Paul I. Ward

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

Abstract 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 females 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.

Superior sperm competitors sire higher-quality young

The evolution of polyandry remains controversial. This is because, unlike males, in many cases multiple mating by females does not increase fecundity and inevitably involves some costs. As a result, a large number of indirect benefit models have been proposed to explain polyandry. One of these, the good sperm hypothesis, posits that high–quality males are better sperm competitors and sire higher–quality offspring. Hence, by mating multiply, females produce offspring of superior quality. Despite being potentially widely applicable across species, this idea has received little attention. In a laboratory experiment with yellow dung flies ( Scathophaga stercoraria ) we found that males that were more successful in sperm competition also had offspring that developed faster. There was no relationship between paternal success in sperm competition and the ability of offspring to survive post–emergence starvation. Since faster development times are likely to be advantageous in this species, our data provide some support for polyandry evolving as a means of producing higher–quality offspring via sperm competition.

Females influence sperm storage and use in the yellow dung fly Scathophaga stercoraria (L.)

SummaryThe influence of the female on the process of sperm storage and use was examined. Copula duration, the condition of the female and whether or not a copula terminated naturally influenced the number of spermathecae (of three) in which once-mated females stored sperm. Females stored more sperm the larger their mate and the sperm from larger males were stored more unevenly amongst the spermathecae than were those from smaller males. Double-mated females had sperm in fewer spermathecae the larger the second of their mates and these spermathecae tended to be the ones which lay together within the female. The P2 values over three successive clutches were constant and sperm precedence was complete when the larger male was second to mate but began low and increased over subsequent clutches when the smaller male mated second. These results suggest females prefer, and are able, to use the sperm of larger males to fertilise their eggs. It is proposed that multiple spermathecae in Diptera have evolved to give females better control over offspring paternity.

Intraspecific variation in sperm size characters

Interspecific variation in quantitative characters of sperm is considerable and can sometimes be related to aspects of sperm competition. In contrast, continuous intraspecific variation is often ignored; for example, many studies concentrate on measuring a species-typical sperm size. However, statistically significant variation amongst males in a variety of sperm size characters, particularly total length and a number of head characters, has been reported in at least 13 species. Furthermore, sperm size has been shown to influence the outcomes of processes involved in sperm competition in two species, with larger sperm being more successful in both cases. It is suggested here that intraspecific variation may be a widespread phenomena, further examination of which could contribute considerably to our understanding of sperm evolution. Size differences between the sperm of competing males could be used in controlled matings to investigate the fates of sperm within female tracts and storage organs, but care should be taken that size characters may not be neutral markers. Furthermore, the genetic determination of sperm size is considerable (h2=0.56–0.92 for total length and a number of head characters) and determinants are at least sometimes sex-linked, with the result that particular care must go into experimental design if genetic effects are to be fully elucidated. Studies combining genetical and behavioural effects of intraspecific variation in sperm morphology should be particularly rewarding.

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.

Co‐evolution of male and female reproductive characters across the Scathophagidae (Diptera)

Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co‐evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.

Developmental stability in yellow dung flies (Scathophaga stercoraria): fluctuating asymmetry, heterozygosity and environmental stress.

The genetic basis for developmental stability, the ability of an organism to withstand genetic and environmental disturbance of development, is poorly understood. Fluctuating asymmetry (FA: small random deviations from symmetry in paired, bilateral traits) is the most widely used measure of developmental stability, and evidence suggests FA is weakly and negatively associated with genome‐wide heterozygosity. We investigated the genetic basis of developmental stability in the yellow dung fly. Fly lines were inbred for 16 generations at which time they were homozygous at the phosphoglucomutase (PGM) loci and PGM appears to influence FA in at least one other taxon. After 16 generations of inbreeding, lines homozygous for different PGM alleles were crossed and levels of FA for four metric traits were compared in the inbred and crossed flies. We also compared FA levels in these flies with previously gathered data on wild‐type (second generation outcrossed) flies, and additionally looked at the effects of two environmental stresses (larval food limitation and increased temperature) on FA. There were no significant differences in any measure of FA, nor in mean FA, in any trait when inbred and crossed flies were compared. Comparison of FA in these and wild flies also revealed no significant differences. Food limitation had no influence on FA, whereas heat stress increased FA of naturally, but not sexually, selected traits. Our results do not show a negative relationship between heterozygosity and FA, but support the notion that FA levels are stress, trait and taxon specific.

Copula in yellow dung flies (Scathophaga stercoraria): investigating sperm competition models by histological observation

While sperm competition has been extensively studied, the mechanisms involved are typically not well understood. Nevertheless, awareness of sperm competition mechanisms is currently recognised as being of fundamental importance for an understanding of many behavioural strategies. In the yellow dung fly, a model system for studies of sperm competition, second male sperm precedence appears to result from a combination of sperm displacement and sperm mixing. Displacement was until recently thought to be directly from the females sperm stores, the spermathecae (i.e. males were thought to ejaculate directly into these stores), and under male control. However, recent work indicates displacement is indirect (i.e. males do not ejaculate directly into the sperm stores) and that it is female-aided, although the evidence was not based on direct observation. Here, we used histological techniques to directly determine interactions during copula and sperm transfer. Our results are consistent with inference and clearly show that males ejaculate into the bursa copulatrix. Our data are also consistent with active female involvement in sperm displacement, which is indirect, and indicate the aedeagus may remove some spermatozoa from the bursa at the end of copula. In addition, evidence suggests females aid sperm transport to and from the spermathecae, possibly by muscular movement of a spermathecal invagination.

Post–copulatory sexual selection and female fitness in Scathophaga stercoraria

Whether sexual selection increases or decreases female fitness is determined by the occurrence and relative importance of sexual–conflict processes and the ability of females to choose high–quality males. Experimentally enforced polyandry and monogamy have previously been shown to cause rapid evolution in the yellow dung fly Scathophaga stercoraria. Flies from polyandrous lines invested more in reproductive tissue, and this investment influenced paternity in sperm competition, but came at a cost to immune function. While some fitness consequences of enforced polyandry or monogamy have been examined when flies mate multiply, the consequences for female fitness when singly copulated remain unexplored. Under a good–genes scenario females from polyandrous lines should be of higher general quality and should outperform females from monogamous lines even with a single copulation. Under sexual conflict, costly adaptations will afford no advantages when females are allowed to mate only once. We investigate the lifetime reproductive success and longevity of females evolving under enforced monogamy or polyandry when mating once with males from these selection regimes. Females from polyandrous lines were found to have lower fitness than their monogamous counterparts when mating once. They died earlier and produced significantly fewer eggs and offspring. These results suggest that sexual conflict probably drove evolution under enforced polyandry as female fitness did not increase overall as expected with purely good–genes effects.

A female nervous system is necessary for normal sperm storage in Drosophila melanogaster: a masculinized nervous system is as good as none

A male Drosophila melanogaster deposits many more sperm in a females bursa copulatrix than are stored in her ventral receptacle or paired spermathecae soon after copula has ended. The remaining sperm are expelled by the female. These observations suggest a sexual conflict over the processes involved in sperm storage. We used genetically manipulated flies to study the role of the central nervous system in sperm storage. Flies with female bodies but masculinized nervous systems, or isolated female abdomens, stored significantly fewer sperm than did control females. Furthermore, compared with control flies, there were relatively more sperm in the ventral receptacle and relatively fewer in the spermathecae. These results suggest that the female nervous input counteracts the males attempts to force sperm into the ventral receptacle during copula and promotes active transport of sperm to the spermathecae during and after copula. The female is clearly a very active partner in influencing processes involved in sperm competition, especially as only stored sperm can be used later to fertilize eggs. To our knowledge, this is the first study to show directly the involvement of the female nervous system in sperm storage.