Variation in ovariole number and egg size of species of Dacus (Diptera; Tephritidae) and their relation to host specialization

Abstract. 1. An analysis of the number of ovarioles/ovary is presented for fourteen species of Dacus ranging from broad generalists (D.tryoni (Frogg.)) to strict host specialists (D.aglaia Hardy). For eight species data on egg size and the incubation time of eggs is also presented. Interrelationships among these life history variables and their relationships to host specialization were examined. 2. There was a broad positive relationship between the breadth of the natural host range (i.e. excluding cultivated fruits) and ovariole number (and hence potential fecundity). Highly polyphagous species had thirty-five to forty ovarioles/ovary while the specialists ranged from eight to twenty. 3. D.musae (Tryon), a specialist on banana, was an exception having an ovariole number similar to that of the most polyphagous species. 4. There was no clear relationship between egg size and ovariole number across eight species, though monophages were not fully represented in this analysis. Among these eight species there was no significant relationship between wing length and egg length, nor was there any such relationship among individuals within species. 5. There was considerable variation among species in the incubation time (at 25°C) for eggs, ranging from 24 h for D.cucumis French to 53 h for D.cacuminatus (Hering). This variation was only weakly related to egg size, though D.cucumis produced the largest eggs of any species examined. 6. Possible explanations for a relationship between potential fecundity and host range are discussed in terms of the balance between the abundance of host resources for generalists and their predictability for specialists. However, the observed relationship between ovariole number and host range must remain tentative, given that only fourteen species from one genus have been examined. Considerably more data is required for other dacines, for species in other tephritid subfamilies, and for other phytophagous insect groups to determine whether the relationship is generally applicable.     

Reproductive variation and adult size in two co-occurring grasshopper species

ABSTRACT. 1. Egg sizes and clutch sizes of the grasshoppers Chorthippus brunneus (Thunb.) and Myrmeleotettix maculatus (Thunb.) were compared among three years and among three sites less than 1.3 km apart. Relationships between these reproductive traits and date of egg laying, body size and body condition were sought.
2. M.maculatus , the smaller species, laid fewer but larger eggs; and only the eggs of this species showed significant differences between sites and years.
3. A negative correlation between egg size and number per clutch was evident between species and years, but generally not among sites and among individuals of a population.
4. However, a hidden negative correlation between egg size and number was uncovered within populations when the relationship was examined for females of a given mature weight.
5. Variation in the number of eggs per clutch was explained statistically by a positive relationship between female body weight and egg number. Also, both interpopulation and intrapopulation comparisons revealed that for M.maculatus , but not for C.brunneus , females with long hind femurs laid large eggs.     

Ecological correlates and heritability of reproductive variation in two co-occurring grasshopper species

ABSTRACT. 1. Relationships are examined between mean egg size and possible selection pressures acting on two grasshopper species, Chorthippus brunneus (Thunb.) and Myrmeleotettix maculatus (Thunb.), co-occurring at three sites in a sand dunes area.
2. Heavy eggs were laid where the mean crowding of hatchlings was high in relation to the amount of grasses with thin-edged leaves. To a lesser extent, large eggs were also found in more desiccating conditions. Mean egg dry weights did not correlate with measures of crowding or measures of crowding in relation to total grass abundance.
3. Significant differences in egg size and clutch sizes were observed between the stocks of second generation, laboratory-reared M.maculatus from three study sites. The significant difference in egg size (site 2 stock > site 1 stock) ran counter to the difference observed in the field but the difference in clutch size (site 3< site 2) was consistent with that observed between field populations.
4. We argue that egg size is primarily determined by selection pressures on the size of hatchlings but that predictably poor feeding conditions for juveniles and adults at site 2 results in the production of smaller adults which, in M.maculatus , then lay smaller eggs. We further argue that M. maculatus from site 2 have been selected to lay larger eggs for their body size and body condition in order to compensate partially for this environmental constraint.     

Fertilization selection on egg and jelly-coat size in the sand dollar Dendraster excentricus

Organisms with external fertilization are often sperm limited, and in echinoids, larger eggs have a higher probability of fertilization than smaller eggs. This difference is thought to be a result of the more frequent sperm-egg collisions experienced by larger targets. Here we report how two components of egg target size, the egg cell and jelly coat, contributed to fertilization success in a selection experiment. We used a cross-sectional analysis of correlated characters to estimate the selection gradients on egg and jelly-coat size in five replicate male pairs of the sand dollar Dendraster excentricus. Results indicated that eggs with larger cells and jelly coats were preferentially fertilized under sperm limitation in the laboratory. The selection gradients were an average of 922% steeper for egg than for jelly-coat size. The standardized selection gradients for egg and jelly-coat size were similar. Our results suggest that fertilization selection can act on both egg-cell and jelly-coat size but that an increase in egg-cell volume is much more likely to increase fertilization success than an equal change in jelly-coat volume. The strengths of the selection gradients were inversely related to the correlation of egg traits across replicate egg clutches. This result suggests the importance of replication in studies of selection of correlated characters.     

Seasonal variation of egg size and number in a Daphnia pulex population

Seasonal variation of egg size and number was examined in a Daphnia pulex population inhabiting a vernal pond. In this population, size at maturity declines at midseason, probably as an adaptive response to size-selective predation by larvae of the salamander Ambystoma. The larger early season individuals produce more and larger eggs than the smaller late season individuals. Age at maturity does not vary between seasons. Laboratory experiments indicate that temperature may affect egg size, egg number and size at maturity. However, field data suggest that temperature accounts for only a small fraction of the total variation in egg size and number. Indirect measures of nutrition indicate that food limitation does not cause the seasonal decline in egg size and number. The seasonal change in reproductive traits is well correlated with changes in invertebrate and vertebrate predation. Examination of predator feeding preferences and their impact on Daphnia mortality indicate that variation of reproductive traits is most likely a complex adaptation to changing predation regimes.     

Sexually antagonistic selection on primate size

Abstract Male intrasexual selection in haplorhine primates has previously been shown to increase male size and to a lesser degree also female size. I address the following questions: (1) why does female size increase when the selection is on males, and (2) why does female size not increase to the same extent as that of males. The potential for correlational selection on females through increased resource competition was analysed with independent contrasts analyses. No such effect was found, nor did matched pairs comparisons reveal females to increase in size because of selection to bear larger male offspring. Instead further matched pairs analyses revealed higher female postpartum investment, as indicated by a longer lactation period, in more sexually selected species, also after correcting for body weight. Concerning the second question, independent contrast analyses showed that large size has had negative effects on female reproductive rate across the primate order. Matched‐pairs analyses on haplorhines revealed that females of species in more polygynous clades have lower reproductive rates than females of species in less polygynous clades. This is also true after the effects of body weight are removed. These results, both when correcting for body weight and when not, suggest that sexual selection has shifted female size from one favouring female lifetime fecundity to one favouring male success in competition. This depicts antagonistic selection pressures on female size and a trade‐off for females between the ecologically optimal size of their foremothers and the larger size that made their forefathers successful.     

Influence of egg size differences within egg clutches on larval parameters in nine libellulid species (Odonata)

Abstract.  1. In libellulids, egg size differs between species and populations. There are also size differences within egg clutches of individual females.
2. Past experiments suggest that there are two different types of egg clutches in libellulids. Egg size decreases significantly during oviposition in species that perform non-contact guarding during oviposition. In contrast, in species ovipositing in tandem, egg size is randomly distributed.
3. This study deals with the possible consequences of egg size variation within the different egg clutch types. The study examined whether there is a correlation between egg development time, offspring sex or larval size and egg size.
4. The current experiments were conducted in Namibia and Germany. Five non-contact guarding and four tandem guarding libellulid species were used.
5. In some species larger eggs needed more time to develop, in some species no correlation between egg size and egg development time could be found, whereas in other species larger eggs developed faster.
6. The sex ratio was biased towards females in Leucorrhinia dubia and in Sympetrum striolatum and egg size was not associated with gender.
7. In both egg clutch types larger eggs resulted in larger larvae. In this study, evidence was found that the effects of egg size diminished with progressing larval development under good conditions. However, it is possible that the effects may have a greater influence under harsh circumstances.     

The role of meiofauna in the marine ecosystem: evolutionary considerations*

The meiobenthos are regarded as the first metazoans to appear on earth This paper considers some aspects of their role in the marine ecosystem over evolutionary time. It is speculatively suggested that they have had a profound influence on many structural and functional attributes of the marine ecosystem as we see it today, both in the benthic and pelagic realms. Certain features such as size spectra, the life-history characteristics of the macrobenthos, and the energetic balance between ecological compartments of the marine ecosystem are considered to result from evolution away from negative interactions with the meiobenthos.     

Egg size,embryonic development time and ovoviviparity in Drosophila species

Lengths, widths and volumes of eggs from 11 species of Drosophila whose genomes have been fully sequenced exhibit significant variation that is not explained by their phylogenetic relationships. Furthermore, egg size differences are unrelated to embryonic development time in these species. In addition, two of the species, Drosophila sechellia and, to a lesser degree, D. yakuba, both ecological specialists, exhibit ovoviviparity, suggesting that female control over oviposition in these species differs from what is observed in D. melanogaster. The interspecific differences in these reproductive characters, coupled with the availability of whole genome sequences for each, provide an unprecedented opportunity to examine their evolution.     

Information and allometry

Many life-history parameters have condition-dependent optima, but individuals are often required to set the values of such parameters relatively early in development, before the relevant conditions can be assessed with full accuracy. If cues are available that predict such future conditions, then the condition-dependent parameter should evolve to assume values that deviate from the mean in the direction implied by the cues, but these deviations should regress towards the mean to the degree that the cues are less than fully reliable. Under mild assumptions, the slopes of the resulting relationships between condition-dependent life-history parameters and the variable conditions on which their optima depend will be the ideal slopes (those that would maximize fitness if the parameter could be chosen on the basis of full information) devalued by the squared correlation between the condition and the parameter.     

Growth pattern of four strains of a Bdelloid rotifer species: egg size and numbers

Life-history traits and growth patterns of four strains of Macrotrachela quadricornifera (Rotifera, Bdelloidea) were studied to assess the influence of maternal traits on egg size. There were two small (S, Va) and two large strains (H, G) with similar patterns of life cycle and body growth. They allocated to reproduction similar relative amounts of resources partitioned into eggs of similar relative size. All strains started reproduction while growing and, although their final sizes differed, at maturity had similar large or small sizes but different ages. Their egg sizes were unaffected by the clutch size, but were positively correlated with mother's body size.     

Changes in feeding habit as caterpillars grow

Abstract.

• 1 200 of 1137 species of British microlepidoptera make a single marked change in feeding habit as they grow.
• 2 Most numerous are species that change from leaf mining to one of case bearing, spinning, tying and rolling or concealed feeding, and species that change from concealed feeding to case bearing or spinning, tying and rolling.
• 3 Although in some instances numbers of closely related species make similar changes in behaviour, most changes observed are scattered among subfamilies.
• 4 Leaf miners have considerably smaller adults than concealed feeders, which themselves are smaller than external feeders and spinners, tiers and rollers. When case bearers are excluded from the analyses, species that remain as leaf miners or concealed feeders throughout larval development are on average smaller as adults than those that start out with these feeding habits but later change.
• 5 We suggest a number of reasons why larvae might make such fundamental switches.