Estimating the size at maturity in field populations of Daphnia (Cladocera)

• 1 Two methods for estimating the average body size at the onset of reproduction were applied to populations of Daphnia hyalina. The first, ‘average maturation size’, assesses maturity by the relative length of abdominal processes of Daphnia; the second, ‘size at first reproduction’ (SFR), calculates the size class in which 50% of the maximum proportion of egg-bearing females is reached.
• 2 The influence of environmental factors on the expression of the difference in length between the first and the second abdominal process was tested in the laboratory. Neither temperature nor chemical cues from predators affected the difference in length, but the processes were indistinguishable at very low food concentrations.
• 3 During one season, D. hyalina exhibited pronounced changes in fecundity, proportion of egg-bearing adults and SFR. The pattern obtained by the two methods was similar in many cases, although maturation size estimated by the first method was slightly smaller than SFR obtained by the second method. Neither method gave reliable results during times of starvation (clear-water phase).

     

Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Field data from seven alpine lakes in Serra da Estrela. Portugal.show that reproduction in Daphnia may be as efficiently controlledby fish predation and copepod predation on eggs in brood cavitiesas it is by food limitation. Body length and clutch size estimatesin Daphnia pulicaria revealed high inter- and intra-populationvariability in maturation size (body size at first reproduction).and in number of eggs per clutch. Daphnia at first maturationin lakes stocked with rainbow trout were half the size of thosefound in fishless lakes (body length of 0.86–0.95 and1.55–1.81 mm. respectively). The mean number of eggs perclutch was reduced to a similar degree by food limitation, predationby fish and copepod predation on eggs in brood cavities, butthe underlying mechanisms of this reduction were different.Food limitation caused smaller clutch sizes in all individuals,so variation remained the same. Fish predation caused the selectiveremoval of individuals with maximum clutches, so variation decreased.Copepod predation caused removal of eggs from brood cavitiesof randomly infested females, so that variation increased, particularlyat a high food level when non-infested females carried largeclutches of eggs.     

Growth patterns,and age and size at maturity in femaleCottus hangiongensis,with special reference to their life-history variation

Growth patterns of the 1982 year-class, individual growth patterns, age and size at sexual maturity and longevity in females of the river-sculpin,Cottus hangiongensis (Cottidae), were examined along the course of the Daitobetsu River of southern Hokkaido, Japan. Growth of females slightly varied both along the river course and among individual fishes: slow growth occurs in females from the lower reaches, while more rapid growth occurs in females from upstream areas. Body size and age at the first sexual maturity of females slightly increased towards the upstream, from 52 mm SL and 2 years in the most downstream area to 72 mm SL and 2–3 years in the uppermost site. Longevity was estimated to be 7 years in the downstream areas and 8–9 years in the upstream sites. These results suggest that female life history varies along the course of the river and thus allow us to consider the following alternative reproductive tactics: when females stay in the lower reaches, they attain sexual maturity at a smaller body size and younger age, and have a small clutch size, but when females migrate into the upper reaches, their maturity is delayed until they reach a larger body size and older age, and have a greater clutch size.     

A new look at relationships between size at maturity and asymptotic size

Comparative studies have revealed positive correlations between size at maturity and asymptotic size in several taxa with asymptotic growth after maturity. Using a simple growth model, we show that positive correlations between size at maturity and asymptotic size are predicted for different individuals in the same species if growth costs of reproduction are inversely related to size at maturity. Several processes might lead to higher growth costs of reproduction for smaller individuals; these include effects of body size on competition for resources required for breeding, on the space available within the body cavity for food processing in gravid individuals, and on the costs of transporting eggs or young in relation to the total energy budget. We confirm several key elements of the growth model using data from female Iguana iguana lizards, including the novel assumption that instantaneous growth rates of adults of the same length will be positively related to their length at maturity. These analyses suggest a simple and possibly general explanation for positive correlations between size at maturity and asymptotic size within-and perhaps also among-species that continue to grow after maturity.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Both costs and benefits of sex correlate with relative frequency of asexual reproduction in cyclically parthenogenic Daphnia pulicaria populations

Sexual reproduction is generally believed to yield beneficial effects via the expansion of expressed genetic variation, which increases the efficiency of selection and the adaptive potential of a population. However, when nonadditive gene action is involved, sex can actually impede the adaptive progress of a population. If selection promotes coupling disequilibria between genes of similar effect, recombination and segregation can result in a decrease in expressed genetic variance in the offspring population. In addition, when nonadditive gene action underlies a quantitative trait, sex can produce a change in trait means in a direction opposite to that favored by selection. In this study we measured the change in genotypic trait means and genetic variances across a sexual generation in four populations of the cyclical parthenogen Daphnia pulicaria, which vary predictably in their incidence of sexual reproduction. We show that both the costs and benefits of sex, as measured by changes in means and variances in life-history traits, increase substantially with decreasing frequency of sex.     

The evolution of reaction norms: simple models for age and size at maturity

This paper presents a simple model for the evolution of reaction norms for age and size at maturity that predicts reaction norms with a variety of shapes. Using realistic parameter values the model predicts reaction norms close to those observed in Drosophila. The major assumptions of the model are: 1) that net reproductive rate is maximized, 2) that growth is determinate, and 3) that mortality rates are independent of age and size at maturity. If, additionally, juvenile mortality is uncorrelated with a growth coefficient, k, the model predicts that selection favors maturation later at a smaller size when k is reduced by environmental factors and that decreased juvenile mortality leads to delayed maturity. These two predictions conform with those found by previous models using other measures of fitness. Correlations between k and juvenile mortality can change the shape of the predicted reaction norm. Depending on the precise form of the correlation, the model can predict done- or bowl-shaped reaction norms and can predict delayed or earlier maturity as k decreases. These shapes are qualitatively different from those predicted by previous models that used different fitness measures. Systematic estimates of the parameter values for this and for related models are required to determine the appropriate fitness measure for models of reaction norms.     

Correlated evolution of genome size and seed mass

Previous investigators have identified strong positive relationships between genome size and seed mass within species, and across species from the same genus and family. Here, we make the first broad-scale quantification of this relationship, using data for 1222 species, from 139 families and 48 orders. We analyzed the relationship between genome size and seed mass using a statistical framework that included four different tests. A quadratic relationship between genome size and seed mass appeared to be driven by the large genome/seed mass gymnosperms and the many small genome size/large seed mass angiosperms. Very small seeds were never associated with very large genomes, possibly indicating a developmental constraint. Independent contrast results showed that divergences in genome size were positively correlated with divergences in seed mass. Divergences in seed mass have been more closely correlated with divergences in genome size than with divergences in other morphological and ecological variables. Plant growth form is the only variable examined thus far that explains a greater proportion of variation in seed mass than does genome size.     

Developmental thresholds and the evolution of reaction norms for age and size at life-history transitions

It is quite common in studies of life-history plasticity to find a negative relationship between the age at which various life-history transitions occur and the growth conditions under which individuals develop. In particular, high growth typically results in earlier transitions, often at a larger size. Here, we use a relatively general optimization model for age and size at life-history transitions to argue that current life-history theory cannot adequately explain these results. Specifically, most such theory requires key assumptions that are unlikely to be generally met. This suggests that some important component of the biology of many organisms must be missing from many of the models in life-history theory. We suggest that this missing component might be the phenomenon of developmental thresholds. There are at least two different types of developmental thresholds possible, and we incorporate these into our general optimality model to demonstrate how they can cause a negative relationship between growth conditions and age at a transition. If developmental thresholds are common throughout taxa, then this might explain the empirical results. Our model formulation and analysis also formalizes the popular Wilbur-Collins hypothesis for age and size at metamorphosis in amphibians. The results demonstrate that optimal combinations of age and size, and the slope of the reaction norm connecting them, depend on the existence and type of threshold assumed. Our results also provide an evolutionary framework that can be used to view the data and many of the proximate submodels derived from the Wilbur-Collins hypothesis.     

Adaptive changes in harvested populations: plasticity and evolution of age and size at maturation

We investigate harvest-induced adaptive changes in age and size at maturation by modelling both plastic variation and evolutionary trajectories. Harvesting mature individuals displaces the reaction norm for age and size at maturation toward older ages and larger sizes and rotates it clockwise, whereas harvesting immature individuals has the reverse qualitative effect. If both immature and mature individuals are harvested, the net effect has approximately the same trend as when harvesting immature individuals only. This stems from the sensitivity of the evolutionary response, which depends on the maturity state of harvested individuals, but also on the type of harvest mortality (negatively or positively density dependent, density independent) and the value of three life-history parameters (natural mortality, growth rate and the trade-off between growth and reproduction). Evolutionary changes in the maturation reaction norm have strong repercussions for the mean size and the density of harvested individuals that, in most cases, result in the reduction of biomass--a response that population dynamical models would overlook. These results highlight the importance of accounting for evolutionary trends in the long-term management of exploited living resources and give qualitative insights into how to minimize the detrimental consequences of harvest-induced evolutionary changes in maturation reaction norms.     

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.     

Variation in phenoloxidase activity and its relation to parasite resistance within and between populations of Daphnia magna

Estimates of phenoloxidase (PO) activity have been suggested as a useful indicator of immunocompetence in arthropods, with the idea that high PO activity would indicate high immunocompetence against parasites and pathogens. Here, we test for variation in PO activity among clones of the planktonic crustacean Daphnia magna and its covariation with susceptibility to infections from four different microparasite species (one bacterium and three microsporidia). Strong clonal variation in PO activity was found within and among populations of D. magna, with 45.6% of the total variation being explained by the clone effect. Quantitative measures of parasite success in infection correlated negatively with PO activity when tested across four host populations. However, these correlations disappeared when the data were corrected for population effects. We conclude that PO activity is not a useful measure of resistance to parasites or of immunocompetence within populations of D. magna. We further tested whether D. magna females that are wounded to induce PO activity are more resistant to infections with the bacterium Pasteuria ramosa than non-wounded controls. We found neither a difference in susceptibility nor a difference in disease progression between the induced group and the control group. These results do not question the function of the PO system in arthropod immune response, but rather suggest that immunocompetence cannot be assessed by considering PO activity alone. Immune response is likely to be a multifactorial trait with various host and parasite characteristics playing important roles in its expression.     

Variation of body size within and between wild populations of Drosophila buzzatii

Four populations of the cactophilous species D. buzzatii have been compared with respect to the phenotypic variation of thorax and wing length of wild versus laboratory reared flies. Three of the strains were intercrossed to provide parent, F₁ and F₂ comparisons as a test of co-adaptation. The genetic contribution to phenotypic variation of laboratory reared flies was estimated from the correlation between sibs derived from random pair mating and reared individually in separate cultures. The average natural temperature during development was estimated from the relations between the wing/thorax ratio and temperature in laboratory tests.The variance of thorax and wing length of wild flies was several times greater than that of laboratory reared flies and the increase was attributed primarily to variation in larval food supply although temperature fluctuation is also important. There was no evidence of heterosis or F₂ break-down in the crosses. For two of the populations the heritability of thorax length was high, 60–70%, and substantially lower for the third. The average temperature estimated from the wing/thorax/temperature relationship differed between sites. The reduction of body size below the potential maximum averaged 30% for two and 20% for the other population, with a wide spread about these values. The evidence is discussed in relation to assessing the nature of ecological variation by comparing the variation of morphological traits in wild and laboratory reared flies.     

Larval budding, metamorphosis, and the evolution of life-history patterns in echinoderms

Abstract. The oral surface and mouth of juvenile asteroids and echinoids with indirect development forms on the lower left side of the larval body, thus establishing a new axis of body symmetry. In contrast, the juvenile mouth of ophiuroids and holothuroids develops from the larval one, and the larval and adult body axes roughly coincide. Explaining how two such disparate modes of development arose in evolution has been a perennial problem for echinoderm biologists, but recent observations on larval budding in asteroids may provide an answer. The juvenile mouth of asteroids forms near the base of the left posterolateral lobe. The posterolateral lobes are also the principal site of bud formation in asteroid larvae that propagate asexually, and buds form mouths. By accelerating the development of oral and ectodermal structures belonging to the bud, and combining these with internal organs derived from the parent larva, a composite individual could be constructed with the same orientation and positioning as the juvenile rudiment in asteroids. Whether this also explains the position of the juvenile rudiment in echinoids is a more complex question, depending in part on whether asexual propagation is derived, and restricted to asteroids and ophiuroids, or is more primitive and hence widespread among stem echinoderms.     

Niche-driven evolution of metabolic and life-history strategies in natural and domesticated populations of Saccharomyces cerevisiae

Background  

Variation of resource supply is one of the key factors that drive the evolution of life-history strategies, and hence the interactions between individuals. In the yeast Saccharomyces cerevisiae, two life-history strategies related to different resource utilization have been previously described in strains from different industrial origins. In this work, we analyzed metabolic traits and life-history strategies in a broader collection of yeast strains sampled in various ecological niches (forest, human body, fruits, laboratory and industrial environments).     

Relationship between genome size and maturity group in soybean

Previous studies have reported a correlation between genome size and relative maturity among plant species. The objective of this study was to determine whether such a relationship exists in soybean. Twenty cultivars, representing maturity groups ranging from 000 to IX, were analyzed using flow cytometric procedures. A 15% difference in genome size was observed ranging from BSR 201 at 2.88 pg to Maple Presto at 2.51 pg. A highly significant correlation (r = 0.55) was observed between maturity and the genome size of the 20 cultivars.     

Effect of previous feeding conditions on survival in Daphnia pulicaria (Crustacea: Cladocera)

Survival of natural Daphnia populations without previous acclimatization in filtered water under laboratory conditions (starvation) was used as a parameter describing the actual competitive ability of populations in periods of food limitation. Two populations (1 and 2) of Daphnia pulicaria Forbes coming from two enclosures with a low (< 1 mgC l^–1) and a high (about 3 mgC l^–1) seston level were examined in their resistance to starvation. Juveniles of both populations survived better than adult females in the conditions without seston as well as in the control experiment, despite their much lower biomass. Animals from population 1 had lower mortality than those from population 2 in all variants of the experiment. The results confirm the hypothesis that the ability of daphnids to starve depends also on adaptation to previous feeding conditions.