The maxillary palp of Drosophila : ultrastructure and physiology depends on the lozenge gene

The ultrastructure and physiology of the maxillary palp of Drosophila melanogaster have been studied in wild-type and lozenge mutants. Olfactory physiology in the maxillary palp is shown to depend upon the lozenge(lz) gene. Reduced response amplitudes were recorded for all odorants tested, and the physiological defect was shown to map to the lz locus. The structure of the maxillary palp sensilla is described by scanning electron microscopy (SEM) at high magnification, initially in the wild-type. A linear arrangement of pores, connected by furrows, was found in one class of sensilla, the basiconic sensilla. In the lz ³ mutant, morphological alterations in the basiconic sensilla and duplications of sensilla are documented by SEM. The correlation of structural abnormalities in the lz sensilla and physiological abnormalities in odorant response are consistent with an olfactory role for the basiconic sensilla of the maxillary palp. Accepted: 10 September 1996     

Multiple excitatory receptor types on individual olfactory neurons: implications for coding of mixtures in the spiny lobster

The aim of our paper was to investigate whether single olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus functionally express more than one type of receptor, examine the consequences of this on coding of mixtures, and compare principles of odorant mixture coding by spiny lobsters with that by the channel catfish, which has been studied extensively using the same experimental and analytical procedures (Caprio et al. 1989; Kang and Caprio 1991). We examined responses of individual taurine-sensitive ORNs to binary mixtures of excitatory compounds, either competitive agonists (taurine, β-alanine, hypotaurine) or non-competitive agonists (taurine, l-glutamate, ammonium chloride, adenosine-5′-monophosphate). Responses to mixtures were compared to two indices: mixture discrimination index (MDI) and independent component index (ICI). Binary mixtures of competitive agonists had MDI values close to 1.0, as expected for competitors. Mixtures of non-competitive agonists had ICI values averaging 0.83, indicating the effects of the components are not independent. We conclude that individual olfactory cells of spiny lobsters can express more than one type of receptor mediating excitation, one of which typically has a much higher density or affinity, and that spiny lobster and catfish olfactory cells encode mixtures of two excitatory agonists using similar rules. Accepted: 20 December 1996     

Response of American cockroach (Periplaneta americana) olfactory receptors to selected alcohol odorants and their binary combinations

The cockroach is known to possess several morphologically distinct types of sensilla on its antenna, each of which contain a couple or a few receptor cells that respond to an array of compounds. We recorded the response of cells exclusively from one type of sensillum to evaluate the variation in the response of the cells in these sensilla to three closely related alcohols and their binary mixtures. Our results indicate that cells within the class of those responsive to aliphatic alcohols are otherwise variable in their response to particular aliphatic alcohols and not easily classifiable into subclasses. They also indicate that patterns of responses among cells are not robust with respect to concentration. Finally, a considerable level of inhibition is indicated in the response of the receptor cells to binary mixtures compared with the response to pure odorants. The data suggest that discrimination of alcohols (and other odorants of general but not special significance) by the cockroach cannot be understood simply in terms of labeled lines or linear filters. Accepted: 20 December 1996     

Physiological characterization of antennular flicking reflexes in the crayfish

The cellular substrates of antennular flicking behavior in the crayfish Procambarus clarkii were investigated. Flicking involves fast downward movements of the external filament of each biramous antennule (1st antenna), and is mediated by phasic contractions of a short muscle, the external filament depressor. Phasic contractions of the external filament depressor depend upon stereotyped impulse bursts in a single motorneuron (P1). These bursts have a characteristic impulse frequency profile that is consistent upon successive occurrences. The temporal characteristics of the impulse burst suggest that the central depolarizations generating each burst may be similar to driver potentials described for motor neurons in crustacean cardiac ganglia. Responses of the external filament to odorants have a long latency and are characterized by repetitive bursts and tonic activity in some external filament depressor fibers. Tonic activity in a slowly contracting muscle, the antennular depressor muscle, is also evoked by chemical stimulation. Flicking is consistently evoked only by mechanical or hydrodynamic stimulation of the cephalothorax, antennae and antennules. The sensitivity and short latency of the hydrodynamic antennule-generated flick reflex is consistent with the sensitivity of rapidly conducting, hydrodynamically activated mechanoreceptor neurons in both antennular filaments. I propose that antennular flicking, which has been shown to enhance the dynamic response characteristics of olfactory receptor neurons on the external antennular filament, has evolved as a response to the turbulence associated with fluid movement, within which chaotic odorant concentration fronts may be imbedded. Accepted: 23 October 1996     

Highly independent olfactory receptor sites for naturally occurring bile acids in the sea lamprey, Petromyzon marinus

(1) Electro-olfactogram recording was used to determine whether the olfactory epithelium of adult sea lamprey is specifically sensitive to bile acids, some of which have been hypothesized to function as pheromones. Ten bile acids were selected from 38 which had already been pre-screened for olfactory activity. These compounds were first tested on their own, then as adapting stimuli, and finally as components of mixtures (2) The lamprey-specific bile acids, petromyzonol sulfate and allocholic acid, were the most potent compounds tested. Five other bile acids were also detectable at picomolar concentrations. Petromyzonol sulfate had a distinctive dose-response curve. (3) Cross-adaptation demonstrated that sensitivity to bile acids is attributable to at least four independent classes of olfactory receptor sites and that both the nature and position of conjugating group(s) are critical to receptor specificity. Notably, petromyzonol sulfate has its own highly specific and independent receptor site. The situation for unconjugated bile acids was more complex and there appeared to be several sub-classes of receptor sites for these compounds. (4) Mixture studies largely confirmed the cross-adaptation results, describing receptor site independence for the same four sets of odorants. Mixture enhancement was also seen when expected and there was no evidence of mixture suppression. (5) Together, these data demonstrate that conspecific bile acids are discriminated by the olfactory epithelium of the sea lamprey, supporting the possibility that these compounds may function as migratory pheromones. Accepted: 23 November 1996     

Aluminum-sensitive mutants of Saccharomyces cerevisiae

We are developing budding yeast, Saccharomyces cerevisiae, as a genetic system for the study of tolerance to the trivalent aluminum cation (Al³⁺). We have isolated eight mutants that are more sensitive to Al³⁺ than the wild type. Each mutant represented a different complementation group. A number of the mutants were pleiotropic, and showed defects in other stress responses, changes in tolerance to other metal cations, or abnormal morphology. Two mutants also showed increased dependence on supplemental Mg²⁺ and Ca²⁺. One mutant with a relatively specific sensitivity to Al³⁺ was chosen for molecular complementation. Normal Al³⁺ tolerance was restored by expression of the MAP kinase gene SLT2. Strains carrying deletions of the SLT2 gene, or of the gene for the corresponding MAP kinase–kinase SLK1, showed sensitivity to Al³⁺. These results indicate that the SLT2 MAP kinase signal transduction pathway is required for yeast to sense and respond to Al³⁺ stress. Received: 17 April 1996 / Accepted: 21 October 1996     

Modulation of Drosophila heartbeat by neurotransmitters

The heart of Drosophila melanogaster is a simple muscular tube with a posterior pulsatile portion and a thoracic-cranial vessel. The pacemaker, located caudally, is myogenic. Its rate of firing is modulated by neurotransmitters. Serotonin, octopamine, norepineph-rine, dopamine, and acetylcholine accelerate the heart, in that order of potency. Dihydroxyphenylalanine, γ-aminobutyric acid, glutamate, and glycine have no effect. Generally, the regularity of the heartbeat is not adversely affected by treatment with any of these neurotransmitters. We show here that amnesiac, a neurological mutation, and Dihydroxyphenylalanine decarboxylase ^{temperature sensitive}, a mutation that interferes with synthesis of dopamine, norepinephrine, and serotonin, result in slower heart rate and reduced regularity across a normal range of temperatures for these flies. Dopamine-N-acetyltransferase, which is on the catabolic route to dopamine, serotonin, and octopamine, has no effect. hypoactiveC reduces the rate of the heart, but its mechanism of action is unknown. Accepted: 5 August 1996     

Genetic analysis of sterile mutants in the dpy-5 unc-13 (I) genomic region of Caenorhabditis elegans

Essential genes were identified in the 1.5-map unit dpy-5 unc-13 region of chromosome I in the Caenorhabditis elegans genome by rescuing lethal mutations using the duplication sDp2. In this paper, we report the mapping and complementation testing of lethal mutations, 45 of which identify 18 new, essential genes. This analysis brings the number of essential genes defined by the sDp2 rescue of lethal mutants to 97; 64 of these map between dpy-5 and unc-13. 61% of these essential genes are identified by more than one allele. Positioning of the mutations was done using the breakpoints of six duplications. The mutant phenotypes of 14 loci essential for fertility were characterized by Nomarski microscopy and DAPI staining. None of the mutants were rescued by wild-type male sperm. The cytological data showed that four genes produced mutants with defects in gonadogenesis, let-395, let-603, let-605 and let-610. Mutations in seven genes, let-355, let-367, let-384, let-513, let-544, let-545 and let-606, affected germ cell proliferation or gametogenesis. Mutants for the remaining three genes, let-370, let-599 and let-604, produced eggs that failed to develop or hatch, thereby acting as maternal effect lethals. We observed a nonrandom distribution of arrest phenotypes with regard to map position. Received: 8 May 1996 / Accepted : 27 January 1997     

The role of carbohydrates in the induction of flowering in Arabidopsis thaliana: comparison between the wild type and a starchless mutant

In order to test whether an increased export of carbohydrates by leaves and starch mobilization are critical for floral transition in Arabidopsis thaliana, the Columbia ecotype as well as its starchless mutant pgm and starch-in-excess mutant sex1 were investigated. Induction of flowering was achieved by exposure of plants to either one long day (LD) or one displaced short day (DSD). The following conclusions were drawn: (i) Both the pgm and sex1 mutants have a late-flowering phenotype in days shorter than 16 h. (ii) When inductive treatments cause a large percentage of induced plants, there is always a large, early and transient increase in carbohydrate export from leaves. By contrast, when an inductive treatment results in only a low percentage of induced plants (pgm plants exposed to one DSD), the export of carbohydrates from leaves is not increased, supporting the idea that phloem carbohydrates have a critical function in floral transition. (iii) Starch mobilization is not required to obtain an increased carbohydrate export when induction is by one LD (extended period of photosynthesis), but is absolutely essential when induction is by one DSD (period of photosynthesis unaffected). (iv) Floral induction apparently increases the capability of the leaf phloem-loading system. Received: 27 August 1997 / Accepted: 6 March 1998     

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 heat increment of feeding in house wren chicks: magnitude, duration, and substitution for thermostatic costs

The heat increment of feeding (HIF), a transient postprandial increase in metabolic rate, is the energy cost of processing a meal. We measured HIF in house wren chicks (Troglodytes aedon) ranging in mass from 1.6 to 10.3 g. This mass range (age 2–10 days) spanned a transition from blind, naked, ectothermic chicks through alert, endothermic birds with nearly complete feathering. We fed chicks crickets (2.7–10% of chick body mass) and determined HIF from continuous measurements of oxygen consumption rate (O₂) before and after meals. At warm ambient temperatures (T _a) of 33–36 °C, the magnitude of HIF (in ml O₂ or joules) was linearly related to meal mass and was not affected by chick mass. HIF accounted for 6.3% of ingested energy, which is within the range of results for other carnivorous vertebrates. The duration of HIF was inversely related to chick mass; 10-g chicks processed a standard meal approximately twice as fast as 2-g chicks. HIF duration increased with increasing meal mass. The peak O₂ during HIF, expressed as the factorial increase above resting metabolism, was independent of body mass and meal mass. In large, endothermic chicks ( > 8 g), HIF substituted for thermoregulatory heat production at low T _a. Accepted: 11 December 1996     

The effect of exercise and restraint on pectoral muscle metabolism in pigeons

The effect of various activity regimes on metabolism of pigeon pectoralis was examined by measurement of blood lactate following exercise, total lactate dehydrogenase activity of pectoral muscle, and proportions of specific isoenzymes of pectoral muscle lactate dehydrogenase. Sprint-trained birds had the highest pectoral muscle lactate dehydrogenase activity (1409 IU · g⁻¹ wet tissue), while endurance-trained birds had the highest peak lactate levels (287 mg · dl⁻¹, extrapolated from decay curves) and fastest half-time of the lactate response (4.8 min) following exercise, but the lowest lactate dehydrogenase activity (115 IU · g⁻¹ wet tissue). Immobilization of one wing for 3 weeks following endurance training produced a marked increase in lactate dehydrogenase activity of the immobilized muscle, compared to that in the contralateral pectoralis and endurance-trained muscle. Aerobic forms of the lactate dehydrogenase enzyme (that favor conversion of lactate to pyruvate) predominated in pectoral muscle of endurance-trained birds, while cage-confined birds exhibited primarily the anaerobic isoenzymes. These results demonstrate that conversion of pectoral muscle lactate dehydrogenase isoenzymes, total lactate dehydrogenase activity, and half-time of lactate response after exercise is dependent on activity regime in pigeons. In this respect, pigeon pectoral muscle responds to training and disuse in a manner similar to that of mammalian skeletal muscle. Accepted: 10 September 1996     

Developing compound eye in lozenge mutants of Drosophila: lozenge expression in the R7 equivalence group

 The lozenge locus is genetically complex, containing two functionally distinct units, cistrons A and B, that influence the structure of the compound eye. Extreme mutations of either cistron produce adult phenotypes that share similarities and that have striking differences. We have analyzed the expression of several developmentally important eye genes including boss, scabrous, rhomboid, seven-up, and Bar in lozenge mutant backgrounds representing both cistrons. This analysis follows the progressive recruitment of photoreceptor neurons during eye development and has confirmed that the initial development of photoreceptors is normal up to the five cell precluster stage (R8, R2/5 and R3/4). However, when lozenge is mutant, further eye development is perturbed. As cells R1, R6 and R7 are recruited, patterns of gene expression for seven-up and Bar become abnormal. We have also characterized the expression of two different enhancer trap alleles of lozenge. The lozenge product(s) appear to be first expressed in the eye disc in undifferentiated cells shortly after the five cell precluster forms. Then, as distinct cells are recruited to a fate, lozenge expression persists and is refined in those cells. Our data suggests that lozenge functions in cone cells and pigment cells as well as in specific glia. With respect to photoreceptor neurons, lozenge biases the developmental potential of cells R1, R6 and R7, by directly influencing the expression of genes important for establishing cell fate. Received: 26 July 1996 / Accepted: 6 January 1997     

Magnesium chelatase: association with ribosomes and mutant complementation studies identify barley subunit Xantha-G as a functional counterpart of Rhodobacter subunit BchD

Magnesium chelatase catalyses the insertion of Mg²⁺ into protoporphyrin and is found exclusively in organisms which synthesise chlorophyll or bacteriochlorophyll. Soluble protein preparations containing >10 mg protein/ml, obtained by gentle lysis of barley plastids and Rhodobacter sphaeroplasts, inserted Mg²⁺ into deuteroporphyrin IX in the presence of ATP at rates of 40 and 8 pmoles/mg protein per min, respectively. With barley extracts optimal activity was observed with 40 mM Mg²⁺. The activity was inhibited by micromolar concentrations of chloramphenicol. Mutations in each of three genetic loci, Xantha-f, -g and -h, in barley destroyed the activity. However, Mg-chelatase activity was reconstituted in vitro by combining pairwise the plastid stroma protein preparations from non-leaky xantha-f, -g and -h mutants. This establishes that, as in Rhodobacter, three proteins are required for the insertion of magnesium into protoporphyrin IX in barley. These three proteins, Xantha-F, -G and -H, are referred to as Mg-chelatase subunits and they appear to exist separate from each other in vivo. Active preparations from barley and Rhodobacter yielded pellet and supernatant fractions upon centrifugation for 90 min at 272 000 × g. The pellet and the supernatant were inactive when assayed separately, but when they were combined activity was restored. Differential distribution of the Mg-chelatase subunits in the fractions was established by in vitro complementation assays using stroma protein from the xantha-f, -g, and -h mutants. Xantha-G protein was confined to the pellet fraction, while Xantha-H was confined to the supernatant. Reconstitution assays using purified recombinant BchH, BchI and partially purified BchD revealed that the pellet fraction from Rhodobacter contained the BchD subunit. The pellet fractions from both barley and Rhodobacter contained ribosomes and had an A₂₆₀:A₂₈₀ ratio of 1.8. On sucrose density gradients both Xantha-G and BchD subunits migrated with the plastid and bacterial ribosomal RNA, respectively. Received: 9 September 1996 / Accepted: 22 October 1996     

The tarso-pretarsal chordotonal organ as an element in cockroach walking

Many types of sense organs have been demonstrated to show repetitive discharges during walking that could provide informational cues about leg movements and other parameters of locomotion. We have recorded activities of receptors of the distal (tarsal) segments of the cockroach hindleg in restrained and freely moving animals while they were videotaped. These recordings show peaks of activities at the onset and termination of the stance phase. We have morphologically and physiologically identified a joint angle receptor, the tarso-pretarsal chordotonal organ, that contributes to the discharges seen late in stance, prior to the onset of leg flexion in swing. This sense organ encodes the angle and rate of change of the most distal leg joint and specifically discharges when the claws are disengaged from the substrate. Applied displacements of the claws in restrained preparations elicit reflex activation of the tibial flexor muscle and a crossed extensor reflex in the opposite hindleg. These reflexes could function to insure that leg flexion in swing does not occur until the claws are disengaged and to enhance support by the opposite hindleg. Thus, the regular discharges of the chordotonal organ could assure efficient and coordinated muscle contractions and movements during normal, unperturbed walking. Accepted: 2 January 1997     

Analyses of Saccharomyces cerevisiae Cdc7 kinase point mutants: dominant-negative inhibition of DNA replication on overexpression of kinase-negative Cdc7 proteins

Saccharomyces cerevisiae Cdc7 kinase is required for initiation of S phase, and its kinase activity, which is positively regulated by Dbf4 protein, reaches maximum at the G1/S boundary. In this study, we constructed Cdc7 point mutants (T281E, T281A, D182N, D163N, and T167E) and examined the effect of each mutant on growth. All the mutants lost the ability to complement temperature-sensitive growth of cdc7(ts) mutants at a low protein level, whereas T281A (putative target of phosphorylation) and T167E (residue involved in substrate recognition) restored the growth of cdc7(ts) when overproduced to a high level. Three putative kinase-negative mutants (T281E, D182N, and D163N) inhibited growth when overexpressed in a wild-type strain. Analyses of DNA content and morphology revealed that most cells were arrested as dumbbells with 1C DNA, indicative of a block in the G1 to S transition. This growth inhibition was suppressed by co-overexpression of the wild-type Cdc7 or Dbf4 protein. Furthermore, deletion of the Dbf4 protein-binding region in each Cdc7 mutant resulted in loss of growth inhibitory effect. Thus, dominant-negative effects of T281E, D182N, and D163N on growth can be best explained by inactivation of the wild-type Cdc7 function through titration of Dbf4 by these inactive kinases. Our results are consistent with the notion that association of Dbf4 with Cdc7 is essential for the G1 to S transition in S. cerevisiae. Received: 17 September 1996 / Accepted: 6 January 1997     

The impact of vertebrate and invertebrate predators on a stream benthic community

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m² (20 leeches/enclosure) and 1.3 trout/m² (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators. Received: 23 June 1997 / Accepted: 4 May 1998     

Explaining curd and spear geometry in broccoli, cauliflower and `romanesco': quantitative variation in activity of primary meristems

Brassica oleracea L. is highly polymorphic and includes varieties which exhibit a headed phenotype (a large preinflorescence): the curd of cauliflower and `romanesco' (var. botrytis), and the spear of broccoli (var. italica). This headed phenotype results from highly iterative patterns of activity at the primary meristems. Differences in the morphology of curds and spears are accounted for by three quantitative variables: the rate of production of branch primordia on the flanks of the apical meristems (RPP); the number of branch primordia produced before the first formed begin producing their own branch primordia (the iteration interval, ITI); and the duration of the preinflorescence stage (before production of flower primordia). Relatively stable iteration parameters (RPP and ITI) during curd development lead to the production of semi-spherical curds with a smooth surface in cauliflower and broccoli, whereas in `romanesco' RPP and ITI increase throughout curd development, inducing a pyramidal curd with an angular surface. A relatively long preinflorescence stage in cauliflower and `romanesco' results in the curd surface being composed largely of branch primordia, whereas in broccoli this stage is short and the spear surface is made up of flower buds. Simplified growth models for these three headed types are presented. The implications for the genetic control of the B. oleracea L. headed phenotype and the relationships between shoot apical meristem size, phyllotaxis and curd/spear morphology are discussed. Received: 11 September 1997 / Accepted: 12 November 1997     

Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995     

Interspecific interactions and co-existence in dabbling ducks: observations and an experiment

We studied the possible role of resource limitation and interspecific competition in assemblages of dabbling ducks on breeding lakes in Finland and Sweden with observational and experimental data. After initial vegetation mapping and yearly censuses of ducks in 1985–1990, we collected observational data in 1991–1994 from 28 lakes with natural populations of mallard Anas platyrhynchos and teal A. crecca. Mallard and teal co-occur over vast areas in the Holarctic and they are the only breeding dabbling ducks on many oligotrophic lakes. Both species are migratory in our study regions, teal arriving later in spring than mallards. Log-linear model analysis of observational presence/absence data revealed a positive, not a negative, association between the species. This association was independent of habitat diversity as well as of lake size. Mallard-teal interaction was also studied in a cross-over introduction experiment in 32 other lakes in two years. Wing-clipped mallards were introduced to breeding lakes before the arrival of teal to induce resource limitation and interspecific competition, hypothesized to reduce lake use by teal. The density of mallard pairs on experimental lakes was 2.9–8.0 times higher than on controls, but there was no negative response by teal to the treatment. This is the first combined observational-experimental demonstration of lack of interspecific competition in waterfowl. Our results indicate that heterospecific attraction may affect species co-existence in dabbling ducks. Received: 28 October 1996 / Accepted: 13 January 1997