Production Of Diapausing Eggs Research Articles

M6A-dependent mevalonate kinase in juvenile hormone synthesis pathway regulates the diapause process of bivoltine silkworm (Bombyx mori).

Research has shown that epigenetic modification are involved the regulation of diapause in bivoltine silkworms (Bombyx mori), but it remains unclear how epigenetic modification in response to environmental signals precisely to regulate the diapause processing of bivoltine B. mori. In this study, the diapause terminated eggs of bivoltine B. mori, Qiufeng (QF) were divided into two groups: a QFHT group incubated at 25°C with a natural day/night cycle to produce diapause eggs, and a QFLT group incubated at 16.5°C in darkness to produce non-diapause eggs. On the 3rd day of the pupal stage, the total RNAs of the eggs were extracted and their N6-adenosine methylation (m6A) abundances were analyzed to explore the effects of m6A methylation on diapause in the silkworm. The results showed that 1984 m6A peaks are shared, 1563 in QFLT and 659 in QFHT. The m6A methylation level of the QFLT group was higher than that of the QFHT one in various signaling pathways. The m6A methylation rate of mevalonate kinase (MK) in the insect hormone synthesis pathway was significantly different between the two groups. The knockdown of MK by RNA interference in the pupae of QFLT resulted in females laying diapause eggs rather than non-diapause eggs after mating. m6A methylation involves in the diapause regulation of bivoltine B. mori by changing the expression levels of MK. This result provides a clearer image of the environmental signals on the regulation of diapause in bivoltine silkworms.

Genome-wide screening of genes involved in programming diapause in the next generation in silkworm, Bombyx mori (Lepidoptera: Bombycidae)

Maternal silkworms (Bombyx mori) of bivoltine strains are destined to produce either diapause or non-diapause eggs depending on environmental factors, such as, temperature and photoperiod experienced during the egg and larval stages. However, the molecular mechanisms that program diapause, which depend on information about the environment, remain unclear. We aimed to identify genes that are involved in programming diapause in the next generation in bivoltine silkworms. We therefore screened differentially expressed genes (DEGs) in the larval brains of diapause- and non-diapause-egg producers kept under three different diapause-inducing conditions using cap analysis of gene expression. Under each condition, only temperature, illumination or photoperiod was changed during the egg or larval stage as a diapause-controlling stimulus to induce the production of diapause or non-diapause eggs. We then verified the expression of DEGs that were common to all the three conditions using real-time quantitative PCR. We investigated the functional involvement of candidate genes in programming diapause using double-stranded RNA interference (RNAi) for gene knockdown. The results showed more abundant juvenile hormone acid methyltransferase (Jhamt) and proton-coupled folate transporter (Pcft) gene expression in the brains of fifth instar larvae of producers of diapause eggs than those of non-diapause eggs under the three conditions. Furthermore, RNAi against either of these genes significantly decreased the incidence of diapause in the next generation. These findings indicate that both Jhamt and Pcft are involved in the programming of diapause in the silkworm brain. These genes could function in retaining information that leads to diapause in the next generation.

Maternally Instigated Diapause in Aedes albopictus: Coordinating Experience and Internal State for Survival in Variable Environments.

The Asian tiger mosquito, Aedes albopictus, is one of the most dangerous invasive species in the world. Females bite mammalian hosts, including humans, to obtain blood for egg development. The ancestral range of Ae. albopictus likely spanned from India to Japan and this species has since invaded a substantial portion of the globe. Ae. albopictus can be broadly categorized into temperate and tropical populations. One key to their ability to invade diverse ecological spaces is the capacity of females to detect seasonal changes and produce stress-resistant eggs that survive harsh winters. Females living in temperate regions respond to cues that predict the onset of unfavorable environmental conditions by producing eggs that enter maternally instigated embryonic diapause, a developmentally arrested state, which allows species survival by protecting the embryos until favorable conditions return. To appropriately produce diapause eggs, the female must integrate environmental cues and internal physiological state (blood feeding and reproductive status) to allocate nutrients and regulate reproduction. There is variation in reproductive responses to environmental cues between interfertile tropical and temperate populations depending on whether females are actively producing diapause vs. non-diapause eggs and whether they originate from populations that are capable of diapause. Although diapause-inducing environmental cues and diapause eggs have been extensively characterized, little is known about how the female detects gradual environmental changes and coordinates her reproductive status with seasonal dynamics to lay diapause eggs in order to maximize offspring survival. Previous studies suggest that the circadian system is involved in detecting daylength as a critical cue. However, it is unknown which clock network components are important, how these connect to reproductive physiology, and how they may differ between behavioral states or across populations with variable diapause competence. In this review, we showcase Ae. albopictus as an emerging species for neurogenetics to study how the nervous system combines environmental conditions and internal state to optimize reproductive behavior. We review environmental cues for diapause induction, downstream pathways that control female metabolic changes and reproductive capacity, as well as diapause heterogeneity between populations with different evolutionary histories. We highlight genetic tools that can be implemented in Ae. albopictus to identify signaling molecules and cellular circuits that control diapause. The tools and discoveries made in this species could translate to a broader understanding of how environmental cues are interpreted to alter reproductive physiology in other species and how populations with similar genetic and circuit organizations diversify behavioral patterns. These approaches may yield new targets to interfere with mosquito reproductive capacity, which could be exploited to reduce mosquito populations and the burden of the pathogens they transmit.

Effects of temperature and phytoplankton community composition on subitaneous and resting egg production rates of Acartia omorii in Tokyo Bay

To clarify the effects of temperature and phytoplankton community composition on Acartia omorii (Copepoda: Calanoida) egg production, its abundance and egg production rates were investigated from 2016 to 2018 in Tokyo Bay, Japan. Abundance was high from March to May (> 3.0 × 104 individuals m−3) and low or undetected from late June to December (≤ 0.4 × 104 individuals m−3). In 2018, most eggs were subitaneous until April; diapause eggs appeared in May when the water temperature exceeded 20 °C. The weight-specific egg production rate (SEPR, Cegg Cfemale−1 day−1) had two peaks. In the first peak in January, > 90% of eggs were subitaneous; in contrast, in the second peak in May, 60% of eggs were unhatched, including diapause eggs. The first peak of subitaneous eggs may contribute to planktonic population development from March to May. In contrast the second peak of diapause eggs probably enhances their recurrence in the next winter. Multiple regression analysis revealed that subitaneous SEPR showed a negative response, whereas diapause SEPR showed a positive response to temperature. Subitaneous SEPR positively correlated with the proportion of small diatoms in phytoplankton carbon biomass, whereas unhatched SEPR positively correlated with the proportion of inedible preys in large diatoms and dinoflagellates. Edible diatoms may induce subitaneous egg production, whereas low-food availability may induce diapause egg production. These results suggest that phytoplankton composition and water temperature have strong impacts on the dynamics of A. omorii via egg production.

Temperature-driven shifts in the reproductive strategies of Centropages abdominalis Sato in Xiangshan Bay, China

Centropages abdominalis Sato, a common cold-water copepod that is distributed widely in the coastal waters of the North Pacific, appears seasonally in winter and spring as the key zooplankton in the Xiangshan Bay, a typical subtropical semi-closed bay in China. Here, we hypothesised that C. abdominalis could produce diapause eggs as an over-summering strategy in late spring as a response to the increasing temperature and day length. We conducted 11 cruises and in situ reproduction experiments from April 2017 to June 2018 to reveal the variation in the reproductive strategies of C. abdominalis and the related environmental factors. The results showed that the revival of diapause eggs in the inner and middle part of the Xiangshan Bay should contribute to the initial population recruitment in early winter. Sustaining subitaneous egg production supported the instantaneous population recruitment in later winter and spring, and diapause eggs were mainly produced at the upper thermal distribution of C. abdominalis (16.12–21.32 °C) to over-summer in late spring. Temperature was the essential environmental factor that affected the shift in reproductive strategy for C. abdominalis, with an optimum temperature of 19.51 °C for diapause egg production and 11.63 °C for subitaneous one in the Xiangshan Bay. Furthermore, the long-day photoperiod possibly acted as a seasonal change signal for C. abdominalis to produce diapause eggs in late spring in the Xiangshan Bay. The timing of diapause egg production for C. abdominalis could be earlier in lower-latitude waters with shorter day length, as an acclimatisation to warmer habitats. Besides, salinity and Chl a concentration had less effects on diapause eggs production of C. abdominalis in this case. Confronted with ocean warming, the diapause egg period of cold-water copepod (like C. abdominalis) would be prolongated, leading to a reduced population appearance period, especially in lower-latitude waters.

Feeding and reproduction including diapausing egg production as cold-water adaptations for overwintering of <i>Acartia steueri</i> (Copepoda, Calanoida) in Okkirai Bay, Sanriku, northern Japan

Feeding and reproduction including diapausing egg production as cold-water adaptations for overwintering of <i>Acartia steueri</i> (Copepoda, Calanoida) in Okkirai Bay, Sanriku, northern Japan

Seasonal occurrence and development of three closely related Oligonychus species (Acari: Tetranychidae) and their associated natural enemies on fagaceous trees.

We compared the life cycles and diapause attributes among three closely related spider mites, Oligonychus castaneae on Castanea crenata, and O. gotohi and O. amiensis on Lithocarpus edulis. The lower thermal thresholds from egg to egg were 10.5, 8.5 and 8.9°C, respectively, and the thermal constants were 177.8, 229.5 and 232.5 degree-days, respectively. The cumulative hatching rates of diapause eggs of O. castaneae and O. gotohi increased as the season progressed in and after early-to-mid January, which indicates diapause termination. In contrast, O. amiensis showed higher hatching rates in December and January, but hatchability gradually decreased in and after February because some of the eggs died from the cold. Oligonychus castaneae and O. gotohi females produced diapause eggs in response to the short photoperiod in late September to early October and in early-to-late October, respectively, which corresponded to the times predicted by the critical photoperiods (at 15°C) of 12h 15min and 11h 15min for the respective species. Oligonychus castaneae showed at least a single population peak over the 3-year observation period, but the time of peak population varied from mid-July to mid-September. The population of O. gotohi was higher between November and May when diapause eggs were present on host plants in early winter and the first-generation females laid eggs on leaves in spring. The population of O. amiensis, which is a non-diapause species, was only high between September and December, because eggs were laid on leaves in autumn to winter and then gradually disappeared and/or died during winter. Natural enemies were observed as the number of spider mites declined, and the density suppression effect by natural enemies was confirmed in the field.

Switch from production of subitaneous to delayed-hatching and diapause eggs in Acartia japonica Mori, 1940 (Copepoda: Calanoida) from Sagami Bay, Japan

To clarify temporal variation of in situ egg production of the copepods, Acartia japonica Mori, 1940, in Sagami Bay, central Japan, the species’ abundance, egg morphology and hatching patterns were studied during 2015. Adults of A. japonica were observed in the water column from June to October. Results showed that adults reared in the laboratory produced three types of eggs: subitaneous, delayed-hatching and diapause. All egg types had a surface covered with short spines; diapause eggs had a thicker chorion than subitaneous eggs. During the early period of the species occurrence in the bay, most females produced only subitaneous eggs, which hatched within 72 h. From the end of September, a large proportion of females began to produce diapause eggs. Linear regression analysis showed that the ratio of diapause eggs was negatively correlated with day length and water temperature (P < 0.05). Delayed-hatching eggs generally hatched over a broad time span and were also produced in conjunction with diapause eggs. This is the evidence that individual females of a marine copepod could produce all three-egg types simultaneously. Diapause eggs of A. japonica had the capacity to survive at least 300 days at ambient temperatures in the laboratory. The production of diapause eggs in this species probably functions to connect temporally separate populations in each year. Production of all three-egg types likely insures the species’ viability as a copepod that is well adapted to seasonal and sudden environmental changes.

Using Proales similis (Rotifera) for toxicity assessment in marine waters.

There is a need to develop more animal species for assessing toxicity in marine environments. Cyst‐based toxicity tests using invertebrates are especially fast, technically simple, cost‐effective, and sensitive to a variety of toxicants. Over the past 30 years, a variety of toxicity endpoints have been measured using the marine rotifer Brachionus plicatilis hatched from cysts, including mortality, reproduction, ingestion, swimming, enzyme activity, and gene expression. A consensus has developed that the most ecologically relevant toxicity measurements should be made using more than one species. Furthermore, it has been noted that the rotifer species toxicant sensitivity distribution is much broader than which endpoint is measured. This implies that toxicity should be measured with the simplest, fastest, least expensive test available on as many species as feasible. If a battery of test species is to be used to estimate toxicity, diapause egg‐based toxicity tests that do not require culturing of test animals will be key. In this paper, we describe how diapause eggs of a new marine rotifer, Proales similis, can be produced, stored and hatched under controlled conditions to produce animals for toxicity tests. Methods are described for quantifying the toxicity of copper, mercury and cadmium based on mortality, ingestion, reproduction, and diapause egg hatching endpoints. We found that reproduction and ingestion endpoints were generally more sensitive to the metals than mortality or diapause egg hatching. When the copper sensitivity of P. similis was compared to Brachionus manjavacas and B. plicatilis using an ingestion test, similar EC50s were observed. In contrast, the B. rotundiformis ingestion EC50 for copper was about 4× more sensitive. Although diapause egg hatching was not the most sensitive endpoint, it is the most ecologically relevant for assessing sediment toxicity. Our discovery of diapausing eggs in the P. similis life cycle has created a conundrum. We have not observed males or sex in P. similis populations, which is a direct contradiction to the orthodox view of the monogonont rotifer life cycle. Work is needed to clarify how diapause egg production is accomplished by P. similis and whether sexual reproduction is involved.

Rotifer adaptation to the unpredictability of the growing season

Sexual reproduction in cyclically parthenogenetic rotifers results in the production of diapausing eggs that tolerate harsh conditions. Theory predicts that the adaptation to environmental unpredictability can occur by the evolution of bet-hedging strategies. When facing unpredictability in the length of rotifer growing season, patterns of diapausing egg production and hatching can disperse risks and act as bet-hedging strategies. Here, we identified the methodologies to test bet-hedging theory and summarized predictions on rotifer diapause traits. We assessed these predictions by reviewing and reanalyzing results from other studies on field and experimentally evolving populations of Brachionus plicatilis from Spanish ponds. These studies include (1) a quantification of the unpredictability in field populations, (2) the measurement of diapause-related traits in laboratory conditions, and (3) genomic data. The view arising is that these rotifers adaptively respond to the unpredictability in their localities. As expected, propensity for sex increased with unpredictability. Hatching fractions of diapausing eggs were intermediate and decreased with unpredictability only in experimentally evolving populations. In field populations, genetic variance in the focal traits did not increase with unpredictability. Finally, environmental features and diapause-related traits are associated to suites of specific genes, although the molecular mechanisms linking these genes with life history are still unknown.

Utilizing Brachionus biodiversity in marine finfish larviculture

Mouth gape sizes of marine finfish larvae determine the size of rotifer prey that can be ingested, so providing appropriate size rotifers controls the success of larval rearing. It is difficult for hatcheries to maintain pure cultures of multiple rotifer species by long-term serial dilution without cross-contamination. In this paper, we describe a technique for utilizing multiple rotifer species in larviculture that uses rotifer diapausing eggs as inocula, enabling hatcheries to supply larval predators with optimally sized rotifers. Biodiversity in the Brachionus plicatilis species complex is extensive, including considerable variation in body size. In this paper, we show that large quantities of diapausing eggs can be produced from many Brachionus species. Using the full range of body size diversity from seven Brachionus species, we can produce adult female rotifers with mean body lengths as small as 137 μM, or as large as 305 μM, and neonate females with body lengths ranging from 103 to 206 μM. The length of diapausing eggs for small Brachionus species averaged 100 μM, and 166 μM for large species. Diapausing egg production varied by 3.6 fold among 14 strains. Using diapausing eggs of the very small rotifer Proales similis (88 μM) to inoculate mass cultures also is described.

A cryptic diapause strategy in Halotydeus destructor (Tucker) (Trombidiformes: Penthaleidae) induced by multiple cues.

The polyphagous mite pest, Halotydeus destructor, typically has three generations during the cool moist season in Australia and produces over-summering diapause eggs in spring. Diapause eggs have a distinct thick and dark chorion and can survive heat, desiccation and the application of pesticides. Farmers suppress mites producing diapause eggs by a carefully timed spring pesticide application using Timerite® , which predicts the onset of diapause egg production based largely on day length. We investigated the association between diapause induction and other environmental factors that may complicate diapause predictions. Diapause in H. destructor induction was influenced by three interacting environmental factors, namely day length, temperature and soil moisture. A cryptic type of diapause egg that lacked a thick chorion and was instead morphologically similar to non-diapause eggs was also discovered. Like diapause eggs, this newly discovered egg type could also survive hot and dry summer conditions. There is an opportunity to refine the Timerite® spring spray by incorporating knowledge of other environmental factors inducing diapause in H. destructor. Compared with typical diapause eggs, the production of cryptic diapause eggs could reflect a strategy to deal with diversifying environmental stresses and/or represent a bet-hedging strategy to adapt to unpredictable environments. © 2018 Society of Chemical Industry.

Experimental evolution of bet hedging in rotifer diapause traits as a response to environmental unpredictability

The adaptive response of organisms to unpredictable environments is increasingly recognized as a central topic in fundamental and applied evolutionary ecology. Selection due to environmental unpredictability can act on multiple traits of an organism's life cycle to reduce the impact of high environmental variance. The aim of this research was to study how unpredictability selects for diapause traits: 1) the timing of sex (a proxy of the timing of diapausing egg production), and 2) the diapausing egg hatching fraction (a proxy of diapause duration). We used an experimental evolution approach with the facultative sexual rotifer Brachionus plicatilis. Laboratory populations experiencing two contrasting regimes of environmental fluctuation (predictable versus unpredictable) evolved divergently over a short time span (&lt; 77 days). The populations under the unpredictable regime showed an earlier initiation of sexual reproduction and a lower hatching fraction of diapausing eggs than populations under the predictable regime. These findings demonstrate empirically the existence of bet‐hedging strategies in B. plicatilis regarding both traits, consistent with theoretical predictions of bet‐hedging evolution under conditions of unpredictable environmental variance. Given that scenarios of increased environmental variability are expected to occur in the near future, a comprehensive understanding of the role of bet‐hedging strategies is necessary for predicting population responses to environmental change.

Metal stress in zooplankton diapause production: post-hatching response.

Aquatic organisms commonly respond to harsh conditions by forming diapausing stages, which enable populations to survive adverse periods forming egg banks. Production of diapausing eggs is frequently observed in monogonont rotifers, previously changing from asexual to partial sexual reproduction (mixis). In despite that zooplankton are frequently used in ecotoxicological assessment because of their sensitivity to various toxicants and their important role in the ecosystems, toxicity evaluations often consider the directly exposed population produced by parthenogenetic reproduction, exclusively. We assessed experimentally effects of exposure to metals on mixis delay and fitness of hatchlings of the rotifer Brachionus plicatilis obtained from a brackish water lagoon with high metal content, especially copper. We show that sub-lethal concentrations of copper affected traits related to sexual reproduction and diapausing egg production in the rotifer. Copper addition did not delay the start of mixis, suggesting that rapid initiation of mixis is promoted in risky environments, according to the hypothesis of mixis as an escape strategy. Higher investment in mixis was obtained when individuals were exposed to metal. Addition of copper negatively affected the hatching success of diapausing eggs and performance of hatchlings. Nevertheless, these effects were greater for individuals formed in non-metal conditions, suggesting an adaptive advantage of populations from natural sediments exposed to copper. These results highlight the ecological and evolutionary consequences of the presence of metals in freshwater environments by modulating diapause adaptive efficacy and the selective process in egg banks.

Convergent evolution of alternative developmental trajectories associated with diapause in African and South American killifish.

Annual killifish adapted to life in seasonally ephemeral water-bodies exhibit desiccation resistant eggs that can undergo diapause, a period of developmental arrest, enabling them to traverse the otherwise inhospitable dry season. Environmental cues that potentially indicate the season can govern whether eggs enter a stage of diapause mid-way through development or skip this diapause and instead undergo direct development. We report, based on construction of a supermatrix phylogenetic tree of the order Cyprinodontiformes and a battery of comparative analyses, that the ability to produce diapause eggs evolved independently at least six times within African and South American killifish. We then show in species representative of these lineages that embryos entering diapause display significant reduction in development of the cranial region and circulatory system relative to direct-developing embryos. This divergence along alternative developmental pathways begins mid-way through development, well before diapause is entered, during a period of purported maximum developmental constraint (the phylotypic period). Finally, we show that entering diapause is accompanied by a dramatic reduction in metabolic rate and concomitant increase in long-term embryo survival. Morphological divergence during the phylotypic period thus allows embryos undergoing diapause to conserve energy by shunting resources away from energetically costly organs thereby increasing survival chances in an environment that necessitates remaining dormant, buried in the soil and surrounded by an eggshell for much of the year. Our results indicate that adaptation to seasonal aquatic environments in annual killifish imposes strong selection during the embryo stage leading to marked diversification during this otherwise conserved period of vertebrate development.

Embryonic diapause in the Australian plague locust relative to parental experience of cumulative photophase decline

The Australian plague locust Chortoicetes terminifera (Walker) exhibits facultative embryonic diapause during autumn. To approximate natural photoperiod changes during late summer and autumn, locust nymphs were reared under different total declines in laboratory photophase (−0.5, −0.75, −1.0, −1.25, −1.5, −1.75, −2h each lowered in 15min steps) in a 24h photoperiod to quantify any effect on the subsequent production of diapause eggs. Induction of diapause eggs was significantly affected by accumulated photoperiod decline experienced by the parental generation throughout all development stages from mid-instar nymph to fledgling adult. The incidence of embryonic diapause ranged from nil at −0.5h to 86.6% diapause at −2h. Continued declines in photoperiod for post-teneral locusts (transitioned from −1h until fledging to −1.75h) produced a further increase in the proportion of diapause eggs. The results were unaffected by time spent at any given photoperiod, despite a previously indicated maximal inductive photoperiod of 13.5h being used as the mid-point of all treatments. Implications for the seasonal timing processes of photoperiodism in C. terminifera, which has a high migratory capacity and a latitudinal cline in the timing of diapause egg production across a broad geographic range, are discussed.

Effects of photoperiod on egg production in Eurytemora affinis Poppe, 1880 (Copepoda: Calanoida) from the Seine Estuary (France)

The calanoid copepod Eurytemora affinis Poppe, 1880 is the most abundant species of the mesozooplanktonic community of the Seine estuary (France) in the lower salinity zone. Diapausing eggs of this species have been found in the sediment of this estuary but they are few in comparison with those found for other calanoid species. We thus decided to test the effect of photoperiod on diapausing egg production in E. affinis from the Seine estuary. For this, an experimental device has been developed. A combination of five photoperiods and two temperatures was used and the production of eggs was studied for the first and the second clutch. The production of diapausing eggs was demonstrated under photoperiodic conditions less than or equal to 12 h of light either at 12 or 16°C with a maximum under constant darkness (6%). A significant increase in the number of non-viable eggs was also observed between the first and the second clutch for all tested conditions and this may be due to the non-fertilization of these eggs. We also verified that the late naupliar instars are sensitive to the photoperiodic signal.The experimental approach has revealed that the production of diapausing eggs in the key species to the downstream Seine estuary, E. affinis, could be induced by short days, but photoperiod was probably not the only factor responsible for the induction of diapausing egg production. The percentage of diapausing eggs obtained under the induction experiment is similar to that obtained from the eggs extracted from the sediment of the Seine estuary (7%). The strategy of E. affinis in this estuary would be to constitute an egg bank, acting as long-term survival or in the case of adverse conditions. The role of geographical position (mainly latitude) and local conditions in the reproductive strategy of E. affinis is discussed.

Comparative oxygen consumption rates of subitaneous and delayed hatching eggs of the calanoid copepod Acartia tonsa (Dana)

Life history strategies can be adaptations to existence in e.g. unstable environments. Under stressful environmental conditions mobile species tend to migrate to more suitable places, but migration in time by dormancy is another durable strategy. More than 50 species of calanoid copepods can produce diapause eggs requiring a long refractory phase. Delayed hatching eggs (DHE; maternally determined oligopause eggs hatching within >72h to a month) have been described as a state between diapause and subitaneous (hatching within 24–72h) because of their very short refractory phase. We used nanorespirometry to monitor initial oxygen consumption rate of individual eggs of the ubiquitous neritic calanoid copepod Acartia tonsa to distinguish between subitaneous and DHE. We hypothesized that subitaneous eggs exhibit higher initial oxygen consumption rates than DHE, and that initial egg oxygen consumption rate is correlated to the time for the individual egg to hatch. Subitaneous eggs exhibited higher initial oxygen consumption rates than DHE and there were no pattern in initial oxygen consumption rates vs. time to hatch or die from the eggs. Variability in initial oxygen consumption rates within batches of both subitaneous and DHE, as well as between these egg types, is prevalent. There was a continuum from sluggish- to fast metabolising eggs considering initial oxygen consumption rates most likely reflecting phenotypic variation within cohorts. No matter the individual initial egg oxygen consumption rate, embryogenesis took place with unpredicted rates and most eggs eventually hatched. Based on our observations and theoretical considerations we suggest that DHE follow a U shape metabolism pattern with time as described for true diapause eggs. DHE strategy (oligopause) is an important life history trait allowing the spread of hatching over a period of time (a month or two), increasing the likelihood that some offspring encounter suitable environmental conditions and can contribute to the pelagic population of neritic calanoids in environmental fluctuating environments.

Does genetic diversity reduce intraspecific competition in rotifer populations?

As a result of reduced intraspecific competition, genetically diverse populations may have higher relative fitness than genetically uniform populations. To test this hypothesis, we compared polyclonal (i.e., genetically diverse) versus monoclonal (i.e., composed of a single clonal genotype) experimental populations of the rotifer Brachionus plicatilis (Muller, 1786) growing separately and in competition. We estimated the following fitness components: intrinsic growth rate; carrying capacity; proportion of sexual females; diapausing egg production per sexual female and total egg production. Polyclonal populations showed similar dynamics to monoclonal populations and no statistical difference between their fitness components was detected. Therefore, results do not support the hypothesis that genetically diverse populations reduce competition through diversification in resource use. Instead, results suggest that B. plicatilis is a generalist consumer whose polyphagy does not depend on genetic differences, but on the broad diet of each genotype. However, clones showed significant differences in almost all fitness components demonstrating among-clone variation in life-history traits. We found a trade-off between sexual ratio and carrying capacity, highlighting the cost of sex in cyclical parthenogenetic rotifers. We discuss the mechanisms that could maintain the observed among-clone genetic variation in natural populations, and speculate on results implication for sex maintenance in rotifers.

Seasonal variation in the egg morphology of the copepod Eurytemora americana and its relationship with reproductive strategy in a temperate estuary in Argentina

Abstract Berasategui, A. A., Hoffmeyer, M. S., Dutto, M. S., and Biancalana, F. 2012. Seasonal variation in the egg morphology of the copepod Eurytemora americana and its relationship with reproductive strategy in a temperate estuary in Argentina. – ICES Journal of Marine Science, 69: 380–388. Seasonal variation in the morphology of Eurytemora americana eggs and its relationship with reproductive strategy were studied in the Bahía Blanca Estuary. Eggs from field females were analysed morphologically by optical (OM) and electron microscopy [scanning electron microscopy (SEM)–transmission electron microscopy (TEM)] during the population pulse. Additionally, egg-carrying females were incubated simulating in situ environmental conditions to evaluate the resting egg character. Subitaneous and resting eggs were indistinguishable by OM, but showed different chorion appearance under SEM and TEM. Sinuous patterned chorion eggs, mainly produced during the period of population growth, were classed as subitaneous eggs based on their high level of hatching success. Eggs with a sponge-like appearance, produced after the population peaked, were classed as diapause eggs based on their inability to hatch even under favourable conditions. There were significant differences in egg size through the population pulse and diapause eggs were significantly bigger than subitaneous eggs. The observation of both morphotypes in egg-carrying females after peak population abundance confirms the existence of a transitional reproductive stage before the exclusive production of diapause eggs at the end of the pulse. Diapause egg production enables E. americana to resist adverse conditions, ensuring its survival in temperate estuaries.