Life-strategy shift by intraspecific interaction in Daphnia magna : change in reproduction from quantity to quality

In Daphnia magna, high animal density causes a mutual intraspecific influence, called life-strategy shift by intraspecific interaction (LiSSII). We investigated mothers (F₀), living singly or in groups of four per 50 ml, and their F₁ offspring. Clutch sizes of F₀ daphnids living in groups were smaller than those of singly living ones. However, the F₁ neonates were bigger in size, had higher dry weights and contained more lipids. They were also able to survive longer starvation periods. If these F₁␣daphnids were again kept in groups of four, the effects of LiSSII on some life-history parameters were stronger. As compared to group-living F₀ daphnids, juvenile development time was prolonged from 7.5 to 10.0 days. In contrast, body size was reduced and the further reduction in offspring number was slight. The F₂␣neonates exhibited the same change towards bigger size and higher dry weight as observed in the F₁. In contrast F₁ daphnids, that were the offspring of daphnids living in groups but were kept singly were bigger and produced more offspring than single daphnids that were the offspring of daphnids kept singly. The neonates of the former were bigger in size and heavier. It is suggested that for the F₀ generation the ecological significance of the LiSSII effects is an increase in both the parent and offspring survival probability when food is limited. LiSSII acts before food conditions deteriorate seriously. Daphnids affected by LiSSII invest less of their biomass into offspring. Offspring release is well-timed and quality is better, at the expense of quantity. Under favourable environmental conditions, F₂ daphnids that are the offspring of mothers affected by LiSSII appeared well adapted to fast recolonisation when living at low density. Received: 18 December 1995 / Accepted: 21 November 1996     

Carapace epithelia are rich in large filamentous actin bundles in Daphnia magna,Daphnia pulex,and Sida crystallina (Crustacea: Cladocera)

Cladocerans (water fleas) are planktonic crustaceans that typically have a bivalved carapace. Each valve of the carapace consists of two cuticle‐secreting epithelial layers that are separated by a hemolymphatic chamber and joined by pillar structures. Ultrastructural analyses in several species of Cladocera have shown that the carapace epithelia and pillars contain filamentous structures of unknown composition. In the present study we used a fluorescent phalloidin conjugate to show that the carapaces of three cladocerans, Daphnia magna, D. pulex, and Sida crystallina, are rich in large bundles of filamentous actin (F‐actin). In D. magna we employed confocal microscopy and orthogonal views of three‐dimensional reconstructions to show that these bundles extend radially from foci in the pillars towards the integument surfaces, and their structure is consistent with that of contractile stress fibers. Using a fluorescent lipophilic stain, DiOC₆(3), we show that the F‐actin bundles are distributed in membrane‐rich regions within the carapace epithelia, and that, in the superficial epithelium, these may be large membrane‐bound organelles. In D. magna, the F‐actin bundles are present in embryonic, juvenile instar, and adult, developmental stages, and through development the bundles become larger, contain more F‐actin, and become more widely spaced. We present an alignment of the deduced amino acid sequences of six putative D. pulex actin genes, and discuss the implications that their respective sequences have on the likelihood of their inclusion into the F‐actin bundles of the carapace. Our identification of these large F‐actin bundles within the pillars of three cladocerans provides new insight into the role these structures play in influencing carapace dynamics within this order.     

Sexual dimorphism and species differences in the neurophysiology and morphology of the acoustic communication system of two neotropical hylids

We examined auditory tuning and the morphology of the anatomical structures underlying acoustic communication in female Hyla microcephala and H. ebraccata and compared our findings to data from a previous study (Wilczynski et al. 1993) in which we showed species differences in the traits that in males relate to differences in the species-typical calls. Female species differences in the best excitatory frequency (BEF) of the basilar papilla (BP) were similar to the differences seen in males, and females had a significantly lower BEF in H. ebraccata, but not H. microcephala. In both species, females had lower BP thresholds. Snout-vent length, head width, and tympanic membrane diameters were sexually dimorphic in both species and larger in females, whereas laryngeal components were sexually dimorphic and larger in males. Middle and inner ear volumes were not sexually dimorphic. Despite the significant species differences in laryngeal morphology seen in males, female larynges are not significantly different. Furthermore, the interaction of species and sex differences resulted in significantly different degrees of sex dimorphism in the species, particularly for the larynx, which is more sexually dimorphic in H. microcephala, and measures of body size, which are more dimorphic in H. ebraccata. Accepted: 6 December 1996     

The Behavior of a Daphnia pulex Transposable Element in Cyclically and Obligately Parthenogenetic Populations

Using Southern blot analysis, we have characterized restriction fragment patterns of a transposable element, Pokey, in obligately and cyclically parthenogenetic populations of the cladoceran crustacean Daphnia pulex. We show that the element is most likely active in cyclically parthenogenetic populations but is, for the most part, inactive in obligate parthenogens. This result is consistent with theory suggesting that transposable element dynamics are likely to change with a change in reproductive mode. Such changes could have important consequences for the long-term evolutionary potential of obligate parthenogens and may also be informative with regard to the underlying mechanisms that regulate transposable element frequencies in sexual organisms. Received: 29 August 2000 / Accepted: 1 March 2001     

The birch tube-maker Acrobasis betulella in a fragmented habitat: the importance of patch isolation and edges

We examined the response of the birch tube-maker Acrobasis betulella Hulst (Lepidoptera: Pyralidae) to habitat patch isolation and edges. Density of A.␣betulella larva was higher on distant islands than on islands close to the shore. Following experimental removal of all larvae from the islands, adults were able to recolonize even the most distant islands, and larval density was again positively correlated with the degree of isolation. Larval density was not correlated with island size or the amount of birch present on the islands. Larvae on more distant islands did not have lower mortality than those in less isolated sites. Larvae were found more often on edges than in the interior of birch stands and developed faster on edges. This positive edge effect, coupled with the ability of the adults to disperse to even our most distant islands, suggests that A. betulella would be favored in an environment fragmented at the same scale as our island system. Received: 15 April 1996 / Accepted: 30 September 1996     

The influence of pollen addition and ramet isolation on current sexual reproduction in a clonal herb

Canada mayflower (Maianthemum canadense Desf.), a rhizomatous, perennial herb, was the subject of a 2-year field experiment that examined two factors potentially affecting fruit and seed production: pollen addition and ramet isolation. Ramets were either open pollinated or overpollinated by hand to supplement natural levels. Rhizomes of the ramets were either severed, to prevent resource supplementation from the genet, or left intact. Ramets that were overpollinated matured more fruits and more seeds than ramets that were open pollinated. Thus, mayflower appears to have been pollen limited in both years. Ramets that were open pollinated and whose rhizomes were severed matured as many fruits, seeds, and seeds/fruit as ramets that were open pollinated and whose rhizomes were left intact. Ramets that were overpollinated and whose rhizomes were severed matured fewer fruits, seeds, and seeds/fruit than overpollinated ramets whose rhizomes were left intact. It appears that at natural levels of pollination, mayflower ramets are physiologically independent but as the level of pollen increases, mayflower ramets receive support from other parts of the genet. Received: 13 May 1996 / Accepted: 1 November 1996     

Populus tremuloides photosynthesis and crown architecture in response to elevated CO2 and soil N availability

We tested the hypothesis that elevated CO₂ would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO₂. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO₂ and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g⁻¹ day⁻¹ net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO₂ resulted in significantly higher maximum leaf photosynthesis (A _max) at both soil N levels. A _max was higher at high N than at low N soil in elevated, but not ambient CO₂. Photosynthetic N use efficiency was higher at elevated than ambient CO₂ in both soil types. Elevated CO₂ resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO₂ and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V _cmax/J _max) than at ambient CO₂ and/or low N availability. From the top to the bottom of the tree crowns, V _cmax/J _max increased in ambient CO₂, but it decreased in elevated CO₂ indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO₂. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A _leaf for each segment. This analysis indicated that increased A _leaf at elevated CO₂ did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO₂. Received: 12 August 1996 / Accepted: 12 November 1996     

Chemical communication in heliothine moths. VII. Correlation between diminished responses to point-source plumes and single filaments similarly tainted with a behavioral antagonist

Addition of (Z)-11-hexadecenyl acetate (Z11-16:Ac) into a normally attractive binary blend of Heliothis virescens pheromone components resulted in a suppression of upwind flight and source location by males. Male response was reduced even at the lowest dosages of Z11-16:Ac tested but upwind flight and source location were most clearly reduced when the loading of Z11-16:Ac reached 10% or more of the (Z)-11-hexadecenal (Z11-16:Ald) loading (the major component present in the binary blend). Similar patterns of suppression in response were noted when Z11-16:Ac was added to binary blends of pheromone components at both 10 and 100 μg loadings of Z11-16:Ald. Males in casting flight following upwind flight in a mechanically generated pulsed plume, responded to the interception of a subsequent, single binary-blend filament by making a toward-source upwind surge. Responses of males to a single filament that was tainted by a level of Z11-16:Ac that had allowed some reduced level of upwind flight and source location to occur in the previous plume experiments were diminished compared with their control counterparts. Analysis of the flight tracks revealed that the surges in response to single tainted filaments were stunted because males made fewer significant changes in course angles steered, airspeeds generated, and in the tempo of counterturns executed. Accepted: 28 December 1996     

Evolutionary history of alpine and subalpine Daphnia in western North America

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Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests. Received: 2 November 1996 / Accepted: 8 February 1997     

The role of hybridization in the origin and spread of asexuality in Daphnia

The molecular mechanisms leading to asexuality remain little understood despite their substantial bearing on why sexual reproduction is dominant in nature. Here, we examine the role of hybridization in the origin and spread of obligate asexuality in Daphnia pulex, arguably the best‐documented case of contagious asexuality. Obligately parthenogenetic (OP) clones of D. pulex have traditionally been separated into ‘hybrid’ (Ldh SF) and ‘nonhybrid’ (Ldh SS) forms because the lactase dehydrogenase (Ldh) locus distinguishes the cyclically parthenogenetic (CP) lake dwelling Daphnia pulicaria (Ldh FF) from its ephemeral pond dwelling sister species D. pulex (Ldh SS). The results of our population genetic analyses based on microsatellite loci suggest that both Ldh SS and SF OP individuals can originate from the crossing of CP female F₁ (D. pulex × D. pulicaria) and backcross with males from OP lineages carrying genes that suppress meiosis specifically in female offspring. In previous studies, a suite of diagnostic markers was found to be associated with OP in Ldh SS D. pulex lineages. Our association mapping supports a similar genetic mechanism for the spread of obligate parthenogenesis in Ldh SF OP individuals. Interestingly, our study shows that CP D. pulicaria carry many of the diagnostic microsatellite alleles associated with obligate parthenogenesis. We argue that the assemblage of mutations that suppress meiosis and underlie obligate parthenogenesis in D. pulex originated due to a unique historical hybridization and introgression event between D. pulex and D. pulicaria.     

Sperm number assessed by flow cytometry in species of Daphnia (Crustacea,Cladocera)

Daphnia magna and Daphnia pulex are two important model species in ecotoxicology. In daphniids, studies of the effects of contaminants have mostly focused on female life history traits, yet it would also be important to examine male reproductive traits, particularly in relation to endocrine disruptors. In this study, we developed a protocol that uses flow cytometry to measure sperm number in individual males of different species of Daphnia. We tested our protocol on 114 males from several clones of three common species of Daphnia. Sperm count varied widely among individuals and reached high numbers (up to 1.45 × 10⁵). Positive relationships between male length and sperm number were observed in D. pulex and Daphnia pulicaria, but not in D. magna. Important inter‐clonal differences in sperm production were observed in all species, with some clones producing very little sperm. Duplicated sperm samples showed on average only 6% difference in sperm counts. Sperm counts were stable at least over a 2‐hr period and up to 5 hr for most samples. This sperm isolation protocol and flow cytometric enumeration approach will be of major interest to ecotoxicologists.     

Microsporidiosis in a Daphnia pulex population

The impact of parasitism by Thelohania sp., an intracellular microsporidian, on Daphnia pulex inhabiting a vernal pond was studied for three years. Three issues were considered: the distribution of parasites among hosts, the effect of parasites on individuals, and the impact of parasitism on population growth. Each year, Thelohania infected members of at least half of the generations of Daphnia produced in the pond. When the frequency of infection was low, parasites were found only in large adults. As infection frequency increased, parasitism spread downward through host size classes. However, parasites were rarely found in juveniles. Parasitism reduced clutch size drastically, increased mortality to a variable extent, had little impact on egg size or on per instar growth and none on molt frequency. Interaction with other stresses, such as food limitation, exacerbated some of these effects. Parasitism reduced instantaneous birth rate much more than it elevated instantaneous death rate. Population growth was reduced significantly but it is unlikely that Thelohania alone regulates the growth of this Daphnia population.     

Sexual reproduction in Daphnia pulex (Crustacea: Cladocera): observations on male mating behaviour and avoidance of inbreeding

1.  Mating behaviour in Daphnia appears to rely on random contact between males and sexual females rather than diffusible pheromones. Males may be able to discriminate sexually receptive females from females in other developmental stages and increase their mating efficiency. Males may also use chemical signals to avoid mating with females from the same clone and avoid the severe inbreeding depression that has been documented for intraclonal mating. The present study used experiments to test for the avoidance of intraclonal mating and assess male mating efficiency in D. pulex .
2.  Three clones were examined for the avoidance of intraclonal mating by providing males with an opportunity to mate with females of the same or two different clones. The proportion of intraclonal matings did not differ from the proportion of interclonal matings, suggesting that D. pulex males do not use kin discrimination to avoid mating with females from the same clone.
3.  The proportion of mated females decreased with increasing numbers and density of sexual females when exposed to a single male. This observation suggests that a male spends more time pursuing and copulating with sexually receptive females than non-receptive females and there is insufficient time to mate with all sexual females. The decrease in proportion of females mated could also be the result of sperm depletion in the male. Sperm depletion is unlikely to occur in nature because sexually receptive females are much rarer than in the experimental conditions.     

Adaptive reconfiguration of a reflex circuit during different motor programmes in the locust

The cuticle strain which develops in the hindleg tibiae when a locust prepares to kick, or when the tibia thrusts against an obstacle, is detected by two campaniform sensilla, which reflexly excite the fast extensor tibiae motoneuron, some of the flexor tibiae motoneurons and nonspiking interneurons. The reflex excitation is adaptive for the extensor motoneuron during both co-activation and thrusting, but is only adaptive for the flexor motoneurons during co-activation, and is maladaptive during thrusting. We show that the femoral chordotonal organ, which monitors tibial position, controls the efficacy of the strain feedback. The campaniform sensilla-induced depolarization in the extensor motoneuron is about twice as large when the tendon is in mid position (reflecting a tibial-femoral angle of 90°) than when fully stretched (reflecting tibial flexion), while in the flexors the reverse is true. The amplitudes of excitatory postsynaptic potentials evoked by single campaniform sensilla spikes, are, however, not affected. Our data suggests that the chordotonal organ modulates the gain of the strain feedback onto the motoneurons by exciting interneuronal circuits whose output sums with the former. Thrusting typically occurs with the tibia partially extended, therefore the actions of the chordotonal organ support the production of a maximal thrusting force. Accepted: 27 December 1996     

Feature-detecting auditory neurons in the brain of a sound-producing fish

The mormyrid fish Pollimyrus adspersus has auditory specializations for sound pressure detection and uses acoustic displays in its natural social behavior. In this paper it is shown that auditory neurons in the mesencephalon (torus semicircularis) are activated selectively by temporal features of complex sounds. Single neurons were recorded while presenting sounds to fish underwater. The stimuli were acoustic click trains, 400 ms in duration, and were synthesized with differing inter-click-intervals (ICIs). The natural sounds of this species are composed similarly and the range of ICIs synthesized overlapped with the natural range (5–40 ms). One-third of the neurons studied were strongly selective for a narrow range of ICIs, increasing spike rate by ten fold or more at the best ICI compared to the minimum response observed. The best ICI for interval selective neurons remained stable when the sound pressure of the stimulus was changed. Neurons that were selective gave phasic responses to tone bursts, and most had non-monotonic rate level functions. The origin of interval selectivity is discussed and a time-based computational mechanism is proposed. Accepted: 20 December 1996     

Acorn selection by Mexican jays: a test of a tri-trophic symbiotic relationship hypothesis

By caching acorns, jays serve as important dispersal agents for oak (Quercus) species. Yet little is known about which acorn characteristics affect selection by jays. In the traditional model of jay/oak symbiosis, large, brown, ripe acorns free of invertebrate parasites (e.g., Curculio acorn weevils) are selected by jays. Recently, it has been suggested that a tri-trophic relationship between oaks, jays, and weevils may have evolved to counter the negative dietary effects of acorn tannins. Under the tri-trophic model, jays would preferentially select acorns containing weevil larvae. We tested the assumptions that (1) acorns containing curculionid larvae exist in sufficient quantities to support jay populations and (2) jays can detect, and preferentially select, acorns containing weevil larvae, and investigated the cues by which jays select acorns. Captive Mexican jays (Aphelocomaultramarina) were presented Emory oak (Quercusemoryi) acorns in aviary feeding trials. Large, dense, viable acorns free of curculionid larvae were preferentially selected. Contrary to results of previous research, color did not affect selection. Acorn viability increased and curculionid larval occupancy decreased in adjacent savannas and isolated stands relative to existing oak woodland, perhaps favoring oak recruitment into adjacent lower-elevation grasslands. Our results compel us to reject the tri-trophic model for this system, and are consistent with the traditional jay/oak symbiosis model. Relatively long-distance dispersal of viable acorns favors Emory oak replacement, and spatial patterns of acorn viability and curculionid parasitism suggest expansion of Emory oak into adjacent low-elevation semi-arid grasslands. Received: 29 February 1996 / Accepted: 26 September 1996