Local adaptation of sex induction in a facultative sexual crustacean: insights from QTL mapping and natural populations of Daphnia magna

Dormancy is a common adaptation in invertebrates to survive harsh conditions. Triggered by environmental cues, populations produce resting eggs that allow them to survive temporally unsuitable conditions. Daphnia magna is a crustacean that reproduces by cyclical parthenogenesis, alternating between the production of asexual offspring and the sexual reproduction of diapausing eggs (ephippia). Prior to ephippia production, males (necessary to ensure ephippia fertilization) are produced parthenogenetically. Both the production of ephippia and the parthenogenetic production of males are induced by environmental factors. Here, we test the hypothesis that the induction of D. magna resting egg production shows a signature of local adaptation. We postulated that Daphnia from permanent ponds would produce fewer ephippia and males than Daphnia from intermittent ponds and that the frequency and season of habitat deterioration would correlate with the timing and amount of male and ephippia production. To test this, we quantified the production of males and ephippia in clonal D. magna populations in several different controlled environments. We found that the production of both ephippia and males varies strongly among populations in a way that suggests local adaptation. By performing quantitative trait locus mapping with parent clones from contrasting pond environments, we identified nonoverlapping genomic regions associated with male and ephippia production. As the traits are influenced by two different genomic regions, and both are necessary for successful resting egg production, we suggest that the genes for their induction co‐evolve.     

Diapause dynamics of two diaptomid copepod species in a large lake

Skistodiaptomus oregonensis and Leptodiaptomus minutus produce diapausing eggs in a large lake, Oneida Lake, in New York State. The timing of the switch from production of subitaneous (immediately hatching) eggs to diapausing eggs for both species is in October. This timing is consistent with a pattern, reviewed here, for other populations of diaptomid copepods: populations living in large lakes tend to begin production of diapausing eggs later in the season than those living in small lakes. Populations living in temporary ponds tend to switch still earlier in the season. All populations reviewed here live in the north temperate zone. The sediments of Oneida Lake contain densities of diaptomid diapausing eggs on the order of 10⁵ m^–2 per cm below the sediment surface down to 5 cm. Below this sediment depth, egg densities decline. The highest egg densities were found in sediments under the deepest water. Diapausing eggs of L. minutus survive in the sediments at least two years, as shown by the recovery of the population after a year in which no new diapausing eggs were produced, and probably for two or more decades. Long-term dormancy can have the effect of ensuring the continuation of a population through periods of poor recruitment, and can help create conditions for the coexistence of competing species. Other investigators have suggested that S. oregonensis and L. minutus are competitors in other lakes when they co-occur.     

Salinity tolerance of diapausing eggs of freshwater zooplankton

1. Many freshwater zooplankton produce diapausing eggs capable of withstanding periods of adverse environmental conditions, such as anoxia, drought and extreme temperature. These eggs may also allow oligostenohaline species to survive increased salinity during periods of tidal flux or evaporation, and here we test the ability of diapause eggs to withstand such conditions. 2. Salinity tolerance may also enable organisms to invade new environments. The increased rate of introduction of non‐indigenous species to the Laurentian Great Lakes since 1989, when ballast water exchange regulations (to replace fresh/brackish water at sea with full seawater) were first implemented for transoceanic vessels, has stimulated studies that explore mechanisms of introduction, other than of active animals, in ballast water. One hypothesis proposes that freshwater organisms transported in ballast tanks as diapausing eggs may be partially responsible for the increased rate of species introduction, as these eggs may tolerate a wide array of adverse environmental conditions, including exposure to saline water. 3. We collected ballast sediments from transoceanic vessels entering the Great Lakes, isolated diapausing eggs of three species (Bosmina liederi, Daphnia longiremis and Brachionus calyciflorus), and measured the effect of salinity on hatching rate. In general, exposure to salinity significantly reduced the hatching rate of diapausing eggs. However, as non‐indigenous species can establish from a small founding population, it is unclear whether salinity exposure will be effective as a management tool.     

Evidence of concurrent local adaptation and high phenotypic plasticity in a polar microeukaryote

Here we investigated whether there is evidence of local adaptation in strains of an ancestrally marine dinoflagellate to the lacustrine environment they now inhabit (optimal genotypes) and/or if they have evolved phenotypic plasticity (a range of phenotypes). Eleven strains of Polarella glacialis were isolated and cultured from three different environments: the polar seas, a hyposaline and a hypersaline Antarctic lake. Local adaptation was tested by comparing growth rates of lacustrine and marine strains at their own and reciprocal site conditions. To determine phenotypic plasticity, we measured the reaction norm for salinity. We found evidence of both, limited local adaptation and higher phenotypic plasticity in lacustrine strains when compared with marine ancestors. At extreme high salinities, local lake strains outperformed other strains, and at extreme low salinities, strains from the hyposaline lake outperformed all other strains. The data suggest that lake populations may have evolved higher phenotypic plasticity in the lake habitats compared with the sea, presumably due to the high temporal variability in salinity in the lacustrine systems. Moreover, the interval of salinity tolerance differed between strains from the hyposaline and hypersaline lakes, indicating local adaptation promoted by different salinity.     

Hatching and viability of rotifer diapausing eggs collected from pond sediments

1. Planktonic rotifers inhabiting variable environments produce diapausing eggs that accumulate in the sediment of lakes and ponds, forming egg banks that may withstand adverse periods. A common assumption in zooplankton diapausing egg bank studies is to count as viable all eggs in the sediment that look healthy. This assumption should be challenged by asking how effectively ‘healthy‐looking’ eggs represent viable eggs. 2. In this study, viability of more than 1100 ‘healthy‐looking’ diapausing eggs belonging to the Brachionus plicatilis species complex was assessed in a laboratory hatching experiment. Eggs were collected at different depths from sediment cores obtained from 15 ponds located in coastal and inland areas of Eastern Spain. 3. Only approximately one half of the ‘healthy‐looking’ diapausing eggs hatched after incubation in experimental conditions. Almost all the hatchlings (99.4%) survived to maturity. The proportion of ‘healthy‐looking’ diapausing eggs that hatched varied among areas and among ponds within area, and substantially declined with sediment depth. Most of the hatchlings (88%) were obtained from the uppermost 2 cm of sediment. ‘Healthy‐looking’ eggs from upper sediment layers hatched after significantly shorter incubation times than eggs recovered from deeper layers. 4. Both decreased hatching success and increased incubation time for hatching with sediment depth suggest that older ‘healthy‐looking’ eggs are less responsive to hatching stimuli and could become unviable. However, the strong correlation found between the number of ‘healthy‐looking’ eggs and the number of hatchlings indicates that the abundance of ‘healthy‐looking’ eggs is a good index of egg bank viability.     

Particle transport by benthic invertebrates: its role in egg bank dynamics

The ecological and evolutionary dynamics of zooplankton is in part a function of the numbers and ages of dispausing eggs hatching from aquatic sediments. Successful recruitment from this egg bank must depend upon the eggs being present at or near the sediment surface. Often, however, zooplankton diapausing eggs are found as deep as 15 to 30 cm in the mud. Bioturbation may provide a mechanism for the regular return of buried eggs to the sediment surface. A substantial portion of the population of the copepod, Diaptomus sanguineus, living in Bullhead Pond, a small lake in Rhode Island, USA, is present as diapausing eggs. To study the role of bioturbation in egg-bank dynamics, we introduced polystyrene beads, the same size and specific gravity as copepod eggs, at two depths in large-diameter sediment chambers in the laboratory. Treatments included chambers with natural and reduced densities of benthos. Consistent with other studies, our results show that the joint activities of tubificid oligochaetes and chironomid larvae are responsible for bidirectional (up and down) transport of beads in the top 2 cm of the sediment. We observed no bead movement below this depth. Thus, eggs in the top two centimeters of sediment in this lake are exposed with some regularity to conditions that stimulate hatching at the sediment-water interface. In Bullhead Pond, these eggs have a mean age of 12.2 years (based on ²¹⁰Pb-dating). Eggs buried more deeply will only be returned to the sediment surface by relatively rare, localized disturbances. This return of old eggs to the surface affects ecological and evolutionary dynamics in a complex way.     

Deterioration patterns in diapausing egg banks of Brachionus (Müller, 1786) rotifer species

Rotifers are cyclical parthenogens that produce sexual diapausing eggs at some stage in their life cycle. These eggs are encysted embryos that remain viable for extended periods in lake and pond sediments, thus acting as an egg bank with many ecological and evolutionary consequences. Despite its importance to rotifer evolution, there are no studies on resting egg deterioration and associated processes in natural environments. In this study, more than 4000 diapausing eggs of species from the Brachionus plicatilis complex, which includes several closely related cryptic species, were collected from different sediment depths in 15 ponds in eastern Spain and were classified according to three features thought to be related to their viability: shell integrity, embryo size, and embryo colour. A positive association was found between embryo size and hatching success in those eggs having an intact shell. Diapausing eggs that showed good shell integrity and no more than a 25% reduction of multinuclear embryo maximum size were classified as healthy and 98.9% hatched. Darkening of diapausing egg embryo was an indicator of viability loss. A decreasing frequency of healthy-looking diapausing eggs was observed with increasing sediment depth, although some exceptions were found.     

Response of diapausing eggs hatching to changes in temperature and the presence of fish kairomones

It has been hypothesized that the production of diapausing eggs in Daphnia can be induced by fish kairomones. A population of Daphnia could survive severe predation using this predator avoidance strategy. However, in changing environments, diapausing eggs experience various temperature conditions, and hatchlings at emergence may be exposed to the same predation risks as their mothers. Therefore, staying in diapause or an immediate response upon hatching to available environmental information could be important for hatchling survival. For this study, we investigated the impact of water temperature (10, 15, 20, and 25°C) in the presence and absence of fish kairomones (Lepomis macrochirus) on the hatching success of resting eggs (D. galeata). Results show that no diapausing eggs hatched at the lowest temperature (10°C), and the highest hatch percentage occurred at 15°C. Although higher water temperatures reduced hatching success, diapausing eggs hatched more quickly. The number of hatchlings was significantly higher after exposure to fish kairomones, and this was more noticeable at higher temperatures (20 and 25°C). The present results suggest that the diapausing eggs were produced as a predator avoidance strategy in Daphnia; however, the presence of fish works as a positive signal to increase hatchlings when the diapausing stage is terminated.     

EVOLUTIONARY POTENTIAL AND LOCAL GENETIC DIFFERENTIATION IN A PHENOTYPICALLY PLASTIC TRAIT OF A CYCLICAL PARTHENOGEN,DAPHNIA MAGNA

The frequent use of neutral markers to quantify genetic variation in natural populations emphasizes the role of stochastic events in explaining genetic differentiation between populations. Complementary studies on ecologically relevant traits are needed to assess the role of natural selection acting on adaptive variation in the development of local genetic differentiation. To test the hypothesis of local adaptation in the cyclical parthenogenetic species Daphnia magna, the phototactic behavior in the absence and presence of fish kairomone was assayed for 30 clones derived from resting eggs isolated from three habitats characterized by a different predation pressure by fish. Clones derived from populations in which fish are present were, on average, more negatively phototactic in and more responsive to the presence of fish kairomone than clones derived from a fishless habitat. In addition, the results show a high genetic diversity for the traits studied in all three gene pools investigated, indicating a high potential for microevolutionary changes in behavior of these Daphnia populations in the face of changes in predation pressure. The results of the present study indicate that working with large samples at the expense of having less precise estimates of genotypic values for specific genotypes may result in a loss of information with regard to the evolutionary potential of local populations.     

Thermal tolerance in the keystone species Daphnia magna—a candidate gene and an outlier analysis approach

Changes in temperature have occurred throughout Earth's history. However, current warming trends exacerbated by human activities impose severe and rapid loss of biodiversity. Although understanding the mechanisms orchestrating organismal response to climate change is important, remarkably few studies document their role in nature. This is because only few systems enable the combined analysis of genetic and plastic responses to environmental change over long time spans. Here, we characterize genetic and plastic responses to temperature increase in the aquatic keystone grazer Daphnia magna combining a candidate gene and an outlier analysis approach. We capitalize on the short generation time of our species, facilitating experimental evolution, and the production of dormant eggs enabling the analysis of long‐term response to environmental change through a resurrection ecology approach. We quantify plasticity in the expression of 35 candidate genes in D. magna populations resurrected from a lake that experienced changes in average temperature over the past century and from experimental populations differing in thermal tolerance isolated from a selection experiment. By measuring expression in multiple genotypes from each of these populations in control and heat treatments, we assess plastic responses to extreme temperature events. By measuring evolutionary changes in gene expression between warm‐ and cold‐adapted populations, we assess evolutionary response to temperature changes. Evolutionary response to temperature increase is also assessed via an outlier analysis using EST‐linked microsatellite loci. This study provides the first insights into the role of plasticity and genetic adaptation in orchestrating adaptive responses to environmental change in D. magna.     

Molecular mechanisms of tolerance to cyanobacterial protease inhibitors revealed by clonal differences in Daphnia magna

Protease inhibitors of primary producers are a major food quality constraint for herbivores. In nutrient‐rich freshwater ecosystems, the interaction between primary producers and herbivores is mainly represented by Daphnia and cyanobacteria. Protease inhibitors have been found in many cyanobacterial blooms. These inhibitors have been shown (both in vitro and in situ) to inhibit the most important group of digestive proteases in the daphnid's gut, that is, trypsins and chymotrypsins. In this study, we fed four different Daphnia magna genotypes with the trypsin‐inhibitor‐containing cyanobacterial strain Microcystis aeruginosa PCC 7806 Mut. Upon exposure to dietary trypsin inhibitors, all D. magna genotypes showed increased gene expression of digestive trypsins and chymotrypsins. Exposure to dietary trypsin inhibitors resulted in increased activity of chymotrypsins and reduced activity of trypsin. Strong intraspecific differences in tolerance of the four D. magna genotypes to the dietary trypsin inhibitors were found. The degree of tolerance depended on the D. magna genotype. The genotypes' tolerance was positively correlated with the residual trypsin activity and the different IC₅₀ values of the trypsins. On the genetic level, the different trypsin loci varied between the D. magna genotypes. The two tolerant Daphnia genotypes that both originate from the same lake, which frequently produces cyanobacterial blooms, clustered in a neighbour‐joining phylogenetic tree based on the three trypsin loci. This suggests that the genetic variability of trypsin loci was an important cause for the observed intraspecific variability in tolerance to cyanobacterial trypsin inhibitors. Based on these findings, it is reasonable to assume that such genetic variability can also be found in natural populations and thus constitutes the basis for local adaptation of natural populations to dietary protease inhibitors.     

Effects of salinity on the life history and fitness of Daphnia magna: variability within and between populations

The life history traits of Daphnia magna were studied in laboratory experiments under freshwater and brackish (5 salinity) conditions. The variability of responses within and between populations was examined by comparing 11 clones from a brackish lake and 10 clones from a freshwater pond. Experimental clones were hatched from ephippia collected from the sediment and thus represent random samples of the clone banks of each population.Most clones with a high salinity tolerance were from the population of the brackish habitat, but some were also found in the freshwater population. Thus, freshwater populations appear to have the potential to invade brackish habitats. A proportion of clones from the brackish population had very low fitness (measured as e^r) under freshwater conditions. This unexpected result means that freshwater adaptation can be lost by the freshwater cladoceran Daphnia magna. The effects of unfavourable conditions on growth and reproduction varied among clones and were not correlated. This clonal variation in growth and reproduction indicates that the environmental sensitivities of these traits are independent. The pattern of fitness reaction norms showed no trade-off between fitness under brackish and under freshwater conditions for either population. Thus, euryhaline generalists should be favoured in habitats with salinity fluctuations between freshwater and brackish conditions.     

Inbreeding and outbreeding depression in Daphnia

Egg-to-adult viability of sexual offspring in Daphnia magna is lower for selfed (average: 43.0%) than for outcrossed families (average: 74.7%). This suggests that intraclonal mating is not the rule in Daphnia populations. For a given family, hatching rate of eggs resulting from interpopulation crosses is lower than for intrapopulation crosses. This breakdown in hatching responses may result in the effective gene flow between Daphnia populations being severely reduced, offering an explanation for the apparent paradox of genetic differentiation of Daphnia populations in spite of efficient dispersal.     

Constitutive but no Triops-induced differences in bet-hedging strategies for hatching in Daphnia

Since cladocerans from the genus Daphnia are known to have evolved several inducible defenses (morphological and life history shifts) against the notostracan predator Triops, we investigated whether hatching was also altered in response to Triops. We tested whether dormant eggs of Daphnia magna are able to detect Triops cancriformis kairomones in the water as a signal of predation pressure and alter their hatching response accordingly to avoid predation. We predicted that, in the presence of Triops kairomones, hatching fractions might be reduced (postponing hatching to a next growing season) and/or that hatching might peak earlier (increasing chances to reproduce before Triops becomes predatory). We also tested whether this response depended on the origin of the population. Ephippia from three D. magna populations, originating from one permanent lake and two temporary pond systems, were exposed to Triops kairomone and control treatments. We observed significant population differences in hatching patterns, both in terms of the fraction of eggs that hatch as well as the timing of hatching, with evidence for within-season bet-hedging through delayed hatching in the populations inhabiting temporary habitats. However, no indication was found that the populations also adjust their hatching pattern to the presence of Triops kairomones.     

Patterns in rotifer diapausing egg banks: Density and viability

Here we present a quantitative study on the density, age and viability of the diapausing egg banks of the rotifer species complex Brachionus plicatilis in the sediments of 15 water bodies from Eastern Spain. Sampled ponds, located in coastal and inland areas, varied in salinity and ranged in size, depth and permanence. By identifying ‘hatched’, ‘deteriorated’ and ‘viable’ diapausing eggs in the sediment samples, we estimated production, hatching and deterioration in relation to the habitat properties of each pond. Our results indicate the presence of large numbers of diapausing eggs in the sediments of almost all of the ponds studied (2-115 eggs cm^− 2). Inland ponds tended to have higher densities than coastal lagoons. The vertical distribution of eggs in the sediments frequently showed a non-decreasing pattern, which suggested a high among-year variation in egg production. Despite maximum age of eggs of 60-80 years, the median age (3-30 years) suggests that rotifer egg banks are young in the studied ponds. Egg senescence is suggested by the declining abundance of ‘healthy-looking’ eggs with depth. The proportion of ‘deteriorated’ eggs ranged 75-99% suggesting that deterioration rates in the sediments are high and vary among habitats. Hatching and deterioration rates, as estimated from the counts of ‘hatched’, ‘deteriorated’ and ‘healthy-looking’ eggs in the sediments, largely varied among ponds. An association between hatching and deterioration rates is suggested by our data. This is in agreement with the hypothesis that hatching rates of diapausing eggs depended not only on the risks associated with the water column, as initially expected by the general theory on diapause, but they are also related to the incidence of deterioration processes in the sediment.     

The effect of diapause on the genetic structure of Daphnia magna populations

For intermittent Daphnia magna populations embryonic diapause following bisexual reproduction is the means of surviving in harsh environmental conditions. Another advantage of bisexuality is genetic recombination which produces a variety of genotypes. Crowding and short-day photo-periods promote bisexual reproduction, but photoperiod, at least, can not be an obligatory factor in D. magna since some bisexual females have been found also during the periods of long daylength. Because the catastrophe dates vary considerably, the most advantageous reproductive strategy must be one with some flexibility in reacting to irregular catastrophes by producing ephippial resting eggs whenever the probability of a catastrophe exceeds a certain threshold value. Extensive genetic changes occurring during diapause suggest that there are considerable genetic differences in ephippial egg production and egg survival. Genetic drift alone can not account for the great genetic changes observed, although in populations reacting slowly to changing conditions the number of diapausing ephippia can be relatively low. If the ephippia hatch over a long period of time, they can act as a stabilizing mechanism for genotype frequencies.     

Temporal dispersal: ecological and evolutionary aspects of zooplankton egg banks and the role of sediment mixing

Zooplankton egg banks are the accumulation of diapausing embryosof planktonic animals buried in the sediments of many aquaticecosystems. These eggs, which are analogous life history stagesto the seeds of many plants, can survive in a ready-to-hatchstate for periods ranging from a few years to greater than acentury. Their presence in ponds, lakes and near-shore marineenvironments has substantial implications for understandingtrajectories of ecological and evolutionary change. When thesediments of lakes are structured in historical sequence, diapausingeggs extracted from different sediment ages can provide a meansof studying past changes in community or population-geneticstructure. A completely different aspect of egg banks derivesfrom the fact that hatching of diapausing eggs can influence,through what can be thought of as temporal dispersal, populationand community response to environmental change. Eggs hatchingfrom diapause introduce to current environments species or genotypeslaid at times in the distant past. In addition, egg banks createextended generation overlap that can play an important rolein maintaining diversity in a fluctuating environment when differenttypes (species or genotypes) are favored at different times.These distinct aspects of egg banks (i.e., their direct impacton ecological and evolutionary processes versus their usefulnessin reconstructing historical changes), are potentially in conflictbecause for old eggs to hatch, the sediments must be at leastpartially mixed. This same mixing, however, degrades the accuracyof the historical record. Both aspects are possible, however,even within a single lake when sediment-mixing intensity isspatially heterogeneous.     

Role of parthenogenetic natality and emergence from diapausing eggs in the dynamics of some rotifer populations

There are few quantitative data on the role of emergence from diapausing eggs in population dynamics of natural populations of zooplankton species; to our knowledge, all these concern copepods and cladocerans. We present here direct estimates of emergence from bottom resting eggs for another important category of freshwater zooplankton, namely rotifers. Three populations of rotifers of the genus Brachionus were studied in a lake. During the study period 10 population increases, each corresponding to an individual sampling interval, were detected. For each interval, emergence from immediately hatching, parthenogenetic eggs calculated on the basis of the Edmondson-Paloheimo model and emergence from diapausing bottom eggs determined in short-term experiments were estimated and compared to each other. We found that three of the population increases observed are entirely explained by parthenogenetic natality. In contrast, emergence from diapausing eggs can, on its own, account for none of population increases. For two population increases, however, it accounts for that part of population growth which remains unexplained by the parthenogenetic natality. For rotifer populations studied, emergence from diapausing eg eggs is generally less important than parthenogenetic births, when both are regarded as an immediate cause of population growth. This is in sharp contrast to the data available for some crustaceans (De Stasio, 1990) where the role of emergence from diapausing eggs in population dynamics has been clearly shown.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Status of Daphnia Resting Egg Banks in Bohemian Forest Lakes Affected by Acidification

We studied the egg banks of Daphnia gr. longispina in four acidified lakes in the Bohemian Forest (Czech Republic, Europe). Daphnia had become extinct in three of the studied lakes due to anthropogenic acidification and we assessed the possibility of autochthonous recovery of the population after recent amelioration of the lake environment. We determined the ephippia distribution in eleven cores 10–30 cm long, and the state of Daphnia resting eggs in over 13 000 ephippia from eight cores. Apparently well-preserved eggs were used for hatching experiments and for DNA amplification. Vertical profiles of ephippia densities in several cores did not agree with historical data on the Daphnia presence in the lakes, as the sediment had been repeatedly disturbed by human activities in the past. Ephippia are present near the surface of the sediment in all lakes, and they might therefore receive hatching cues. We were not able, however, to prove that viable eggs are present in the ephippia banks of the lakes. The percentage of the well-preserved eggs in all but one core was below 1%. In addition, no egg hatched in the hatching experiments and we were not able to amplify DNA from the preserved eggs of lakes where Daphnia is extinct, although amplification from relatively young eggs from the fourth lake was successful in 60% of the cases. We conclude that the recovery of Daphnia populations in studied lakes from autochthonous sources is unlikely.