The courtship song of African Drosophila melanogaster

For many years it was thought that Drosophila melanogaster was relatively panmictic, without differentiation in the Mate Recognition System. Recent studies have demonstrated that flies from Africa vary in pheromones and assortative mating. Strains from Zimbabwe show strong sexual isolation from others. We show that the interpulse interval (IPI) of courtship song, an important mating signal, is unusually short among African flies. Zimbabwean flies have the shortest IPI, but there is no correlation with assortative mating, suggesting little direct role in sexual isolation. Chromosome replacements show that the IPI difference is largely due to genes on chromosome III, with significant interactions involving other chromosomes. Several traits potentially influencing sexual isolation among the melanogaster group of Drosophila seem to be localized to this chromosome. A concentration of important genetic differences might mean that the interaction effects reflect secondary coadaptation of the genetic background to changes associated with chromosome III.     

Incipient sexual isolation among cosmopolitan Drosophila melanogaster populations

Understanding the biological conditions and the genetic basis of early stages of sexual isolation and speciation is an outstanding question in evolutionary biology. It is unclear how much genetic and phenotypic variation for mating preferences and their phenotypic cues is segregating within widespread and human-commensal species in nature. A recent case of incipient sexual isolation between Zimbabwe and cosmopolitan populations of the human-commensal fruit fly Drosophila melanogaster indicates that such species may initiate the process of sexual isolation. However, it is still unknown whether other geographical populations have undergone evolution of mating preferences. In this study we present new data on multiple-choice mating tests revealing partial sexual isolation between the United States and Caribbean populations. We relate our findings to African populations, showing that Caribbean flies are partially sexually isolated from Zimbabwe flies, but mate randomly with West African flies, which also show partial sexual isolation from the United States and Zimbabwe flies. Thus, Caribbean and West African populations seem to exhibit distinct mating preferences relative to populations in the United States and in Zimbabwe. These results suggest that widespread and human-commensal species may harbor different types of mating preferences across their geographical ranges.     

Limits to environmental masking of genetic quality in sexual signals

There is considerable debate over the value of male sexual ornaments as signals of genetic quality. Studies alternately report that environmental variation enhances or diminishes the genetic signal, or leads to crossover where genotypes perform well in one environment but poorly in another. A unified understanding is lacking. We conduct a novel experimental test examining the dual effects of distinct categories of genetic (inbred vs. crossed parental lines) and environmental quality (low, through high to extreme larval food stress) on a condition‐dependent male ornament. We find that differences in genetic quality signalled by the ornament (male eyespan in Diasemopsis meigenii stalk‐eyed flies) become visible and are amplified under high stress but are overwhelmed in extreme‐stress environments. Variance among independent genetic lines increases with environmental stress in both genetic quality classes, but at a slower rate in high quality outcrossed flies. Individual genetic lines generally maintain their ranks across environments, except among high quality lines under low environmental stress, where low genetic variance among lines precludes differentiation between ranks. Our results provide a conceptual advance, demonstrating a unified pattern for how genetic and environmental quality interact. They show when environmental conditions lead to the amplification of differences in signals of genetic quality and thereby enhance the potential indirect genetic benefits gained by female mate choice.     

Contrasting population genetic patterns and evolutionary histories among sympatric Sonoran Desert cactophilic Drosophila

We studied population genetic differentiation in the sympatric Sonoran Desert cactophilic flies Drosophila pachea, D. mettleri and D. nigrospiracula across their continental and peninsular ranges. These flies show marked differences in ecology and behaviour including dispersal distances and host cactus specialization. Examination of a fragment of the mitochondrial cytochrome oxidase subunit I gene (mtCOI) reveals that the Sea of Cortez has constituted an effective dispersal barrier for D. pachea, leading to significant genetic differentiation between the continental and peninsular ranges of this species. No genetic differentiation was detected, however, within its continental and peninsular ranges. In contrast, our mtCOI-based results for D. mettleri and D. nigrospiracula are consistent with a previous allozyme-based study that showed no significant genetic differentiation between continental and peninsular ranges of these two species. For D. mettleri, we also found that the insular population from Santa Catalina Island, California, is genetically differentiated with respect to continental and peninsular localities. We discuss how differences in the genetic structure patterns of D. pachea, D. mettleri and D. nigrospiracula may correspond to differences in their dispersal abilities, host preferences and behaviour.     

Sex,death, and genetic variation: natural and sexual selection on cricket song

Male field crickets, Gryllus integer, in Texas, USA, produce a trilled calling song that attracts female crickets, resulting in enhanced mating success. Gravid female parasitoid flies, Ormia ochracea, are also attracted to male cricket calling song, resulting in the death of the male within about seven days. Using playbacks of field-cricket calling song in the natural habitat, we show that both female crickets and female parasitoid flies prefer male calling song with average numbers of pulses per trill. Thus female crickets exert stabilizing sexual selection, whereas flies exert disruptive natural selection on male song. Disruptive natural selection will promote genetic variation and population divergence. Stabilizing sexual selection will reduce genetic variation and maintain population cohesiveness. These forces may balance and together maintain the observed high levels of genetic variation (ca. 40%) in male calling song.     

Genetic differentiation associated with host plants and geography among six widespread species of South American Blepharoneura fruit flies (Tephritidae)

Tropical herbivorous insects are astonishingly diverse, and many are highly host‐specific. Much evidence suggests that herbivorous insect diversity is a function of host plant diversity; yet, the diversity of some lineages exceeds the diversity of plants. Although most species of herbivorous fruit flies in the Neotropical genus Blepharoneura are strongly host‐specific (they deposit their eggs in a single host plant species and flower sex), some species are collected from multiple hosts or flowers and these may represent examples of lineages that are diversifying via changes in host use. Here, we investigate patterns of diversification within six geographically widespread Blepharoneura species that have been collected and reared from at least two host plant species or host plant parts. We use microsatellites to (1) test for evidence of local genetic differentiation associated with different sympatric hosts (different plant species or flower sexes) and (2) examine geographic patterns of genetic differentiation across multiple South American collection sites. In four of the six fly species, we find evidence of local genetic differences between flies collected from different hosts. All six species show evidence of geographic structure, with consistent differences between flies collected in the Guiana Shield and flies collected in Amazonia. Continent‐wide analyses reveal – in all but one instance – that genetically differentiated flies collected in sympatry from different host species or different sex flowers are not one another's closest relatives, indicating that genetic differences often arise in allopatry before, or at least coincident with, the evolution of novel host use.     

Gene duplication, tissue-specific gene expression and sexual conflict in stalk-eyed flies (Diopsidae)

Gene duplication provides an essential source of novel genetic material to facilitate rapid morphological evolution. Traits involved in reproduction and sexual dimorphism represent some of the fastest evolving traits in nature, and gene duplication is intricately involved in the origin and evolution of these traits. Here, we review genomic research on stalk-eyed flies (Diopsidae) that has been used to examine the extent of gene duplication and its role in the genetic architecture of sexual dimorphism. Stalk-eyed flies are remarkable because of the elongation of the head into long stalks, with the eyes and antenna laterally displaced at the ends of these stalks. Many species are strongly sexually dimorphic for eyespan, and these flies have become a model system for studying sexual selection. Using both expressed sequence tag and next-generation sequencing, we have established an extensive database of gene expression in the developing eye-antennal imaginal disc, the adult head and testes. Duplicated genes exhibit narrower expression patterns than non-duplicated genes, and the testes, in particular, provide an abundant source of gene duplication. Within somatic tissue, duplicated genes are more likely to be differentially expressed between the sexes, suggesting gene duplication may provide a mechanism for resolving sexual conflict.     

Variation in fertility of two wild type strains of Drosophila melanogaster meigen

Initial observations of low fertility, reduced sexual activity of males, and a high frequency of abnormalities in sperm differentiation of the wild type strain (Sevelen, Zürich) of Drosophila melanogaster, normally used in this laboratory, have lead to a study of this phenomenon and its causes. The abnormalities occur during all spermiogenetic stages and are not unique but are found with much lower frequency in normally fertile flies (Oregon R). Growth of Sevelen flies at high but sublethal temperatures (30°C) results in complete sterility, highly abnormal sperm differentiation, and a failure to recover fertility after return to normal temperatures (25° C) in the time period in which normal flies recover. — The principal factor, or factors, controlling normal sperm differentiation are located on the Y chromosome, but are thus far not precisely localized. Expression of the phenotype is modified by genetic background in which the Y chromosome is found.     

Genetic structure of cherry fruit fly (Rhagoletis cingulata) populations across managed,unmanaged, and natural habitats

The cherry fruit fly (CFF), Rhagoletis cingulata Loew (Diptera: Tephritidae: Trypetini), is endemic to eastern North America and Mexico, where its primary native host is black cherry [Prunus serotina Ehrh. (Rosaceae)]. Cherry fruit fly is also a major economic pest of the fruit of cultivated sweet (Prunus avium L.) and tart (Prunus cerasus L.) cherries. Adult CFF that attack wild black cherry and introduced, domesticated cherries in commercial and abandoned orchards are active at different times of the summer, potentially generating allochronic isolation that could genetically differentiate native from sweet and tart CFF populations. Here, we test for host‐related genetic differences among CFF populations in Michigan attacking cherries in managed, unmanaged, and native habitats by scoring flies for 10 microsatellite loci. Little evidence for genetic differentiation was found across the three habitats or between the northern and southern Michigan CFF populations surveyed in the study. Local gene flow between native black cherry, commercial, and abandoned orchards may therefore be sufficient to overcome seasonal differences in adult CFF activity and prevent differentiation for microsatellites not directly associated with (tightly linked to) genes affecting eclosion time. The results do not support the existence of host‐associated races in CFF and imply that flies attacking native, managed, and unmanaged cherries should be considered to represent a single population for pest management purposes.     

An analysis of genetic differentiation among assortatively mating Drosophila melanogaster in Zimbabwe

African Drosophila melanogaster populations, and those from Zimbabwe in particular, have attracted much interest recently. African flies differ genetically from 'cosmopolitan' populations and were found to exhibit discriminative mating behaviour against individuals from 'cosmopolitan' populations. It has therefore been proposed that Zimbabwean and some other African populations are in an 'incipient stage of speciation'. However, whether the mating behaviour is an effective barrier against gene flow from other populations, and whether intra-population genetic differentiation has already evolved in sympatry is not known. Here, we took a population-based approach to test whether the well-characterized mating type differences have resulted in a genome-wide differentiation at the population level. Using 122 polymorphic microsatellite loci mapping to the third chromosome, we demonstrate a significant genetic differentiation between Zimbabwean flies differing in their mating behaviour. We also provide evidence to suggest that this difference is unlikely to be attributable to population structure within Zimbabwe. However, the analysis of individual microsatellite loci did not indicate more loci differentiating these two groups than expected by chance. Our data suggest that the 'Z'-'M' mating behaviour is strong enough to result in a small but significant genetic differentiation. Thus, future studies based on a larger population sample of flies characterized for their mating behaviour and using more markers are expected to provide more information on the genetic basis of the mating traits in the Zimbabwe flies.     

Cryptic diversity and gene flow among three African agricultural pests: Ceratitis rosa,Ceratitis fasciventris and Ceratitis anonae (Diptera,Tephritidae)

The ‘Ceratitis FAR complex’ is a species complex of African fruit flies (Diptera, Tephritidae) including the major agricultural pest Ceratitis rosa and the morphologically similar Ceratitis fasciventris and Ceratitis anonae. To resolve their intra‐ and interspecific genetic relationships and to estimate gene flow within this complex, we surveyed allelic variation at 16 microsatellite loci in 27 African populations of the three morphospecies. Interpopulation genetic distances and individual Bayesian assignments distinguished five genotypic clusters: two involving C. rosa (R1, R2; that may occur in sympatry), two involving C. fasciventris (F1, F2; with parapatric distributions) and one involving C. anonae (A). Intra‐ and interspecific patterns of genetic differentiation were not hierarchically structured and genetic differentiation between conspecific clusters (F1–F2 and R1–R2) was higher or comparable with differentiation between heterospecific clusters (e.g. F1‐A or R2‐A). In some cases, gene flow estimates among morphospecies or among heterospecific genotypic clusters were significantly different from zero, showing the lack of reproductive isolation. Genetic differentiation between genotypic clusters was partly supported by morphological differences observed a posteriori in male secondary sexual characters. These results suggest important revisions to current models of ecological niche requirements and invasion risk of the major agricultural pest C. rosa and provide a basis for a taxonomic re‐interpretation of the FAR complex.     

Sexual differentiation of the zebra finch song system: Positive evidence,negative evidence,null hypotheses,and a paradigm shift

Permanent sex differences in the brain are found in many vertebrates, and are thought to be induced by sex differences in secretion of gonadal steroid hormones during critical periods of early development. This theory has received support primarily from many experiments conducted on mammals, but also from studies on other vertebrate classes, including birds. The only avian neural dimorphism that has allowed extensive tests of this hypothesis is the neural circuit for song in passerine birds, which is much larger in males than in females. Experiments in zebra finches have yielded contradictory results. Although it is relatively easy to induce masculine patterns of development in genetic females with estrogen, it has not been possible to induce feminine patterns of development in males with any treatments, including antiestrogens and inhibitors of estrogen synthesis. Moreover, genetic females that develop with large amounts of functional testicular tissue but with virtually no ovarian tissue nevertheless have a feminine song circuit. The latter studies fail to support the idea of steroid induction of sexual differentiation. An alternative to the steroidal control hypothesis is that nonhormonal gene products expressed in the brain early in development trigger sexually dimorphic patterns of development. Although current evidence in several neural and nonneural systems indicates that sexual differentiation of some somatic phenotypes cannot be explained by the actions of gonadal steroids, the idea of direct genetic (nonhormonal) induction of sexual differentiation has yet to be proved. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 572–584, 1997     

Sex and differentiation: population genetic divergence and sexual dimorphism in Mexican goodeid fish

Genetic differentiation arises due to the interaction between natural and sexual selection, migration and genetic drift. A potential role of sexual selection in speciation has received much interest, although comparative studies are inconsistent in finding supporting evidence. A poorly tested prediction is that species subject to a higher intensity of sexual selection should show greater genetic differentiation amongst populations because females from these populations should be more choosy in mate choice. The Goodeinae is a group of endemic Mexican fishes in which female choice has driven some species to be morphologically sexually dimorphic, whereas others are relatively monomorphic. Here, we measured population divergence, using microsatellite loci, within four goodeid species which show contrasting levels of sexual dimorphism. We found higher levels of differentiation between populations of the more dimorphic species, implying less gene flow between populations. We also found evidence of higher levels of genetic differences between the sexes within populations of the dimorphic species, consistent with greater dispersal in males. Adjusted for geographic distance, the mean F(ST) for the dimorphic species is 0.25 compared with 0.16 for the less dimorphic species. We conclude that population differentiation is accelerated in more sexually dimorphic species, and that comparative phylogeography may provide a more powerful approach to detecting processes, such as an influence of sexual selection on differentiation, than broad-scale comparative studies.     

Sexual differentiation in three unconventional mammals: spotted hyenas, elephants and tammar wallabies

The present review explores sexual differentiation in three non-conventional species: the spotted hyena, the elephant and the tammar wallaby, selected because of the natural challenges they present for contemporary understanding of sexual differentiation. According to the prevailing view of mammalian sexual differentiation, originally proposed by Alfred Jost, secretion of androgen and anti-Mullerian hormone (AMH) by the fetal testes during critical stages of development accounts for the full range of sexually dimorphic urogenital traits observed at birth. Jost's concept was subsequently expanded to encompass sexual differentiation of the brain and behavior. Although the central focus of this review involves urogenital development, we assume that the novel mechanisms described in this article have potentially significant implications for sexual differentiation of brain and behavior, a transposition with precedent in the history of this field. Contrary to the "specific" requirements of Jost's formulation, female spotted hyenas and elephants initially develop male-type external genitalia prior to gonadal differentiation. In addition, the administration of anti-androgens to pregnant female spotted hyenas does not prevent the formation of a scrotum, pseudoscrotum, penis or penile clitoris in the offspring of treated females, although it is not yet clear whether the creation of masculine genitalia involves other steroids or whether there is a genetic mechanism bypassing a hormonal mediator. Wallabies, where sexual differentiation occurs in the pouch after birth, provide the most conclusive evidence for direct genetic control of sexual dimorphism, with the scrotum developing only in males and the pouch and mammary glands only in females, before differentiation of the gonads. The development of the pouch and mammary gland in females and the scrotum in males is controlled by genes on the X chromosome. In keeping with the "expanded" version of Jost's formulation, secretion of androgens by the fetal testes provides the best current account of a broad array of sex differences in reproductive morphology and endocrinology of the spotted hyena, and androgens are essential for development of the prostate and penis of the wallaby. But the essential circulating androgen in the male wallaby is 5alpha androstanediol, locally converted in target tissues to DHT, while in the pregnant female hyena, androstenedione, secreted by the maternal ovary, is converted by the placenta to testosterone (and estradiol) and transferred to the developing fetus. Testicular testosterone certainly seems to be responsible for the behavioral phenomenon of musth in male elephants. Both spotted hyenas and elephants display matrilineal social organization, and, in both species, female genital morphology requires feminine cooperation for successful copulation. We conclude that not all aspects of sexual differentiation have been delegated to testicular hormones in these mammals. In addition, we suggest that research on urogenital development in these non-traditional species directs attention to processes that may well be operating during the sexual differentiation of morphology and behavior in more common laboratory mammals, albeit in less dramatic fashion.     

Investigation on the founder effect

The fitness of threeDrosophila melanogaster populations (A,B,C) and the crosses between them (A×B; A×C; B×C) has been studied in conditions of overlapping generations. Each line was subdivided into 4 sub-populations (1,2,3,4) differing in the phenotype of the founder flies (vestigial or wild type) and of which sub-populations 2 and 3 had the same gene composition. Fitness was estimated as the number of adult flies present in each population. The results suggest that: (1) a differentiation of fitness values is detectable between sub-populations founded from groups of flies having the same gene composition; (2) isolation does not seem to be a necessary condition for the observed differentiation. These results are discussed in connection with the evolutionary effects of random genetic drift. A working hypothesis has been advanced which would explain the observed results as dependent on complex interactions involving sexual dimorphism and interactions between genes.     

Genetics of house flies. Variability studies with North Dakota, Texas, and Florida populations.

Genetic data were used to compare the structure of native house fly populations collected in North Dakota, Texas, and Florida. Recombination studies with mutant markers on chromosomes 3 and 4 indicated a lack of inversion polymorphism among the three populations in those areas of the genetic map studied. Significant differences were observed among flies from the three regions with regard to the frequency of 1) females that produced only male progeny, and 2) male-determining 3rd chromosomes (IIIm chromosomes). However, the North Dakota and Texas flies were more similar to each other than to the Florida flies since populations from the two former areas possessed a low frequency of both male-producing females and IIIm chromosomes; in contrast, the Florida population was void of females that produced males only and a high percentage if not all Florida males appeared to possess the IIIm male-determining mechanism. Tests for recessive lethal 3rd chromosomes showed that there was no significant difference in the frequency of lethal factors recovered from the North Dakota and Texas flies; the presence of IIIm chromosomes in Florida males precluded the recovery of lethal factors from this population by the method employed. The data suggest that house fly strains to be employed in genetic control programs should 1) originate from target control areas to avoid possible behavioral differences existing among flies from different locales, 2) be initiated with as many flies as possible to provide a background for the maintenance of variability, and 3) be renewed periodically with field-collected material since the genotype may be capable of rapid reorganization in response to laboratory selection pressures.     

Genetic differences among populations in sexual dimorphism: evidence for selection on males in a dioecious plant

Genetic variation among populations in the degree of sexual dimorphism may be a consequence of selection on one or both sexes. We analysed genetic parameters from crosses involving three populations of the dioecious plant Silene latifolia, which exhibits sexual dimorphism in flower size, to determine whether population differentiation was a result of selection on one or both sexes. We took the novel approach of comparing the ratio of population differentiation of a quantitative trait (Q(ST) ) to that of neutral genetic markers (F(ST) ) for males vs. females. We attributed 72.6% of calyx width variation in males to differences among populations vs. only 6.9% in females. The Q(ST) /F(ST) ratio was 4.2 for males vs. 0.4 for females, suggesting that selection on males is responsible for differentiation among populations in calyx width and its degree of sexual dimorphism. This selection may be indirect via genetic correlations with other morphological and physiological traits.     

Sequence polymorphism of candidate behavioural genes in Drosophila melanogaster flies from 'Evolution canyon'

This study focuses on the molecular features of three candidate behavioural genes in Drosophila melanogaster from the opposite slopes of Nahal Oren Canyon, Mount Carmel, Israel. These slopes display dramatic physical and biotic contrasts. Spatial variation of microclimatic conditions leads to adaptive differentiation and partial sexual isolation of populations, as suggested by our previous studies. The chosen candidate genes presumably contributing to genetic variation in sexual behaviour of Drosophila in the Canyon were desaturase, period, and no-on-transient A. These genes are known to include polymorphic repeated sequences, insertions/deletions, or nucleotide substitutions. The idea was that their polymorphism might be one of the determinants of behavioural peculiarities of flies derived from the opposite slopes. Indeed, interslope differences in the sequence encoding the (Thr-Gly)n repeat (exon 5) of the period gene were established, suggesting evolutionary functional importance. In particular, we unraveled variation in the length and composition of this region in different NFS (north-facing slope) and SFS (south-facing slope) lines. The 'European' allele (n = 20) was a 2.6-fold more abundant on the NFS compared to the SFS. This predominance probably gives some advantages for flies inhabiting wet and less warm conditions of the NFS. We suggest that repeat length/composition may influence the functional features of flies, i.e. habitat choice, nonrandom mating, and temperature adaptation. A series of female single-mate-choice tests show that females derived from NFS distinguish between males with specific per alleles (n = 17 vs. n = 20), as well as between males originated from the opposing slopes. Females from SFS were less discriminating and did not manifest significant deviation from random mating.     

Developmental and age-specific effects of selection on divergent virgin life span on fat content and starvation resistance in Drosophila melanogaster

Investigations into the genetic basis of longevity variation have shown life span to be positively correlated with starvation resistance and negatively with female fecundity, both of which rely on lipid content. To assess the firmness of this relation, we assayed correlated responses in age-specific relative fat content (RFC) and starvation resistance in lines successfully selected for divergent virgin life span. We have previously demonstrated that genetic differentiation in female fecundity between our selection lines had disappeared during relaxation of selection. Therefore, we also expected genetic differences in lipid content and starvation resistance to have disappeared. However, RFC and starvation resistance were still significantly lower in short-lived flies than in control flies. Surprisingly, also in long-lived flies RFC and starvation resistance were mostly, but not invariably, found to be significantly lower than in control flies. These results indicate that the genetic correlation of RFC and starvation resistance with reproduction has broken down. Furthermore, the relationship between life span and starvation resistance appears to be more complex than previously anticipated. Also, we could demonstrate that differences in RFC were not brought about by differences in lipid accumulation during adult life, but were already present at eclosion. These findings suggest that pre-adult developmental pathways already impact on the rate of ageing of the adult fly.     

Genetic and morphological differentiation in Tephritis bardanae (Diptera: Tephritidae): evidence for host-race formation

The fruit fly Tephritis bardanae infests flower heads of two burdock hosts, Arctium tomentosum and A. minus. Observations suggest host-associated mating and behavioural differences at oviposition indicating host-race status. Previously, flies from each host plant were found to differ slightly in allozyme allele frequencies, but these differences could as well be explained by geographical separation of host plants. In the present study, we explicitly test whether genetic and morphological variance among T. bardanae are explained best by host-plant association or by geographical location, and if this pattern is stable over a 10-year period. Populations of A. tomentosum flies differed significantly from those of A. minus flies in (i) allozyme allele frequencies at the loci Pep-A and Pgd, (ii) mtDNA haplotype frequencies and (iii) wing size. In contrast, geographical location had no significant influence on the variance estimates. While it remains uncertain whether morphometric differentiation reflects genotypic variability or phenotypic plasticity, allozyme and mtDNA differentiation is genetically determined. This provides strong evidence for host-race formation in T. bardanae. However, the levels of differentiation are relatively low indicating that the system is in an early stage of divergence. This might be due to a lack of time (i.e. the host shift occurred recently) or due to relatively high gene flow preventing much differentiation at loci not experiencing selection.