Genetic divergence does not predict change in ornament expression among populations of stalk-eyed flies

Stalk-eyed flies (Diptera: Diopsidae) possess eyes at the ends of elongated peduncles, and exhibit dramatic variation in eye span, relative to body length, among species. In some sexually dimorphic species, evidence indicates that eye span is under both intra- and intersexual selection. Theory predicts that isolated populations should evolve differences in sexually selected traits due to drift. To determine if eye span changes as a function of divergence time, 1370 flies from 10 populations of the sexually dimorphic species, Cyrtodiopsis dalmanni and Cyrtodiopsis whitei, and one population of the sexually monomorphic congener, Cyrtodiopsis quinqueguttata, were collected from Southeast Asia and measured. Genetic differentiation was used to assess divergence time by comparing mitochondrial (cytochrome oxidase II and 16S ribosomal RNA gene fragments) and nuclear (wingless gene fragment) DNA sequences for c. five individuals per population. Phylogenetic analyses indicate that most populations cluster as monophyletic units with up to 9% nucleotide substitutions between populations within a species. Analyses of molecular variance suggest a high degree of genetic structure within and among the populations; > 97% of the genetic variance occurs between populations and species while < 3% is distributed within populations, indicating that most populations have been isolated for thousands of years. Nevertheless, significant change in the allometric slope of male eye span on body length was detected for only one population of either dimorphic species. These results are not consistent with genetic drift. Rather, relative eye span appears to be under net stabilizing selection in most populations of stalk-eyed flies. Given that one population exhibited dramatic evolutionary change, selection, rather than genetic variation, appears to constrain eye span evolution.     

Condition dependence of sexual ornament size and variation in the stalk-eyed fly Cyrtodiopsis dalmanni (Diptera: Diopsidae)

We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection.     

Sperm survival in female stalk-eyed flies depends on seminal fluid and meiotic drive

Sperm competition is common in many insect species; however, the mechanisms underlying differences in sperm precedence are not well understood. In the stalk-eyed fly, Cyrtodiopsis whitei (Diptera, Diopsidae), sperm precedence is influenced by the presence of sex chromosome meiotic drive. When drive-carrying males compete with non-driving males for fertilizations within a female, the number of progeny sired by drive males is significantly fewer than predicted by sperm mixing alone. Thus, drive males apparently suffer not only a reduction in the number of viable sperm produced, but also a reduction in sperm competitive ability. In this study, we manipulated the amount and source of seminal fluid and sperm received by females by interrupting copulations before sperm, but after seminal fluid, was transferred. We find that seminal fluid from another male influences the number of progeny sired by a drive-carrying male when both males mate with the same female. Sperm viability staining reveals that sperm from drive males are incapacitated by seminal fluid from other males within the female reproductive tract. These results suggest that multiple mating by females enables seminal fluid products to interact differentially with sperm and may reduce the transmission advantage of the drive chromosome.     

Low cost of reproduction in female stalk-eyed flies, Cyrtodiopsis dalmanni

We investigated the effect of exposure to males on female longevity and egg production in the stalk-eyed fly, Cyrtodiopsis dalmanni. In this species, females typically mate several times each morning over a lifetime that can span several months. We hypothesised that high costs of mating with males would be incompatible with the life history of this species. We predicted that 1) female longevity costs of exposure to males would be low, and 2) that the magnitude of mating-induced fecundity increases in C. dalmanni, if present, would also be low. We tested the predictions by comparing the longevity and lifetime fecundity of virgin and mated females. In line with prediction 1), the longevity difference between virgin and fully reproductive females was small and of borderline significance. In line with prediction 2), egg production was not significantly higher in females continually exposed to males than for virgin females. Our results suggest that costs of reproduction resulting from exposure to males are low in species that mate promiscuously at high rates.     

The costs and benefits of high early mating rates in male stalk-eyed flies, Cyrtodiopsis dalmanni

Cyrtodiopsis dalmanni stalk-eyed flies have extremely high mating rates. In this paper, we examine the fitness costs and benefits of high mating rates, using flies with low or high early mating rates. We show that high early mating rates are beneficial as they increase the total number of matings gained by a male for a given lifespan. However, males with high early mating rates suffer a cost through lowered survival. There was no consistent effect of male mating rate on female egg production or on male fertility. As higher mating rates are associated with larger accessory gland size, we hypothesize that the extra production of accessory gland products in high early mating males is responsible for much of the cost of reduced longevity.     

RAPID EVOLUTION OF ASYMMETRIC REPRODUCTIVE INCOMPATIBILITIES IN STALK‐EYED FLIES

The steps by which isolated populations acquire reproductive incompatibilities remain poorly understood. One potentially important process is postcopulatory sexual selection because it can generate divergence between populations in traits that influence fertilization success after copulation. Here we present a comprehensive analysis of this form of reproductive isolation by conducting reciprocal crosses between variably diverged populations of stalk‐eyed flies (Teleopsis dalmanni). First, we measure seven types of reproductive incompatibility between copulation and fertilization. We then compare fertilization success to hatching success to quantify hybrid inviability. Finally, we determine if sperm competition acts to reinforce or counteract any incompatibilities. We find evidence for multiple incompatibilities in most crosses, including failure to store sperm after mating, failure of sperm to reach the site of fertilization, failure of sperm to fertilize eggs, and failure of embryos to develop. Local sperm have precedence over foreign sperm, but this effect is due mainly to differences in sperm transfer and reduced hatching success. Crosses between recently diverged populations are asymmetrical with regard to the degree and type of incompatibility. Because sexual conflict in these flies is low, postcopulatory sexual selection, rather than antagonistic coevolution, likely causes incompatibilities due to mismatches between male and female reproductive traits.     

The development of a long,coiled, optic nerve in the stalk-eyed fly <Emphasis Type="Italic">Cyrtodiopsis whitei</Emphasis>

In the stalk-eyed fly Cyrtodiopsis whitei (Diopsidae; Diptera), the relatively long optic nerve develops within the tight lumen of a very short eyestalk. Axonal growth is generally considered in terms of path finding, selective fasciculation, and towing. Physical forces that are necessary for axon lengthening are generated either by the growth cone or by the growth of surrounding tissues. Therefore, it is surprising to encounter a loosely coiled nerve apparently lacking any attachments that could allow for pull, or towing, of the nerve. In this study, we used histological sections and whole-mount preparations to confirm that the optic nerve of the stalk-eyed fly indeed elongates without the external application of tension to the nerve. Secondly, we examined the distribution of cytoskeletal elements and selected proteins that may be involved in axon extension. Staining against the vesicle fusion proteins SNAP-24 and SNAP-25 consistently results in stronger staining in the rapidly extending optic nerve than in a control nerve, suggesting a possible role of these proteins in the extension process. On a gross morphological level, SNAP-24/25 as well as the cytoskeletal elements actin and tubulin are uniformly distributed throughout the lengths of the growing nerve, suggesting that nerve elongation is distributed rather than localized. Finally, we identified glia as a possible source for tension within the nerve bundle. Glia proliferate rapidly in the optic nerve but not in the control nerve. Much work continues to focus on the growth of axons in culture, but this study is one of the few that considers the dynamics of nerve bundle extension as a whole.This research is supported by the National Science Foundation (IBN-9974512 and IBN-211770).     

Size matters,but so does shape: quantifying complex shape changes in a sexually selected trait in stalk‐eyed flies (Diptera: Diopsidae)

The elaborate morphologies of sexually selected ornaments are ubiquitous across the animal kingdom. In studies investigating these traits, ornament size is frequently the focus, and empirical evidence supports its positive correlation with fitness. Yet shape plays an important role and, surprisingly, is often overlooked. Shape frequently changes with size, influenced by biomechanical, developmental, or performance constraints. Therefore, shape can provide additional insights into the morphological differences between individuals and the potential limits on sexual trait exaggeration. Here, we used landmark‐based geometric morphometric methods on a sexually dimorphic species of stalk‐eyed fly (Teleopsis dalmanni) to examine patterns of sexual shape dimorphism. Our analyses reveal a significant difference in head shape between the sexes, with males exhibiting smaller eye bulbs, thinner stalks, and smaller heads than females. Additionally, as eyestalk length increases within each sex, a similar pattern of shape change was observed as that observed between sexes. This pattern of shape change may be the result of constraints acting against further ornament exaggeration, and we suggest that this may significantly impact the whole‐organism performance in stalk‐eyed flies. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 104–113.     

Heightened condition dependence is not a general feature of male eyespan in stalk-eyed flies (Diptera: Diopsidae)

Stalk-eyed flies are exemplars of sexual selection leading to the evolution of exaggerated male ornaments (eyespan). In Sphyracephala beccarri, there is no evidence for female mate choice for exaggerated male eyespan and only minor sex differences in eyespan. We used S. beccarri to test whether heightened condition dependence only evolves when male eyespan becomes sexually exaggerated. Male eyespan showed heightened condition dependence under food stress compared with a control trait (wing length). However, female eyespan displayed a similar pattern and there was no sex difference in the degree of increased eyespan sensitivity. The finding that eyespan is a sensitive indicator of food stress, even in an unexaggerated state, suggests that this may have acted as a pre-adaptation to the role of eyespan in sexual signalling in other Diopsid species. These results are consistent with handicap theory and Fisher's view of how sexual selection is initiated.     

Accessory gland size influences time to sexual maturity and mating frequency in the stalk-eyed fly, Cyrtodiopsis dalmanni

Age at first reproduction is an extremely important life-historytrait. Several factors such as nutritional state and age-specificfecundity have been shown to influence time to sexual maturity;however, little work has been done in insects. We addressedthis in a stalk-eyed fly (Cyrtodiopsis dalmanni), by testingthe hypothesis that time to sexual maturity is associated withthe development of male internal reproductive structures. Wefound that sexual maturity was attained after an increased rateof growth in the accessory glands, several days after maturesperm bundles, and motile sperm were observed in the testes.Although testis development is essential, the results suggestthat accessory gland growth is more closely associated withthe time taken to reach sexual maturity than is testis growth.When we manipulated the growth of testes and accessory glandsvia a dietary manipulation, we found that delayed growth ratesincreased the time taken to reach sexual maturity. Among thedelayed individuals, sexually mature males had larger accessoryglands, but not testes, than did immature males. In adult males,mating frequency was significantly positively correlated withaccessory gland size, but not with testis length or body size.We conclude that accessory gland size is a critical determinantof sexual maturity and male mating frequency in this species.