotide sites along a new greenish warbler consensus genome assembly. Genomic regions of low within‐group variation are remarkably consistent between the three populations. These regions show high relative differentiation but low absolute differentiation between populations. Comparisons with outgroup species show the locations of these peaks of relative differentiation are not well explained by phylogenetically conserved variation in recombination rates or selection. These patterns are consistent with a model in which selection in an ancestral form has reduced variation at some parts of the genome, and those same regions experience recurrent selection that subsequently reduces variation within each subspecies. The degree of heterogeneity in nucleotide diversity is greater than explained by models of background selection, but is consistent with selective sweeps. Given the evidence that greenish warblers have had both population differentiation for a long period of time and periods of gene flow between those populations, we propose that some genomic regions underwent selective sweeps over a broad geographic area followed by within‐population selection‐induced reductions in variation. An important implication of this ‘sweep‐before‐differentiation’ model is that genomic regions of high relative differentiation may have moved among populations more recently than other genomic regions.     

A study of 28 Elymus species using repetitive DNA sequences.

Four repetitive DNA sequences cloned from the barley (Hordeum vulgare) genome and common for different Triticeae species were used for a molecular study of phylogenetic relationships among 28 Elymus species. Two wild Hordeum species (H genome), two Pseudoroegneria species (S genome), Agropyron cristatum (P genome), and Australopyrum velutinum (W genome) were included as genomic representatives for the genomes that supposedly were involved in the evolution of the genus Elymus. Our results are essentially congruent with the genomic classification system. This study demonstrates that Elymus is not a monophyletic genus. Based on an analysis of Southern blot hybridization we could discriminate between SY and SH species owing to the strong specific hybridization pattern of the H genome. Hexaploid SYH species gave a hybridization pattern similar to SH species for the same reason. The results support the genomic composition of Elymus batalinii as SYP and also indicated the presence of at least one H genome in Elymus enysii with a hitherto unknown genomic constitution. Elymus erianthus had a hybridization pattern distinctly different from all other species in the investigation. Key words : Elymus, RFLP, phylogeny, repetitive DNA.     

Long perfect dinucleotide repeats are typical of vertebrates, show motif preferences and size convergence

Microsatellites are simple sequence repeats (SSRs) showing complex patterns of length, motif sizes, motif sequences, and repeat perfection. We studied the structure of the dinucleotide SSR population at the genome level by analyzing assembled DNA sequence across species. Three dinucleotide populations were distinguished when SSR genome frequency was analyzed as a function of repeat length and repeat perfection. A population of low-perfection SSRs was identified, which is constituted by short repeats and represents the vast majority of genomic dinucleotide SSRs across eukaryotic genomes. In turn, the highly perfect repeats are 30 to 50 times less frequent and, in addition to short repeats, also contain a long repeat population that is uniquely represented in vertebrate species. Distinctive features of this population include the modal peak in the frequency distribution of repeat length and the strong preferential usage of the repeat motifs AC and AG. These results raise the hypothesis that the ability of carrying a distinct population of long, highly perfect dinucleotide repeats in the genome is a late acquisition in chordate evolution. Our analysis also suggests that different dinucleotide repeat populations have different dynamics and are likely to be underlined by different molecular mechanisms of generation and maintenance in the genome. Thus, these observations imply that caution should be taken in extrapolating results from studies on SSR mutability and on SSR phylogenetic comparisons that do not take into account the stratification of dinucelotide populations in the eukaryotic genome.     

The pigmentary system of planaria

The pigmentary system of the planaria, Dugesia gonocephala s.l. (Platyhelminthes, Turbellaria, Tricladida), consists of granules contained in chromatophore cells distributed in the parenchyma tissue. The administration of MSH release-inhibiting Factor (M.I.F.) leads to an easily observable general decolouration of the animal due to the migration of the pigment granules towards the deeper-lying cell nucleus. In planarians bisected transversely through the pharyngeal region, the decolouration occurs only in the cephalic segment, and the caudal segment remains dark. When, however, the decapitated caudal segment regenerates a head region, a decolouration response occurs when exposed to M.I.F. The significance of these results is discussed, and an hypothesis on the hormonal regulation of the pigmentary system is proposed.     

Polymorphism and Divergence of Multilocus DNA Markers in Sibling Species Chironomus riparius Meigen and Chironomus piger Strenzke (Diptera,Chironomidae)

Intra- and interspecific variation and divergence of multilocus markers for genomic DNA of the sibling species from the thimmi group,Chironomus riparius and C. piger, were studied by PCR with arbitrary primers (RAPD). A high level of RAPD polymorphism was determined in both laboratory and natural populations of these species. The genetic distances were estimated between the C. riparius populations and between the sibling species C. riparius and C. piger. The genetic distance between C. riparius andC. piger was 4 to 5 times higher than that between the C. riparius populations. A comparison of the variation and divergence for the RAPD markers with those for other genomic markers—enzyme-coding genes and chromosomes (gene linkage groups)—showed that different components of the genome differed in their contribution to the genome divergence.     

The pigmentary system of planaria

Summary The pigmentary system of the planaria, Dugesia gonocephala s.l. (Platyhelminthes, Turbellaria, Tricladida), consists of granules contained in chromatophore cells distributed in the parenchyma tissue. The administration of MSH release-Inhibiting Factor (M.I.F.) leads to an easily observable general decolouration of the animal due to the migration of the pigment granules towards the deeper-lying cell nucleus.In planarians bisected transversely through the pharyngeal region, the decolouration occurs only in the cephalic segment, and the caudal segment remains dark. When, however, the decapitated caudal segment regenerates a head region, a decolouration response occurs when exposed to M.I.F.The significance of these results is discussed, and an hypothesis on the hormonal regulation of the pigmentary system is proposed.     

What can patterns of differentiation across plant genomes tell us about adaptation and speciation?

Genome scans have become a common approach to identify genomic signatures of natural selection and reproductive isolation, as well as the genomic bases of ecologically relevant phenotypes, based on patterns of polymorphism and differentiation among populations or species. Here, we review the results of studies taking genome scan approaches in plants, consider the patterns of genomic differentiation documented and their possible causes, discuss the results in light of recent models of genomic differentiation during divergent adaptation and speciation, and consider assumptions and caveats in their interpretation. We find that genomic regions of high divergence generally appear quite small in comparisons of both closely and more distantly related populations, and for the most part, these differentiated regions are spread throughout the genome rather than strongly clustered. Thus, the genome scan approach appears well-suited for identifying genomic regions or even candidate genes that underlie adaptive divergence and/or reproductive barriers. We consider other methodologies that may be used in conjunction with genome scan approaches, and suggest further developments that would be valuable. These include broader use of sequence-based markers of known genomic location, greater attention to sampling strategies to make use of parallel environmental or phenotypic transitions, more integration with approaches such as quantitative trait loci mapping and measures of gene flow across the genome, and additional theoretical and simulation work on processes related to divergent adaptation and speciation.     

Heterogeneity and concordance in locus‐specific differentiation and introgression between species of towhees

The maintenance or breakdown of reproductive isolation is an observable outcome of secondary contact between species. In cases where hybrids beyond the F1 are formed, the representation of each species' ancestry can vary dramatically among genomic regions. This genomic heterogeneity in ancestry and introgression can offer insight into evolutionary processes, particularly if introgression is compared in multiple hybrid zones. Similarly, considerable heterogeneity exists across the genome in the extent to which populations and species have diverged, reflecting the combined effects of different evolutionary processes on genetic variation. We studied hybridization across two hybrid zones of two phenotypically well‐differentiated bird species in Mexico (Pipilo maculatus and P. ocai), to investigate genomic heterogeneity in differentiation and introgression. Using genotyping‐by‐sequencing (GBS) and hierarchical Bayesian models, we genotyped 460 birds at over 41 000 single nucleotide polymorphism (SNP) loci. We identified loci exhibiting extreme introgression relative to the genome‐wide expectation using a Bayesian genomic cline model. We also estimated locus‐specific F_ST and identified loci with exceptionally high genetic divergence between the parental species. We found some concordance of locus‐specific introgression in the two independent hybrid zones (6–20% of extreme loci shared across zones), reflecting areas of the genome that experience similar gene flow when the species interact. Additionally, heterogeneity in introgression and divergence across the genome revealed another subset of loci under the influence of locally specific factors. These results are consistent with a history in which reproductive isolation has been influenced by a common set of loci in both hybrid zones, but where local environmental and stochastic factors also lead to genomic differentiation.     

Genomic differentiation and patterns of gene flow between two long‐tailed tit species (Aegithalos)

Patterns of heterogeneous genomic differentiation have been well documented between closely related species, with some highly differentiated genomic regions (“genomic differentiation islands”) spread throughout the genome. Differential levels of gene flow are proposed to account for this pattern, as genomic differentiation islands are suggested to be resistant to gene flow. Recent studies have also suggested that genomic differentiation islands could be explained by linked selection acting on genomic regions with low recombination rates. Here, we investigate genomic differentiation and gene‐flow patterns for autosomes using RAD‐seq data between two closely related species of long‐tailed tits (Aegithalos bonvaloti and A. fuliginosus) in both allopatric and contact zone populations. The results confirm recent or ongoing gene flow between these two species. However, there is little evidence that the genomic regions that were found to be highly differentiated between the contact zone populations are resistant to gene flow, suggesting that differential levels of gene flow is not the cause of the heterogeneous genomic differentiation. Linked selection may be the cause of genomic differentiation islands between the allopatric populations with no or very limited gene flow, but this could not account for the heterogeneous genomic differentiation between the contact zone populations, which show evidence of recent or ongoing gene flow.     

The freshwater planarian Dugesia sicula Lepori from Sardinia (Platyhelminthes,Tricladida)

Fissiparous strains of freshwater triclads of the Dugesia gonocephala group were collected from 12 localities in Sardinia all situated not more than 5 kilometers from the coast. Some environmental factors and the sexual status of the specimens were noted at the time of collection. During the laboratory rearing 30% of individuals of each strain became sexual (ex-fissiparous individuals). All the examined strains showed common karyological and morphological characteristics suggestive of the species Dugesia sicula Lepori. The chromosome complement, which was a constant 27+2–3 B chromosomes, was classified as aneutriploid, due to its clearly documented characteristics. The fissiparous populations of D. sicula appear to have a high degree of tolerance to variations in environmental factors, especially temperature. Within the D. gonocephala group this species has the broadest distribution in the Mediterranean region.