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.     

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.     

Along the speciation continuum: Quantifying intrinsic and extrinsic isolating barriers across five million years of evolutionary divergence in California jewelflowers

Understanding the relative roles of intrinsic and extrinsic reproductive barriers, and their interplay within the geographic context of diverging taxa, remains an outstanding challenge in the study of speciation. We conducted a comparative analysis of reproductive isolation in California Jewelflowers (Streptanthus, s.l., Brassicaceae) by quantifying potential barriers to gene flow at multiple life history stages in 39 species pairs spanning five million years of evolutionary divergence. We quantified nine potential pre‐ and postzygotic barriers and explored patterns of reproductive isolation in relation to genetic distance. Intrinsic postzygotic isolation was initially weak, increased at intermediate genetic distances, and reached a threshold characterized by complete genetic incompatibility. Climatic niche differences were strong at shallow genetic distances, and species pairs with overlapping ranges showed slight but appreciable phenological isolation, highlighting the potential for ecological barriers to contribute to speciation. Geographic analyses suggest that speciation is not regionally allopatric in the California Jewelflowers, as recently diverged taxa occur in relatively close proximity and display substantial range overlap. Young pairs are characterized by incomplete intrinsic postzygotic isolation, suggesting that extrinsic barriers or fine‐scale spatial segregation are more important early in the divergence process than genetic incompatibilities.     

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.     

A biogeographic genetic approach for testing the role of reinforcement: the case of Drosophila pseudoobscura and D. persimilis

Abstract.— The role of reinforcement in speciation can be explained by two distinct models. In model I, two diverged populations hybridize and produce fertile hybrids that successfully backcross (hybridization with gene flow). In model II, two populations hybridize but succeeding backcrosses are unproductive (hybridization without gene flow). Using Drosophila persimilis and D. pseudoobscura , we have tested model I by comparing the extent of heterospecific introgression in sympatric versus allopatric populations. We show that certain expectations of this particular model of reinforcement, which is based on hybridization and gene flow between divergent populations after secondary contact, are not realized in these two species. The evidence consists of the similarity of genetic distances as well as proportions of unique/rare alleles between sympatric and allopatric heterospecific populations and a negative correlation between genetic distance and geographical distance between heterospecific populations, which suggests ecological differentiation. This approach in quantifying differential gene flow has important consequences to studies that compare sympatric and allopatric isolation using genetic distance. Following model I, one would expect a pattern of higher prezygotic isolation in sympatric species compared to allopatric species of the same genetic distance simply as a result of an underestimation of genetic distance due to introgression between sympatric populations. We suggest more parsimonious explanations such as reinforcement without genetic exchange (model II) and ecological differentiation, which require high levels of preexisting reproductive isolation between populations.     

No character displacement for reproductive isolation between Drosophila bipectinata and Drosophila malerkotliana.

To test whether character displacement for reproductive isolation between Drosophila bipectinata and Drosophila malerkotliana exists, the degree of sexual isolation was measured between their sympatric and allopatric populations. Although the isolation indices vary in different crosses, the average isolation index for sympatric populations is very close to that for allopatric populations. This shows no difference in the degree of sexual isolation between sympatric and allopatric populations of D. bipectinata and D. malerkotliana. Thus there is no evidence for the existence of character displacement for sexual isolation between these two closely related sympatric species.     

Ecology and population genetics of Sonoran Desert Drosophila

Three species of cactophilic Drosophila endemic to the Sonoran Desert of North America, D. nigrospiracula, D. pachea and D. mettleri, experience marked differences in spatial resource availability, and the first two of these display significant differences in dispersal behaviour. We employed starch gel and cellulose acetate electrophoresis for eight allozyme loci to test for a relationship between these variables and genetic differentiation among geographical populations of each species. No evidence was found for population structure in any of the three species, populations of which were separated by geographical distances of up to 475 km. Allele frequencies for two loci, Mdh-1 and Est-2, in D. nigrospiracula and D. pachea were very similar to those obtained approximately 30 years ago by other workers, indicating that the polymorphisms are remarkably stable under the stressful and variable conditions of the desert environment. High longevity, dispersal and multiple female remating are likely to contribute to the apparent high level of gene flow in all three species.     

Patterns of reproductive isolation in three angiosperm genera

Analyses among animal species have found that reproductive isolation increases monotonically with genetic distance, evolves more quickly for prezygotic than postzygotic traits, and is stronger among sympatric than allopatric species pairs. The latter pattern is consistent with expectations under the reinforcement hypothesis. To determine whether similar trends are found among plant species, patterns of reproductive isolation (postpollination prezygotic, postzygotic, and "total" isolation) in three plant genera (Glycine, Silene, Streptanthus) were examined using data from previously published artificial hybridization experiments. In Silene, all measures of reproductive isolation were positively correlated with genetic distance. In contrast, in Glycine and Streptanthus, correlations between reproductive isolation and genetic distance were weak or nonsignificant, possibly due to the influence of biologically unusual taxa, variable evolutionary forces acting in different lineages, or insufficient time to accumulate reproductive isolation. There was no evidence that postpollination prezygotic reproductive isolation evolved faster than postzygotic isolation in Glycine or Silene. We also detected no evidence for faster accumulation of postmating prezygotic isolation between sympatric than allopatric species pairs; thus we found no evidence for the operation of speciation via reinforcement. In Silene, which included six polyploid species, results suggest that changes in ploidy disrupt a simple monotonic relationship between isolation and genetic distance.     

Multilocus test for introgression between the cactophilic species Drosophila mojavensis and Drosophila arizonae

Information obtained from laboratory studies regarding the efficacy of barriers to gene flow (reproductive isolation) between species is often incomplete or misleading, so detailed genetic analyses are needed to determine whether hybridization and introgression occur in nature. Previous laboratory studies of the cactophilic species Drosophila mojavensis and Drosophila arizonae suggest that reproductive isolation is incomplete and that gene flow may occur in sympatry. We sampled 18 nuclear and one mitochondrial loci from multiple populations of D. arizonae and D. mojavensis to test for the signature of recent or historic gene flow between these two species. We located chromosomal regions that were inverted between these species and analyzed those regions independently of others. Statistical tests for introgression using all loci or only collinear loci failed to reject expectations of an isolation model. Further tests using average nucleotide differences between species and phylogenetic analyses also failed to find support for introgression between D. mojavensis and D. arizonae. Additional ecological and behavioral studies of these species in their natural habitats are required to explain why the signature of gene flow was not detected at the DNA sequence level in populations when laboratory studies suggest such gene flow should be possible.     

Resource availability and population size in cactophilic Drosophila

1. Four species of Drosophila, Drosophila nigrospiracula ( Patterson & Wheeler 1942 ) , Drosophila mettleri ( Heed 1977 ) , Drosophila pachea ( Patterson & Wheeler 1942 ) , and Drosophila mojavensis ( Patterson & Crow 1940 ) , are endemic to the Sonoran Desert of North America and breed in different species of necrotic columnar cacti. Differences in resource availability have been suggested to explain the interspecific variability in fly population biology, but resource availability for these species has not been quantitatively assessed thoroughly in either spatial or temporal terms. The resource availability was quantified quarterly at three sites for 3 years and population sizes for each Drosophila species were estimated.
2. Spatial and temporal availability of resources differed significantly among species of host cacti, with organpipe cactus ( Stenocereus thurberi ) being the least abundant and senita ( Lophocereus schottii ) the most abundant spatially.
3. Drosophila species differed significantly in population size. The largest population sizes were found for D. nigrospiracula and D. mojavensis and smallest for D. pachea . Populations of D. mettleri were intermediate to these.
4. Population size was greatest for fly species utilizing host species having the largest and longest lasting necroses.
5. Resource availability does not explain the reduction of fly populations in the summer. Necroses were most abundant when flies were absent.     

Population differentiation in plethodontid salamanders: divergence of allozymes and sexual compatibility among populations of Desmognathus imitator and D. ochrophaeus (Gaudata: Plethodontidae)

Plethodontid salamanders of the genus Desmognathus exhibit varying levels of genetic differentiation among and within both allopatric and sympatric taxa. This provides excellent opportunities to study population differentiation and speciation. Two morphologically similar species in this genus, D. imitator and D. ochrophaeus, are genetically well-differentiated from one another and occur in sympatry with no evidence of hybridization and introgression. We report that the degree of sexual isolation between these two species is very high, regardless of whether the populations under comparison are allopatric or sympatric with one another. Neither reinforcement nor reproductive character displacement are required to explain the evolution of sexual incompatibility in sympatry. Sexual behaviour apparently diverges while populations are allopatric with one another. Preliminary study indicates that D. imitator consists of populations among which there may be significant sexual isolation in the absence of detectable genetic differentiation.     

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.     

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.     

Strong assortative mating between allopatric sticklebacks as a by-product of adaptation to different environments

Speciation involves the evolution of reproductive isolation between populations. One potentially important mechanism is the evolution of pre- or postzygotic isolation between populations as a by-product of adaptation to different environments. In this paper, we tested for assortative mating between allopatric stickleback populations adapted to different ecological niches. Our experimental design controlled for interpopulation interactions and non-adaptive explanations for assortative mating. We found that prezygotic isolation was surprisingly strong: when given a choice, the majority of matings occurred between individuals from similar environments. Our results indicate that the by-product mechanism is a potent source of reproductive isolation, and likely contributed to the origin of sympatric species of sticklebacks.     

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.     

WHEN MORPHOLOGY MISLEADS: INTERPOPULATION UNIFORMITY IN SEXUAL SELECTION MASKS GENETIC DIVERGENCE IN HARLEQUIN BEETLE-RIDING PSEUDOSCORPION POPULATIONS

Differences in secondary sexual characteristics of males often provide the most conspicuous means of distinguishing between closely related species. Does this therefore imply that the absence of differentiation in exaggerated male traits between allopatric populations provides evidence of a single, genetically cohesive species? We addressed this question with a comprehensive investigation of two populations (French Guiana and Panama) of the harlequin beetle-riding pseudoscorpion, Cordylochernes scorpioides. This highly sexually dimorphic pseudoscorpion is currently described as a single species, ranging throughout the Neotropics. Our morphometric analyses detected minimal differentiation between the two populations in all nine external morphological characters measured, including sexually dimorphic traits in males. Only in traits of the spermatophore was there any appreciable level of differentiation. Behavior differentiation and prezygotic reproductive isolation were also limited: 78.3% of males successfully transferred sperm to “foreign” females, and in 63.9% of these cases, females' eggs were successfully fertilized. By contrast, extensive divergence existed in two of nine electrophoretic loci, including an essentially fixed-allele difference at the Ldh locus. Most significantly, postzygotic reproductive isolation was complete, with heteropopulation zygotes invariably aborting early in development. These results strongly suggest that the two populations are, in fact, sibling species, a conclusion supported by our recently published findings on their marked divergence in minisatellite DNA. How can such interpopulation homogeneity in male sexually dimorphic traits exist in the face of strong genetic divergence? We propose that sexual selection, oscillating between favoring small and then large males, maintains such high levels of male variability within each population that it has obscured a speciation event in which genetic divergence and postzygotic incompatibility have clearly outpaced the evolution of prezygotic reproductive isolation.     

Postmating-prezygotic isolation is not an important source of selection for reinforcement within and between species in Drosophila pseudoobscura and D. persimilis

Abstract Most work on adaptive speciation to date has focused on the role of low hybrid fitness as the force driving reinforcement (the evolution of premating isolation after secondary contact that reduces the likelihood of matings between populations). However, recent theoretical work has shown that postmating, prezygotic incompatibilities may also be important in driving premating isolation. We quantified premating, postmating-prezygotic, and early postzygotic fitness effects in crosses among three populations: Drosophila persimilis, D. pseudoobscura USA (sympatric to D. persimilis ), and D. pseudoobscura Bogotá (allopatric to D. persimilis ). Interspecific matings were more likely to fail when they involved the sympatric populations than when they involved the allopatric populations, consistent with reinforcement. We also found that failure rate in sympatric mating trials depended on whether D. persimilis females were paired with D. pseudoobscura males or the reverse. This asymmetry most likely indicates differences in discrimination against heterospecific males by females. By measuring egg laying rate, fertilization success and hatching success, we also compared components of postmating-prezygotic and early postzygotic isolation. Postmating-prezygotic fitness costs were small and not distinguishable between hetero- and conspecific crosses. Early postzygotic fitness effects due to hatching success differences were also small in between-population crosses. There was, however, a postzygotic fitness effect that may have resulted from an X-linked allele found in one of the two strains of D. pseudoobscura USA. We conclude that the postmating-prezygotic fitness costs we measured probably did not drive premating isolation in these species. Premating isolation is most likely driven in sympatric populations by previously known hybrid male sterility.     

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.     

Molecular correlates of reproductive isolation

Evolution of reproductive isolation as a byproduct of genetic divergence in isolated populations is the dominant (albeit not exclusive) mode of speciation in sexual animals. But little is known about the factors linking speciation to general divergence. Several authors have argued that allopatric speciation should proceed more rapidly if isolated populations also experience divergent selection. Reproductive isolation between allopatric populations is not subject to direct selection; it can accumulate only by random drift or as a fortuitous byproduct of selection on other traits. Here I present a novel analysis of published data, demonstrating that pre- and postmating isolation of Drosophila species are more tightly correlated with allozyme divergence than with silent DNA divergence. Inasmuch as proteins are more subject to the action of natural selection than are silent DNA polymorphisms, this result provides broad support for a model of selection-mediated allopatric speciation.     

Resistance to thermal stress in desert Drosophila

1. Four species of Drosophila are endemic to the Sonoran Desert of North America where daily and seasonal high temperatures exceed those experienced by other species in this genus. The close association between these species and their cactus hosts means that they reside only in the desert and raises the question as to whether they are better able to survive heat stress than are non-desert species of Drosophila . The tolerance of adult flies of the four desert species D. mojavensis, D. nigrospiracula, D. pachea and D. mettleri and the cosmopolitan D. simulans to acute heat stress was tested.
2. There was considerable variability among the desert endemic species with respect to survival following heat exposure. Two species, D. mojavensis and D. pachea , were more resistant at 44 °C and 46 °C than the others, with D. mettleri exhibiting similar heat stress resistance to D. simulans .
3. While there was no consistent influence of gender on heat resistance, younger flies (1-day-old) showed significantly greater survival than did older flies (7- or 14-days old).