Morphometric variation in a hybrid zone of two subspecies of Gerris costae (Heteroptera: Gerridae) in the Maritime Alps

Two morphologically distinct subspecies of Gerris costae form a contact zone extending from the Maritime to the Western Alps. Within this area canonical trend surface analysis revealed a geographic pattern of morphometric variation consistent with topography. From North to South, and from high to low elevation there was a transition from G. c. costae-like phenotypes to phenotypes resembling pure G. c. fieberi. The same pattern was confirmed with canonical variate analysis not taking geographic location into account; it is therefore not an artifact of trend surface analysis. Comparisons of the pattern of morphometric variation of laboratory-reared offspring with the pattern of their parents sampled from natural populations show that geographic variation is mostly determined genetically. Intermediate individuals from field populations probably are natural hybrids between the two subspecies, because laboratory-reared hybrids were intermediate between the offspring of pure strains. We did not find increased morphometric variation within the contact zone. This suggests unimpeded introgression and is in contrast with an increase in size variability that is predicted to be associated with a transition between uni- and bivoltine forms.     

Does alary dimorphism imply dispersal dimorphism in the waterstrider,Gerris remigis?

ABSTRACT.

• 1 The hypothesis that wing dimorphism reflects dimorphism for dispersal ability is tested in two populations of stream-dwelling waterstriders, Gerris remigis Say, in southern Quebec, Canada.
• 2 Movements were assessed directly by recaptures of marked adults, and indirectly by comparisons of residence times and patterns of disappearance of macropterous and apterous morphs. Sampling was done weekly, over a period of 26 months, and 4828 adult G.remigis were individually marked during this time.
• 3 Movements of >100 m were very rare for both morphs. The movement patterns of the two morphs were very similar, and differed only over the winter, when no macropters moved upstream. This latter observation suggests that macropters may be less successful than apterous individuals at moving overland, or rowing against a current at this time of year.
• 4 Residence times and patterns of disappearance of the two morphs were very similar during all seasons.
• 5 These results suggest that macropterous G.remigis do not, in general, disperse using any means unavailable to the apterous morph, and that macropters in this species should not be simply categorized as long-distance dispersers.

     

Onset of heartbeat revealed by successive structural changes in the developing single embryonic heart of the waterstrider (Gerris paludum insularis)

Videofilm images of the heartbeat in the living embryos of the waterstrider, Gerris paludum insularis, were analyzed to demonstrate successive changes in the width of the contractile heart at the different developmental stages. This information is graphically represented and termed structural cardiogram. Onset of the embryonic heartbeat characterized by anteriorly spreading peristaltic movement of the heart wall occurs at about 55% HL (percent heart length) as early as at about 81 h after katatrepsis (K+81 h embryo). This peristaltic wave occurs almost always following swinging movement of abdominal tip observed exclusively at this stage. Similar peristaltic wave of the heart wall may also be observed at later stages, exclusively in the anterior two-fifth of the heart. Conduction velocity of the peristaltic wave estimated from structural cardiogram of K+81 h embryo was approximately 0.57 mm/s; it was approximately 1.33 mm/s in the K+102 h embryos. In the posterior three-fifth of the heart, however, rhythmic movement was not peristaltic. Development of heartbeat generator in the specific region of the heart was discussed in relation to the onset of embryonic heartbeat.     

Evolutionarily stable dispersal of a waterstrider in a temporally and spatially heterogeneous environment

Summary Evolutionary stable dispersal and wing muscle histolysis strategies are studied in the waterstriderGerris thoracicus. These strategies relate to spreading reproductive risk. Overwintering individuals have the choice of dispersing to either a brackish sea bay or a rock pool habitat. The former is reproductively more favorable than the latter during warm dry years and less favorable during cool wet years. After spring migration, individuals may histolyse their flight muscles and lay all their eggs in one pool or they may retain their flight ability and lay fewer eggs in total but spread them in several pools. We use a simple two-habitat model to examine the question of habitat dispersal. Our results indicate that, although the value of the evolutionary stable dispersal depends on the degree of variability in the environment and on the probability of local extinctions in either habitat, the population always disperses to both habitats as a consequence of density dependent growth. We use a more detailed multiple-rockpool habitat model to examine the question of wing muscle histolysis as a response to density dependence. Our results indicate that a wing muscle histolysis response to population density is an evolutionarily stable strategy when compared with the two alternatives of females always histolysing or never histolysing their flight muscles. The application of evolutionarily stable theory to stochastic problems presents a number of difficulties. We discuss these difficulties in the context of computing evolutionarily stable strategies for the problems at hand.     

GENETIC POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE STREAM DWELLING WATERSTRIDER,AQUARIUS (=GERRIS) REMIGIS (HEMIPTERA: GERRIDAE)

Gene flow, in combination with selection and drift, determines levels of differentiation among local populations. In this study we estimate gene flow in a stream dwelling, flightless waterstrider, Aquarius remigis. Twenty-eight Aquarius remigis populations from Quebec, Ontario, New Brunswick, Iowa, North Carolina, and California were genetically characterized at 15 loci using starch gel electrophoresis. Sampling over two years was designed for a hierarchical analysis of population structure incorporating variation among sites within streams, streams within watersheds, watersheds within regions, and regions within North America. Hierarchical F statistics indicated that only sites within streams maintained enough gene flow to prevent differentiation through drift (Nm = 27.5). Above the level of sites within streams gene flow is highly restricted (Nm ≤ 0.5) and no correlation is found between genetic and geographic distances. This agrees well with direct estimates of gene flow based on mark and recapture data, yielding an N_e of approximately 170 individuals. Previous assignment of subspecific status to Californian A. remigis is not supported by genetic distances between those populations and other populations in North America. Previous suggestion of specific status for south-eastern A. remigis is supported by genetic distances between North Carolina populations and other populations in North America, and a high proportion of region specific alleles in the North Carolina populations. However, because of the high degree of morphological and genetic variability throughout the range of this species, the assignment of specific or subspecific status to parts of the range may be premature.     

Significance of spring migration and flexibility in flight-muscle histolysis in waterstriders (Heteroptera, Gerridae)

Abstract. 1. Individuals of long-winged waterstrider (Gerridae) species were found in spring far from their breeding habitats, which indicates that they fly before reproduction.
2. Field samples and laboratory studies show that once they return to their breeding sites, many individuals of three waterstrider species ( Gerris odontogaster (Zett.), Gerris lacustris (L.) and Limnoporus rufoscutellatus (Lat.)) histolyse wing muscles and lose flight ability during their reproductive period.
3. The extent of flight-muscle histolysis varies with environmental factors. Food scarcity affects flight-muscle histolysis in G.odontogaster females. In G.Lacustris , flight-muscle histolysis was more common in the laboratory than in the field samples. Proportionately more females than males lost their flight ability by the end of the reproductive period.
4. Flight ability had direct costs in reproductive potential with (non-flyer) females, which histolysed their flight muscles, laying more eggs than (flyer) females, which maintained flight ability. This was also the case during food scarcity. Non-flyer males of G.odontogaster survived longer than flyer males.
5. Spring migration was distinguished from dispersal during the reproductive period, because these flights serve different functions. Flight-muscle histolysis of females during reproduction is a qualitative reproductive option, with a trade-off between dispersal ability and reproductive potential. Ability to change reproductive behaviour depending on environmental conditions increases an individual's ability to cope with a large variety of habitats.