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.     

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.     

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.     

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.     

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.     

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.     

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.     

The heritability of sexually dimorphic traits in the yellow dung fly Scathophaga stercoraria (L.)

A precise method was used for estimating the proportion of heritable variation in two life history parameters of the yellow dung fly, whereby environmental components of variance were minimized. Significant heritable variation for body size was revealed for father to son and mother to daughter relationships. Variation in development time was not significantly heritable. There is a marked sexual dimorphism in body size in this species which is discussed in the light of the observed sex-genotype interaction in heritabilities and low genetic correlation for size between the sexes. It is suggested that opposing pressures of sexual and natural selection and/or genetic pleotropy may be responsible for the maintenance of heritable variation, and the evolution of sexual dimorphism in these two traits.     

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.     

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.     

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.     

Female accessory reproductive gland activity in the yellow dung fly Scathophaga stercoraria (L.)

The role of the female accessory reproductive glands has been investigated in relatively few insects. Gland secretion has a number of potential functions, including lubrication during copula, involvement in fertilization and protection of eggs. Female yellow dung flies (Scathophaga stercoraria) have large paired accessory glands whose function(s) prior to this study were unknown. Our study indicated glands were involved in copulation and egg laying. The volume of secretion remaining in glands was negatively associated with copulation duration, and this effect was most pronounced in non-ovipositing females. Gland volume and secretion volume remaining in the glands were significantly smaller in females which were allowed to oviposit. In addition, there was a significant interaction between male size, female size and whether or not females were allowed to oviposit which affected the volume of the secretion remaining in the glands, with changes in secretion volume being greatest when males were large. Sperm were found in the accessory glands of some females and this was apparently not related to age, mating history of either sex, to female nutrition or male size. Our results indicate that either large males stimulate greater secretory responses from females or that females alter their responses based on male size.     

Asymmetry and fitness in female yellow dung flies

Fluctuating asymmetry (FA) has been widely used as a measure of developmental stability, and two recent meta-analyses suggest FA may be associated with several fitness components, including fecundity, growth rate and longevity. However, these studies have been strongly criticised on a number of grounds, and it was suggested that further evidence was required before the importance of any associations could be accurately assessed. Furthermore, studies should be of individual FA and fitness components. Here we investigated associations between individual FA of the mid and hind tibia and several fitness related traits (including fecundity, fertility, longevity and offspring development time) in female yellow dung flies ( Scathophaga stercoraria ). As in several previous studies, asymmetry could be discerned from measurement error and was FA. However, we found no significant associations between any measure of FA and any fitness component regardless of how the data were analysed. Our results therefore do not support any fitness–FA association and suggest that as with other aspects of FA, associations may be trait and species specific.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 557–563.     

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.     

Effects of avermectin residues in cattle dung on yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae) populations in grazed pastures

The effects of avermectin exposure on natural populations of the yellow dung fly, Scathophaga stercoraria Linnaeus, were investigated at the field scale on farms in south-west Scotland. Pastures forming the focus of the study were grazed with either untreated cattle or cattle receiving standard, manufacturer-recommended treatment regimes of an avermectin product. Flies were sampled between April and July in 2002 and 2003 using dung-baited pitfall traps. Abundance and wing asymmetry in S. stercoraria populations were examined in relation to a range of environmental and management variables (including avermectin exposure, pasture management intensity, weather and season). Data used for abundance analyses were collected in fields where treated cattle had been dosed with either doramectin or ivermectin, while the data for the asymmetry analyses were from a subset of fields where treated cattle had been dosed with doramectin only. While abundance of S. stercoraria varied significantly between years and with season, there was no difference in their abundance between fields grazed by avermectin-treated or untreated cattle. Asymmetry was significantly higher in fly populations in fields grazed by doramectin-treated cattle, suggesting that exposure to doramectin during development could have imposed some degree of environmental stress. While these results suggest that exposure to doramectin residues in dung on grazed pastures may have sublethal effects on the insects developing in that dung, there was no evident avermectin effect on the abundance of adult S. stercoraria occurring in the pastures.     

Heteropopulation males have a fertilization advantage during sperm competition in the yellow dung fly (Scathophaga stercoraria)

Sexual conflict occurs whenever there is not strict genetic monogamy. The sexually antagonistic coevolution that potentially occurs because of this conflict involves adaptation by one sex followed by the counter-adaptation by the other, and may be thought of as an evolutionary arms-race. As a result of these cycles of antagonistic coevolution, females from one population may be less resistant to heteropopulation males, at least after short periods of allopatry, as they will not have evolved any resistance to them. We tested this prediction in yellow dung fly (Scathophaga stercoraria) populations from the UK and Switzerland. Males from each population mated as first and second males to females from each population, and the mean numbers of offspring sired by the last male to mate in each situation were compared. We also compared the fertility and fecundity of single females mated to males from both populations, as well as the fertility and fecundity of the F(1) crosses. Both crosses produced viable and fertile offspring and the offspring sex ratios were not skewed. However, the fecundity of F(1)-cross females was greater than that of the parentals. In the sperm-competition experiment, there was a significant interaction between male and female origin influencing the proportion of offspring sired by the second male to mate, with heteropopulation males always outcompeting conpopulation males. This effect was independent of copula duration and the delay between copulations. In a separate experiment, we tested to see whether this was due to female preference for genetically dissimilar males but found no evidence for paternity biasing based on genetic similarity. Our results therefore seem to be best explained by sexually antagonistic coevolution as females appear less resistant to males with which they have not coevolved.     

Some effects of rearing the yellow dung fly Scatophaga stercoraria in cattle dung containing invermectin

Invermectin was added to cattle dung in controlled concentrations like those found in the pats of injected cattle, and the medium was used to rear larvae of the dung fly Scatophaga stercoraria. Ivermectin at 0.036 ppm (wt/wet weight) debilitates 50% of the larvae within 48 h. At 0.015 ppm, 50% of the larvae are unable to pupariate, while at 0.001 ppm, 50% of the larvae fail to reach the adult stage. Adults produced from larvae reared in pats containing 0.0005 ppm invermectin show high levels of fluctuating asymmetry in wing characteristics as well as deformities in the wing veins themselves. The data are discussed in relation to the effects of excreted ivermectin on pastureland biology.