Litter Overlap in Gambusia hubbsi: Superfetation Revisited

The frequency of litter overlap, the simultaneous presence of two litters at different stages of development within a single female, varies among populations of Gambusia hubbsi in different habitats on Andros, Bahamas. In freshwater bluehole and shallow water populations, less than 2% of the females carried two litters, independent of the difference in developmental stage between the litters. In well field populations, 2% of the females carried two litters composed of immature and mature oocytes, whereas 10% carried developing oocytes and late stage embryos, and 16% carried mature oocytes (or older) and late stage embryos. Among females from populations that we introduced into unoccupied well fields 2% carried two litters composed of immature and mature oocytes, about 14% carried developing oocytes and late stage embryos, and less than 6% carried mature oocytes (or older) and late stage embryos. When two litters composed developing oocytes and late stage embryos were present, the two litters were of equal size, and the total number of offspring was 1.5 times larger than expected on the basis of female body length. When two litters composed mature oocytes (or older stages) and late stage embryos were present, the two litters were also of equal size, and the total number of offspring was as expected on the basis of female body length. Litter overlap was independent of female size. A comparison of the consequences of litter overlap for strictly lecithotrophic and strictly matrotrophic modes of reproduction with the patterns of litter overlap observed in G. hubbsi, suggests that litter overlap in G. hubbsi reduces the cost of reproduction, but does not increase the rate of offspring production.     

Life-History Variation in Caribbean gambusia, Gambusia puncticulata puncticulata (Poeciliidae) from the Cayman Islands, British West Indies

We studied seasonal and spatial variability in the reproductive life-history traits of Caribbean gambusia, Gambusia puncticulata puncticulata, using collections representing dry and wet periods from eight pond sites located across the three Cayman Islands. Caribbean gambusia exhibited a seasonal life-history response over the 5-month interval between the relatively dry and wet periods, marked by shifts to larger adult sizes and smaller broods made up of larger offspring. This seasonal shift in the life-history pattern coincided with increased rainfall, lower salinity, lower water temperature, and higher food availability. Overall, there was a reproductive trade-off involving a reciprocal relationship between brood size and mean embryo mass, and a direct relationship between brood size and total embryo mass. Levels of various environmental variables, including salinity, submerged aquatic vegetation cover, and capture depth, were apparently unrelated to the life-history pattern. Furthermore, the life-history pattern did not reflect an island effect. However, a correlation between the seasonal difference in salinity and offspring size suggested that the Cayman Island life-history pattern may correspond in part with the environmental stability hypothesis.     

Responses of Sexual and Apomictic Genotypes of Taraxacum officinale to Variation in Light

Abstract: The mode of reproduction, sexual or asexual, will influence the way populations respond to selective pressures. This can cause genetic and ecological divergence between sexual and asexual forms of the same species. Here we examine differences in morphology and phenology between sexual and apomictic types of dandelion, Taraxacum officinale. Sexual and apomictic dandelions were collected from a mixed population on the banks of the river Rhine, The Netherlands. Clonal copies of both sexual and apomictic genotypes were planted in an experimental garden under two light levels. Sexual plants flowered four days later on average than apomicts, but the number of capitula was the same. Apomicts had longer leaves and were heavier than sexual plants, especially under shaded conditions. In apomicts plasticity for leaf length and height was larger than in sexuals, but for most other measured traits no differences in plasticity were observed. Trait values of apomicts were within the same range as those of sexual plants.     

Effect of age‐based and environment‐based cues on reproductive investment in Gambusia affinis

We examined the multivariate life‐history trajectories of age 0 and age 1 female Gambusia affinis to determine relative effects of age‐based and environment‐based cues on reproductive investment. Age 0 females decreased reproductive investment prior to the onset of fall and winter months, while age 1 females increased reproductive investment as the summer progressed. The reproductive restraint and terminal investment patterns exhibited by age 0 and age 1 females, respectively, were consistent with the predictions from the cost of reproduction hypothesis. Age 0 females responded to environment‐based cues, decreasing reproductive investment to increase the probability of overwinter survival and subsequent reproductive opportunities in the following summer. Age 1 females responded to age‐based cues, or the proximity of death, increasing investment to current reproduction as future reproductive opportunities decreased late in life. Thus, individuals use multiple cues to determine the level of reproductive investment, and the response to each cue is dependent on the age of an individual.     

Phenotypic plasticity in age and size at maturity and its effects on the integrated phenotypic expressions of life history traits of Cardamine flexuosa (Cruciferae)

We analyzed variation in phenotypic plasticity of life history traits between two Cardamine flexuosa populations based on differences in plasticity of age and size at maturity. C. flexuosa (Cruciferae) is a facultative, vernalization-sensitive, long-day annual, and its phenology and the phenotypic expressions of many life history traits are largely controlled by photoperiod and vernalization in natural populations. We used plants from two populations which differed in their responses to chilling and photoperiod treatments. The timing of developmental processes was changed by controlling temperature and photoperiod regimes in growth chambers. Plasticity in size at maturity was analyzed as changes in a growth trajectory using two parameters, age at maturity (Δt) and growth rate (k). Both traits showed plasticity, but differences between the populations were found mostly for Δt. Distinctive differences in size at maturity of individuals in the two populations were mainly due to different amounts of plasticity in Δt. Variations in plasticity of nine other life history traits and their associations to age and size at maturity were also analyzed. Variation for eight of the traits can be described, at least in part, as a function of age and size at maturity for both populations, and most of the variation in the total number of seeds was explained by age and size at maturity. Only age at maturity had any effect on changes in resource allocation. The nine life history traits were integrated through associated character expressions with age and size at maturity. Changes in the association between a trait and age and/or size at maturity were rather conservative compared to changes in the plasticity of a trait between the two populations. Associations with age and size at maturity are mostly explicable in terms of inherent relationships in the developmental processes, and they may limit the ecological range expansion and the adaptive evolution of plasticity in C. flexuosa. The negative correlation between reproductive allocation and age at maturity can be a cost of delaying maturation in C. flexuosa.     

Variation in the latitudinal reproductive patterns of the genus Didelphis (Didelphimorphia:Didelphidae)

Abstract: The data published on reproduction of the species of the genus Didelphis (Didelphimorphia, Didelphidae) were compiled and analysed to identify general patterns. The duration of the breeding season for the Didelphis species varied from 12 months near the Equator to 6 months at latitude 25°S and varied inversely with latitude and litter size. The mean litter size varied from 4.2 in Colombia to 9.4 in New York and was positively associated with latitude and negatively with the duration of breeding season. We propose that the higher investment in production at higher latitudes was compensated by increased mortality owing to the seasonal variation in food availability and environmental variables in theses latitudes.     

Placental efficiency and intrauterine resource allocation strategies in the common marmoset pregnancy

Mothers and fetuses are expected to be in some degree of conflict over the allocation of maternal resources to fetal growth in the intrauterine environment. Variation in placental structure and function may be one way a fetus can communicate need and quality to its mother, potentially manipulating maternal investment in its favor. Whereas common marmosets typically produce twin litters, they regularly give birth to triplet litters in captivity. The addition of another fetus is a potential drain on maternal resource availability and thus a source of elevated conflict over resource allocation. Marmoset littermates share a single placental mass, so that differences in the ratio of fetal to placental weight across litter categories suggest the presence of differential intrauterine strategies of resource allocation. The fetal/placental weight ratio was calculated for 26 marmoset pregnancies, representing both twin and triplet litters, to test the hypothesis that triplet fetuses respond to intrauterine conflict by soliciting placental overgrowth as a means of accessing maternal resources. In fact, relative to fetal mass, the triplet marmoset placenta is significantly undergrown, with individual triplets associated with less placental mass than their twin counterparts, suggesting that the triplet placenta is relatively more efficient in its support of fetal growth. There still may be an important role for maternal-fetal conflict in the programming of placental structure and function. Placental adaptations that solicit potential increases of maternal investment may occur at the microscopic or metabolic level, and thus may not be reflected in the size of the placenta as a whole.     

Life history adaptation along a latitudinal cline in the water strider Aquarius remigis (Heteroptera: Gerridae)

We addressed the general hypothesis that life history differences among eastern populations of the North American water strider, Aquarius remigis (Heteroptera: Gerridae), along a north-south gradient are manifestations of genetic differentiation due to natural selection. We raised offspring of two field-caught populations from each of three latitudes in a common laboratory environment at 20° C and two photoperiods. Nearly all Quebec (PQ) individuals (ca. 46° N) entered diapause to reproduce the following spring (univoltine life cycle), while intermediate proportions of New York (NY; ca. 43° N) and New Jersey (NJ; ca. 41° N) individuals reproduced directly, producing a second generation (bivoltine life cycle). PQ females were smaller, developed faster, and laid smaller eggs than NY and NJ individuals; NY and NJ populations differed little in these variables. NY females had longer life spans than either PQ or NJ females, but lower oviposition rates. Total reproductive output did not differ across latitudes. Photoperiod affected body length, development time, and reproductive pathway, resulting in a latitude by environmental interaction. PQ individuals reproduced directly under 15L : 9D (summer) conditions only, while the NY and NJ populations exhibited more direct reproduction under 13L : 11D (spring or fall) conditions. Some life history characters of the NY and NJ populations displayed the higher variability indicative of phenological transition zones. These results indicate local adaptation of populations to long-term climatic patterns. Water striders appear to adapt to longer seasons by extending development, growing larger, and breeding directly. Larger body size and extended or rapid development are associated with bivoltinism and increase in egg size, but not necessarily with higher fecundity or oviposition rate. The phenological transition zone appears to be unrelated to a transition zone a little further south established by allozyme data and morphology, as all populations studied here could be electrophoretically identified as northern "type".     

黑带食蚜蝇体色变异的研究

通过对黑带食蚜蝇Episyrphus balteatus(De Geer)体色变异的研究,发现黑带食蚜蝇存在深色型和浅色型2种基本形式,体色变异具有明显的季节性,深色型个体出现在温度较低的秋季及早春,浅色型个体发生在温度较高的春夏季节;在腹部色斑方面,深色型和浅色型个体中均存在变异,可划分为8种类型,腹部色斑的变异在浅色型个体中较为丰富,腹部不同体节背板上色斑的变异程度不同;黑带食蚜蝇在体型大小方面同样存在差异,这些变异的多态性可能是食蚜蝇系统学研究中异名产生的一个重要原因。因此,食蚜蝇类群的个体体色变异在系统学研究中是一个必须特别注意的问题。     

Temporal and latitudinal comparisons of reproductive parameters in a heavily exploited shark,the bonnethead,Sphyrna tiburo (L. 1758), in the southern Gulf of Mexico

In the southern Gulf of Mexico, the bonnethead shark, Sphyrna tiburo, is one of the most frequently captured species in landings of small-scale fisheries. Based on the analysis of two fishery-dependent sampling periods (1993–1994 and 2007–2014), this study aimed to determine reproductive parameters and identify temporal differences between the two time periods. In the first sampling period, 776 males and 352 females with a size range of 28.0–120.0 cm total stretched length (L_T) were analysed, and in the second sampling period, 387 males and 432 females with a size range of 28.0–122.0 cm L_T were analysed. The size at 50% maturity in the second sampling period was significantly different between sexes, 82.6 cm L_T for females and 73.8 cm L_T for males (no estimation was possible for the first sampling period). The size at 50% maternity was not different between sampling periods, 97.3 cm L_T for the first sampling period and 99.0 cm L_T for the second sampling period. Litter size varied from 3 to 19 embryos and the average was not statistically different in both periods, 10.1 (S.D. = 3.8) for the first sampling period and 11.3 (S.D. = 3.5) for the second sampling period. The female reproductive cycle is asynchronous, and it seems to be annual, with a gestation period of 5–6 months, and a consecutive ovarian cycle and gestation period. Temporal (between sampling periods) and latitudinal (southern Gulf versus northern regions) variations occur in the synchronicity of the reproductive cycle, temporal variation in the relationship between maternal length and litter size, and latitudinal variation in average size of mature sharks.     

Variation in Caenorhabditis elegans dauer larva formation

Dauer larvae of Caenorhabditis elegans are formed when young larvae experience conditions of low food availability and high conspecific population density; non-dauer, third stage larvae are formed in conditions of plenty. This developmental response to environmental conditions is an example of phenotypic plasticity; that is, an environmentally induced change in phenotype and, as such, a manifestation of a genotype-environment interaction. Extensive variation was found in reaction norms of phenotypic plasticity of dauer formation among wild lines of C. elegans. Recombinant-inbred lines were constructed from parental lines with very different reaction norms of dauer formation. These recombinant-inbred lines had a wide range of reaction norms, of a range greater than that set by the parental lines. The natural variation in reaction norms of dauer formation in C. elegans is, presumably, an adaptation to enhance fitness under the lines' different natural prevailing conditions. The genetic basis of this variation, as well as its phenotypic consequences, are now ripe for further investigation.     

Annual variations in the reproductive traits of Pomatoschistus microps in a Mediterranean lagoon undergoing environmental changes: evidence of phenotypic plasticity

The spawning period of the common goby Pomatoschistus microps from 1993 to 1997 in the Vaccarès lagoon did not vary, except in 1997 when it was longer due to the reproduction of the young-of-the-year. Egg size and number, and reproductive allocation varied greatly with one year to another. Female common gobies increased both their fecundity per spawning act and their egg size from 1993 to 1995. The annual variation in the reproductive effort suggests a high phenotypic plasticity of reproductive traits in P. microps , in the face of environmental perturbations. In winter 1993–1994, a centennial flood of the Rhône River caused major hydrological changes in the lagoon in less than 1 week, affecting many invertebrates and fish for several years. The reproductive investment of the common goby increased, possibly as a consequence of those environmental changes.     

Population density, body size, and phenotypic plasticity of brood size in a burying beetle

Theory predicts that organisms living in heterogeneous environmentswill exhibit phenotypic plasticity. One trait that may be particularlyimportant in this context is the clutch or brood size becauseit is simultaneously a maternal and offspring characteristic.In this paper, I test the hypothesis that the burying beetle,Nicrophorus orbicollis, adjusts brood size, in part, in anticipationof the reproductive environment of its adult offspring. N. orbicollisuse a small vertebrate carcass as a food resource for theiryoung. Both parents provide parental care and actively regulatebrood size through filial cannibalism. The result is a positivecorrelation between brood size and carcass size. Adult bodysize is an important determinant of reproductive success forboth sexes, but only at higher population densities. I testthree predictions generated by the hypothesis that beetles adjustbrood size in response to population density. First, averageadult body size should vary positively with population density.Second, brood size on a given-sized carcass should be larger(producing more but smaller young) in low-density populationsthan in high-density populations. Third, females should respondadaptively to changes in local population density by producinglarger broods when population density is low and small broodswhen population density is high. All three predictions weresupported using a combination of field and laboratory experiments.These results (1) show that brood size is a phenotypically plastictrait and (2) support the idea that brood size decisions arean intergenerational phenomenon that varies with the anticipatedcompetitive environment of the offspring.     

Early exposure to nonlethal predation risk by size-selective predators increases somatic growth and decreases size at adulthood in three-spined sticklebacks

Predation has an important influence on life history traits in many organisms, especially when they are young. When cues of trout were present, juvenile sticklebacks grew faster. The increase in body size as a result of exposure to cues of predators was adaptive because larger individuals were more likely to survive predation. However, sticklebacks that had been exposed to cues of predators were smaller at adulthood. This result is consistent with some life history theory. However, these results prompt an alternative hypothesis, which is that the decreased size at adulthood reflects a deferred cost of early rapid growth. Compared to males, females were more likely to survive predation, but female size at adulthood was more affected by cues of predators than male size at adulthood, suggesting that size at adulthood might be more important to male fitness than to female fitness.     

Age-specific resource investment strategies: evidence from female Richardson's ground squirrels (Spermophilus richardsonii)

To avoid a possible cost to their future survival and/or reproduction, individuals must balance their somatic and reproductive investments. The van Noordwijk and De Jong model of resource investment predicts that investments into reproduction and soma can vary among individuals of a population based on the variation in the total amount of energy that individuals acquire. With principal components analysis (PCA), we created two axes of life history for female Richardson's ground squirrels Spermophilus richardsonii : an index of total energy investment (PC1) and an index of investment tactic (PC2). Using these indices, we examined patterns of resource allocation to reproductive and somatic investments. Because yearling female Richardson's ground squirrels complete growth to adult size during pregnancy and early lactation, their somatic needs exceed those of older, fully grown females. Therefore, we predicted that yearlings would show more evidence of a tradeoff between reproductive and somatic investments compared with older females. Both yearling and older females invested four to five times more mass into their litters than into their own body mass. With increasing total investment, yearling females increased investment in both reproduction and themselves, whereas older females invested relatively more in reproduction than themselves. Regardless of age, females that emerged heavier from hibernation invested fewer resources into themselves and more into their litters. Variation in total energy investment and investment tactic indices was similar for yearling and older females. Contrary to our prediction, however, yearling females showed positive associations between reproductive and somatic investments, whereas older females exhibited showed no significant association between reproductive and somatic investments.