Host size selection and sex allocation in Leptomastix dactylopii, a parasitoid of Planococcus citri

Host size selection and sex allocation in Leptomastix dactylopii Howard were studied (1) to determine whether invulnerable size classes exist, (2) to establish whether the size range of hosts attacked differs from its potential competitor Leptomastidea abnormis (Girault). It was found that L. dactylopii attacks mainly the larger host sizes (i.e. 3rd and 4th instars and reproducing females). Proportionally more female parasitoids were reared from larger hosts (i.e. 4th instars and reproducing females), while from size class 3 (i.e. mainly 3rd and smaller 4th instars) predominantly male wasps were reared. The adaptive meaning of the host selection behaviour of L. dactylopii is discussed.

---

Zusammenfassung Es wurde untersucht, wie die Schlupfwespe, Leptomastix dactylopii Howard zwischen den verschiedenen Grössenklassen ihres Wirtes, der Schmierlaus Planococcus citri (Risso), wählt.Der Parasitoid bevorzugt das vierte Stadium des Wirtes, aber parasitiert auch Wirte im 3. Stadium und adulte Weibchen mit Eisack. Wirte in ersten und zweiten Stadium wurden nie befallen. Weibliche Parasitoide entstanden vor allem bei Wirten im 4. Stadium, aber auch bei adulten Weibchen, männliche nahezu nur bei Wirten im 3. Stadium. Wirte im 4. Stadium und adulte Weibchen, die von L. dactylopii befallen worden waren, produzierten noch eine anzahl Eier bevor sie starben. Darum sind diese Wirte teilweise unangreifbar. Diese Eigenschaft kann sich möglicherweise auf die Stabilität der Parasitoid-Wirt-Interaktionen auswirken. L. dactylopii befällt die älteren Stadien und der potentiëlle Konkurrent Leptomastidae abnormia die jüngeren Stadien. Solch ein Unterschied kann Koexistenz beider Parasitoidarten möglich machen.

     

Host selection and sex allocation in Leptomastidea abnormis, a parasitoid of the citrus mealybug Planococcus citri

Leptomastidea abnormis (Girault) and Leptomastix dactylopii (Howard) [Hymenoptera: Encyrtidae] are the two parasitoid species frequently used for biological control of the citrus mealybug Planococcus citri (Risso) [Homoptera: Pseudococcidae]. As part of a programme to determine the best control practice, host size selection and sex allocation were studied in L. abnormis, and compared with those of L. dactylopii. L. abnormis showed a preference for the second size class, but also attacked third and fourth size class and adult mealybugs, while in previous studies L. dactylopii was found to prefer the third and fourth size class and adults, and never attacked second size class mealybugs. Female L. abnormis reared from parasitized mealybugs had a higher mean body mass than males, as is predicted by Charnov's theory on sex allocation. However, sex ratios of emerging L. abnormis did not differ between mealybug size classes. These results imply possibile coexistence of the two parasitoids. As field evidence suggests a more complex interaction between the two species, studying direct competition would be a useful next step.     

Host size induced variation in progeny sex ratio of an aphid parasitoid Lysiphlebia mirzai

A study aimed at determining the effect of host size on the pattern of sex allocation by an aphid parasitoid Lysiphlebia mirzai Shuja-Uddin (Hymenoptera: Braconidae: Aphidiinae) was undertaken to test the validity of the following three hypotheses: (1) larger hosts will yield a greater proportion of daughters, (2) differential mortality of the sexes of the parasitoid during development is a function of host-size, and (3) daughters emerging from larger hosts are more fecund. The results suggested a propensity in the parasitoid to deposit fertilised (diploid) eggs in large hosts (third instar nymph) and unfertilised (haploid) eggs in small hosts (first and second instar nymphs). Unpreferred fourth instar nymphs and apterous aphid adults also received more haploid eggs despite being larger in size than the preferred third instar nymphs. However, the perception of host size by the mother was dependent on the extent of temporal variation in the host size distribution and on her previous experience of host size. Developmental period, longevity, mating potential, fecundity and progeny sex ratio of L. mirzai emerging from small versus large hosts indicated that the host size affected the fitness of the daughters more than that of sons. No differential mortality of the sexes during development of L. mirzai was observed in small versus large hosts. This shows that L. mirzai, while ovipositing in growing stages of the host, adjusts progeny sex ratio according to the host size and by doing so she tends to contribute maximally to progeny fitness without knowing about the future host quality.     

Constrained sex allocation in a parasitoid due to variation in male quality

The theory of constrained sex allocation posits that when a fraction of females in a haplodiploid population go unmated and thus produce only male offspring, mated females will evolve to lay a female-biased sex ratio. I examined evidence for constrained sex ratio evolution in the parasitic hymenopteran Uscana semifumipennis. Mated females in the laboratory produced more female-biased sex ratios than the sex ratio of adults hatching from field-collected eggs, consistent with constrained sex allocation theory. However, the male with whom a female mated affected her offspring sex ratio, even when sperm was successfully transferred, suggesting that constrained sex ratios can occur even in populations where all females succeed in mating. A positive relationship between sex ratio and fecundity indicates that females may become sperm-limited. Variation among males occurred even at low fecundity, however, suggesting that other factors may also be involved. Further, a quantitative genetic experiment found significant additive genetic variance in the population for the sex ratio of offspring produced by females. This has only rarely been demonstrated in a natural population of parasitoids, but is a necessary condition for sex ratio evolution. Finally, matings with larger males produced more female-biased offspring sex-ratios, suggesting positive selection on male size. Because the great majority of parasitic hymenoptera are monandrous, the finding of natural variation among males in their capacity to fertilize offspring, even after mating successfully, suggests that females may often be constrained in the sex allocation by inadequate number or quality of sperm transferred.     

Host selection and sex ratio in a heteronomous hyperparasitoid

Abstract.

• 1 Encarsia tricolor Förster is a heteronomous hyperparasitoid: females develop as primary endoparasitoids of Homoptera, whereas males develop hyperparasitically in primary endoparasitoids, including conspecific females.
• 2 When offered pupae of Encarsia inaron (Walker) or conspecific pupae, E.tricolor showed a distinct preference to exploit E.inaron for male production.
• 3 Parasitoids were given the opportunity to gain different types of ovipositional experience on male and female hosts and were then offered patches containing mixtures of male and female hosts in various ratios. The mean sex ratio which they laid was not dependent on the relative availability of each type of host, whereas prior ovipositional experience had a significant effect.
• 4 Adaptive reasons for the observed oviposition strategies are suggested.

     

Host selection and egg allocation behaviour by Aphytis melinus and A.lingnanensis: comparison of two facultatively gregarious parasitoids

Abstract. 1. The ovipositional and egg allocation behaviour of individual females of Aphytis melinus DeBach and A.lingnunensis Compere were compared.
2. Both Aphytis species exhibit the same behavioural sequence during oviposition.
3. Aphytis melinus laid most of its female eggs on the dorsum of a scale-insect beneath its cover, and most of its male eggs under the scale-insect's body. Aphytis lingnanensis also oviposited both dorsally and ventrally on scale-insect hosts, but female and male progeny arose with equal frequency from eggs laid in both locations.
4. Both A.melinus and A. lingnanensis are facultatively gregarious parasitoids. The degree of gregariousness depends on host size, i.e. the larger the host, the more the Iikelihood that several eggs will be deposited at each visit by the parasitoid.
5. When two eggs were laid during the same host visit, both A.melinus and A.lingnanensis laid one female and one male egg more often than would be expected under an assumption of random allocation of sexes.
6. Because A.melinus successfulIy utilize smaller hosts than A.lingnanensis to produce progeny, these parasitoids should not be considered ecological homo-logues, as suggested by DeBach & Sundby (1963).     

Sex allocation and larval competition in a superparasitizing solitary egg parasitoid: competing strategies for an optimal sex ratio

1. Parasitic Hymenoptera reproduce by arrhenotokous parthenogenesis, and females of these species are able to control their progeny sex ratios. In structured populations of parasitic Hymenoptera, primary sex ratios are often highly biased toward females. However, sex ratio can be adjusted to the quality of encountered patches or hosts or be modified by differential developmental mortality.
2. In this paper, the effects were evaluated of the quality of encountered hosts and developmental mortality on the sex ratio in Anaphes victus , a solitary egg parasitoid whose first instar larvae present a sexual dimorphism and where superparasitism is regulated by larval fights between first instar larvae.
3. The results showed that a female-biased sex ratio is allocated to unparasitized hosts. In the presence of parasitized hosts, the second (superparasitizing) female produced a significantly higher sex ratio than the first female but the tertiary sex ratio (sex ratio at emergence) was not significantly different from the sex ratio produced with unparasitized hosts. The increase in the primary sex ratio produced by the second female was mostly compensated by the higher mortality of male larvae.     

Offspring allocation in externally ovipositing fig wasps with varying clutch size and sex ratio

The simultaneous optimization of clutch size and sex ratio isa tricky problem. Unless parameters such as host size or fecundityexist to pin down the optimal clutch size, this problem remainselusive to analytical analysis. This is because the fitnesslandscape with respect to clutch size and sex ratio does nothave one single evolutionarily stable peak toward which thepopulation can evolve. To solve this problem, I used a computeremulation to optimize both clutch size and sex ratio using externallyovipositing fig wasps as a model taxon. The simulation approachallows the use of integer numbers of eggs rather than assumingthat females can produce any sex ratio between 0 and 1. Whenfemales have no information about the patches on which theyoviposit, they produce either large clutches with a strong femalebias or clutches of a single male egg. When females have completeknowledge of their oviposition site, a set of conditional substrategiesis evolutionarily stable. Again, these substrategies are eitherlarge clutches with a female bias or dutches consisting of asingle male egg. This dichotomous oviposition pattern resultsin unrelated males sharing a fig, a condition conducive to theevolution of fatal fighting. Selection on female ovipositionstrategies may therefore be an important driving force behindhigh levels of fighting observed between male fig wasps.     

Nest-site philopatry and selection for environmental sex determination

The reason for the frequent occurrence of environmental sex determination (ESD) in reptiles is still not well understood, although much effort has been devoted to solving the issue. Stimulated by the occurrence of nest-site philopatry in some species, this paper examines a diploid model of the influence of nest-site philopatry on the evolution of ESD. Analysis shows that nest-site philopatry can lead to ESD because the fitnesses of sons and daughters are not influenced in the same way by nest-site quality. Daughters inherit the nest site and thus benefit more than sons from a high-quality nest site. Conversely, the fitness of daughters at low-quality nest sites is lower compared to the fitness of sons. Therefore, genes causing ESD can spread by causing the production of more sons at low-quality nest sites and more daughters at high-quality nest sites. Suggestions are made to test empirically whether nest-site philopatry led to the evolution of ESD. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fitness effects of sex ratio response to host quality and size in the parasitoid wasp Spalangia cameroni

The parasitoid wasp Spalangia cameroni oviposited a greaterproportion of daughters in stable fly pupae than in house flypupae, even when I controlled for stable flies being smallerthan house flies. Sex ratio manipulation in response to hostquality has been modeled as being adaptive through an effectof host quality on the size and hence offspring production ofdaughters. 5. cameronis response to host species may insteadbe adaptive through an effect on larval survivorship, the developmenttime of daughters, and the size of sons. There was greater survivalof daughters than sons on stable flies. Controlling for hostsize, I found that development time of daughters was about 2%less on stable flies than on house flies. The decrease in developmenttime corresponds to a 2% increase in fitness as estimated byr, the intrinsic rate of increase, and is equivalent to abouta 9% increase in offspring production. Sons were about 2% largerfrom house flies than stable flies, which may increase offspringproduction by up to 3%. Host species had no consistent effecton size of daughters or development time of sons. In additionto the response to host species, mothers oviposited a greaterproportion of daughters in larger stable fly hosts. Whetherthis behavior is adaptive is unclear. Although offspring werelarger when they developed on larger stable flies, the rateof increase was less for daughters dian for sons. Effects ofstable fly size on offspring development time were negligible.     

The influence of host size on sex ratios in the parasitoid Diglyphus begini (Hymenoptera: Eulophidae)

Abstract. 1. Sex ratio in the ectoparasitoid, Diglyphus begini (Ashmead), attacking larvae of the dipterous leafminer Liriomyza trifolii (Burgess) in glasshouse marigolds, is best depicted by a model where sex ratio varies as a function of host size.
2. Male D. begini progeny are produced in hosts significantly smaller in size than those producing female progeny.
3. Female wasps attack and oviposit on the largest leafminer larvae available and whether a host is large or small depends upon the size of the other hosts attacked.
4. Diglyphus begini females adjust the thre:shold size for the change-over in sex allocation relative to the size of hosts attacked; however, the sex ratio is maintained at between 60% and 70% males.
5. The patterns observed in these glasshouse studies are not due to sex-specific differential mortality or superparasitism.     

The influence of female age and host size on the sex ratio of the parasitoid Opius concolor

The sex ratio of the progeny of single females parasitizing large hosts favoured the females (sex ratio=0.26); but on small hosts favoured the males (0.73). No differences in mortality of the sexes were detected. The sex ratio was independent of female age when large hosts were used. The percentage of males observed in the progeny of the first day of female oviposition was significantly greater than the mean, irrespective of the age at which female oviposition began. When females were exposed to small hosts, a greater percentage of females was observed in the progeny from the last days of oviposition.

---

Résumé L'influence de la taille de l'hôteret de l'âge de la femelle sur le taux sexuel de la descendance a été étudiée sur le parasitoïde Opius concolor Szépl.Le taux sexuel de la descendance des femelles isolées est favorable aux femelles (t.s.=0,26) quand elles ont à leur disposition des hôtes de grande taille, tandis qu'avec des hôtes petits le taux sexuel est favorable aux mâles (t.s.=0.73). On n'a pas détecté de mortalité différentielle des sexes.Les pourcentage de mâles obtenu le premier jour de ponte des femelles sur les hôtes de grande taille est significativement différent de la moyenne, indépendamment de l'âge de la femelle à ce moment. Cependant, sur des hôtes petits, bien que restant favorable aux mâles dans l'ensemble, une plus grande proportion de femelles à partir des premiers jours de ponte a pu être observée.

     

Host selection behaviour of deathwatch beetle, Xestobium rufovillosum: Oviposition preference choice assays testing old vs new oak timber, Quercus sp.

In a choice bio-assay adult female deathwatch beetles were offered two dendrochronologically dated wood blocks from oak timber to study oviposition preference behaviour. There was a clear preference for ovipositing on old wood dating from the 13th to 19th centuries rather than new wood from the 20th century. Control, same-century choice, experiments showed that beetles will oviposit on young wood and that the age of the wood does not alter the overall oviposition potential. Oviposition frequency varied with insect age. Fecundity of insects collected from an infested building was similar to that of insects maintained in culture.     

Parental investment, sexual selection and sex ratios

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self‐reinforcing process. The initial asymmetry in pre‐mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in post‐mating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871–1971 (1972), Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre‐mating and post‐mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the ‘Concorde Fallacy’ as optimal decisions should depend on future pay‐offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay‐offs, it remains weak. The factors likely to change future pay‐offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male‐biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequency‐dependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non‐random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female‐biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female‐only care, male‐biased OSR and female‐biased ASR) to an avian type system (biparental care and a male‐biased OSR and ASR).     

Constant relative age and size at sex change for sequentially hermaphroditic fish

A general problem in evolutionary biology is that quantitative tests of theory usually require a detailed knowledge of the underlying trade-offs, which can be very hard to measure. Consequently, tests of theory are often constrained to be qualitative and not quantitative. A solution to this problem can arise when life histories are viewed in a dimensionless way. Recently, dimensionless theory has been developed to predict the size and age at which individuals should change sex. This theory predicts that the size at sex change/maximum size (L50/L(max)), and the age at sex change/age at first breeding (tau/alpha) should both be invariant. We found support for these two predictions across 52 species of fish. Fish change sex when they are 80% of their maximum body size, and 2.5 times their age at maturity. This invariant result holds despite a 60 and 25 fold difference across species in maximum size and age at sex change. These results suggest that, despite ignoring many biological complexities, relatively simple evolutionary theory is able to explain quantitatively at what point sex change occurs across fish species. Furthermore, our results suggest some very broad generalities in how male fitness varies with size and age across fish species with different mating systems.     

Toward evolutionary graphs with two sexes: a kin selection analysis of a sex allocation problem

Evolutionary graphs are used to model the effects of spatial and social structure in social evolutionary problems (e.g. evolutionary games). Recent work has highlighted the fact that evolution on graphs can be understood using kin selection theory. This paper shows how one can use kin selection to study evolutionary graphs inhabited by a diploid sexual organism by means of a simple example. Specifically, we study the well-known sex allocation problem of how best to divide a fixed amount of effort between the production of sons on the one hand and the production of daughters on the other. Like many previous studies, we identify equal investment in sons and daughters as the only phenotype favoured by selection. Our analysis also highlights the advantages and disadvantages of applying kin selection to the study of evolutionary graphs.     

Sex ratio selection and multi-factorial sex determination in the housefly: a dynamic model

Sex determining (SD) mechanisms are highly variable between different taxonomic groups and appear to change relatively quickly during evolution. Sex ratio selection could be a dominant force causing such changes. We investigate theoretically the effect of sex ratio selection on the dynamics of a multi-factorial SD system. The system considered resembles the naturally occurring three-locus system of the housefly, which allows for male heterogamety, female heterogamety and a variety of other mechanisms. Sex ratio selection is modelled by assuming cost differences in the production of sons and daughters, a scenario leading to a strong sex ratio bias in the absence of constraints imposed by the mechanism of sex determination. We show that, despite of the presumed flexibility of the SD system considered, equilibrium sex ratios never deviate strongly from 1 : 1. Even if daughters are very costly, a male-biased sex ratio can never evolve. If sons are more costly, sex ratio can be slightly female biased but even in case of large cost differences the bias is very small (<10% from 1 : 1). Sex ratio selection can lead to a shift in the SD mechanism, but cannot be the sole cause of complete switches from one SD system to another. In fact, more than one locus remains polymorphic at equilibrium. We discuss our results in the context of evolution of the variable SD mechanism found in natural housefly populations.     

Variable host quality, life-history invariants, and the reproductive strategy of a parasitoid wasp that produces single sex clutches

The parasitic wasp Achrysocharoides zwoelferi (Hymenoptera, Eulophidae) produces clutches consisting of only one sex. Moreover,male clutch size is invariably one while female clutches arein the range one to four. We designed field experiments todetermine the effect of host quality on clutch composition.We found that solitary male and solitary female clutches werereared from the same size mines, and that larger mines tendedto produce gregarious female clutches. A higher proportionof male clutches were placed in older hosts, despite theirlarge size. Variation in body size, both between and withinclutches, was measured in order to test the predictions of models that take into account the constraint that clutch size is aninteger trait, something of potential importance when absoluteclutch size is low. Our data supported several predictionsof these models, including the trade-off-invariant rule foroptimal offspring size developed by Charnov and Downhower.However, while most invertebrate clutch size models assume equal resource share among members of the same clutch, we found anincrease in inequality in larger clutches.