Temporal and microspatial variation in the intensities of natural and sexual selection in the yellow dung fly Scathophaga stercoraria

Studies of phenotypic selection in natural populations often concentrate only on short time periods and do not quantify selection intensities. We quantified temporal and microspatial variation in the intensities of natural and sexual selection for body size in the yellow dung fly over 2 years. Female fecundity selection intensity remained approximately constant over the season with an overall mean ± SE of 0.187 ± 0.014. Selection intensity for male reproductive success, defined as eggs obtained by mating males, did not differ from zero, indicating there was no assortative mating by size. Sexual selection intensity for male mating success favouring large males was variable but overall strong in the two years (0.499 ± 0.053 and 0.510 ± 0.051). As theoretically expected for male–male competition, sexual selection intensity increased with competitor density and reached an asymptote at about 250 males per pat; it also decreased with time in spring and increased again in autumn as a function of density. Small males had the best chance of obtaining a female at very low male densities. Greater selection intensity for large size in males than females is consistent with, and might be responsible for, the observed sexual size dimorphism in this species, as males are larger. The seasonal pattern of mean male body size (smallest at the beginning and end of the season) most likely reflects mere environmental (primarily temperature) influences on phenotypic size.     

Cryptic female choice in the yellow dung fly Scathophaga stercoraria (L.)

Both female choice and male-male competition may take place during reproduction in many species. Female choice tends to be less obvious than male-male competition and consequently has received less attention from researchers. The opportunity for cryptic female choice arises after multiple insemination. Through postcopulatory processes, a female could alter the pattern of paternity among her offspring so that it does not directly reflect the different contributions of sperm made by her mates. To be able to determine if a female alters the relative sperm contributions of her mates, the behaviors and influences of the males must therefore be first taken into account. The interest of each male is to father all the offspring, and the interest of each female is to maximize paternal quality. Female yellow dung flies have complex internal reproductive tracts that may give them considerable control over the fertilization success of stored sperm from different males. In laboratory trials to date, the last male to mate has usually been most successful. In the present study, cryptic choice occurred in Scathophaga stercoraria and the pattern of choice was consistent with previously reported results. The fertilization success of a female's second mate (P2) was substantially larger if a female was kept at constant temperature and if the second male was genetically similar to her at the phosphoglucomutase (Pgm) locus. Females from the field normally have three spermathecae, but some have four. Lines were bred to have either three or four spermathecae. Flies from the different lines were crossed to generate females with similar genetic backgrounds that had either three or four spermathecae. P2 was significantly lower for high-quality females, that is, those that laid larger-than-average-clutches, with four spermathecae than for low-quality females with four spermathecae; female quality had no influence on P2 for females with three spermathecae. The results suggest that only large females may benefit from increased spermathecae number by being able to act against male interests. Females may only have three spermathecae, even though genetic variation for more is present, because selection for more spermathecae is weak.     

Temperature effects on egg size and their fitness consequences in the yellow dung fly Scathophaga stercoraria

Organisms and parts of an organism like eggs or individual cells developing in colder environments tend to grow bigger. A unifying explanation for this Bergmann's rule extended to ectotherms has not been found, and whether this is an adaptive response or a physiological constraint is debated. The dependence of egg and clutch size on the mother's temperature environment were investigated in the yellow dung fly Scathophaga stercoraria. Smaller eggs were laid at warmer temperatures in the field and the laboratory, where possible confounding variables were controlled for. As clutch size at the same time was unaffected by temperature, this effect was not due to a trade-off between egg size and number. Temperature-dependent egg sizes even persisted within individuals: when females were transferred to a cooler (warmer) environment, they laid third-clutch eggs that were larger (smaller) than their first-clutch eggs. The fitness consequences of these temperature-mediated egg sizes were further investigated in two laboratory experiments. Neither egg and pre-adult survivorship nor larval growth rate were maximized, nor was development time minimized, at the ambient temperature corresponding to the mother's temperature environment. This does not support the beneficial acclimation hypothesis. Instead, this study yielded some, but by no means conclusive indications of best performance by offspring from eggs laid at intermediate temperatures, weakly supporting the optimal temperature hypothesis. In one experiment the smaller eggs laid at 24 °C had reduced survivorship at all ambient temperatures tested. Smaller eggs thus generally performed poorly. The most parsimonious interpretation of these results is that temperature-mediated variation in egg size is a maternal physiological response (perhaps even a constraint) of unclear adaptive value. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sperm survival in the female reproductive tract in the fly Scathophaga stercoraria (L.)

While sperm competition risk favours males transferring many sperm to secure fertilizations, females of a variety of species actively reduce sperm numbers reaching their reproductive tract, e.g. by extrusion or killing. Potential benefits of spermicide to females include nutritional gains, influence over sperm storage and paternity, and the elimination of sperm bearing somatic mutations that would lower zygote fitness.We investigated changes in sperm viability after in vivo and in vitro exposure to the female tract in the polyandrous fly, Scathophaga stercoraria. Sperm viability was significantly lower in the females' spermathecae immediately after mating than in the experimental males' testes. Males also varied significantly in the proportion of live sperm found in storage in vivo. However, the exact mechanism of sperm degradation remains to be clarified. In vitro exposure to extracts of the female reproductive tract, including female accessory glands, failed to significantly lower sperm viability compared to controls. These results are consistent either with postcopulatory sperm mortality in vivo depending entirely on the male (with individual differences in sperm viability, motility or longevity) or with postcopulatory sperm mortality being subtly affected by female effects which were not detected by the in vitro experimental conditions. Importantly, we found no evidence in support of the hypothesis that female accessory glands contribute to sexual conflict via spermicide. Therefore, female muscular control remains to date the only ascertained mechanism of female influence on sperm storage in this species.     

Sex and immunity in the yellow dung fly Scathophaga stercoraria

The immune system is of increasing interest to evolutionary biologists. Immunity may trade-off against other fitness components, with recent work suggesting reproduction in particular impinges on immune defence. There may also be sex differences in the immune system. Additionally, while life history traits typically have low heritability, little is known about additive genetic variance of immunity. An insect's major defence against multicellular pathogens is to encapsulate the invader. Phenoloxidase (PO) is a key enzyme in the cascade resulting in the melanized capsule, and is often used to estimate resistance to an immune insult. We examined the effects of copulation, egg laying, sex and age on PO in Scathophaga stercoraria. We also measured the heritability of PO activity. The sexes differed in haemolymph PO activity and PO was significantly affected by age, but not by copulation or egg laying. There was significant heritable variation for haemolymph PO.     

Phenoloxidase activity and pathogen resistance in yellow dung flies Scathophaga stercoraria

Phenoloxidase (PO) is an important component of the insect immune system and is frequently used to measure an individual's immune defence ability. However, evidence documenting positive correlations between the immune assay and resistance against pathogens is scarce and contradictory. We used replicate lines of yellow dung flies Scathophaga stercoraria (L.) with different PO levels to investigate whether PO levels affect resistance against parasitic mites and entomopathogenic fungi. Prevalence of flies exposed to pathogens was the same in all selection regimes, although pathogens clearly negatively affected fitness. PO measurements alone therefore do not necessarily predict overall resistance against pathogens. Furthermore, under starvation lines selected for high PO levels did not survive longer than those selected for low PO levels, irrespective of exposure to pathogens. This suggests that even if elevated immune levels increase an individual's ability to combat pathogens, the benefits may not outweigh the costs of increased investment in immunity.     

Oviposition Site Choice in the Yellow Dung Fly Scathophaga stercoraria

Female yellow dung flies Scathophaga stercoraria oviposit on the dung surface, which is extremely uneven and other eggs are often already present. We examined the effects of these features on egg placement. We found that: 1. females preferred to lay on small hills on the dung surface and avoided depressions and points; and 2. they ignored the presence of other eggs. These results were confirmed in a field experiment, where the pattern of egg distribution on artificially formed dung pats was consistent with the predictions from the laboratory results. We further showed that survival to adult emergence was best when laying sites were experimentally manipulated so that the eggs were apparently laid on small hills, avoiding the drying effects of small points in the dung and possible drowning by (simulated) rain when laid in depressions in the dung surface. A female dung fly is clearly capable of making subtle decisions about the placement of her eggs, thus increasing her reproductive success.     

Stage- and sex-specific heat tolerance in the yellow dung fly Scathophaga stercoraria

Thermal tolerance varies at all hierarchical levels of biological organization: among species, populations, individuals, and even within individuals. Age- or developmental stage- and sex-specific thermal effects have received relatively little attention in the literature, despite being crucial for understanding thermal adaptation in nature and responses to global warming. We document stage- and sex- specific heat tolerance in the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), a species common throughout the northern hemisphere that generally favours cool climates. Exposure of eggs to temperatures up to 32 °C did not affect larval hatching rate, but subsequent egg-to-adult survival at a benign temperature was reduced. Permanent transfer from benign (18 °C) to hot temperatures (up to 31 °C) at different larval and pupal stages strongly decreased egg-to-adult survival, though survival continuously improved the later the transfer occurred. Temporary transfer for only two days increased mortality more weakly, survival being lowest when temperature stress was imposed early during the larval or pupal stages. Adult flies provided with sugar and water tolerated 31 °C longer than previously thought (5 days in males to 9 days in females). Eggs were thus less susceptible to thermal stress than larvae, pupae or adults, in agreement with the hypothesis that more mobile stages require less physiological protection against heat because they can behaviourally thermoregulate. The probability of mating, of laying a clutch, and hatching success were generally independently reduced by exposure of females or males to warm temperatures (24 °C) during the juvenile or adult stages, with some interactions evident. High temperature stress thus affects survival differentially depending on when it occurs during the juvenile or the pre-reproductive adult life stage, and affects reproductive success via the mating behaviour of both sexes, female physiology in terms of oviposition, and fertility via sperm and/or egg quality. Our results illustrate that temperature stress, even when moderate and temporary, during early development can have profound lethal and non-lethal fitness-consequences later in life.     

The anatomy of fertilization in the yellow dung fly Scathophaga stercoraria

Female yellow dung flies, Scathophaga stercoraria, can influence the traffic of sperm stored in their spermathecae to the site of fertilization in the bursa copulatrix. However, the anatomical mechanisms employed are largely unknown. We investigated the anatomy of the female genital tract, seeking structures involved in sperm transfer and egg fertilization. We found a membranous structure descending from the ends of the spermathecal and accessory gland ducts into the bursa copulatrix. We call this the prolatus. Sperm accumulate in the prolatus during oviposition. When an egg is in the bursa the egg micropyle, rather than being aligned towards the dorsal openings of the spermathecal ducts, lies on the opposite, ventral side. We also confirm the presence, and suggest a function for, a cuticularized pouch on the ventral wall of the anterior bursa copulatrix. This pouch, plus a previously undescribed chamber, may be homologous to the ventral receptacle/fertilization chamber found in other dipterans. Further, we describe a translucent cap, apparently transversed by channels, covering the micropyle. Sperm were observed to aggregate on and in the micropyle cap, which appears to attract and hold sperm. We interpret the prolatus as a structure that allows an ovipositing female to transfer a few sperm onto the ventral bursal wall and thus, indirectly, onto the micropyle cap. Such anatomy potentially gives the female a large degree of control over sperm traffic from storage to the site of fertilization.     

Altitudinal life history variation in the dung flies Scathophaga stercoraria and Sepsis cynipsea

 Field phenologies of high- (ca. 1500 m) and low- (ca. 500 m) altitude populations of the two most common European species of dung flies, Scathophaga stercoraria and Sepsis cynipsea, differ quite markedly due to differences in climate. To differentiate genetic adaptation due to natural selection and phenotypic plasticity, I compared standard life history characters of pairs of high- and low-altitude populations from three disjunctive sites in Switzerland in a laboratory experiment. The F1 rearing environment did not affect any of the variables of the F2 generation with which all experiments were conducted; hence, there were no carry-over or maternal effects. In Sc. stercoraria, high-altitude individuals were smaller but laid larger eggs; the latter may be advantageous in the more extreme (i.e. more variable and less predictable) high-altitude climate. Higher rearing temperature strongly decreased development time, body size and the size difference between males and females (males are larger), produced female-biased sex ratios and led to suboptimal adult emergence rates. Several of these variables also varied among the three sites, producing some interactions complicating the patterns. In Se. cynipsea, high-altitude females were marginally smaller, less long-lived and laid fewer clutches. Higher rearing temperature strongly decreased development time and body size but tended to increase the size difference between males and females (males are smaller); it also increased clutch size but decreased physiological longevity. Again, interpretation is complicated by variation across sites and some significant interactions. Overall, genetic adaptation to high-altitude conditions appears weak, probably prevented by substantial gene flow, and may be swamped by the effects of other geographic variables among populations. In contrast, phenotypic plasticity is extensive. This may be due to selection of flexible, multi-purpose genotypes. The results suggest that differences in season length between high- and low-altitude locations alone do not explain well the patterns of variation in phenology and body size. Received: 21 March 1996 / Accepted: 1 September 1996     

Size-dependent alternative male mating tactics in the yellow dung fly,Scathophaga stercoraria

Whenever males can monopolize females and/or resources used by females, the opportunity for sexual selection will be great. The greater the variation among males in reproductive success, the greater the intensity of selection on less competitive males to gain matings through alternative tactics. In the yellow dung fly, Scathophaga stercoraria, males aggressively compete for access to receptive, gravid females on fresh dung. Larger males are better able to acquire mates and to complete copulation successfully and guard the female throughout oviposition. Here we demonstrate that when an alternative resource is present where females aggregate (i.e. apple pomace, where both sexes come to feed), smaller males will redirect their searching for females from dung to the new substrate. In addition, we identify a class of particularly small males on the alternative substrate that appears never to be present searching for females on or around dung. Smaller males were found to have a mating ‘advantage’ on pomace, in striking contrast to the pattern observed on dung, providing further support for the existence of an alternative male reproductive tactic in this species.     

Temperature-mediated seasonal variation in phosphoglucomutase allozyme frequency in the yellow dung fly, Scathophaga stercoraria

The allozyme genetic variability of various species is correlated with a variety of morphological, physiological and fitness-related traits. In particular, temperature can affect the fitness of insects through its influence on enzyme function. We examined the seasonal (12 days over 1 year) and daily (nine samples over each day) allozyme variation at the phosphoglucomutase (PGM) locus in one population of yellow dung flies (Scathophaga stercoraria; Diptera: Scathophagidae). PGM is of central functional importance in the mobilization of glycogen reserves for flight, and has been shown to affect larval growth at different temperatures in the laboratory. Based on a sample of over 3000 flies, we found a quadratic relationship, with a minimum at approximately 12 degrees C, between the frequency of the most common allele and temperature, primarily mediated by seasonal temperature variation. This could be caused by behavioural responses over the short-term, but over the year either variable viability or sexual selection probably operates on this locus, maintaining the existing polymorphism. These results call for further work on the functional differences between PGM allozyme genotypes.     

Sperm release and use at fertilization by yellow dung fly females (Scathophaga stercoraria)

Females of many species mate multiple times and store transferred sperm in storage organs. The mechanisms underlying sperm release from the stores at fertilization remain poorly understood, although they are central to an understanding of the female influence on post-copulatory male competition. Using double-mated females of the yellow dung fly, we counted the sperm sticking to the surface of deposited eggs of two successive clutches to obtain insight into the physiological processes associated with fertilization. The number of sperm released to fertilize an egg decreased between the first and second clutches, as well as within clutches from early to late eggs. These results indicate that: (1) sperm are lost from the stores over time independent of egg laying and (2) the number of sperm released depends on the amount of sperm stored. The lower number of sperm on eggs of the second clutches was accompanied by a strong increase of the proportion of sperm adhering to the micropyle region, suggesting that sperm use is more efficient and sperm release better controlled when sperm supply is substantially reduced. Finally, our approach indicates that sperm storage capacity of the female is higher than assumed from counts of spermathecal sperm.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 511–518.     

On the copulation duration of the yellow dung fly (Scathophaga stercoraria)

We model the optimal copulation duration in the yellow dungflyScathophaga stercoraria, assuming that males optimize their reproductive success per day. The independent state-variables of a male are the actual sperm reserves, the female encounterrate and the time of the day. We used stochastic dynamic programming to predict the optimal copulation duration. The model predicts that copulation duration should increase (i) for larger males, (ii) for males with a better previous diet (iii) for males accepting more females (iv) for males staying away from the dung during a whole day. It also predicts that (v) males on a restricted diet should accept fewer matings, (vi) testes size should shrivel during the day, (vii) the correlation coefficient between copulation duration and sequence number should decrease with increasing initial sperm reserve and with increasing number of copulations performed on a given day.There are two essential differences between our model and a model of Parker (1992). (i) Our model assumes that males optimize per day. Parker's model assumes that males optimize per mating bout, one bout comprising many days. (ii) Since all the important state variables depend only on the moment of the copulation, our model does not assume a long-time memory in dungflies, and it is more flexible than Parker's model. Overall, both models explain the data available equally well.     

Effects of temperature on cell size and number in the yellow dung fly Scathophaga stercoraria

(1) A number of hypotheses suggest that the temperature-size rule (larger at cooler temperatures), and consequently Bergmann clines in whole-organism body size (larger at higher latitudes), may be a mere consequence of processes at the cellular level, i.e., a physiological constraint.(2) We show that in the yellow dung fly, Scathophaga stercoraria (Diptera: Scathophagidae), the temperature-size rule holds for wing cell and ommatidia size. Increases in cell number made up two-thirds (eye) to three-quarters (wing) of the increase in organ size. Temperature effects on body size can be fully explained by its effects on cell size and number.(3) Our study adds to the generality of previous results in Drosophila spp. The physiological constraint hypothesis remains viable as a proximate, non-adaptive explanation for the temperature-size rule in ectotherms.     

Trinucleotide microsatellite loci in the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae)

We describe six polymorphic, trinucleotide microsatellite loci that were isolated from the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), a model system for examining the mechanisms behind sperm competition in species with internal fertilization. These microsatellites yielded between 3 and 11 alleles per locus in a sample of 20–48 dung flies collected from several sites in Cheshire, UK. Observed and expected heterozygosities varied between 0.195 and 0.900 and 0.335–0.842, respectively. These markers should allow us to investigate postcopulatory sexual selection processes in this fly much more fully than has been possible in the past.