Sexual size dimorphism in a Drosophila clade, the D. obscura group

Positive correlations in susceptibility to a diverse panel of viruses across Drosophilidae host species

Our ability to predict the emergence of novel viruses relies on there being generalisable patterns in the susceptibilities of hosts to novel infections. Studies investigating variation in susceptibility among host species have consistently shown that closely related hosts share similar susceptibilities to a given virus. However, the extent to which such phylogenetic patterns of susceptibility are correlated amongst diverse sets of viruses is unclear. Here, we investigate phylogenetic correlations in susceptibility among Drosophilidae hosts to a panel of eleven different invertebrate viruses, comprising seven unique virus species, six unique families, and both RNA and DNA viruses. The susceptibility of hosts to each pair of viruses tested was either positively correlated across host species or did not show evidence of correlation. No negative correlations, indicative of evolutionary trade-offs in host susceptibility to different viruses, were detected between any virus pairs. The strength of correlations were generally higher in viruses of the same species and family, consistent with virus phylogenetic patterns in host infectivity. Our results suggest that generalised host susceptibility can result in positive correlations, even between highly diverged viruses, while specialised interactions with individual viruses cause a stepwise decrease in correlation strength between viruses from the within-species, to the within-family, to the across-family level.

Genetic variation for sexual dimorphism in developmental traits in Drosophila melanogaster

Sexual dimorphism in traits of insects during the developmental stages could potentially be the direct or indirect result of sex-specific selection provided that genetic variation for sexual dimorphism is present. We investigated genetic variation in sexual dimorphism in a set of Drosophila melanogaster inbred lines for 2 traits: egg to adult development time and pupation site preference. We observed considerable genetic variation in sexual dimorphism among lines in both traits. The sexual dimorphic patterns remained relatively consistent across multiple trials, despite both traits being sensitive to environmental conditions. Additionally, we measured 2 sexually dimorphic adult morphological traits in 6 sampled lines and investigated correlations in the sexual dimorphism patterns with the 2 developmental traits. The abundance of genetic variation in sexual dimorphism for D. melanogaster developmental traits demonstrated in this study provides evidence for a high degree of evolvability of sex differences in preadult traits in natural populations.

The evolutionary trajectory of drosophilid walking

Neural circuits must both execute the behavioral repertoire of individuals and account for behavioral variation across species. Understanding how this variation emerges over evolutionary time requires large-scale phylogenetic comparisons of behavioral repertoires. Here, we describe the evolution of walking in fruit flies by capturing high-resolution, unconstrained movement from 13 species and 15 strains of drosophilids. We find that walking can be captured in a universal behavior space, the structure of which is evolutionarily conserved. However, the occurrence of and transitions between specific movements have evolved rapidly, resulting in repeated convergent evolution in the temporal structure of locomotion. Moreover, a meta-analysis demonstrates that many behaviors evolve more rapidly than other traits. Thus, the architecture and physiology of locomotor circuits can execute precise individual movements in one species and simultaneously support rapid evolutionary changes in the temporal ordering of these modular elements across clades.

Between virus correlations in the outcome of infection across host species: Evidence of virus by host species interactions

Virus host shifts are a major source of outbreaks and emerging infectious diseases, and predicting the outcome of novel host and virus interactions remains a key challenge for virus research. The evolutionary relationships between host species can explain variation in transmission rates, virulence, and virus community composition between hosts, but it is unclear if correlations exist between related viruses in infection traits across novel hosts. Here, we measure correlations in viral load of four Cripavirus isolates across experimental infections of 45 Drosophilidae host species. We find positive correlations between every pair of viruses tested, suggesting that some host clades show broad susceptibility and could act as reservoirs and donors for certain types of viruses. Additionally, we find evidence of virus by host species interactions, highlighting the importance of both host and virus traits in determining the outcome of virus host shifts. Of the four viruses tested here, those that were more closely related tended to be more strongly correlated, providing tentative evidence that virus evolutionary relatedness may be a useful proxy for determining the likelihood of novel virus emergence, which warrants further research.

Viral susceptibility across host species is largely independent of dietary protein to carbohydrate ratios

The likelihood of a successful host shift of a parasite to a novel host species can be influenced by environmental factors that can act on both the host and parasite. Changes in nutritional resource availability have been shown to alter pathogen susceptibility and the outcome of infection in a range of systems. Here, we examined how dietary protein to carbohydrate altered susceptibility in a large cross-infection experiment. We infected 27 species of Drosophilidae with an RNA virus on three food types of differing protein to carbohydrate ratios. We then measured how viral load and mortality across species was affected by changes in diet. We found that changes in the protein:carbohydrate in the diet did not alter the outcomes of infection, with strong positive inter-species correlations in both viral load and mortality across diets, suggesting no species-by-diet interaction. Mortality and viral load were strongly positively correlated, and this association was consistent across diets. This suggests changes in diet may give consistent outcomes across host species, and may not be universally important in determining host susceptibility to pathogens.

Genetic manipulation of insulin/insulin-like growth factor signaling pathway activity has sex-biased effects on Drosophila body size

In Drosophila raised in nutrient-rich conditions, female body size is approximately 30% larger than male body size due to an increased rate of growth and differential weight loss during the larval period. While the mechanisms that control this sex difference in body size remain incompletely understood, recent studies suggest that the insulin/insulin-like growth factor signaling pathway (IIS) plays a role in the sex-specific regulation of processes that influence body size during development. In larvae, IIS activity differs between the sexes, and there is evidence of sex-specific regulation of IIS ligands. Yet, we lack knowledge of how changes to IIS activity impact body size in each sex, as the majority of studies on IIS and body size use single- or mixed-sex groups of larvae and/or adult flies. The goal of our current study was to clarify the body size requirement for IIS activity in each sex. To achieve this goal, we used established genetic approaches to enhance, or inhibit, IIS activity, and quantified pupal size in males and females. Overall, genotypes that inhibited IIS activity caused a female-biased decrease in body size, whereas genotypes that augmented IIS activity caused a male-specific increase in body size. These data extend our current understanding of body size regulation by showing that most changes to IIS pathway activity have sex-biased effects, and highlights the importance of analyzing body size data according to sex.

An investigation of Y chromosome incorporations in 400 species of Drosophila and related genera

Y chromosomes are widely believed to evolve from a normal autosome through a process of massive gene loss (with preservation of some male genes), shaped by sex-antagonistic selection and complemented by occasional gains of male-related genes. The net result of these processes is a male-specialized chromosome. This might be expected to be an irreversible process, but it was found in 2005 that the Drosophila pseudoobscura Y chromosome was incorporated into an autosome. Y chromosome incorporations have important consequences: a formerly male-restricted chromosome reverts to autosomal inheritance, and the species may shift from an XY/XX to X0/XX sex-chromosome system. In order to assess the frequency and causes of this phenomenon we searched for Y chromosome incorporations in 400 species from Drosophila and related genera. We found one additional large scale event of Y chromosome incorporation, affecting the whole montium subgroup (40 species in our sample); overall 13% of the sampled species (52/400) have Y incorporations. While previous data indicated that after the Y incorporation the ancestral Y disappeared as a free chromosome, the much larger data set analyzed here indicates that a copy of the Y survived as a free chromosome both in montium and pseudoobscura species, and that the current Y of the pseudoobscura lineage results from a fusion between this free Y and the neoY. The 400 species sample also showed that the previously suggested causal connection between X-autosome fusions and Y incorporations is, at best, weak: the new case of Y incorporation (montium) does not have X-autosome fusion, whereas nine independent cases of X-autosome fusions were not followed by Y incorporations. Y incorporation is an underappreciated mechanism affecting Y chromosome evolution; our results show that at least in Drosophila it plays a relevant role and highlight the need of similar studies in other groups.

In contrast to other chromosomes (X and autosomes), which are present in males and females, Y chromosomes spend all time in males. Hence it is not surprising that along evolution they became male specialized, e.g., containing a disproportionate amount of male-fertility genes. Interestingly it was found in 2005 that in Drosophila pseudoobscura the Y chromosome reverted to "male-female existence", being incorporated into an autosome. These "Y chromosome incorporations" have important consequences on sex-chromosome evolution, and allow the study of the evolutionary forces that shaped Y chromosomes as they act backwards. As D. pseudoobscura was the second Drosophila species investigated in this respect, it is likely that other cases exist, and that perhaps it is a common phenomenon. In order to answer this question we studied 400 Drosophila species. We found one additional case of Y incorporation, which occurred in the ancestor of Drosophila montium, and currently affects a large number of species; overall 13% of the species we sampled (52/400) have Y incorporations. We also found that a previously suggested cause of Y incorporations (X-autosome fusions) is not a general explanation. Our results show that in Drosophila Y incorporations play a relevant role and highlight the need of similar studies in other groups.

Changes in temperature alter the potential outcomes of virus host shifts

Host shifts-where a pathogen jumps between different host species-are an important source of emerging infectious disease. With on-going climate change there is an increasing need to understand the effect changes in temperature may have on emerging infectious disease. We investigated whether species' susceptibilities change with temperature and ask if susceptibility is greatest at different temperatures in different species. We infected 45 species of Drosophilidae with an RNA virus and measured how viral load changes with temperature. We found the host phylogeny explained a large proportion of the variation in viral load at each temperature, with strong phylogenetic correlations between viral loads across temperature. The variance in viral load increased with temperature, while the mean viral load did not. This suggests that as temperature increases the most susceptible species become more susceptible, and the least susceptible less so. We found no significant relationship between a species' susceptibility across temperatures, and proxies for thermal optima (critical thermal maximum and minimum or basal metabolic rate). These results suggest that whilst the rank order of species susceptibilities may remain the same with changes in temperature, some species may become more susceptible to a novel pathogen, and others less so.

Effect of genomic deficiencies on sexual size dimorphism through modification of developmental time in Drosophila melanogaster

Sexual size dimorphism (SSD), a difference in body size between sexes, is common in many taxa. In insects, females are larger than males in >70% of all taxa in most orders. The fruit fly, Drosophila melanogaster is one prominent model organism to investigate SSD since its clear and representative female-biased SSD and its growth regulation are well studied. Elucidating the number and nature of genetic elements that can potentially influence SSD would be helpful in understanding the evolutionary potential of SSD. Here, we investigated the SSD pattern caused by artificially introduced genetic variation in D. melanogaster, and examined whether variation in SSD was mediated by the sex-specific modification of developmental time. To map the genomic regions that had effects on sexual wing size and/or developmental time differences (SDtD), we reanalyzed previously published genome-wide deficiency mapping data to evaluate the effects of 376 isogenic deficiencies covering a total of ~67% of the genomic regions of the second and third chromosomes of D. melanogaster. We found genetic variation in SSD and SDtD generated by genomic deficiencies, and a negative genetic correlation between size and development time. We also found SSD and SDtD allometries that are not qualitatively congruent, which however overall at best only partly help in explaining the patterns found. We identified several genomic deficiencies with the tendency to either exaggerate or suppress SSD, in agreement with quantitative genetic null expectations of many loci with small effects. These novel findings contribute to a better understanding of the evolutionary potential of sexual dimorphism.

The causes and consequences of changes in virulence following pathogen host shifts

Emerging infectious diseases are often the result of a host shift, where the pathogen originates from a different host species. Virulence--the harm a pathogen does to its host-can be extremely high following a host shift (for example Ebola, HIV, and SARs), while other host shifts may go undetected as they cause few symptoms in the new host. Here we examine how virulence varies across host species by carrying out a large cross infection experiment using 48 species of Drosophilidae and an RNA virus. Host shifts resulted in dramatic variation in virulence, with benign infections in some species and rapid death in others. The change in virulence was highly predictable from the host phylogeny, with hosts clustering together in distinct clades displaying high or low virulence. High levels of virulence are associated with high viral loads, and this may determine the transmission rate of the virus.

Latitudinal and climatic variation in body size and dorsal scale counts in Sceloporus lizards:a phylogenetic perspective

Squamates often follow an inverse Bergmann's rule, with larger-bodied animals occurring in warmer areas or at lower latitudes. The size of dorsal scales in lizards has also been proposed to vary along climatic gradients, with species in warmer areas exhibiting larger scales, putatively to reduce heat load. We tested for these patterns in the diverse and widespread lizard genus Sceloporus. Among 106 species or populations, body size was associated positively with maximum temperature (consistent with the inverse of Bergmann's rule) and aridity, but did not covary with latitude. Scale size (inferred from the inverse relation with numbers of scales) was positively related to body size. Controlling for body size via multiple regression, scale size was associated negatively with latitude (best predictor), positively with minimum temperature, and negatively with aridity (similar results were obtained using scores from a principal components analysis of latitude and climatic indicators). Thus, lizards with larger scales are not necessarily found in areas with higher temperatures. Univariate analyses indicated phylogenetic signal for body size, scale counts, latitude, and all climate indicators. In all cases, phylogenetic regression models fit the data significantly better than nonphylogenetic models; thus, residuals for log(10) number of dorsal scale rows exhibited phylogenetic signal.

Host phylogeny determines viral persistence and replication in novel hosts

Pathogens switching to new hosts can result in the emergence of new infectious diseases, and determining which species are likely to be sources of such host shifts is essential to understanding disease threats to both humans and wildlife. However, the factors that determine whether a pathogen can infect a novel host are poorly understood. We have examined the ability of three host-specific RNA-viruses (Drosophila sigma viruses from the family Rhabdoviridae) to persist and replicate in 51 different species of Drosophilidae. Using a novel analytical approach we found that the host phylogeny could explain most of the variation in viral replication and persistence between different host species. This effect is partly driven by viruses reaching a higher titre in those novel hosts most closely related to the original host. However, there is also a strong effect of host phylogeny that is independent of the distance from the original host, with viral titres being similar in groups of related hosts. Most of this effect could be explained by variation in general susceptibility to all three sigma viruses, as there is a strong phylogenetic correlation in the titres of the three viruses. These results suggest that the source of new emerging diseases may often be predictable from the host phylogeny, but that the effect may be more complex than simply causing most host shifts to occur between closely related hosts.

Emerging infectious diseases such as SARS, HIV and swine-origin influenza have all been recently acquired by humans from other species. Understanding the reasons why parasites jump between different host species is essential to allow us to predict future threats and understand the causes of disease emergence. Here we ask how host-relatedness might determine when host-shifts can occur in the most important group of emerging diseases—RNA viruses. We show that the relationship between host species is the primary factor in determining a virus's ability to persist and replicate in a novel host following exposure. This can be broken down into two components. Firstly, species closely related to the virus's natural host are more susceptible than distantly related species. Secondly, independent of the distance effect, groups of closely related host species have similar levels of susceptibility. This has important implications for our understanding of disease-emergence, and until now the only large-scale studies of viruses have been correlative rather than experimental. We also found groups of related species that are susceptible to these viruses but are distantly related to the natural hosts, which may explain why viruses sometimes jump between distantly related species.

A contemporary, sex-limited change in body size of an estuarine turtle in response to commercial fishing

Juvenile growth rate and adult body size are important components of life-history strategies because of their direct impact on fitness. The diamondback terrapin (Malaclemys terrapin) is a sexually dimorphic, long-lived turtle inhabiting brackish waters throughout the Atlantic and Gulf coasts of the United States. In parts of its range, terrapins face anthropogenically imposed mortality: juveniles of both sexes inadvertently enter commercial crab traps and drown. For adult females, the carapace eventually grows large enough that they cannot enter traps, whereas males almost never reach that critical size. We compared age structure, carapace dimensions, growth curves, and indices of sexual dimorphism for a Chesapeake Bay population of terrapins (where mortality of turtles is high due to crab traps) with contemporary terrapins from Long Island Sound and museum specimens from Chesapeake Bay (neither group subject to commercial crab traps). We also calculated the allochronic and synchronic rates of evolutionary change (haldanes) for males and females to measure the rate of trait change in a population or between populations, respectively. We found a dramatic shift to a younger male age structure, a decrease in the length of time to terminal female carapace size, a 15% increase in female carapace width, and an increase in sexual dimorphism in Chesapeake Bay. In a new twist, our results implicate a fishery in the selective increase in size of a reptilian bycatch species. These sex-specific changes in life history and demography have implications for population viability that need to be considered when addressing conservation of this threatened turtle.

Sexual dimorphism of skull shape in a lacertid lizard species (Podarcis spp., Dalmatolacerta sp., Dinarolacerta sp.) revealed by geometric morphometrics

Geometric morphometric techniques were used to examine allometric and non-allometric influences on sexual shape dimorphism (SShD) in the ventral cranium (skull base, palate and upper jaw) of four species of lacertid lizards (Podarcis muralis, Podarcis melisellensis, Dalmatolacerta oxycephala, Dinarolacerta mosorensis). These species differ in body shape, ecology and degree of phylogenetic relatedness. The structures of the ventral cranium that were studied are directly involved in the mechanics of feeding and are connected to the jaw musculature; these structures are potentially subject to both sexual and natural selection. Allometry accounted for a considerable degree of cranial shape variation between the sexes. Allometric shape changes between individuals with smaller cranium size and individuals with larger cranium size are mostly related to changes in the skull base showing pronounced negative allometry. The rostral part, however, either scaled isometrically or showed less pronounced negative allometry than the skull base. Non-allometric intersexual shape variation predominantly involved changes related to the jaw adductor muscle chamber, i.e., changes that are associated with biomechanically relevant traits of the jaw system in females and males. Both allometric and non-allometric shape changes appeared to be species-specific. Our results indicate that natural and sexual selection may be involved in the evolution of SShD.

Wing morphometry as a tool for correct identification of primary and secondary New World screwworm fly

Cochliomyia hominivorax and Cochliomyia macellaria are endemic Neotropical Calliphoridae species. The former causes severe myiasis in hosts while the latter is Sarcosaprophagous, but commonly found as a second invader in wounds. Due to the morphological similarity between them and the potential losses that C. hominivorax represents for cattle breeders, the rapid and correct identification of these two species is very important. In addition to a correct identification of these species, a good knowledge of C. hominivorax biology can be helpful for designing control programs. We applied geometric morphometric methods to assess wing differences between C. hominivorax and C. macellaria and conduct a preliminary analysis of wing morphological variation in C. hominivorax populations. Canonical variate analysis, using wing shape data, correctly classified 100% of the individuals analyzed according to sex and species. This result demonstrates that wing morphometry is a simple and reliable method for identifying C. hominivorax and C. macellaria samples and can be used to monitor C. hominivorax. Both species show sexual dimorphism, but in C. hominivorax it is magnified. We suggest that this may reflect different histories of selection pressures operating on males and females. Significant differences in wing size and shape were obtained among C. hominivorax populations, with little correlation with latitude. This result suggests that wing variation is also a good morphological marker for studying population variation in C. hominivorax.

Coevolution of male and female reproductive structures in Drosophila

The morphology of male genitalia whilst stable within species, exhibits huge interspecific variation. This variation is likely to be as a result of sexual selection due to the direct involvement of these reproductive structures in mating and sperm transfer. In contrast, internal soft tissue components of the genitalia are generally poorly investigated as they are not directly involved in physical and mechanical adequacy during sperm transfer. However, these soft tissue structures may also drive differential male-female interactions, particularly in internally fertilising organisms where females have the ability to store sperm and bias male reproductive success. In this paper we use the drosophila model to investigate the role of male and female reproductive elements in sexual selection. Our meta-analysis supplemented with additional new data clearly shows that within species, sperm length versus testis length, and sperm length versus seminal receptacle length, are highly correlated. Thus, independent of the phylogenetic relationship among species, gamete evolution is likely to result in sexual selection interactions that drive the evolution of internal reproductive components in both sexes. Our results and discussion of the literature highlight the importance of considering internal soft structures that may influence fertilisation, when investigating selective forces acting on the evolution of reproductive traits.

Quantitative morphometrical analysis of a North African population of Drosophila melanogaster: sexual dimorphism, and comparison with European populations

Genetic variability of quantitative traits was investigated in a Moroccan population of Drosophila melanogaster, with an isofemale line design. Results were compared with data previously obtained from French populations. Although the environmental and thermal conditions are very different in France and Morocco, only two significant differences were observed: a shorter wing and a lighter abdomen pigmentation in Morocco. It is, therefore, concluded that Moroccan D. melanogaster are quite typical temperate flies, belonging to the Palaearctic region, and very different from the ancestral Afrotropical populations. Almost all traits were genetically variable, as shown by significant intraclass correlations among lines. Genetic correlations were highly significant among three size-related traits, while much lower between size and bristle numbers. Fluctuating asymmetry was greater for abdominal bristles than for sternopleural bristles. Sex dimorphism, analysed as a female/male ratio, was identical in French and Moroccan populations. Examination of the thorax length/thorax width ratio showed that the thorax is more elongated in females. Sexual dimorphism of wing length was significantly more correlated to thorax width than to thorax length. The results illustrate the value of measuring numerous quantitative traits on the same flies for characterizing the genetic architecture of a natural population. In several cases, and especially for genetic correlations, some interesting suggestions could be made, which should be confirmed, or invalidated, by more extensive investigations.

Phenotypic plasticity of abdomen pigmentation in two geographic populations of Drosophila melanogaster: male-female comparison and sexual dimorphism

In Drosophila melanogaster male, the last abdominal tergites (A5-A6) are completely dark due to a strong internal constraint while, in female, all abdominal tergites (A2-A7) are phenotypically variable and highly plastic. Male A2-A4 are quite similar to those of female, but their plasticity was never investigated. In this paper, we compared the phenotypic plasticity of A2-A4 in both sexes in order to know if the major dimorphism (SD) expressed in male A5-A6 also extended toward the more anterior segments. We also compared two geographic populations living under very different climates in order to know if adaptive differences, previously observed in females also existed in males. With an isofemale line design, pigmentation variation according to growth temperature was investigated in the two populations from France and India. Male and female data were compared and sexual dimorphism (SD) analyzed in various ways. Reaction norms were quite similar in both sexes for A2 and A3, but clearly different for A4. Considering the total pigmentation (A2 + A3 + A4) males were darker than females at low temperatures and either identical to them (France) or lighter (India) above 25 degrees C. SD (male-female difference) was genetically variable among lines and significantly different among segments. Reaction norms of SD exhibited an overall decrease with temperature and also a significant difference among populations, suggesting a local adaptation of SD to thermal conditions. The three plastic segments in male (A2-A4) seem to react adaptively to the thermal environment more efficiently than the same segments in female, in agreement with the thermal budget hypothesis. To our knowledge, it is the first time that a SD trait exhibits an adaptive difference between geographic populations.

Grafting the molecular phylogenetic tree with morphological branches to reconstruct the evolutionary history of the genus Zaprionus (Diptera: Drosophilidae)

A molecular phylogeny for the drosophilid genus Zaprionus was inferred using a mitochondrial (CO-II) and a nuclear (Amyrel) gene using 22 available species. The combined molecular tree does not support the current classification, dubbed phylogenetic, based entirely upon a morphocline of forefemoral ornamentation. For species for which DNA was not available, phylogenetic positioning was only assigned using morphological characters. In order to avoid conflict between DNA and morphology in the combined analyses (supermatrix method), we developed a new method in which few morphological characters were sampled according to an a priori homoplasy assessment on the consensus molecular tree. At each internal node of the tree, a number of synapomorphies was determined, and species with no molecular sequences were grafted thereon. Analogously to tree vocabulary, we called our method 'morphological grafting'. New species groups and complexes were then defined in the light of our findings. Further, divergence times were estimated under a relaxed molecular clock, and historical biogeography was reconstructed under a maximum likelihood model. Zaprionus appears to be of recent origin in the Oriental region during the Late Miocene ( approximately 10 MYA), and colonization of Africa started shortly after ( approximately 7 MYA) via the maritime route of the Indian Ocean Islands. Most of the morphological and ecological diversification took place, later, in Western Africa during the Quaternary cyclic climatic changes. Furthermore, some species became recent invaders, with one, Zaprionus indianus, has successfully invaded South and North America during the last decade.

Only half right: species with female-biased sexual size dimorphism consistently break Rensch's rule

BACKGROUND

Most animal species display Sexual Size Dimorphism (SSD): males and females consistently attain different sizes, most frequently with females being larger than males. However the selective mechanisms driving patterns of SSD remain controversial. 'Rensch's rule' proposes a general scaling phenomenon for all taxa, whereby SSD increases with average body size when males are larger than females, and decreases with body size when females are larger than males. Rensch's rule appears to be general in the former case, but there is little evidence for the rule when females are larger then males.

METHODOLOGY/PRINCIPAL FINDINGS

Using comprehensive data for 1291 species of birds across 30 families, we find strong support for Rensch's rule in families where males are typically larger than females, but no overall support for the rule in families with female-biased SSD. Reviewing previous studies of a broad range of taxa (arthropods, reptiles, fish and birds) showing predominantly female-biased SSD, we conclude that Rensch's conjecture is the exception rather than the rule in such species.

CONCLUSIONS/SIGNIFICANCE

The absence of consistent scaling of SSD in taxa with female-biased SSD, the most prevalent direction of dimorphism, calls into question previous general evolutionary explanations for Rensch's rule. We propose that, unlike several other ecological scaling relationships, Rensch's rule does not exist as an independent scaling phenomenon.

Within-Species Variation and Measurement Error in Phylogenetic Comparative Methods

Most phylogenetically based statistical methods for the analysis of quantitative or continuously varying phenotypic traits assume that variation within species is absent or at least negligible, which is unrealistic for many traits. Within-species variation has several components. Differences among populations of the same species may represent either phylogenetic divergence or direct effects of environmental factors that differ among populations (phenotypic plasticity). Within-population variation also contributes to within-species variation and includes sampling variation, instrument-related error, low repeatability caused by fluctuations in behavioral or physiological state, variation related to age, sex, season, or time of day, and individual variation within such categories. Here we develop techniques for analyzing phylogenetically correlated data to include within-species variation, or "measurement error" as it is often termed in the statistical literature. We derive methods for (i) univariate analyses, including measurement of "phylogenetic signal," (ii) correlation and principal components analysis for multiple traits, (iii) multiple regression, and (iv) inference of "functional relations," such as reduced major axis (RMA) regression. The methods are capable of incorporating measurement error that differs for each data point (mean value for a species or population), but they can be modified for special cases in which less is known about measurement error (e.g., when one is willing to assume something about the ratio of measurement error in two traits). We show that failure to incorporate measurement error can lead to both biased and imprecise (unduly uncertain) parameter estimates. Even previous methods that are thought to account for measurement error, such as conventional RMA regression, can be improved by explicitly incorporating measurement error and phylogenetic correlation. We illustrate these methods with examples and simulations and provide Matlab programs.

Proximate Causes of Rensch’s Rule: Does Sexual Size Dimorphism in Arthropods Result from Sex Differences in Development Time?

A prominent interspecific pattern of sexual size dimorphism (SSD) is Rensch's rule, according to which male body size is more variable or evolutionarily divergent than female body size. Assuming equal growth rates of males and females, SSD would be entirely mediated, and Rensch's rule proximately caused, by sexual differences in development times, or sexual bimaturism (SBM), with the larger sex developing for a proportionately longer time. Only a subset of the seven arthropod groups investigated in this study exhibits Rensch's rule. Furthermore, we found only a weak positive relationship between SSD and SBM overall, suggesting that growth rate differences between the sexes are more important than development time differences in proximately mediating SSD in a wide but by no means comprehensive range of arthropod taxa. Except when protandry is of selective advantage (as in many butterflies, Hymenoptera, and spiders), male development time was equal to (in water striders and beetles) or even longer than (in drosophilid and sepsid flies) that of females. Because all taxa show female-biased SSD, this implies faster growth of females in general, a pattern markedly different from that of primates and birds (analyzed here for comparison). We discuss three potential explanations for this pattern based on life-history trade-offs and sexual selection.