Testing the ecological relevance of Daphnia species designations

1. Molecular approaches have increasingly revealed hidden genetic structure within ecologically important species, leading to the creation of sibling species whose ecological relevance is often unclear. A prime example is Daphnia galeata mendotae, which was split into D. dentifera and D. mendotae based on differences at two allozyme loci. 2. In a set of lake populations in Michigan USA, we test the geographical and temporal consistency of the genetic structure underlying this species split. We also test the morphological relevance of this molecular variation and its ecological significance in lakes. In essence, we ask: does recognition of these new species provide valuable information for plankton ecologists? 3. We found that D. dentifera and D. mendotae represent morphologically and ecologically distinct forms that are distributed among lakes in non‐random fashion, which were remarkably stable over 6 years. Key differences between the species concern their body and head shape, vertical habitat use within lakes and distribution among lakes of different size. We hypothesise that these differences represent specialisation to habitats that differ in risk of invertebrate predation. 4. Reproductive barriers alone are insufficient to explain the pattern of genetic structure; in some lakes complete introgression is apparent. However, parent species and hybrids exhibit a stable co‐existence in many lakes, which suggests that ecological specialisation reinforces divergence within this taxon.     

North Sea cod and climate change – modelling the effects of temperature on population dynamics

In order to examine the likely impacts of climate change on fish stocks, it is necessary to couple the output from large‐scale climate models to fisheries population simulations. Using projections of future North Sea surface temperatures for the period 2000–2050 from the Hadley General Circulation Model, we estimate the likely effects of climate change on the North Sea cod population. Output from the model suggests that increasing temperatures will lead to an increased rate of decline in the North Sea cod population compared with simulations that ignore environmental change. Although the simulation developed here is relatively simplistic, we demonstrate that inclusion of environmental factors in population models can markedly alter one's perception of how the population will behave. The development of simulations incorporating environment effects will become increasingly important as the impacts of climate change on the marine ecosystem become more pronounced.     

Inactivation of phosphorylase is a major component of the mechanism by which insulin stimulates hepatic glycogen synthesis.

Multiple signalling pathways are involved in the mechanism by which insulin stimulates hepatic glycogen synthesis. In this study we used selective inhibitors of glycogen synthase kinase-3 (GSK-3) and an allosteric inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a, to determine the relative contributions of inactivation of GSK-3 and dephosphorylation of phosphorylase a as alternative pathways in the stimulation of glycogen synthesis by insulin in hepatocytes. GSK-3 inhibitors (SB-216763 and Li+) caused a greater activation of glycogen synthase than insulin (90% vs. 40%) but a smaller stimulation of glycogen synthesis (30% vs. 150%). The contribution of GSK-3 inactivation to insulin stimulation of glycogen synthesis was estimated to be less than 20%. Dephosphorylation of phosphorylase a with CP-91149 caused activation of glycogen synthase and translocation of the protein from a soluble to a particulate fraction and mimicked the stimulation of glycogen synthesis by insulin. The stimulation of glycogen synthesis by phosphorylase inactivation cannot be explained by either inhibition of glycogen degradation or activation of glycogen synthase alone and suggests an additional role for translocation of synthase. Titrations with the phosphorylase inactivator showed that stimulation of glycogen synthesis by insulin can be largely accounted for by inactivation of phosphorylase over a wide range of activities of phosphorylase a. We conclude that a signalling pathway involving dephosphorylation of phosphorylase a leading to both activation and translocation of glycogen synthase is a critical component of the mechanism by which insulin stimulates hepatic glycogen synthesis. Selective inactivation of phosphorylase can mimic insulin stimulation of hepatic glycogen synthesis.     

The importance of carotenoids in signaling during aggressive interactions between male firemouth cichlids (Cichlasoma meeki)

Male firemouth cichlids, Cichlasoma meeki, have red pigmentationcovering large areas of their ventral surface, which is displayedduring aggressive interactions. We manipulated the levels ofred pigmentation by assigning the fish to one of two diets,which were as similar as possible except that one was high incarotenoids while the other was low in carotenoids. During diadictrials under white light, fish kept on the high carotenoid dietwon a higher proportion of contests than fish kept on the lowcarotenoid diet Under green light, where differences in rednesscannot be discriminated, there was no effect of diet on theoutcome of contests. These experiments demonstrate that it isthe effect of the diet on red pigmentation that is importantrather than some confounding variable such as differential growthrates. The weight of the two fish was also important; therewas a tendency for the heavier fish to win more contests. Themass effect was subordinate to color under white light but wasthe dominant factor under green light The nature of the contestsunder the different light conditions also varied; the displayin which the red pigmentation is most obvious was not used undergreen light, but was common under white light This suggeststhat the display strategies are flexible and can be alteredaccording to which displays are most effective in a given environmentPrevious studies of other species of fish and birds have shownthat the degree of redness influences mate choice and is affectedby parasite infestations. We propose that carotenoid pigmentationis likely to reflect a general quality, influenced by severalfactors, rather than a context-specific quality such as fightingability.     

An experimental analysis of mate choice in the wren: a monomorphic, polygynous passerine

Male wrens (Troglodytes troglodytes) construct nests that areused in their display to females. Previous work has suggestedthat the number of vacant nests may be used as a mate choicecue. Correlational data from 1992 confirmed that females appearedto be assessing die number of vacant nests on a male's territoryand preferentially mating with males with more nests. Male taillengdi was also correlated widi mating success. In 1993 thenumbers of nests on territories was experimentally manipulated,the female setdement patterns confirmed that die number of vacantnests did mediate mate choice. Male tail length failed to explainadditional variance in mating success when die variance explainedby the experimental manipulation was removed, suggesting diatdie original correlation arose because both tail length andmating success were correlated widi a confounding variable.The structure of the vegetation in a male's territory influencedmating success. This appeared to be due to nests surviving betterin territories widi dense vegetation. Males on territories inwhich nests survive well had longer tails. Male-male competitionfor good territories may explain die observed effects of malemorphology on mating success. Furdier analysis of die nest choicedata showed diat all nests had an equal chance of being usedby a female. The fact diat all nests had an equal probabilityof being chosen by a female means diat each additional nestbuilt by a male wren results in die same increase in matingsuccess. This suggests diat die benefits to males of nest buildingincrease linearly. The number of nests on a territory will beaffected by various factors such as predation pressure, nestbuilding rate, and vegetation structure. The information diatfemales are getting by assessing such a signal is discussed.     

Altered SOS induction associated with mutations in recF, recO and recR

The SOS system of Escherichia coli aids survival following damage to DNA by promoting DNA repair while cell division is delayed. Induction of the SOS response is dependent on RecA and also on the product of recF. We show that normal induction also requires the products of recO and recR. SOS induction was monitored using a sfiA-lacZ fusion strain. Induction was delayed to a similar degree by mutation in recF, recO or recR. A similar effect was observed following overexpression of RecR from a recombinant recR ⁺plasmid. We show that the overexpression of RecR also reduces the UV resistance of a recBC sbcBC strain and of a sfiA strain, but not of a rec ⁺ sfiA ⁺strain. The implications of these data for the kinetics of DNA repair are discussed.