INFLUENCE OF THE INDIRECT EFFECTS OF GUPPIES ON LIFE‐HISTORY EVOLUTION IN RIVULUS HARTII

Early theories of life‐history evolution predict that increased predation on young/small individuals selects for delayed maturation and decreased reproductive effort, but such theory only considers changes in mortality. Predators reduce prey abundance and increase food to survivors. Theory that incorporates such indirect effects yields different predictions. Trinidadian killifish, Rivulus hartii, inhabit communities with and without guppies. Guppies prey on young Rivulus and Rivulus densities decline and growth rates increase when guppies are present. Prior work showed that Rivulus phenotypes from communities with guppies matured earlier and had higher fecundity, consistent with theories that incorporate indirect effects. Here we examined the genetic basis of these differences by rearing 2nd generation, laboratory‐born Rivulus from sites with and without guppies under two food levels that match natural differences in growth. Many locality × food interactions were significant, often reversing the relationship between communities. Such interactions imply that there are fitness trade‐offs associated with adaptation to high or low resource environments. On high food, Rivulus from localities with guppies matured earlier, produced many small eggs, and exhibited increased reproductive investment; these differences reversed on low food. Our results suggest that indirect effects mold Rivulus evolution and thereby highlight connections between community processes and evolutionary change.     

LIFE-HISTORY EVOLUTION IN GUPPIES (POECILIA RETICULATA): 1. PHENOTYPIC AND GENETIC CHANGES IN AN INTRODUCTION EXPERIMENT

Previous investigations (Reznick and Endler, 1982; Reznick, 1982a, 1982b) demonstrated that genetic differences in guppy life histories were associated with differences in predation. Guppies from localities with the pike cichlid Crenicichla alta and associated predators matured earlier, had greater reproductive efforts, and produced more and smaller offspring than did guppies from localities with only Rivulus harti as a potential predator. Crenicichla preys primarily on large, sexually mature size-classes of guppies, while Rivulus preys primarily on small, immature size-classes. These patterns of predation are hypothesized to alter mean age-specific survival. Theoretical treatments of such differences in survival predict the observed trends in age at maturity and reproductive effort. We are using introduction experiments to evaluate the role of predators in selecting for these life-history patterns. The experiment whose results are presented here was conducted in a tributary to the El Cedro River (Trinidad), where a waterfall was the upstream limit to the distribution of all fish except Rivulus. Guppies collected from the Crenicichla locality immediately below the waterfall (the downstream control) were introduced over the waterfall in 1981. This introduction released the guppies from Crenicichla predation, exposed them instead to Rivulus predation only, and also introduced them to a different environment, since the introduction site has greater canopy cover than the site of origin. Changes in guppy life-history patterns can be attributed to predation and/or the environment. Evidence from fish collected and preserved in the field demonstrated that, by mid-1983, guppies from the introduction site above the waterfall matured at larger sizes and produced fewer, larger offspring. There were no consistent differences in reproductive allotment (weight of offspring/total weight). With the exception of reproductive allotment, these patterns are identical to previous comparisons between Rivulus and Crenicichla localities. A laboratory genetics experiment demonstrated that males from the introduction site matured at a later age and at a larger size than did males from the control site downstream, as predicted from the “age-specific predation” hypothesis. No differences between localities were observed for female age and size at maturity or for reproductive effort. The trends for fecundity and offspring size were the reverse of those observed in the field. Because only the males changed in the predicted fashion, it is not possible either to reject or to accept the hypothesis of age-specific predation at this time. We discuss the possible causes for these patterns and the high degree of plasticity in the life history, as evidenced by the differences in fecundity and offspring size between the field and laboratory results.     

THE LIFE HISTORIES OF RHODOCHORTON PURPUREUM AND R. TENUE IN CULTURE

The life histories of the red algae Rhodochorton purpureum and R. tenue were studied in unialgal culture. Telrasporophytes produced sporangia in short day regimes (8–12 hr) with 50–200 ft-c cool white light at 10 or 15 C. Mature gametophytes were not observed in the Washington and Alaska clones of R. purpureum. Tetraspores from R. tenue and the California clones of R. purpureum give rise to unisexual gametophytes that are reproductive when smaller than the tetrasporophytes. The tetrasporophytes develop directly from the gonimoblast cells. Because of similarities in morphology and life histories, it is proposed that R. tenue be placed in synonomy with R. purpureum.     

EFFECTS OF AGE AND SIZE ON LIFE HISTORIES AND POPULATION GROWTH OF RHODODENDRON MAXIMUM SHOOTS

Age- and size-specific shoot life histories were studied with population censuses in June 1984 and June 1985 in an evergreen understory shrub. Rhododendron maximum. Most shoots (65%) survived without branching or flowering, and lesser numbers branched (2%), flowered (20%), or died (23%) during the year between censuses. The probabilities of surviving, branching, flowering or dying were both age- and size-dependent. Small, young shoots increased in leaf area. Flowering occurred most prominently in 3- to 6-year old shoots that had exceeded a leaf area of 200 cm², and the rate of flowering increased proportionately with size above this threshold. Branching normally occurred in the year following flowering. The age and size distributions of the population shifted significantly between years, indicating a nonequilibrium population. The survival schedule was Deevey Type I, indicating a high degree of “parental care” of young shoots. Age- and age + size-based demographic models predicted a rapid decline of the shoot population over a decade, while a size-based model predicted a much slower decline in shoot numbers. A sensitivity analysis of the models showed that overall shoot population growth was positively influenced by branching shoots and shoots that added leaf area, and negatively influenced by shoots that lost leaf area, died, or flowered. The role of shoot life histories in determining individual plant fitness and ecological dominance is discussed.     

EVOLUTION OF TRADE-OFFS BETWEEN SEXUAL AND ASEXUAL PHASES AND THE ROLE OF REPRODUCTIVE PLASTICITY IN THE GENETIC ARCHITECTURE OF APHID LIFE HISTORIES

Life‐history theory postulates that evolution is constrained by trade‐offs (i.e., negative genetic correlations) among traits that contribute to fitness. However, in organisms with complex life cycles, trade‐offs may drastically differ between phases, putatively leading to different evolutionary trajectories. Here, we tested this possibility by examining changes in life‐history traits in an aphid species that alternates asexual and sexual reproduction in its life cycle. The quantitative genetics of reproductive and dispersal traits was studied in 23 lineages (genotypes) of the bird cherry‐oat aphid Rhopalosiphum padi, during both the sexual and asexual phases, which were induced experimentally under specific environmental conditions. We found large and significant heritabilities (broad‐sense) for all traits and several negative genetic correlations between traits (trade‐offs), which are related to reproduction (i.e., numbers of the various sexual or asexual morphs) or dispersal (i.e., numbers of winged or wingless morphs). These results suggest that R. padi exhibits lineage specialization both in reproductive and dispersal strategies. In addition, we found important differences in the structure of genetic variance–covariance matrices ( G ) between phases. These differences were due to two large, negative genetic correlations detected during the asexual phase only: (1) between fecundity and age at maturity and (2) between the production of wingless and winged parthenogenetic females. We propose that this differential expression in genetic architecture results from a reallocation scheme during the asexual phase, when sexual morphs are not produced. We also found significant G × E interaction and nonsignificant genetic correlations across phases, indicating that genotypes could respond independently to selection in each phase. Our results reveal a rather unique situation in which the same population and even the same genotypes express different genetic (co)variation under different environmental conditions, driven by optimal resource allocation criteria.     

LABORATORY EVOLUTION OF LIFE-HISTORY TRAITS IN THE BEAN WEEVIL (ACANTHOSCELIDES OBTECTUS): THE EFFECTS OF DENSITY-DEPENDENT AND AGE-SPECIFIC SELECTION

Four types of laboratory populations of the bean weevil (Acanthoscelides obtectus) have been developed to study the effects of density-dependent and age-specific selection. These populations have been selected at high (K) and low larval densities (r) as well as for reproduction early (Y) and late (O) in life. The results presented here suggest that the r- and K-populations (density-dependent selection regimes) have differentiated from each other with respect to the following life-history traits: egg-to-adult viability at high larval density (K > r), preadult developmental time (r > K), body weight (r > K), late fecundity (K > r), total realized fecundity (r > K), and longevity of males (r > K). It was also found that the following traits responded in statistically significant manner in populations subjected to different age-specific selection regimes: egg-to-adult viability (O > Y), body weight (O > Y), early fecundity (Y > O), late fecundity (O > Y), and longevity of females and males (O > Y). Although several life-history traits (viability, body weight, late fecundity) responded in similar manner to both density-dependent and age-specific selection regimes, it appears that underlying genetic and physiological mechanisms responsible for differentiation of the r/K and Y/O populations are different. We have also tested quantitative genetic basis of the bean weevil life-history traits in the populations experiencing density-dependent and age-specific selection. Among the traits traded-off within age-specific selection regimes, only early fecundity showed directional dominance, whereas late fecundity and longevity data indicated additive inheritance. In contrast to age-specific selecton regimes, three life-history traits (developmental time, body size, total fecundity) in the density-sependent regimes exhibited significant dominance effects. Lastly, we have tested the congruence between short-term and long-term effects of larval densities. The comparisons of the outcomes of the r/K selection regimes and those obtained from the low- and high-larval densities revealed that there is no congruence between the selection results and phenotypic plasticity for the analyzed life-history traits in the bean weevil.     

PHENOTYPIC DIVERSIFICATION ACROSS AN ENVIRONMENTAL GRADIENT: A ROLE FOR PREDATORS AND RESOURCE AVAILABILITY ON THE EVOLUTION OF LIFE HISTORIES

Changes in age/size‐specific mortality, due to such factors as predation, have potent evolutionary consequences. However, interactions with predators commonly impact prey growth rates and food availability and such indirect effects may also influence evolutionary change. We evaluated life‐history differences in Trinidadian killifish, Rivulus hartii, across a gradient in predation. Rivulus are located in (1) “high predation” sites with large piscivores, (2) “Rivulus/guppy” sites with guppies, and (3) “Rivulus‐only” sites with just Rivulus. Rivulus suffer higher mortality with large predators, and guppies may prey upon small/young Rivulus in Rivulus/guppy environments. In turn, population densities decline while growth rates increase in both localities compared to Rivulus‐only sites. To explore how the direct and indirect effects of predators and guppies influence trait diversification in Rivulus, we examined life‐history phenotypes across five rivers. High predation phenotypes exhibited a smaller size at reproduction, a greater number of eggs that were smaller, and increased reproductive allotment. Such changes are consistent with a direct response to predation. Rivulus from Rivulus/guppy sites were intermediate; they exhibited a smaller size at reproduction, increased fecundity, smaller eggs, and larger reproductive allotment than Rivulus‐only fish. These changes are consistent with models that incorporate the impacts of growth and resources.     

卵圆前殖吸虫与透明前殖吸虫生活史研究——幼虫在中间宿主体内发育的观察

本文报道了卵圆前殖吸虫(Prosthogonimus ovatus)与透明前殖吸虫(P.pellucidus)在赤豆螺(Bithynia fuchsianus)、蜻蜓幼虫及家禽体内,从卵发育到成虫的全过程,从而为防治工作提供了理论基础。     

OBSERVATIONS ON THE LIFE HISTORY AND BIOLOGY OF TIPULA LATERALIS MEIG.

The occurrence of large numbers of larvae of Tipula lateralis Meig. in watercress beds has not hitherto been recorded. Observations on commercial beds and laboratory experimental work show this species to be primarily a saprophyte feeding on rotting submerged cress leaves, though it can develop during the final instar on green cress. Populations as high as 250,000/acre do not affect cress beds adversely.
There are two generations a year and probably a partial third. The life cycle can be completed in 63 days. Mating and oviposition are described, the preferred site for egg laying being on wet soil. The average number of eggs is over 500. The egg and first-instar larva are described for the first time. There are four instars, the anal segment of the first being radically different from that of the remainder. It has been shown that pupation cannot take place under water as previously stated.     

LOCAL ADAPTATION AND THE EVOLUTION OF PHENOTYPIC PLASTICITY IN TRINIDADIAN GUPPIES (POECILIA RETICULATA)

Divergent selection pressures across environments can result in phenotypic differentiation that is due to local adaptation, phenotypic plasticity, or both. Trinidadian guppies exhibit local adaptation to the presence or absence of predators, but the degree to which predator‐induced plasticity contributes to population differentiation is less clear. We conducted common garden experiments on guppies obtained from two drainages containing populations adapted to high‐ and low‐predation environments. We reared full‐siblings from all populations in treatments simulating the presumed ancestral (predator cues present) and derived (predator cues absent) conditions and measured water column use, head morphology, and size at maturity. When reared in presence of predator cues, all populations had phenotypes that were typical of a high‐predation ecotype. However, when reared in the absence of predator cues, guppies from high‐ and low‐predation regimes differed in head morphology and size at maturity; the qualitative nature of these differences corresponded to those that characterize adaptive phenotypes in high‐ versus low‐predation environments. Thus, divergence in plasticity is due to phenotypic differences between high‐ and low‐predation populations when reared in the absence of predator cues. These results suggest that plasticity might initially play an important role during colonization of novel environments, and then evolve as a by‐product of adaptation to the derived environment.     

两种盾盘吸虫的生活史及吸虫纲系统发生的讨论

本文记述分布在我国的盾盘吸虫六种:贝居腹盾吸虫(Aspidogaster conchicola K.Baer);黑龙江腹盾吸虫(Aspidogaster amurensis Achmerov,1956);饭岛腹盾吸虫(Aspidogasterijimai Kawamura,1913);印度腹盾吸虫(Aspidogaster indica Dayal,1943);中华杯盾吸虫(Cotylaspis sinensis Faust and Tang,1936)及东方簇盾吸虫(Lophotaspis orientalis Faust and Tang,1936)。对各虫种的形态特征作简单描述。       

CHAOBORUS PREDATION AND LIFE-HISTORY EVOLUTION IN DAPHNIA PULEX: TEMPORAL PATTERN OF POPULATION DIVERSITY,FITNESS, AND MEAN LIFE HISTORY

The effect of predation by the aquatic dipteran larva Chaoborus americanus on genetic diversity and life-history evolution in the cladoceran Daphnia pulex was investigated in large replicate laboratory populations. Instantaneous daily loss rates of clonal diversity and genetic variance for fitness indicate that 93–99% of initial genetic diversity can be removed from populations during the 8–12 generations of clonal reproduction that occur each year in natural populations. In the absence of predation, the principal evolved changes in mean population life history were smaller immature body size and increased and earlier fecundity. In the presence of size-selective Chaoborus predation, populations evolved toward larger body size and increased and earlier reproduction. The difference between these two trajectories is an estimate of the direct additive effect of Chaoborus predation. This effect was manifested as evolution toward larger body size with a trend toward earlier and increased reproduction.