A test of the hypothesis of compensatory upstream dispersal using a stream-dwelling waterstrider,Gerris remigis Say

Summary This paper reports the results of 2 experiments designed to determine if adult Gerris remigis (Hemiptera, heteroptera), a stream-dwelling waterstrider, tend to disperse preferentially upstream, as predicted by the colonization cycle hypothesis summarized by Muller (1982). Markrecapture observations and experimental removals were used to assess the distance and direction of movement of adult G. remigis along a small mountain stream, over a full year. These experiments indicated that adult G. remigis show a significant upstream bias in movement distance, but not in numbers of animals moving. This upstream bias is characteristic of pre-reproductive, sexually immature adults of both sexes, and occurs primarily in association with movements to and from diapause sites. Although the existence of a significant upstream bias in movement distances tends to support the colonization cycle hypothesis, the data from the removal experiments clearly show that upstream dispersal is not sufficient to compensate entirely for downstream drift.     

The origins of allometry: size and shape polymorphism in the common waterstrider, Gerris remigis Say (Heteroptera, Gerridae)

Changes in size, whether ontogenetic or phylogenetic, tend to be associated with changes in shape. This allometry can arise through two different evolutionary mechanisms: (1) selection acting primarily on overall size may be associated with changes in shape because of physiological and mechanical constraints or differential responses of different body components; or (2) selection acting primarily on shape (on the size of specific body components) may be associated with changes in overall size because of genetic correlations, and thus correlated responses, of other body components. To assess the relative importance of these two mechanisms, shape polymorphism is examined along two axes of size dimorphism (sex and wing morphology) in the common waterstrider, Gerris remigis Say. Eight measurements were made of body and appendage components of 234 adults, from three independent populations. Univariate and multivariate analyses reveal that both sexes and wing morphs differ significantly in size and shape. Shape differentiation along the two axes of size dimorphism is found to be dissimilar, partially independent of size, and strongly correlated with the ecological specialization of the various morphs. These observations suggest that selection is acting directly on shape, and thus that allometry in this species primarily reflects shape-mediated changes in size (mechanism 2), rather than size-mediated changes in shape. The role of developmental processes in facilitating this shape differentiation is discussed.     

Adaptive significance of wing dimorphism in the absence of dispersal: a comparative study of wing morphs in the waterstrider, Gerris remigis

ABSTRACT.

• 1 Gerris remigis Say (Hemiptera; Gerridae) is primarily apterous, but populations with up to 33% macropters have been reported. The macropters seldom fly, and field studies have revealed no detectable differences between wing morphs in movement or survival at any time of year.
• 2 In this paper, life history traits of macropterous and apterous G. remigis are compared in an attempt to determine the mechanisms responsible for the maintenance of macroptery in this species in spite of the very low flight capacity and infrequent flight of macropters.
• 3 Development time, proportion breeding without diapause, and overwinter survival do not differ between morphs. However, pre-diapause macropterous females have a significantly shorter pre-oviposition period than apterous females. In contrast, post-diapause macropters begin reproducing later than apters, and have a lower cumulative fecundity.
• 4 These results suggest that macropters may be at a selective advantage in warm habitats which favour pre-diapause reproduction, but that apters should be favoured in the preferred, cool, lotic habitats.
• 5 However, crossing and rearing experiments indicate that wing morphology is primarily environmentally controlled in this species, and that the heritability of wing morphology is low, at best. In light of this, the relative impacts of purely phenotypic (environmental) variation, random effects, and the observed fitness differences on the maintenance of macroptery in this species are discussed.

     

Costs of loading associated with mate-carrying in the waterstrider, Aquarius remigis

When one sex carries the other during some phase of courtshipor mating, the associated loading may entail a significant costto the carrier. This paper presents a series of laboratory experimentsdesigned to identify the costs of mate-carrying in Aquariusremigis. Female A. remigis mate repeatedly and carry each matefor several hours. Dead males and lead weights were used tosimulate normal mating and loading associated with mate-carrying,respectively. Females carrying weights equivalent to the weightof an average male showed no detectable reductions in survival,lipid reserves, or foraging success, and maintained themselveson the water surface for more than 10 days without access toresting sites. Weights equivalent to two males were supportedfor 6.1 ± 4.9 days. Thus, female A. remigis appear tobe very well adapted to carrying their mates and are unlikelyto be near their load limits when carrying a single mate. However,females carrying males or equivalent weights suffered a significantreduction in maximum mobility (stride length and speed), andan increased risk of predation by frogs (Rana clamitans). Femalescarrying weights were more susceptible to predation than unburdenedfemales but were less susceptible than females carrying males,suggesting that loading contributes significantly to, but doesnot fully explain, the increased predation risk. This risk probablyresults from both reduced mobility due to loading and greatervisibility (size). Possible influences of the costs of loadingon mating behavior and sexual size dimorphism are discussed.     

Dispersion and spatial orientation of Gerris remigis in response to water current: a comparison of pre- and post-diapause adults

ABSTRACT. The movements and orientation of adult Gerris remigis Say (Hemiptera; Gerridae) were examined in an artificial stream to determine the mechanisms by which these stream-dwelling waterstriders compensate for and/or avoid downstream drift, and to compare the movement tendency of pre- and post-diapause adults. G. remigis respond to current by turning to face directly into the current (rotational orientation), intermittently rowing against the current (translational orientation), and moving onto land to avoid the current. These rotational and translational responses enable G. remigis to maintain their positions in currents =S10 cm s^-1. However, avoidance increases with current speed, and few animals are found where currents exceed 10 cm s^-1. These observations correlate well with the observed distribution of adult G. remigis in a natural stream.
Comparisons between life stages reveal that post-diapause adults move more, and are more susceptible to drift than pre-diapause adults. Post-diapause adults also avoid even slight currents (<5 cm s^_1) by seeking land, while pre-diapause adults do not show a significant preference for land until currents exceed 10 cm s^-1. These data suggest that previously reported differences in the movement tendency of pre- and post-diapause adults in the field may reflect physiologically-based behavioural differences between these two life stages rather than merely direct responses to environmental cues.     

The histology and histochemistry of oogenesis in the water strider, Gerris remigis Say

In each ovariole of Gerris remigis, nurse cells arise by mitotic divisions at the anterior end of the germarium. These cells enlarge as they move posteriorly. This size increase is possibly caused by fusion of cells, but probably by endopolyploidy as well. The nurse cells then establish connections with a central trophic core, which receives the products of subsequent nurse cell degradation. Two possible pathways of nuclear degradation are suggested: one involves the condensation of chromatin within the nucleus; the other, the release of DNA as fine granules into the cytoplasm. Cytoplasmic areas containing such DNA are also rich in proteinaceous granules, but have a meager content of RNA. The remainder of the cytoplasm of the mature nurse cells contains a high concentration of RNA, as do the nucleoli. Posteriorly the trophic core connects via nutritive cords with each developing oocyte in the prefollicular region and in the anterior vitellarium. RNA is apparently contributed to the ooplasm via the trophic stream. Patches of cytoplasmic DNA are present in the young oocytes; the origin and fate of this DNA is uncertain. During early oocyte maturation chromosomal stainability decreases, and the nucleolus enlarges. In previtellogenic stages, numerous proteinaceous bodies appear in association with the nucleolus-chromosome complex. These bodies, like the nucleolus, have only a low RNA content. They may pass to the cytoplasm, but cannot be traced with certainty. During the latter part of this period a complex population of small proteinaceous and lipid preyolk bodies accumulates peripherally in the oocyte. Definitive protein and lipid yolk are probably derived by the enlargement and inward migration of these bodies. The oocytes are each surrounded by a layer of follicle cells proliferated in the prefollicular region. These become binucleate and enlarge as the enclosed oocytes grow and elongate. RNA also increases in the nucleoli and cytoplasm of the follicle cells as they move posteriorly in the vitellarium. There is no evidence of transfer of nucleic acids or protein from the follicle cells to the oocyte. The nurse cells are therefore implicated as the major source of nucleic acids for the maturing oocyte.     

Sex‐specific selection and sexual size dimorphism in the waterstrider Aquarius remigis

We estimated selection on adult body size for two generations in two populations of Aquarius remigis, as part of a long‐term study of the adaptive significance of sexual size dimorphism (SSD). Net adult fitness was estimated from the following components: prereproductive survival, daily reproductive success (mating frequency or fecundity), and reproductive lifespan. Standardized selection gradients were estimated for total length and for thorax, abdomen, genital and mesofemur lengths. Although selection was generally weak and showed significant temporal and spatial heterogeneity, patterns were consistent with SSD. Prereproductive survival was strongly influenced by date of eclosion, but size (thorax and genital lengths in females; total and abdomen lengths in males) played a significant secondary role. Sexual selection favoured smaller males with longer external genitalia in one population. Net adult fitness was not significantly related to body size in females, but was negatively related to size (thorax and total length) in males.     

Fine structure of the chromatoid body during spermatogenesis in the water-strider,Gerris remigis (SAY)

Summary During spermatogenesis in Gerris remigis, chromatoid bodies appear in the spermatocytes and persist to the-mid-spermatid stage. These structures consist of numerous, parallel tubules, which measure approximately 500 Å in diameter. The tubules are arranged in hexagonal array, and contain dense granules that resemble ribosomes. The chromatoid body may be secretory in function, or may be involved in intracellular transport.The technical assistance of Mr. Roy R. Keppie and Mrs. Mona Brandreth is gratefully acknowledged.     

Body size and fecundity in the waterstrider Aquarius remigis: a test of Darwin's fecundity advantage hypothesis

The general female bias in body size of animals is usually attributed to fecundity selection. While many studies have demonstrated a positive relationship between body size and fecundity, the most common interpretation of fecundity selection is that larger females have larger abdomens and can hold more eggs, yet the relationship between abdomen size and fecundity has rarely been examined. For the waterstrider, Aquarius remigis, we find a significant relationship between body size and fecundity and demonstrate that the target of fecundity selection is abdomen size. Thus, larger females have higher fecundities because they have larger abdomens and not because of their total size per se. The rate at which fecundity increases with increasing abdomen size exceeds that which would be expected due to a simple volume constraint and suggests that other factors, such as increased ability to obtain resources, may contribute to the increase in fecundity with body size. Selection intensities estimated from our data indicate that fecundity selection could be a significant selective force on both total and abdomen lengths. Previous studies have found that abdomen size increased faster than body size and thus, larger females had relatively larger abdomens. The relationship of abdomen length and thorax length in A. remigis is hypoallometric and indicates that larger females have relatively smaller abdomens. We hypothesize that this may reflect conservation of abdomen size in females developing under poor conditions. Finally, while egg size is not directly related to body size, we find a trade-off between egg size and number when female abdomen length is held constant, suggesting that selection on egg size may influence abdomen length only indirectly through its effects on fecundity.     

Glinal variation in morphology among eastern populations of the waterstrider, Aquarius remigis Say (Hemiptera: Gerridae)

We examined morphological variation among populations of the stream-dwelling waterstrider, Aquarius remigis. A previous analysis of allozyme variation along a transect from southern Quebec to southern North Carolina revealed two distinct 'types' of A. remigis, north and south of a transition zone in southern New York and northern Pennsylvania. To assess the concordance of morphological and allozyme differentiation, adults were sampled from 15 populations along the original transect, as well as 11 additional populations in New York, Connecticut and New Jersey. Eight measurements of leg and body size were taken of preserved individuals. Univariate and multivariate analyses (multiple regression, PCA, discriminant analysis) reveal significant differentiation between northern and southern populations with a transition zone between latitudes 39^oand 42^onorth. Strong clines within each region and a clear transition between regions support previous suggestions that there are two distinct 'types' of this species in eastern North America. The transition zone, in morphology coincides with that revealed by the allozyme variation, but extends further south. The covariance of allozyme frequency and morphometric traits suggests that differentiation has occurred through vicariance and secondary contact, and that in this system, cladistic and adaptive differentiation are coupled.     

The sexual arms race and phenotypic correlates of mating success in the waterstrider,Aquarius remigis (Hemiptera: Gerridae)

Intersexual conflicts over mating decisions may have an important impact on the evolution of mating behaviours and strategies and may develop into an arms race between the sexes. In waterstrider species, where intersexual conflict is known to occur, the evolution of male traits that allow them to overcome female reluctance to mate is expected. Reproductively active waterstriders,Aquarius remigis, were videotaped in the laboratory to examine the influence of total body length and front femoral width of males, male: female body size ratio, and female reproductive condition (number of mature eggs) on three variables associated with mating success: duration of the premating struggle, duration of mating, and number of successful matings. None of these behaviors was significantly correlated with the size ratio of the mating pair. However, total body length was negatively correlated with premating struggle duration, male front femoral width was positively correlated with number of successful matings, and number of mature eggs in females was positively correlated with duration of both the premating struggle and the mating itself. The relative influence of male sexual armaments and female choice on the outcome of mating interactions is discussed.     

Formation and structure of the surface coat in the midgut of a waterstrider,Gerris najas deg. (Heteroptera : Gerridae)

The midgut epithelium of Gerris najas (Heteroptera : Gerridae) consists of a single cell type in different differentiating and functioning states. Cells elongate from nests of regenerative cells, forming together with their differentiating neighbours an interepithelial cavity, which contains coat material of membrane-like structure, and aggregations of cubic crystals visible only at this stage. Having reached a fold, the contents of the cavity are discharged into the lumen, and the young cells join the columnar cells. Exocytosis of coat material continues, while the cells gradually change to typical digestive columnar cells. The membrane-like material spreads from the folds into the lumen in whorls without forming a continuous cover of the luminal surface. Therefore, a peritrophic membrane does not exist in Gerris.     

Fitness consequences of foraging success in water striders (Gerris remigis; Heteroptera: Gerridae)

Foraging theory's central premise assumes that behavioral actionsthat affect the acquisition of food also ultimately affect fitness.However, very few investigations in behavioral ecology actuallydemonstrate ultimate fitness consequences of any particularbehavior. Many studies focus on short-term estimates of fitnessin animals with considerable life spans. I investigated whetherdifferences in foraging performance, as related to patch choiceand behavioral dominance, show consistent patterns over extendedperiods of an animal's life such that they translate into differencesin reproductive success. In one experiment, I focused on theability of single female water strider (Gerris remigis) to discriminatebetween foraging patches of differing prey abundances in laboratorystreams simulating natural conditions. In another experiment,I focused on competitive interactions within groups of threefemale water striders foraging in laboratory streams where resourceavailability varied with position in a patch. The first experimentshowed that, overall, the individuals detected and preferredto forage in the richer patch, which directly increased theirforaging success and their lifetime fecundity. However, therewas marked variability among individuals in their ability torespond to the differences in food availability, ranging fromno tracking to fast tracking when the patch qualities were switched.In the second experiment, both the foraging position of individualsin the stream and their dominance rank were reasonably consistentover their entire reproductive life. Foraging position and dominancerank were significant predictors of lifetime fecundity, theformer being the better predictor due to an imperfect correlationbetween the two variables. In both experiments, the increasein fecundity was achieved by higher oviposition rates ratherthan by extending the oviposition period or by inducing firstreproduction at an earlier age. [Behav Ecol 1991; 2: 46-55)     

Relative impacts of mortality factors in field populations of the waterstrider Gerris buenoi Kirkaldy (Heteroptera: Gerridae)

Summary Impacts of predators, food levels and cannibalism on population growth of G. buenoi were studied in two experiments using field exclosures. In the first experiment, experiments using field exclosures. In the first experiment, impacts of (1) predation by freshwater invertebrates and (2) food limitation on gerrid populations were considered in a 2 x 2 factorial design, using food supplements and elimination of predators as the experimental treatments. In the second experiment, the possible contribution of intraspecific predation to fitness of gerrid cannibals was assessed.Presence of invertebrate predators decreased egg-adult survivorship 2–3 fold and decreased the range of juvenile development times. The main predators noted in this study were fishing spiders (Dolomedes), backswimmers (Notonecta), larvae of predaceous diving beetles (Dytiscus), and dragonfly naiads (Aeshna). Food supplements, at 50–200% (by weight) of average natural surface fall, did not significantly effect survivorship but were associated with decreases in mean development time and with increases in whole body dry mass of teneral adults. Increases were greater for females than for males, suggesting that females are more likely to be protein limited under field conditions. Absence of predators was associated with smaller body size among teneral adults of G. buenoi, suggesting that screening out aquatic predators also had significant impact on food available to semi-aquatic bugs.Results of the second experiment demonstrate that the food cache hypothesis (Polis 1980) does not hold for G. buenoi. Neither survival to the adult stage nor dry mass of teneral males differed significantly between groups with or without access to early stages as potential prey. Dry mass of teneral females with access to younger stages during their own development was significantly less than for those without access to gerrid prey, suggesting that competition among stages for food was more important than cannibalism in, this experiment.A more comprehensive version of a paper presented at the XVII Int Cong Entomol, Hamburg, Federal Republic of Germany as part of a symposium entitled Phylogeny, Bionomics and Ecology of Waterstriders (Hemiptera, Gerridae)     

Sexual size dimorphism and selection in the wild in the waterstrider Aquarius remigis: Body size,components of body size and male mating success

Sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necessary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary force. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordance between patterns of sexual selection and patterns of sexual dimorphism. We examined sexual selection in the wild, acting on male body size, and components of body size, in the waterstrider Aquarius remigis, as part of a long term study examining net selection pressures on the two sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavioral, we estimated reliabilities to determine if males perform consistently. Reliabilities were measured as ? statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate analysis of components of body size revealed that sexual selection for larger males was not acting directly on total length but on genital length. Sexual selection for larger male body size was opposed by direct selection favoring smaller midfemoral lengths. While males of this species are smaller than females, they have longer genital segments and wider forefemora. Patterns of contemporary sexual selection and sexual size dimorphism agree only for genital length. For total length, and all other components of body size examined, contemporary sexual selection was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the species may still act to maintain sexual size dimorphism, sexual selection alone does not.