Dormant Eggs Research Articles

Hide, rest or die: a light‐mediated diapause response in Daphnia magna to the threat of fish predation

Summary1. In a laboratory batch culture experiment, a diapause response of Daphnia magna to a simulated threat of fish predation was tested at various light intensities, which under natural conditions determine potential vulnerability of Daphnia to visual planktivorous fish.2. Under moderate light intensity (1.4 μmol m−2 s−1) that allows effective predation by fish, the proportion of females producing dormant eggs was significantly higher than under dim light conditions (0.001 μmol m−2 s−1) that are not favourable for visual detection of prey. Production of dormant eggs was not observed in complete darkness or in treatments missing fish kairomones, irrespective of tested light conditions.3. The observed phenomenon is interpreted as a flexible response of prey to the conditional risk of predation assessed by Daphnia according to the presence of fish‐derived cues on the one hand and the presence of dark refugia on the other. Irrespective of the presence of fish kairomones, Daphnia may not produce resting eggs as long as a safe, dark, bottom zone is accessible.

Induction of embryonic dormancy in the calanoid copepod Acartia hudsonica: heritability and phenotypic plasticity in two geographically separated populations

The planktonic copepod Acartia hudsonica apparently requires summer diapause or dormancy to persist in Narragansett Bay, RI. In estuaries to the north, however, active A. hudsonica populations are present year-round and may not express dormancy. Using a full-sibling rearing design and analysis of variance (ANOVA), I assessed the importance of phenotypic and genetic sources of variation in two geographically separate populations, one from Maine and one from Rhode Island. Both populations showed phenotypic plasticity in the percentage of dormant eggs produced. Moreover, experiments revealed significant sibship–environment interactions in both populations, indicating that the phenotypic plasticity has a genetic component. Both populations also revealed a significant amount of genetic variation in the percentage of dormant eggs produced. For the Maine population, broad sense heritability was high (0.91, 1.10) in two short-day (12L:12D) treatments and near zero (0.08) in the long-day treatment (15L:9D). For a Rhode Island population, broad sense heritability was higher (0.95) in a low-temperature short-day treatment (13.5 °C, 12L:12D) than in a high-temperature short-day treatment (17.5 °C, 12L:12D; h 2=0.25).

Induction of embryonic dormancy in the Calanoid copepod Acartia hudsonica: proximal cues and variation among individuals

In this study of Acartia hudsonica, the change in egg types from subitaneous eggs to dormant eggs was investigated. Temperature and photoperiod were hypothesized to act as the proximal environmental cues inducing this population to produce dormant eggs, and the variation in the dormant egg responses of individual females was quantified. Some field-collected females produced some dormant eggs early in the spring (March and April), but most of the switch to dormant eggs occurred in late June. Short-term experiments showed that increased temperatures increased the percentage of dormant eggs produced, whereas increased length of day did not. Long-term experiments showed a different pattern from short-term experiments, indicating that duration of exposure to particular temperatures or photoperiods may influence dormant egg production. Under certain conditions, it appeared that nearly all copepods were capable of producing dormant eggs, although identical experimental conditions did not affect all individuals similarly. In several experiments that revealed a significant treatment effect, between 42% and 85% of the variance detected could be attributed to variation among individuals. Also, the fraction of individuals producing dormant eggs generally increased with increasing water temperature. In Narragansett Bay, A. hudsonica appears to produce dormant eggs primarily in response to increased water temperatures although an effect of photoperiod could not be ruled out. Furthermore, individual responses vary greatly. Appreciation of such variation in an important life history trait, such as embryonic dormancy, is important in understanding how this population may be adapted to its environment.

Use of ephippial morphology to assess richness of anomopods: potentials and pitfalls

Zooplankton species richness is typically assessed through analysis of active community samples. These samples ought to be collected at many different locations in the lake and at multiple occasions throughout the year so as to cover the spatial and temporal heterogeneity in active community structure. A number of studies have shown that high numbers of species can be retrieved with a limited effort through hatching of dormant eggs isolated from lake sediments. However, dormant eggs of different species differ in their propensities to hatch, resulting in biased assessments of species composition, abundance and richness. In this paper, we explore the potentials and pitfalls of a third method to assess cladoceran species richness. For twenty European lakes, we identified the number of ephippium morphotypes in sediment samples taken on a single occasion. The morphotype richness was well correlated with species richness as assessed through hatching of dormant forms and through analysis of active community samples covering a six month period. However, not all species had a species-specific ephippial morphotype, consequently resulting in an underestimation of true species richness.

Critical review of the vector status of Aedes albopictus.

The mosquito Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae), originally indigenous to South-east Asia, islands of the Western Pacific and Indian Ocean, has spread during recent decades to Africa, the mid-east, Europe and the Americas (north and south) after extending its range eastwards across Pacific islands during the early 20th century. The majority of introductions are apparently due to transportation of dormant eggs in tyres. Among public health authorities in the newly infested countries and those threatened with the introduction, there has been much concern that Ae. albopictus would lead to serious outbreaks of arbovirus diseases (Ae. albopictus is a competent vector for at least 22 arboviruses), notably dengue (all four serotypes) more commonly transmitted by Aedes (Stegomyia) aegypti (L.). Results of many laboratory studies have shown that many arboviruses are readily transmitted by Ae. albopictus to laboratory animals and birds, and have frequently been isolated from wild-caught mosquitoes of this species, particularly in the Americas. As Ae. albopictus continues to spread, displacing Ae. aegypti in some areas, and is anthropophilic throughout its range, it is important to review the literature and attempt to predict whether the medical risks are as great as have been expressed in scientific journals and the popular press. Examination of the extensive literature indicates that Ae. albopictus probably serves as a maintenance vector of dengue in rural areas of dengue-endemic countries of South-east Asia and Pacific islands. Also Ae. albopictus transmits dog heartworm Dirofilaria immitis (Leidy) (Spirurida: Onchocercidae) in South-east Asia, south-eastern U.S.A. and both D. immitis and Dirofilaria repens (Raillet & Henry) in Italy. Despite the frequent isolation of dengue viruses from wild-caught mosquitoes, there is no evidence that Ae. albopictus is an important urban vector of dengue, except in a limited number of countries where Ae. aegypti is absent, i.e. parts of China, the Seychelles, historically in Japan and most recently in Hawaii. Further research is needed on the dynamics of the interaction between Ae. albopictus and other Stegomyia species. Surveillance must also be maintained on the vectorial role of Ae. albopictus in countries endemic for dengue and other arboviruses (e.g. Chikungunya, EEE, Ross River, WNV, LaCrosse and other California group viruses), for which it would be competent and ecologically suited to serve as a bridge vector.

Incidence of diapause varies among populations of Daphnia pulicaria.

Dormancy is a common way in which organisms survive environmental conditions that would be lethal to the active individual. However, while dormant, individuals forego reproduction. Hence theory suggests an optimal time in which to enter dormancy, depending on risks associated with both remaining active and entering dormancy. When these relative risks differ among habitats, dormancy strategies are predicted to vary as well. For freshwater zooplankton, it has been suggested that sensitivity to the cues that initiate dormancy should be selected against when females have the opportunity to remain in the water column year round. We tested this prediction with 12 populations of lake-dwelling Daphnia pulicaria (Crustacea: Cladocera). Differences among lakes in basin morphometry, predators and resources create a gradient of risk for Daphnia in the water column. Some populations persist in high numbers year round while others are abundant only in spring. We used this difference in persistence ability as an estimate of risk in the water-column. For 3 years of field sampling we found consistent differences among the lake populations in the incidence of dormancy. In some populations, only a small fraction of females switched to producing dormant eggs each year whereas in others the majority of eggs produced in the late spring were dormant. In general, populations that experienced predictably low abundances in the active form exhibited higher incidence of dormancy than did populations that persisted in high abundance year round, but there were exceptions. Our results confirm that the incidence of dormancy varies considerably among populations in a fashion consistent with general theory, but suggest that persistence in the water column is not the sole predictor of the diapause strategy found in any particular lake.

Differentiation in sex investment by clones and populations of Daphnia

AbstractDormancy is an ecological strategy by which organisms avoid stressful environments, but it also can have genetic consequences. Many facultative parthenogens shift from asexual to sexual reproduction to enter dormancy. Hence, conditions that favour dormancy are predicted to select for more sex, which should increase clonal diversity. We examined lake populations of Daphnia that face different ecological risks to remaining active year‐round, and quantified the extent to which they have differentiated in their investment in dormancy and sex. There was substantial genetic variation among populations and clones for sex induction and production of dormant eggs, and striking evidence of gender specialization. We also observed a positive association between the magnitudes of population‐level investment in dormancy and of variance among clones in sex induction. These results document an ecological gradient in dormancy that is manifest as a genetic gradient in clonal variation for the propensity to engage in sex.

Influence of Environmental Conditions on Hatching Activity of Youth from Dormant Eggs of Polyphemus pediculus L.

Influence of Environmental Conditions on Hatching Activity of Youth from Dormant Eggs of Polyphemus pediculus L.

Genetic versus environmental influence on pigment variation in the ephippia ofDaphnia pulicaria

Summary1. The dormant eggs ofDaphnia(Crustacea: cladocera) are encased in a protective, chitonous casing known as an ephippium. Ephippia are pigmented with melanin, and the degree of pigmentation ranges from transparent to opaque. Variation in pigmentation exists within and across populations and species, raising questions about the factors that influence the natural distribution of pigmentation.2. We used image analysis to quantify pigmentation in ephippial casings ofDaphnia pulicariathat were produced both in the field and in the laboratory. The percentage of the surface area of ephippia that is darkly pigmented ranged from 0.5 to 99.5%. The range, mean and variance of ephippial pigmentation varied across our 11 study lakes.3. Laboratory experiments compared clonal variation (five clones/lake) and population variation (five lakes) at three temperatures (15 °C, 20 °C and 25 °C). The degree of pigment variation between clones was much greater than pigment variation between temperatures, indicating that there is a genetic component to pigmentation in the ephippia which is stronger than the effect of temperature.4. Comparisons of pigment levels and lake characteristics were used to identify physical and biological factors associated with ephippial pigmentation. Investment in ephippial production was the strongest predictor with darker ephippia occurring more often in lakes with the highest ephippial production.

HOW LONG TO REST: THE ECOLOGY OF OPTIMAL DORMANCY AND ENVIRONMENTAL CONSTRAINT

Dormancy is a common mechanism employed by short-lived organisms for persistence in a variable environment. Theory suggests that the fraction of propagules that terminate dormancy each year should be <100% when recruitment success varies temporally. Moreover, the fraction of propagules that resumes development should vary across habitats that differ in the probability of successful recruitment or the probability of survival during dormancy. We tested these predictions by using dormant eggs from five populations of the freshwater cladoceran Daphnia pulicaria that differ in their ability to recruit to and persist in the water column. In two separate experiments, newly produced dormant eggs were incubated in situ for one year at various sites on the bottom of the lakes. A series of reciprocal transplants among four of these populations separated the effects of lake-specific environmental cues from the genetic and maternal effects of the different populations. Additional eggs were incubated in the laboratory under photoperiod–temperature combinations representative of those in the field. We found that the annual hatching fraction ranged from 6% to 50% among lakes, and that hatching fraction was primarily driven by environmental cues rather than being a result of the source of the eggs. However, laboratory incubations demonstrated significant differences among populations in the trajectories of the hatching curves, and a much higher rate of hatching than the field incubations. Our results suggest that variation in dormancy strategies within these systems is likely influenced both by the seasonal risk experienced by the active individuals and by risks associated with entering the dormant egg bank.

Food Threshold for Diapause in Daphnia under the Threat of Fish Predation

The chances of prey to survive and leave viable offspring may be determined by many environmental constraints, of which food conditions may be of key importance. When food is not limited, the high reproductive effort of a well-fed prey individual dis- playing various defensive responses may assure successful reproduction, whereas low food supplies may prevent it by limiting energy gains and expenditures on defense and procre- ation. Under low chances for survival of active forms in a local environment, production of diapausing forms of enhanced resistance to unfavorable conditions or dispersion to more favorable places may assure persistence. The laboratory batch-cultured experiments detailed here sought to test whether food concentration may influence probability of producing resting eggs by Daphnia when under chemically simulated predation pressure. In fish-free conditions, no production of resting eggs was observed with either the tested low (0.4 mg C/L) or high (1.6 mg C/L) food concentrations, whereas in fish treatments, resting eggs were produced by various proportions of females according to food concentration. With the limited food (0.4 mg C/ L) there was production of ephippial (dormant) eggs in the early broods of almost all females in the fish treatment. Where food concentrations were higher, proportions of ephippial females were progressively lower, with females postponing the formation of resting eggs until later broods. At the highest food concentration (1.6 mg C/L), only about 17% of females produced ephippia when exposed to cues. These results are considered to indicate an adaptive response of prey individuals to a simulated danger of predation in various food conditions, determining relative gains of producing active vs. dormant off- spring. It seems that D. magna may assess their chances of survival and successful reproduction in accordance with the anticipated danger of predation and food conditions, and enter diapause when the expected gains from diapause exceed those of active life. A graphical model explaining observed production of active vs. dormant offspring by D. magna under predation pressure at various food concentration is proposed.

FOOD THRESHOLD FOR DIAPAUSE INDAPHNIAUNDER THE THREAT OF FISH PREDATION

The chances of prey to survive and leave viable offspring may be determined by many environmental constraints, of which food conditions may be of key importance. When food is not limited, the high reproductive effort of a well-fed prey individual displaying various defensive responses may assure successful reproduction, whereas low food supplies may prevent it by limiting energy gains and expenditures on defense and procreation. Under low chances for survival of active forms in a local environment, production of diapausing forms of enhanced resistance to unfavorable conditions or dispersion to more favorable places may assure persistence. The laboratory batch-cultured experiments detailed here sought to test whether food concentration may influence probability of producing resting eggs by Daphnia when under chemically simulated fish predation pressure. In “fish-free” conditions, no production of resting eggs was observed with either the tested low (0.4 mg C/L) or high (1.6 mg C/L) food concentrations, whereas in “fish” treatments, resting eggs were produced by various proportions of females according to food concentration. With the limited food (0.4 mg C/L) there was production of ephippial (dormant) eggs in the early broods of almost all females in the “fish” treatment. Where food concentrations were higher, proportions of ephippial females were progressively lower, with females postponing the formation of resting eggs until later broods. At the highest food concentration (1.6 mg C/L), only about 17% of females produced ephippia when exposed to fish cues. These results are considered to indicate an adaptive response of prey individuals to a simulated danger of predation in various food conditions, determining relative gains of producing active vs. dormant offspring. It seems that D. magna may “assess” their chances of survival and successful reproduction in accordance with the anticipated danger of predation and food conditions, and enter diapause when the expected gains from diapause exceed those of active life. A graphical model explaining observed production of active vs. dormant offspring by D. magna under predation pressure at various food concentration is proposed.

Reproduction, molting, and growth of two Mexican uniparental forms of the tadpole shrimp Triops (Branchiopoda: Notostraca) under a recirculating culture system

Two forms (here called short and long) of the tadpole shrimp Triops from the Baja California peninsula (Mexico) exhibit interesting features for aquacultural purposes; rapid growth, early maturation (six days) and uniparental reproduction via cysts (i.e. drought-resistant dormant eggs). The use of Triops for aquaculture depends on the standard production of viable cysts. Basic information on the reproductive potential of Triops is scarce. Using a recirculating system, we studied, through two culture tests, the cyst production, frequency of molting, and growth rate of the two Mexican forms. For each form, individual data were obtained from five specimens in Test 1 over 15 days, and from 10 specimens in Test 2 over 20 days. Hatching response of cysts produced in Test 1 was also studied. The short-form showed a high fecundity depositing groups of cysts from the ovisacs several times a day. The mean cyst production per day was 156–306 (Test 1 & 2), and the mean total cyst production was 2028 (range 728–3193) (Test 1), and 5821 (range 4136–7554) (Test 2). The maximum number of cysts deposited by one short-form individual in a day was 1231 (Test 2). The reproductive performance of the long-form was poor. The mean cyst production per day was only 4.2–7.9 (Test 1 & 2). The short-form molted every 2.5–2.8 days and the long-form molted every 2.8–3.7 days. The mean growth rate calculated from the standard length (mm d−1) of the short-form was 0.43–0.84 (Test 1 & 2), and in the long-form the values were 0.84–1.25 (Test 1 & 2). The poor cyst production of the long-form may be explained by inadequate food resources that did not fulfill the nutritional requirements for reproduction. Given its prolific uniparental reproduction (vs. biparental reproduction), the short-form Triops appears as a good potential candidate for aquaculture.

Paleogenetic evidence for a past invasion of Onondaga Lake, New York, by exotic Daphnia curvirostris using mtDNA from dormant eggs

Cladocerans possess traits such as resistant diapausing eggs and rapid parthenogenetic reproduction that make them efficient invaders of new habitats. Nearly all known invasions have been successful, perhaps because failed invasions are difficult to detect. It is possible, however, to identify past failed invasions, by studying the diapausing egg bank. Daphnia ephippia were found in the sediments of Onondaga Lake, New York that could neither be hatched nor identified using egg‐case morphology. Instead, we used sequences of the 12S ribosomal ribonucleic acid (rRNA) gene of mitochondrial deoxyribonucleic acid (mtDNA) extracted from diapausing eggs to identify the unknown Daphnia. We compared these DNA sequences with those generated from morphologically identified Daphnia species collected in Onondaga Lake, and with published sequences for other North American Daphnia species. The invader was identified as Daphnia curvirostris, a Eurasian species that has been only reported once before from North America, in extreme northwestern Canada. The discovery of it in Onondaga Lake signifies a greater than 4,500‐km range extension for this species. On the basis of the sediment ephippial data, D. curvirostris first appeared in the lake about 1952, reached maximum abundance during the period of peak pollution (1950s–1980s), and then essentially disappeared after 1983 when lake water quality improved. As with the finding of another exotic cladoceran, D. exilis, in Onondaga Lake (Hairston et al. 1999a), it is likely that chemical industry activities on the lakeshores were the original source of invading D. curvirostris, that pollution allowed this species to become established in the lake, and that the reduction in pollution ultimately led to its disappearance from the water column.

Rapid evolution revealed by dormant eggs

Natural selection can lead to rapid changes in organisms, which can in turn influence ecosystem processes1. A key factor in the functioning of lake ecosystems is the rate at which primary producers are eaten, and major consumers, such as the zooplankton Daphnia2, can be subject to strong selection pressures when phytoplankton assemblages change. Lake Constance in central Europe experienced a period of eutrophication (the biological effects of an input of plant nutrients) during the 1960s–70s3, which caused an increase4 in the abundance of nutritionally poor or even toxic5 cyanobacteria. By hatching long-dormant eggs6 of Daphnia galeata found in lake sediments, we show that the mean resistance of Daphnia genotypes to dietary cyanobacteria increased significantly during this eutrophication. This rapid evolution of resistance has implications for the ways that ecosystems respond to nutrient enrichment through the impact of grazers on primary production.

Egg dormancy in the Australian estuarine-endemic copepods Gippslandia estuarina and Sulcanus conflictus, with reference to dormancy of other estuarine fauna

The presence of dormant life history stages was investigated for the zooplankton of an annually flooding salt-wedge estuary. Such stages are seen as a potential mechanism for population persistence following environmental adversity. Laboratory incubation experiments were conducted on estuarine sediments. As a result, dormant eggs in Australian estuarine-endemic copepods are reported for the first time. Nauplii of the dominant estuarine-endemic calanoids Gippslandia estuarina and Sulcanus conflictus commonly hatched from the sediments. Manipulation of the salinity and temperature of experimental media indicated that temperature was the more important hatching trigger for S. conflictus, and that both high salinity and high temperature were important for G. estuarina. Results of the incubation experiments, including those of ‘conversion’ experiments (i.e. from freshwater to saline conditions or low temperature to high temperature), help to elucidate the type of dormancy characteristic of each species; it appears that S. conflictus may have diapause eggs and G. estuarina may have quiescent eggs, although this is yet to be confirmed. Other estuarine fauna developed from the mud during the incubation experiments, most notably the harpacticoids Onychocamptus chathamensis, an ectinosomatid and Schizopera sp., and the medusa Australomedusa baylii. Ecological and evolutionary consequences of dormancy in these estuarine-endemic zooplankton are briefly discussed.

A Study of &lt;i&gt;Artemia&lt;/i&gt; in Lagkor Co，Tibet,I：Biological Feature

西藏拉果错位于阿里高原南部,湖面海拔4470m,水化学类型属于弱度碳酸盐亚型.湖水盐度为5.5%-6.5%.湖区属于高原亚寒带干旱区.拉果错每年12月至翌年3月为湖水冰冻期,4月下旬-5月初卤虫冬卵大量孵化.5—9月为卤虫早繁衍期.9月底-10月大量产生休眠卤虫卵,卤虫卵主要分布于湖东北部和西北部.卤虫和卤虫卵呈紫红色；虫卵属深色卵壳.卤虫干卵径264.5μm.卤虫克卵量约12万粒.无节幼体长607.7μm,成虫体长0.7-1.2cm.与其它13个国内外卤虫品系比较,为较大型卤虫品系.;Lagkor Co is a saline lake located in the subfrigid—plateau arid zone on the Ali Plateau. Tibet.at 89°02-89°12 E and 31°59-32°04 N.with an elevation of 4470m above sea 1eve1. The lake water belongs hydrochemically to the weak carbonate sub—typic and its salinity is 5.5-6.5. The lake water is frozen from December to M arch of the next year.W inter eggs of brine shrimp are hatched mostly from late April to the beginning of June.The brine shrimps are reproduced from M ay to September. Dormant eggs of brine shimp are produced abundatly from late September to October,Eggs of Artemia are distributed chiefly in the northeastern and northwestern parts of the lake. The brine shrimp and eggs of Artemia of the Lagkor Co in Tibet are purplish red.and egg of Amemia belongs to the dark colored shell type.The diameter of the dry Aretemia egg is 264.5m. The number of eggs per gram is about 120000. The length of nauplii is 607.7Ⅲn and that of adult brine shrimp 0.7-1.2cm. Com pared with other 13 brine shrim p strains at hom e and abroad·the Artemia of Lagkor Co belongs to the larger type of Artemia strain.

Eggs and Their Incubation as Factors in the Ecology of Planktonic Crustacea

ABSTRACT Most crustaceans carry their embryos in a brood chamber or ovisac. However, among the pelagic Crustacea nearly all halocyprid ostracods, most calanoid copepods, euphausiids, and dendrobranchiate decapods (Penaeoidea and Sergestoidea) shed their eggs into the water column. Planktonic Branchiopoda produce resting eggs which remain dormant in the sediment as part of their life cycle. Eggs of calanoids of species within the superfamily Diaptomoidea may also survive dormant in the sediment. The eggs of some Aetideidae adhere to suitable substrates. The freely spawned eggs of other calanoids do not appear to be able to survive in the sediment and their spawning strategies and egg characteristics, and those of Euphausiacea, minimize the risk of their eggs reaching the sea bed. The eggs of free-spawning decapods are buoyant, but demersal development has been suggested for some penaeids. The distributions of brooders are not directly limited by the egg or incubation characteristics, but those of free spawners may be thus limited. The production of dormant eggs enables populations to survive when conditions are unfavorable for the pelagic stages, but only in depths in which newly hatched nauplii or juveniles can return to the strata where they can feed to continue development. In contrast, free spawners with nonresistant eggs are at a disadvantage in shallow waters. Free spawning enables populations to reproduce rapidly where food supply is intermittent, particularly in high latitudes.

Commercial feasibility of semi-intensive larviculture of atlantic halibut ( Hippoglossus hippoglossus L.)

Larval production of Atlantic halibut is performed in 75 m 3 large floating units in a pond in Western Norway. The production and supply of larval food are based mainly on the regular Zooplankton production starting with the dormant eggs of copepods hatching in March. At the time of metamorphosis in June, Zooplankton production is declining, possibly due to the seasonal increase in temperature. Artemia is supplied until metamorphosed juveniles are transferred for weaning in land-based tanks near the pond, at the end of June. The 1994 production was 315 000 juveniles at metamorphosis, with an average survival rate of 35% from first feeding to metamorphosis, and a production of more than 200 000 juveniles for further ongrowing. The considerable weight variation at metamorphosis is probably due to individual variations in larval growth conditions in large outdoor tanks where the light regime might cause great patchiness and variability both in larval and Zooplankton distribution. Growth can further be restricted by the size of the prey, especially in periods with a dominance of small developmental copepodid stages. In larviculture production faculties like this, continuity in production is maintained by the existence of a regular cycle of Zooplankton abundance. A low production cost is assumed possible by combining weaning and ongrowing facilities at one site.

Analysis of LaCrosse virus S mRNA 5' termini in infected mosquito cells and Aedes triseriatus mosquitoes

Nucleotide sequences were determined for the 5' termini of La Crosse virus (LAC) S segment mRNA from persistently infected mosquito cell cultures (C6/36 from Aedes albopictus) and embryos (Aedes triseriatus). LAC primes transcription of its mRNA with "scavenged" 5' caps and adjacent oligonucleotides from host mRNAs, and these non-virus-encoded 5'-terminal extensions are heterogeneous in infected mammalian cells. The nature of mosquito host-derived primers has not been previously investigated. During early C6/36 cell infection, LAC mRNA 5'-terminal sequences were heterogeneous, but variability decreased as infection persisted. One predominant sequence, 5' CCACTCGCCACT (sequence 1), was observed throughout C6/36 cell infection but was more prevalent after 15 days postinfection. This LAC mRNA 5'-terminal sequence comprised 81% of the scavenged host oligonucleotides from vertically infected A. triseriatus eggs during embryogenesis. As these embryos progressed in the dormant overwintering stage (diapause), the predominant scavenged sequence became 5' AGGAAAAGATGGT (sequence 2), and sequence 1 became less prevalent. As the eggs emerged from diapause, the LAC mRNA 5' termini were more variable; 33% had sequence 1, and the remainder were heterogeneous. In post-diapausing eggs, 100% of viral mRNAs had sequence 1 at their 5' termini. Molecular analyses thus revealed continuous but selective LAC cap scavenging during persistent C6/36 cell infection and during embryogenesis and diapause in A. triseriatus eggs. The variety of host-derived sequences was limited in both biosynthetically active (embryonating) and dormant (diapausing) eggs.