Cell death in lake phytoplankton communities

1. The fraction of living and dead phytoplankton cells in seven Florida lakes was assessed by using the cell digestion assay, a non‐staining membrane permeability test. The cell digestion assay is an effective method to analyse cell viability in complex natural phytoplankton communities. 2. The lakes examined ranged widely in phytoplankton abundance and community composition. The variability in the percentage of living cells (% LC) was high among the taxonomic groups forming the different phytoplankton communities, ranging from 19.7% to 98% LC. 3. All cells within single cyanobacteria filaments were determined to be either dead or alive, suggesting physiological integration of the cells within colonies. 4. Within each lake, the dominant taxa generally exhibited the highest proportion of living cells. A high proportion of living cells was found to be a characteristic of the different taxa forming the communities of eutrophic lakes. The average value for the % LC for all groups comprising the phytoplankton communities in each of the lakes ranged from 29.9 ± 7.2 to 80.4 ± 4.0 (mean ± SE) and varied strongly and positively with chlorophyll a concentration. 5. These results suggest phytoplankton cell death to be an important process structuring phytoplankton communities in lakes, particularly in oligotrophic ones.     

Population history and genetic structure of a circumpolar species: the arctic fox

The circumpolar arctic fox Alopex lagopus thrives in cold climates and has a high migration rate involving long-distance movements. Thus, it differs from many temperate taxa that were subjected to cyclical restriction in glacial refugia during the Ice Ages. We investigated population history and genetic structure through mitochondrial control region variation in 191 arctic foxes from throughout the arctic. Several haplotypes had a Holarctic distribution and no phylogeographical structure was found. Furthermore, there was no difference in haplotype diversity between populations inhabiting previously glaciated and unglaciated regions. This suggests current gene flow among the studied populations, with the exception of those in Iceland, which is surrounded by year-round open water. Arctic foxes have often been separated into two ecotypes: 'lemming' and 'coastal'. An analysis of molecular variance suggested particularly high gene flow among populations of the 'lemming' ecotype. This could be explained by their higher migration rate and reduced fitness in migrants between ecotypes. A mismatch analysis indicated a sudden expansion in population size around 118 000 BP, which coincides with the last interglacial. We propose that glacial cycles affected the arctic fox in a way opposite to their effect on temperate species, with interglacials leading to short-term isolation in northern refugia.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 79–89.     

Seasonal clutch size variation of multi-brooded bird species: comparisons between breeding season and latitudes

Single- and multi-brooded species of birds differ in their seasonal patterns of clutch size. Single-brooded species start with a maximum clutch size that declines continuously as the season progresses, whereas the clutch sizes of multi-brooded species usually increase to a mid-season maximum peak and then decrease progressively until the end of the breeding season. Previous studies have shown that multi-brooded migrant species present seasonal patterns that are similar to single-brooded species at high latitudes but similar to multi-brooded non-migratory species at lower latitudes. We studied the Greenfinch Carduelis chloris and Goldfinch C. carduelis populations in eastern Spain (Sagunto, Valencia) between 1975 and 2002 to compare seasonal variations in clutch sizes between years with early and late starts to the breeding season. The period over which clutch sizes increase was longer when the breeding season started earlier. The Goldfinch population showed no pattern of initial increase in clutch size when there was a late start to the breeding season: a late start shortens the season giving them less time to breed, and may also coincide with maximum food availability. Thus, the pattern of single-brooded species was observed. In the Greenfinch population, a trend toward the seasonal pattern of single-brooded species was also observed when the following indices were compared: clutch size increase, modal timing, initial slope and timing of maximum clutch size. We have also compared the seasonal patterns of clutch size of both species in eastern Spain with the patterns observed in Britain. Our results show that for both Goldfinches and Greenfinches, the non-migrant southern populations of Sagunto in eastern Spain do not tend towards a more multi-brooded seasonal pattern of clutch size than the migrant Goldfinches of Britain.     

Effects of genetics and light environment on colour expression in threespine sticklebacks

The genetic basis of traits that are under sexual selection and that are involved in recognizing conspecific mates is poorly known, even in systems in which the phenotypic basis of these traits has been well studied. In the present study, we investigate genetic and environmental influences on nuptial colour, which plays important roles in sexual selection and sexual isolation in species pairs of limnetic and benthic threespine sticklebacks ( Gasterosteus aculeatus species complex). Previous work demonstrated that colour differences among species correlate to differences in the ambient light prevalent in their mating habitat. Red fish are found in clear water and black fish in red-shifted habitats. We used a paternal half-sib split-clutch design to investigate the genetic and environmental basis of nuptial colour. We found genetic differences between a red and a black stickleback population in the expression of both red and black nuptial colour. In addition, the light environment influenced colour expression, and genotype by environment interactions were also present. We found evidence for both phenotypic and genetic correlations between our colour traits; some of these correlations are in opposite directions for our red and black populations. These results suggest that both genetic change and phenotypic plasticity underlie the correlation of male colour with light environment.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 663–673.     

Turbidity-mediated interactions between invasive filter-feeding mussels and native bioturbating mayflies

1. Invasive dreissenid mussels are known to cause large ecosystem changes because of their high filter‐feeding capacity, while native bioturbators may interfere with the mussels filter feeding. In this experiment, we investigated indirect environmental interactions between invasive filter‐feeding dreissenid mussels (zebra and quagga mussels) and native recolonizing bioturbating hexagenid mayflies (Hexagenia) at two mussel densities and two Hexagenia densities in a 2‐month long laboratory experiment. 2. Mean turbidity increased with increasing density of Hexagenia and decreased with increasing density of mussels. Turbidity showed the fastest decline at the highest mussel density, and no decline or a lower rate of decline at the low mussel density, dependent on Hexagenia density. 3. Mussel growth decreased with increasing Hexagenia density at low but not at high mussel density. Moreover, growth of mussels decreased as a function of increased mean turbidity at low mussel density but not at high mussel density. Filtering activity at the highest mussel density increased after introduction of food at the lower two densities of Hexagenia, but was constantly high at the highest Hexagenia density. 4. There was no difference in emergence of Hexagenia among the treatments, but mortality of Hexagenia was higher in the presence of mussels than in their absence. 5. Our results indicate that interactions between dreissenids and hexagenids are mediated through the sediment, and depend on density of both dreissenids and hexagenids. As the natural densities of these animals vary considerably within lakes, their growth and survival because of indirect environmental interactions is expected to vary spatially.     

Rapid diversification by recombination in Bartonella grahamii from wild rodents in Asia contrasts with low levels of genomic divergence in Northern Europe and America

Bartonella is a genus of vector‐borne bacteria that infect the red blood cells of mammals, and includes several human‐specific and zoonotic pathogens. Bartonella grahamii has a wide host range and is one of the most prevalent Bartonella species in wild rodents. We studied the population structure, genome content and genome plasticity of a collection of 26 B. grahamii isolates from 11 species of wild rodents in seven countries. We found strong geographic patterns, high recombination frequencies and large variations in genome size in B. grahamii compared with previously analysed cat‐ and human‐associated Bartonella species. The extent of sequence divergence in B. grahamii populations was markedly lower in Europe and North America than in Asia, and several recombination events were predicted between the Asian strains. We discuss environmental and demographic factors that may underlie the observed differences.     

Behaviour and stress response during capture and handling of the red-billed chough Pyrrhocorax pyrrhocorax (Aves: Corvidae)

We studied the effect of capture and handling on free-living red-billed choughs Pyrrhocorax pyrrhocorax . We analysed the association between bird behavioural response and individual health, breeding status, sex, and age. Active responses (aggression towards the ringer, screaming) were more common in the breeding period, and in individuals with a high heterophils to lymphocytes ratio, indicating poor physiological condition and/or high chronic stress. Adults were more aggressive than juveniles and yearlings, and females were more aggressive than males. Sex, age, and condition differences were also recorded in the spectrotemporal output of distress calls. Birds with a screaming/active response appeared to be more stress-susceptible than passive and silent individuals, and this response was stronger during the energy demanding period of reproduction. The results obtained suggest that the response of the red-billed chough during capture might primarily reflect stress-susceptibility, although a number of potential alternative explanations are discussed.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 846–855.     

THE EVOLUTION OF HETEROMORPHIC SEX CHROMOSOMES

The facts and ideas which have been discussed lead to the following synthesis and model. 1. Heteromorphic sex chromosomes evolved from a pair of homomorphic chromosomes which had an allelic difference at the sex-determining locus. 2. The first step in the evolution of sex-chromosome heteromorphism involved either a conformational or a structural difference between the homologues. A structural difference could have arisen through a rearrangement such as an inversion or a translocation. A conformational difference could have occurred if the sex-determining locus was located in a chromosomal domain which behaved as a single control unit and involved a substantial segment of the chromosome. It is assumed that any conformational difference present in somatic cells would have been maintained in meiotic prophase. 3. Lack of conformational or structural homology between the sex chromosomes led to meiotic pairing failure. Since pairing failure reduced fertility, mechanisms preventing it had a selective advantage. Meiotic inactivation (heterochromatinization) of the differential region of the X chromosome in species with heterogametic males and euchromatinization of the W in species with heterogametic females are such mechanisms, and through them the pairing problems are avoided. 4. Structural and conformational differences between the sex chromosomes in the heterogametic sex reduced recombination. In heterogametic males recombination was reduced still further by the heterochromatinization of the X chromosome, which evolved in response to selection against meiotic pairing failure. 5. Suppression of recombination resulted in an increase in the mutation rate and an increased rate of fixation of deleterious mutations in the recombination-free chromosome regions. Functional degeneration of the genetically isolated regions of the Y and W was the result. In XY males this often led to further meiotic inactivation of the differential region of the X chromosome, and in this way an evolutionary positive-feedback loop may have been established. 6. Structural degeneration (loss of material) followed functional degeneration of Y or W chromosomes either because the functionally degenerate genes had deleterious effects which made their loss a selective advantage, or because shorter chromosomes were selectively neutral and became fixed by chance. 7. The evolutionary routes to sex-chromosome heteromorphism in groups with female heterogamety are more limited than in those with male heterogamety. Oocytes are usually large and long-lived, and are likely to need the products of X- or Z-linked genes. Meiotic inactivation of these chromosomes is therefore unlikely. In the oocytes of ZW females, meiotic pairing failure is avoided through euchromatinization of the W rather than heterochromatinization of the Z chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)