Genetics of incipient speciation in Drosophila mojavensis. I. Male courtship song, mating success, and genotype x environment interactions

Few studies have examined genotype by environment (GxE) effects on premating reproductive isolation and associated behaviors, even though such effects may be common when speciation is driven by adaptation to different environments. In this study, mating success and courtship song differences among diverging populations of Drosophila mojavensis were investigated in a two-environment quantitative trait locus (QTL) analysis. Baja California and mainland Mexico populations of D. mojavensis feed and breed on different host cacti, so these host plants were used to culture F2 males to examine host-specific QTL effects and GxE interactions influencing mating success and courtship songs. Linear selection gradient analysis showed that mainland females mated with males that produced songs with significantly shorter L(long)-IPIs, burst durations, and interburst intervals. Twenty-one microsatellite loci distributed across all five major chromosomes were used to localize effects of mating success, time to copulation, and courtship song components. Male courtship success was influenced by a single detected QTL, the main effect of cactus, and four GxE interactions, whereas time to copulation was influenced by three different QTLs on the fourth chromosome. Multiple-locus restricted maximum likelihood (REML) analysis of courtship song revealed consistent effects linked with the same fourth chromosome markers that influenced time to copulation, a number of GxE interactions, and few possible cases of epistasis. GxE interactions for mate choice and song can maintain genetic variation in populations, but alter outcomes of sexual selection and isolation, so signal evolution and reproductive isolation may be slowed in diverging populations. Understanding the genetics of incipient speciation in D. mojavensis clearly depends on cactus-specific expression of traits associated with courtship behavior and sexual isolation.     

Premating isolation is determined by larval-rearing substrates in cactophilic Drosophila mojavensis. V. Deep geographic variation in epicuticular hydrocarbons among isolated populations

Adult epicuticular hydrocarbon variation of 14 geographically isolated populations of cactophilic Drosophila mojavensis was assessed to further investigate mechanisms of sexual isolation. Hydrocarbon transfer experiments demonstrated that these compounds are part of the mate recognition system in this species. Sixteen of the 23 epicuticular hydrocarbon components studied differed in amounts between males and females, and 13 differed in quantity between the geographic regions encompassing Baja California and mainland Mexico (Sonora and Sinaloa). Eight hydrocarbon components, seven of which differed in quantity between sexes, showed significant sex-by-region interactions, indicating region-specific sex reversals in hydrocarbon quantities. Such regional variation in epicuticular hydrocarbon profiles suggests that these hydrocarbon differences have also evolved in D. mojavensis since this species invaded mainland Sonora and Sinaloa from Baja California by switching host plants, in addition to a number of key genetic, behavioral, and life-history characters.     

Divergence and reproductive isolation in the early stages of speciation

To understand speciation we need to identify the factors causing divergence between natural populations. The traditional approach to gaining such insights has been to focus on a particular theory and ask whether observed patterns of reproductive isolation between populations or species are consistent with the hypothesis in question. However, such studies are few and they do not allow us to compare between hypotheses, so often we cannot determine the relative contribution to divergence of different potential factors. Here, I describe a study of patterns of phenotypic divergence and premating and postmating isolation between populations of the grasshopper Chorthippus parallelus. Information on the phylogeographic relationships of the populations means that a priori predictions from existing hypotheses for the evolution of reproductive isolation can be compared with observations. I assess the relative contributions to premating isolation, postmating isolation and phenotypic divergence of long periods of allopatry, adaptation to different environments and processes associated with colonisation (such as population bottlenecks). Likelihood analysis reveals that long periods of allopatry in glacial refugia are associated with postmating reproductive isolation, but not premating isolation, which is more strongly associated with colonisation. Neither premating nor postmating isolation is higher between populations differing in potential environmental selection pressures. There are only weak correlations between patterns of genetic divergence and phenotypic divergence and no correlation between premating and postmating isolation. This suggests that the potential of a taxon to exercise mate choice may affect the types of factor that promote speciation in that group. I discuss the advantages and disadvantages of the general approach of simultaneously testing competing hypotheses for the evolution of reproductive isolation.     

Population divergence in chemical signals and the potential for premating isolation between islet- and mainland populations of the Skyros wall lizard (Podarcis gaigeae)

When sexually selected traits diverge because of different local selective environments, premating isolation might arise as a correlated response. However, sexually selected traits might also diverge by stochastic forces. Here, we show that odour-based mate preferences and scent composition have diverged between islet- and mainland populations of Skyros wall lizard, Podarcis gaigeae. We quantified the degree of scent-mediated premating isolation between populations. Islet lizards preferred scent from islet lizards, whereas the mainland populations were less discriminatory. The pheromone compositions differed more between islets than between islet- and mainland populations and did not differ significantly between mainland populations. There was a tendency for population divergence in pheromones to be positively correlated with neutral genetic divergence. This might indicate a role for genetic drift in evolutionary change in these signals and partial decoupling between signals and preferences. Our results suggest that chemical signals and associated mate preferences can diverge through stochastic and selective forces and influence premating isolation.     

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. VIII. Mating success mediated by epicuticular hydrocarbons within and between isolated populations

We tested the hypothesis that intrademic sexual selection has caused sexual isolation between populations of geographically isolated populations of cactophilic Drosophila mojavensis, and was mediated by epicuticular hydrocarbons (EHCs), contact pheromones in this system. Sexual selection and sexual isolation were estimated using a Baja California and mainland population by comparing the number of mated and unmated males and females in each of four pairwise population mating trials. EHC profiles were significantly different in mated and unmated males in the interdemic (Bajafemale symbol x Mainlandmale symbol and Mainlandfemale symbol x Bajamale symbol), but not the intrademic mating trials. A small number of EHCs was identified that best discriminated among mated and unmated males, mostly alkadienes with 34 and 37 carbons. Females showed population-specific preferences for male EHC profiles. However, EHC profiles between mated and unmated males in the intrademic mating trials were not significantly different, consistent with undetectable sexual selection estimated directly from numbers of copulating pairs vs. unmated adults. Thus, sexual isolation among populations was much stronger than sexual selection within these populations of D. mojavensis.     

Sympatry, allopatry and sexual isolation between Drosophila mojavensis and D. arizonae

Populations of the North American cactophilic fruitfly Drosophila mojavensis and its sibling species D. arizonae exist both in sympatry and in allopatry. Females of D. arizonae, regardless of their population of origin, are effectively completely isolated behaviorally from D. mojavensis males. On the other hand, females of D. mojavensis from the sympatric populations in Sonora, Mexico exhibit significantly stronger premating isolation from D. arizonae males than do D. mojavensis females from allopatric populations from the Baja California peninsula. Earlier studies interpreted these limited observations as support for reinforcement. Since the time of those studies, additional allopatric populations of D. mojavensis have been collected from southern California and from Santa Catalina Island, off the coast of southern California. Here, we tested the prediction that if sympatry is in fact associated with increased isolation in D. mojavensis, these additional allopatric populations also should show, relative to the sympatric ones, less isolation from D. arizonae. Our results are consistent with this prediction and suggest that isolation is in fact stronger in sympatry.     

GENETICS OF INCIPIENT SPECIATION IN DROSOPHILA MOJAVENSIS. III. LIFE‐HISTORY DIVERGENCE IN ALLOPATRY AND REPRODUCTIVE ISOLATION

We carried out a three‐tiered genetic analysis of egg‐to‐adult development time and viability in ancestral and derived populations of cactophilic Drosophila mojavensis to test the hypothesis that evolution of these life‐history characters has shaped premating reproductive isolation in this species. First, a common garden experiment with 11 populations from Baja California and mainland Mexico and Arizona reared on two host species revealed significant host plant X region and population interactions for viability and development time, evidence for host plant adaptation. Second, replicated line crosses with flies reared on both hosts revealed autosomal, X chromosome, cytoplasmic, and autosome X cactus influences on development time. Viability differences were influenced by host plants, autosomal dominance, and X chromosomal effects. Many of the F₁, F₂, and backcross generations showed evidence of heterosis for viability. Third, a QTL analysis of male courtship song and epicuticular hydrocarbon variation based on 1688 Baja × mainland F₂ males also revealed eight QTL influencing development time differences. Mainland alleles at six of these loci were associated with longer development times, consistent with population‐level differences. Eight G × E interactions were also detected caused by longer development times of mainland alleles expressed on a mainland host with smaller differences among Baja genotypes reared on the Baja host plant. Four QTL influenced both development time and epicuticular hydrocarbon differences associated with courtship success, and there was a significant QTL‐based correlation between development time and cuticular hydrocarbon variation. Thus, the regional shifts in life histories that evolved once D. mojavensis invaded mainland Mexico from Baja California by shifting host plants were genetically correlated with variation in cuticular hydrocarbon‐based mate preferences.     

Population genetics and geographic variation of alcohol dehydrogenase (Adh) paralogs and glucose-6-phosphate dehydrogenase (G6pd) in Drosophila mojavensis

Populations of Drosophila mojavensis from the deserts of the Baja California peninsula and mainland Mexico utilize different cactus hosts with different alcohol contents. The enzyme alcohol dehydrogenase (ADH) has been proposed to play an important role in the adaptation of Drosophila species to their environment. This study investigates the role of ADH in the adaptation of the cactophilic D. mojavensis to its cactus host. In D. mojavensis and its sibling species, D. arizonae, the Adh gene has duplicated, giving rise to a larval/ovarian form (Adh-1) and an adult form (Adh-2). Studies of sequence variation presented here indicate that the Adh paralogs have followed different evolutionary trajectories. Adh-1 exhibits an excess of fixed amino acid replacements, suggesting adaptive evolution, which could have been a result of several host shifts that occurred during the divergence of D. mojavensis. A 17-bp intron haplotype polymorphism segregates in Adh-2 and has markedly different frequencies in the Baja and mainland populations. The presence of the intron polymorphism suggests possible selection for the maintenance of pre-mRNA structure. Finally, this study supports the proposed Baja California origination of D. mojavensis and subsequent colonization of the mainland accompanied by a host shift.     

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. IX. Host plant and population specific epicuticular hydrocarbon expression influences mate choice and sexual selection

Sexual signals in cactophilic Drosophila mojavensis include cuticular hydrocarbons (CHCs), contact pheromones that mediate female discrimination of males during courtship. CHCs, along with male courtship songs, cause premating isolation between diverged populations, and are influenced by genotype × environment interactions caused by different host cacti. CHC profiles of mated and unmated adult flies from a Baja California and a mainland Mexico population of D. mojavensis reared on two host cacti were assayed to test the hypothesis that male CHCs mediate within‐population female discrimination of males. In multiple choice courtship trials, mated and unmated males differed in CHC profiles, indicating that females prefer males with particular blends of CHCs. Mated and unmated females significantly differed in CHC profiles as well. Adults in the choice trials had CHC profiles that were significantly different from those in pair‐mated adults from no‐choice trials revealing an influence of sexual selection. Females preferred different male CHC blends in each population, but the influence of host cactus on CHC variation was significant only in the mainland population indicating population‐specific plasticity in CHCs. Different groups of CHCs mediated female choice‐based sexual selection in each population suggesting that geographical and ecological divergence has the potential to promote divergence in mate communication systems.     

Reproductive isolation driven by the combined effects of ecological adaptation and reinforcement

Recent years have seen a resurgence of interest in the process of speciation but few studies have elucidated the mechanisms either driving or constraining the evolution of reproductive isolation. In theory, the direct effects of reinforcing selection for increased mating discrimination where interbreeding produces hybrid offspring with low fitness and the indirect effects of adaptation to different environments can both promote speciation. Conversely, high levels of homogenizing gene flow can counteract the forces of selection. We demonstrate the opposing effects of reinforcing selection and gene flow in Timema cristinae walking-stick insects. The magnitude of female mating discrimination against males from other populations is greatest when migration rates between populations adapted to alternate host plants are high enough to allow the evolution of reinforcement, but low enough to prevent gene flow from eroding adaptive divergence in mate choice. Moreover, reproductive isolation is strongest under the combined effects of reinforcement and adaptation to alternate host plants. Our findings demonstrate the joint effects of reinforcement, ecological adaptation and gene flow on progress towards speciation in the wild.     

SPECIES HYBRIDS IN THE LABORATORY BUT NOT IN NATURE: A REANALYSIS OF PREMATING ISOLATION BETWEEN DROSOPHILA ARIZONAE AND D. MOJAVENSIS

Understanding speciation relies critically on the identification of mechanisms responsible for maintaining species integrity (i.e., reproductive isolation) especially when closely related species are sympatric in nature. Studies of reproductive isolation in Drosophila often involve laboratory mating experiments that assume that patterns of mate choice in the laboratory are similar to those in the wild. Two sibling species, Drosophila arizonae and D. mojavensis , known to exhibit low levels of interspecific hybridization in the laboratory, but not in nature, were used in multiple-choice mating trials using various mating chamber designs as well as synthetic and natural media for developing larvae and courting adults. Sympatric populations were more sexually isolated than allopatric ones, consistent with past studies, and all experimental variables tested (chamber size, host plant presence and rearing substrates) had significant effects on levels of premating isolation between these species. Flies reared on cactus showed increased premating isolation versus those reared on synthetic laboratory food as did providing fermenting host plant tissue during mating trials. Also, surprisingly, smaller mating chambers led to an increase in premating isolation versus larger containers. The design of these types of mating trials is thus critical to understanding how mating behaviors in the laboratory are related to those in natural populations.     

Mate preference across the speciation continuum in a clade of mimetic butterflies

Premating behavioral isolation is increasingly recognized as an important part of ecological speciation, where divergent natural selection causes the evolution of reproductive barriers. A number of studies have now demonstrated that traits under divergent natural selection also affect mate preferences. However, studies of single species pairs only capture a snapshot of the speciation process, making it difficult to assess the role of mate preferences throughout the entire process. Heliconius butterflies are well known for their brightly colored mimetic warning patterns, and previous studies have shown that these patterns are also used as mate recognition cues. Here, we present mate preference data for four pairs of sister taxa, representing different stages of divergence, which together allow us to compare diverging mate preferences across the continuum of Heliconius speciation. Using a novel Bayesian approach, our results support a model of ecological speciation in which strong premating isolation arises early, but continues to increase throughout the continuum from polymorphic populations through to "good," sympatric ecologically divergent species.     

Evolution of premating reproductive isolation among conspecific populations of the sea rock-pool beetle Ochthebius urbanelliae driven by reinforcing natural selection

How natural selection might be involved in speciation remains a fundamental question in evolutionary biology. When two or more species co-occur in the same areas, natural selection may favor divergence in mating traits. By acting in sympatric but not allopatric populations, natural selection can also affect mate choice within species and ultimately initiate speciation among conspecific populations. Here, we address this potential effect in the sea rock-pool beetles Ochthebius quadricollis and O. urbanelliae. The two species, which inhabit the Mediterranean coasts, co-occurr syntopically in an area along the Italian Tyrrhenian coast and completed reproductive isolation by reinforcement. In this article, through mating trials under laboratory conditions between conspecific populations, we found in O. quadricollis no deviations from random mating. Conversely, in O. urbanelliae, we found a clear pattern of premating isolation between the reinforced populations sympatric with O. quadricollis and those nonreinforced allopatric. This pattern is consistent with the view that natural selection, which completed the reproductive isolation between the two species in sympatry, led incidentally also to partial premating reproductive isolation (I(PSI) estimator from 0.683 to 0.792) between conspecific populations of O. urbanelliae. This case study supports an until recently underappreciated role of natural selection resulting from species interactions in initiating speciation.     

Divergent environments and population bottlenecks fail to generate premating isolation in Drosophila pseudoobscura

While the feasibility of bottleneck-induced speciation is in doubt, population bottlenecks may still affect the speciation process by interacting with divergent selection. To explore this possibility, I conducted a laboratory speciation experiment using Drosophila pseudoobscura involving 78 replicate populations assigned in a two-way factorial design to both bottleneck (present vs. absent) and environment (ancestral vs. novel) treatments. Populations independently evolved under these treatments and were then tested for assortative mating and male mating success against their common ancestor. Bottlenecks alone did not generate any premating isolation, despite an experimental design that was conducive to bottleneck-induced speciation. Premating isolation also did not evolve in the novel environment treatment, neither in the presence nor absence of bottlenecks. However, male mating success was significantly reduced in the novel environment treatment, both as a plastic response to this environment and as a result of environment-dependent inbreeding effects in the bottlenecked populations. Reduced mating success of derived males will hamper speciation by enhancing the mating success of immigrant, ancestral males. Novel environments are generally thought to promote ecological speciation by generating divergent natural selection. In the current experiment, however, the novel environment did not cause the evolution of any premating isolation and it reduced the likelihood of speciation through its effects on male mating success.     

Microsatellite variation among diverging populations of Drosophila mojavensis

Divergence and speciation may occur by various means, depending on the particular history, selective environments, and genetic composition of populations. In Drosophila mojavensis, a good model of incipient speciation, understanding the population genetic structure within this group facilitates our ability to understand the context in which reproductive isolation among populations is developing. Here we report the genetic structure and relationships of D. mojavensis populations at nuclear loci. We surveyed 29 populations throughout the distribution of D. mojavensis for four microsatellite loci to differentiation among populations of this species. These loci reveal four distinct geographical regions of D. mojavensis populations in the south-western United States and north-western Mexico--(i) Baja California peninsula (Baja), (ii) Sonora, Mexico-southern Arizona, United States (Sonora), (iii) Mojave Desert and Grand Canyon (Mojave), and (iv) Santa Catalina Island (Catalina). While all regions show strong isolation, Mojave and Catalina are highly diverged from other regions. Within any region, populations are largely homogenous over broad geographical distances. Based on the population structure, we find clear geographical barriers to gene flow appear to have a strong effect in isolating populations across regions for this species.     

Evolutionary relationships of Drosophila mojavensis geographic host races and their sister species Drosophila arizonae

The cactophilic Drosophila mojavensis species group living in the deserts and dry tropical forests of the southwestern United States and Mexico provides a valuable system for studies in diversification and speciation. Rigorous studies of the relationships between host races of D. mojavensis and the relationships among the members of the species group (D. mojavensis, Drosophila arizona, and Drosophila navojoa) are lacking. We used mitochondrial CO1 sequence data to address the phylogenetics and population genetics of this species group. In this study we have found that the sister species D. mojavensis and D. arizonae share no mitochondrial haplotypes and thus show no evidence for recent introgression. We estimate the divergence time between D. mojavensis and D. arizonae to be between 1.91 and 2.97 million years ago. D. arizonae shows little structure in our population genetic analyses but there is phylogenetic differentiation between southeastern and northern populations of D. arizonae. Drosophila mojavensis shows significant population and phylogenetic structure across the four geographic regions of its distribution. The mitochondrial data support an origin of D. mojavensis on the mainland with early differentiation into the populations now found in the Mojave Desert and the Mainland Sonoran Desert and later colonization of the Baja Peninsula, in contrast to previous models. Also, the sister clade to D. mojavensis/D. arizonae includes D. navojoa and Drosophila huaylasi. By defining the genetic relationships among these populations, we provide a foundation for more sophisticated hypothesis testing regarding the timing of early speciation events and host switches in this species group.     

Pleiotropic effect, clock genes, and reproductive isolation

 The mechanism by which a clock gene pleiotropically controlling life history and behavioral traits causes reproductive isolation is explained using a model species, the melon fly, Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae). Melon flies mate once a day, at dusk. The population selected for life history traits exhibits correlated responses in the time of mating during the day. For example, the fly populations selected for faster (slower) development have an earlier (later) time of mating. A circadian rhythm controls the time of mating. The circadian periods in constant darkness were about 22 h in lines selected for a short developmental period and about 31 h in lines selected for a long developmental period. The data on crosses between the selected lines indicated that the developmental period is controlled by a polygene, whereas the circadian period may be controlled by a single clock gene. These results suggest a clock gene pleiotropically controls developmental and circadian periods in the melon fly. Reproductive isolation may often evolve as an indirect (pleiotropic) consequence of adaptation to different environments or habitats. For example, niches that are temporally or seasonally offset can select organisms with different developmental characteristics. These developmental differences can inadvertently cause reproductive isolation by a variety of means including shifts in mating activity patterns. The difference in time of mating between populations selected for developmental period translated into significant prezygotic isolation, as measured by mate choice tests. If the mating time between populations differed more than 1 h, the isolation index was significantly higher than zero. These findings indicate that premating isolation can be established by a pleiotropic effect of a clock gene. There are many examples in which the difference in timing of reproduction prevents gene flow between populations, such as the egg spawning time in marine organisms, the flowering time in angiosperms, and the time of mating in insects. In such organisms, if genetic correlations between circadian rhythm and reproductive traits exist, multifarious divergent selection for life history traits would often accelerate the evolution of reproductive isolation through clock genes. Natural populations may diverge in reproduction time through drift, direct natural selection for time of reproduction, or as a by-product effect of genetic correlations. In any case, clock genes are keys in reproductive isolation. Received: January 31, 2002 / Accepted: July 29, 2002 Acknowledgments I am grateful to Tetsuo Arai, Akira Matsumoto, Takashi Matsuyama, Toru Shimizu, Aya Takahashi, Teiichi Tanimura, Tetsuya Toyosato, and Yasuhiko Watari for useful discussion, and to the responsible editor and two anonymous reviewers for helpful suggestions. I also thank Yoshihiko Chiba, Norio Ishida, Emi Koyama, Kazuhiko Sakai, and Takaomi Sakai for useful information. My work on speciation has been supported by a Grant-in-Aid for Scientific Research (KAKENHI 14340244) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.     

Pre-mating isolation is determined by larval rearing substrates in cactophilicDrosophila mojavensis. II. Effects of larval substrates on time to copulation,mate choice and mating propensity

Summary It has been hypothesized that reproductive character displacement has evolved in mainland Sonora, Mexico populations of cactophilicD. mojavensis due to the presence of a sympatric sibling speciesD. arizonae. In laboratory tests using ancestral Baja California populations and derived, sympatric mainland populations, asymmetrical sexual isolation has been observed among populations ofD. mojavensis where mainland females discriminate against Baja males. Effects of different pre-adult rearing environments on adult mating behaviour were assessed by comparing fermenting cactus tissues like those used in nature for breeding with laboratory media because previous studies have employed synthetic growth media for fly growth and development. Significant behavioural isolation was evident in all cases when larvae were reared on laboratory food, but was non-significant when flies were reared on fermenting cactus, except for the cactus used by most mainland populations, consistent with previous studies. Time to copulation of Baja females was greater than mainland females over all substrates, but male time to copulation did not differ between populations. Time to copulation for both sexes was significantly greater when flies were reared on laboratory food with one exception. The degree of behavioural isolation was weakly correlated with time to copulation across food types (Spearman rank correlation = 0.58,p = 0.099). Therefore, use of laboratory media in this and previous studies exaggerated adult pre-mating isolation and time to copulation in comparison to natural breeding substrates. These experiments suggest that a change in host substrates by saprophagous insects (where chemical differences exist between hosts) may have subtle effects on mating behaviour in a manner which promotes low levels of sexual isolation as a by-product of their utilization of a particular substrate during larval development. ForD. mojavensis, these results suggest that over evolutionary time, radiation into a new environment (from Baja to the mainland) allowed utilization of new host plants that may have incidentally promoted the sexual isolation patterns that have been observed within this species.See Etges (1992) for the first paper in this series.     

ADAPTATION TO DESICCATION FAILS TO GENERATE PRE‐ AND POSTMATING ISOLATION IN REPLICATE DROSOPHILA MELANOGASTER LABORATORY POPULATIONS

Many laboratory speciation experiments have raised allopatric populations in different environments to determine whether reproductive isolation evolves as a byproduct of adaptation (a form of ecological speciation). Few, however, have addressed the evolution of both pre‐ and postmating isolation or investigated the conditions affecting the process. We present results of an evolution experiment in which 12 lines of Drosophila melanogaster were derived from a common population and then independently evolved for more than 57 generations under alternative selection treatments. Six “desiccation” populations had access to food and water removed during a period of their adult lives generating strong desiccation selection, and six “starvation” populations had access to food but not water removed for the same period, generating a mild starvation stress. Substantial divergence of cuticular hydrocarbons occurred between the desiccation and starvation populations, key traits that have been implicated in sexual isolation in Drosophila. Despite this divergence, there was no detectable premating isolation between desiccation and starvation populations and postmating isolation was asymmetrical: the fitness of F1 hybrids was reduced in the desiccation but not the starvation environment. This asymmetry was likely caused by the absence of divergent selection: adaptation to desiccation appears to have come at no cost to performance in the starvation environment. Novel environments are thought to promote the evolution of reproductive isolation. Understanding the conditions that favor or hamper this remains a key challenge for speciation research.     

Model-based comparisons of phylogeographic scenarios resolve the intraspecific divergence of cactophilic Drosophila mojavensis

The cactophilic fly Drosophila mojavensis exhibits considerable intraspecific genetic structure across allopatric geographic regions and shows associations with different host cactus species across its range. The divergence between these populations has been studied for more than 60years, yet their exact historical relationships have not been resolved. We analysed sequence data from 15 intronic X-linked loci across populations from Baja California, mainland Sonora-Arizona and Mojave Desert regions under an isolation-with-migration model to assess multiple scenarios of divergence. We also compared the results with a pre-existing sequence data set of eight autosomal loci. We derived a population tree with Baja California placed at its base and link their isolation to Pleistocene climatic oscillations. Our estimates suggest the Baja California population diverged from an ancestral Mojave Desert/mainland Sonora-Arizona group around 230,000-270,000years ago, while the split between the Mojave Desert and mainland Sonora-Arizona populations occurred one glacial cycle later, 117,000-135,000years ago. Although we found these three populations to be effectively allopatric, model ranking could not rule out the possibility of a low level of gene flow between two of them. Finally, the Mojave Desert population showed a small effective population size, consistent with a historical population bottleneck. We show that model-based inference from multiple loci can provide accurate information on the historical relationships of closely related groups allowing us to set into historical context a classic system of incipient ecological speciation.