Optimal rates of dispersal I. Haploid populations

The problem of how an individual should divide its progeny into dispersed and non-dispersed descendants is investigated for a number of haploid models. In each model, a unique optimal ratio of dispersed to non-dispersed has been found, where the type with this optimal strategy has a selective advantage over all other types. Although the survival ability of a dispersed offspring is reduced (compared to that of a non-dispersed offspring), the optimal strategy implies that a substantial part of the progeny will be dispersed.     

Marriage patterns in a Mesoamerican peasant community are biologically adaptive

Differential investment in offspring by parental and progeny gender has been discussed and periodically analyzed for the past 80 years as an evolutionary adaptive strategy. Parental investment theory suggests that parents in poor condition have offspring in poor condition. Conversely, parents in good condition give rise to offspring in good condition. As formalized in the Trivers-Willard hypothesis (TWH), investment in daughters will be greater under poor conditions while sons receive greater parental investment under good conditions. Condition is ultimately equated to offspring reproductive fitness, with parents apparently using a strategy to maximize their genetic contribution to future generations. Analyses of sex ratio have been used to support parental investment theory and in many instances, though not all, results provide support for TWH. In the present investigation, economic strategies were analyzed in the context of offspring sex ratio and survival to reproductive age in a Zapotec-speaking community in the Valley of Oaxaca, southern Mexico. Growth status of children, adult stature, and agricultural resources were analyzed as proxies for parental and progeny condition in present and prior generations. Traditional marriage practice in Mesoamerican peasant communities is patrilocal postnuptial residence with investments largely favoring sons. The alternative, practiced by ～25% of parents, is matrilocal postnuptial residence which is an investment favoring daughters. Results indicated that sex ratio of offspring survival to reproductive age was related to economic strategy and differed significantly between the patrilocal and matrilocal strategies. Variance in sex ratio was affected by condition of parents and significant differences in survival to reproductive age were strongly associated with economic strategy. While the results strongly support TWH, further studies in traditional anthropological populations are needed.     

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above‐mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two‐dimensional spatial model female‐biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male‐biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.     

Dispersal, Infectivity and Sex Ratio of Early- or Late-Emerging Infective Juveniles of the Entomopathogenic Nematode Steinernema carpocapsae

Differences in activity between infective juveniles (IJ) of the entomopathogenic nematode Steinernema carpocapsae that emerged directly from cadavers onto either a sand or agar substrate compared with those emerging from a cadaver into water and then being placed on the same substrate are known to occur. Differences between S. carpocapsae IJ that emerged directly from a cadaver vs. those that emerged from a cadaver and held in water were further elucidated. Dispersed and non-dispersed IJ from a cadaver were compared with those held in water between two time periods designated as early- (first two days) or late-emerging IJ (seventh day). A significantly greater proportion of early-emerging IJ from the cadaver treatment dispersed, compared with late-emerging IJ from a cadaver or either group of emerging IJ held in aqueous suspension. Moreover, IJ from cadavers were more infectious than those from the aqueous suspensions, and IJ that dispersed were less infectious than those that did not disperse. IJ that emerged early were mostly males, whereas those that emerged late were mostly females. For the non-dispersed IJ, most that emerged early were males, and those that emerged later were females, but among dispersing IJ, there was no difference in sex ratio between early- and late-emerging nematodes.     

Maternal investment and size-number trade-off in a bee, Megachile apicalis, in seasonal environments

Maternal investment in offspring size and number differed between spring- and summer-emerging individual females of Megachile apicalis, a solitary multivoltine bee. Data from experimentally initiated female populations indicated that spring-emerging females produced a relatively large number of progeny but allocated a small amount of food to each, resulting in small progeny. Adult females of larger body sizes provisioned food at a greater rate than did smaller females, and this body-size effect was significant in spring-emerging females. The large body size of these females allowed them to increase the number of progeny produced under the abundant floral resources that occurred during the spring. Conversely, summer-emerging females produced fewer progeny under the diminishing resources for brood production, but allocated each with more food, producing larger progeny, most of which emerged in the spring of the following year. Field data using trap-nests also indicated the same pattern of seasonal offspring size allocation found in the experimental populations. This maternal investment strategy entails a trade-off between the size and number of progeny, so that the daughters upon emergence can best perform in their brood production under the seasonally variable environments where they reproduce.     

Open-cell parasitism shapes maternal investment patterns in the Red Mason bee Osmia rufa

Brood cell parasitism inflicts high fitness costs on solitary,nest-constructing bees. Many of these parasites enter open cellsduring its provisioning, when the mother bee is absent. Therefore,females can reduce the risk of open-cell parasitism by limitingthe time they are away from the nest. However, provisioningefficiency (provisioning time per unit of progeny body mass)decreases due to aging. To limit the increasing risk of open-cellparasitism as the nesting season progresses, female bees couldoptimize their maternal investment strategy by shifting thesex ratio and the body size of offspring during the nestingseason. This prediction was tested in the Red Mason bee Osmiarufa (O. bicornis), a stem- or hole-nesting, polylectic, univoltinemegachilid bee. In O. rufa, the risk of open-cell parasitismwas found to be correlated with cell provisioning time. Additionally,the provisioning efficiency of females declined during the nestingseason to one-fourth of the initial value. However, cell-provisioningtime did not increase correspondingly. Bees dealt with theirdecreasing provisioning efficiency by reducing the amount ofstored larval food, leading to a reduction of offspring sizeand a seasonal shift toward males in the offspring sex ratio.The influence of provisioning efficiency and risk of open-cellparasitism on optimal offspring size was analyzed by means ofa statistical model. The observed maternal investment patternof Red Mason bees is an adaptive strategy to reduce open-cellparasitism.     

Alternative Predictions for Optimal Dispersal in Response to Local Catastrophic Mortality

What dispersal strategy should be employed by an organism in response to local catastrophic mortality? Here we contrast predictions from an analytical solution derived from an ESS model which optimizes competitive ability (Comins et al., 1980) with those from a stochastic, branching process model (Karlson and Taylor, 1992) which maximizes survivorship of a clonal lineage. The optimal dispersal fraction varies directly with the probability of local extinction in the ESS model, yet varies inversely with this probability over much of the parameter space in the latter model. In order to conform more closely with the assumptions of the ESS model, we have modified the branching process model to have a random, Poisson-distributed number of offspring and compared the predictions of these models. Both models invoke dispersal as an escape from local extinction and predict mixed dispersal strategies over a wide range of conditions. However, increasing local catastrophic mortality favors more dispersal in the ESS model, but it can be so severe in the branching process model that no dispersal strategy is adaptive. In this model, the predicted optimal proportion of dispersed offspring is highest at low to intermediate levels of catastrophic mortality depending on the total number of offspring produced. We suggest that this observed discrepancy is sufficiently large to warrant empirical tests of these qualitatively different predictions.     

Seed dispersal by sympatric tamarins,Saguinus mystax and Saguinus fuscicollis: diversity and characteristics of plant species

In a comparative study of Saguinus mystax and Saguinus fuscicollis, we examined the spectrum of plant species whose seeds are dispersed by these two tamarin species. We characterize these plants in terms of life form, fruit colour, pulp consistency and seed dimensions. The tamarins disperse a much broader spectrum of plant species than previously reported (88 of the total of 155 species exploited for fruit). While the distribution over plant life form, fruit colour and pulp consistency is identical between dispersed plant species and the overall spectrum of consumed plant species, clear differences exist in all seed parameters (length, width, height, volume, mass) except specific weight between dispersed and non-dispersed plant species for both tamarin species. Plant and fruit characteristics and seed parameters of dispersed plant species do not differ between S. mystax and S. fuscicollis, suggesting that their ecological and evolutionary interaction with plants is very similar or identical.     

How to put all your eggs in one basket: empirical patterns of offspring provisioning throughout a mother's lifetime

Maternal effects arise when a mother's phenotype or the environment she experiences influences the phenotype of her progeny. Most studies of adaptive maternal effects are a "snapshot" of a mother's lifetime offspring provisioning and do not generally consider the effects of earlier siblings on those produced later. Here we show that in soil mites, offspring provisioning strategies are dynamic, changing from an emphasis on egg number in young females to egg size in older females. This pattern may be adaptive if it increases the survival of younger offspring that must compete with older, larger siblings. The dynamic shift in egg provisioning was greater in high-food environments in which females lived longer, creating increasing asymmetry in offspring competitive abilities. Females reared in isolation and in the presence of a high-density colony had identical provisioning strategies, suggesting that, unlike males in this species, females do not use pheromones to assess colony size. Our findings suggest that the adaptive significance of maternal effects may be misinterpreted when studies consider only a snapshot of a female's offspring provisioning strategy or when components of the offspring provisioning strategy are studied in isolation.     

The effects of relatedness on progeny sex ratio in spider mites

Theory predicts that optimal sex allocation in subdivided populations is dependent on the genetic relatedness among competing offspring such that when relatedness is high, progeny sex allocation should be more biased than when relatedness is low. In the laboratory we compared the progeny sex ratio of four groups of five ovipositing females of various presumed degrees of relatedness (sisters of inbred laboratory population, sisters of outbred field population, non-sisters of the same field population and non-sisters each from a different, geographically distant field population). We found a greater female bias in the progeny of inbred sisters than for sisters from the field population; the progeny sex ratio was also more strongly female biased for sisters than for non-sisters from the field population. These differences in sex ratio are in line with theoretical predictions. Our results indicate that spider mites are capable of some method of discrimination between kin and non-kin.     

Alteration of nucleolar dispersion in prophase by 5-FUdR treatment.

The action of 5-Fluorodeoxyuridine (FUdR) used as an inhibitor of RNA synthesis on the nucleolar evolution during mitosis, has been studied in meristematic cells. Under FUdR treatment the nucleolar dispersion appears as a continuous process, but generally it is not completed and nucleolar remnants remain throughout the whole mitosis. The nucleolar material which was dispersed is transported by the mitotic chromosomes, and in telophase contributed to the formation of the new nucleolus. The non-dispersed part persisted in the cytoplasm during telophase, coexisting with both the prenucleolar bodies and the new nucleolus which was being formed. Our results suggest the necessity of some kind of RNA synthesis, preferentially blocked by FUdR, for nucleolar dispersion to take place.     

MOLECULAR GENETIC DISSECTION OF SPAWNING,PARENTAGE, AND REPRODUCTIVE TACTICS IN A POPULATION OF REDBREAST SUNFISH,LEPOMIS AURITUS

Despite a great diversity of reproductive behaviors in fishes, few studies have examined the genetic consequences of alternative reproductive tactics. Here we develop and employ microsatellite markers to assess genetic paternity and maternity of progeny cohorts in a population of redbreast sunfish (Lepomis auritus), a species in which males build and tend nests. Nearly 1000 progeny from 25 nests, plus nest-attendant males and nearby adults, were genotyped at microsatellite loci that displayed more than 18 alleles each. The genetic data demonstrate that multiple females (at least two to six) spawned in each nest, their offspring were spatially dispersed across a nest, and more than 90% of the young were sired by the attendant male. However, about 40% of the nests also showed genetic evidence of low-level reproductive parasitism, and two nests were tended by males that had fathered none of the sampled offspring. Genetically deduced reproductive behaviors in this population of redbreast sunfish contrast with those reported previously in bluegill sunfish (L. macrochirus) wherein heteromorphic males specialized for parasitism or for parental care coexist in high frequency. Thus, nest-parasitic reproductive behaviors in fishes appear to be evolutionary labile.     

Natural Selection of Optimal Reproductive Tactics

Acting through differential reproductive success, natural selectionhas produced a great diversity of existing reproductive tactics,each of which presumably corresponds to a local optimum thatmaximizes an individual organism's lifetime reproductive successin its particular environment. A body of theory on so-calledreproductive "strategies" has yet to be adequately related toan independent theoretical framework on optimal foraging tactics.Some of the possible interactions and constraints between ananimal's input of matter and energy via foraging and its outputin offspring using these same materials are briefly considered.For example, storage and utilization of lipids allow an organismto gather and sequester matter and energy during a period thatis not suitable for successful reproduction, but enable theorganism to expend those materials at a later, more satisfactorytime. Such interactions between foraging and reproduction leadto a sort of temporal integration, which greatly complicatesestimation of reproductive effort (current investment in seed,eggs, or progeny) in variable environments. An optimal reproductivetactic maximizes an individual's reproductive value (the sumof all present plus the expected probable number of all futureoffspring) at every age. Reproductive effort should vary inverselywith residual reproductive value (expectation of future offspring);moreover, the precise form of the trade-off between presentoffspring versus future progeny, which is itself sensitive toa multitude of environmental influences including resource availabilityand the immediate environmental conditions for reproductionand survival, dictates the optimal tactic at any given age.Simple graphical models of optimal reproductive tactics arepresented and discussed. Finally, some promising directionsfor future work, as well as certain potential difficulties,are noted.     

Transgenerational effects of parental larval diet on offspring development time, adult body size and pathogen resistance in Drosophila melanogaster

Environmental conditions experienced by parents are increasingly recognized to affect offspring performance. We set out to investigate the effect of parental larval diet on offspring development time, adult body size and adult resistance to the bacterium Serratia marcescens in Drosophila melanogaster. Flies for the parental generation were raised on either poor or standard diet and then mated in the four possible sex-by-parental diet crosses. Females that were raised on poor food produced larger offspring than females that were raised on standard food. Furthermore, male progeny sired by fathers that were raised on poor food were larger than male progeny sired by males raised on standard food. Development times were shortest for offspring whose one parent (mother or the father) was raised on standard and the other parent on poor food and longest for offspring whose parents both were raised on poor food. No evidence for transgenerational effects of parental diet on offspring disease resistance was found. Although paternal effects have been previously demonstrated in D. melanogaster, no earlier studies have investigated male-mediated transgenerational effects of diet in this species. The results highlight the importance of not only considering the relative contribution each parental sex has on progeny performance but also the combined effects that the two sexes may have on offspring performance.     

Removal of repeated sequences from hybridisation probes.

Pre-reassociation of human clone probes, containing dispersed highly repeated sequences, (e.g. Alu and KpnI families), with a large excess of sonicated total human DNA allows signal from single and low copy number components to be detected in transfer hybridisations. The signal from non-dispersed repeated sequences is reduced to single copy levels. The procedure, which is simple and quick, is illustrated using model combinations of well characterised cloned probes, and is applied to a sample of randomly chosen cosmid clones. A theoretical assessment is presented which may be useful to those wishing to use this procedure.     

Reproductive skew and indiscriminate infanticide

In communally breeding animals, there is an evolutionary conflict over the partitioning of reproduction within the group. If dominant group members do not have complete control over subordinate reproduction, this conflict may favour the evolution of infanticidal behaviour (by either subordinates or dominants or both). Elimination of offspring, however, is likely to be constrained by the difficulty of discriminating between an individual's own progeny and those of cobreeders. Here, we develop an evolutionarily stable strategy (ESS) model of reproductive partitioning, which demonstrates that killing of young can be favoured, even if such discrimination is not possible. The model predicts that infanticide will typically be associated with elevated levels of offspring production, and is most likely to prove evolutionarily stable when the coefficient of relatedness between cobreeders is low, and offspring are cheap to produce. The effect of infanticide is to release subordinates from the reproductive restraint they would otherwise be forced to exercise, leading to reduced reproductive skew. When infanticide is possible, addition of numerous young to the joint brood will not lower overall productivity, because progeny in excess of the most productive brood size are eliminated. Subordinates are thus free to contribute more young to the brood than would otherwise be the case. In addition, we show that the possibility of infanticide may influence the pattern of reproduction within a group even if no offspring are actually killed at equilibrium. Copyright 1999 The Association for the Study of Animal Behaviour.     

Conditional dispersal under kin competition: extension of the Hamilton-May model to brood size-dependent dispersal

I consider a site-based model with contest competition among siblings, and assume that dispersal is conditional on the number of offspring in the natal site. Evolutionarily stable populations contain threshold dispersal strategies, which retain a certain number of offspring in the natal site and disperse the rest (if the actual number of offspring is less than the threshold, then all offspring are retained). Due to the discrete nature of the strategy set (the threshold must be integer), the ESS may not be unique or may not exist. In the latter case, two neighboring threshold strategies coexist in the evolutionarily stable population. Dispersal first decreases and then increases as a function of dispersal mortality, such that all but one offspring should be dispersed both when dispersal mortality is very small or very high. Population-level dispersal fractions are often similar to the unconditional ESS, but differ strongly when fecundity is small and dispersal mortality is high.     

Adaptive social and maternal induction of antipredator dorsal patterns in a lizard with alternative social strategies

Maternal effects facilitate adaptation to changing environments because they alter individual offspring traits to match current conditions. We show that maternal effects can also resolve context-dependent, correlational selection on multiple offspring traits, promoting adaptation to more complex environments. In side-blotched lizards ( Uta stansburiana ), two alternative pathways of dorsal pattern induction involve maternal oestradiol and alleles for throat colouration (∼social strategy). In one pathway, females increased yolk oestradiol when mated to yellow-throated sires; oestradiol induced dorsal barring in yellow-throated progeny of both sexes. In another pathway, females elevated yolk oestradiol in response to a high frequency of orange alleles in experimental social neighbourhoods. When the sire lacked yellow alleles, this secondary pathway resulted in striped, orange sons and striped, non-orange daughters. All maternally induced types had high fitness in the wild. These results illustrate a (previously undescribed) mechanism for females to simultaneously resolve differing correlational selection pressures on different progeny.     

Dispersal polymorphisms in subdivided populations

Price's method for analyzing natural selection in subdivided populations is applied to the problem of dispersal polymorphism strategies in a stable habitat. The results agree with the more traditional Mendelian models for this same problem that have recently been published. Further, by using Price's method, the results obtained are simpler and more general, and the causal evolutionary mechanisms underlying the predicted patterns are more easily recognized. The most interesting new result is that the equilibrium proportion of dispersed individuals is a simple function of the risk of dispersing and the regression coefficient of relatedness among individuals who, in the absence of dispersal, would compete for a limited, local resource. This regression coefficient refers to the genotypes that control the dispersal phenotype. For example, when mothers control the phenotype of their progeny, then the regression is from the mother onto an offspring chosen randomly from the local group before dispersal; while when offspring control their own phenotype, the regression is taken directly from offspring onto a randomly chosen cohort member before dispersal. This use of controlling genotypes to calculate regressions explains the form of the parent-offspring conflict over dispersal noted by previous authors. The simplicity and generality of these results suggest that Price's method is a useful approach for studying the class of phenomena known as "games among relatives".