Role In Reproductive Isolation Research Articles

Genetic System and Evolutionary Trends in <I>Solanum nigrum</I> Complex II: Cytomorphological Studies of the Hybrids between Hexaploid Indian <I>Solanum nigrum</I> Linn. and <I>Solanum nodiflorum</I> Jacq.

Cytomorphological characteristics of Indian hexaploid S. nigrum and S. nodiflorum and of their F1hybrids were studied. The parents differed significantly in several morphological and cytological characters. The hybrids were highly sterile and did not set seed.They showed a wide range of meiotic irregularities. In several pollen mother cells, trivalents and quadrivalents were recorded, although in a very low frequency, in addition to a large number of univalents and bivalents. Occasionally bridges with or without fragments, were recorded. The results of these investigations show that chromosomal differences as well as genic differences have played an important role in reproductive isolation and morphological differentiation of the two species.

Hybridization and gene expression: Beyond differentially expressed genes.

Gene expression has a key role in reproductive isolation, and studies of hybrid gene expression have identified mechanisms causing hybrid sterility. Here, we review the evidence for altered gene expression following hybridization and outline the mechanisms shown to contribute to altered gene expression in hybrids. Transgressive gene expression, transcending that of both parental species, is pervasive in early generation sterile hybrids, but also frequently observed in viable, fertile hybrids. We highlight studies showing that hybridization can result in transgressive gene expression, also in established hybrid lineages or species. Such extreme patterns of gene expression in stabilized hybrid taxa suggest that altered hybrid gene expression may result in hybridization-derived evolutionary novelty. We also conclude that while patterns of misexpression in hybrids are well documented, the understanding of the mechanisms causing misexpression is lagging. We argue that jointly assessing differences in cell composition and cell-specific changes in gene expression in hybrids, in addition to assessing changes in chromatin and methylation, will significantly advance our understanding of the basis of altered gene expression. Moreover, uncovering to what extent evolution of gene expression results in altered expression for individual genes, or entire networks of genes, will advance our understanding of how selection moulds gene expression. Finally, we argue that jointly studying the dual roles of altered hybrid gene expression, serving both as a mechanism for reproductive isolation and as a substrate for hybrid ecological adaptation, will lead to significant advances in our understanding of the evolution of gene expression.

Flower orientation and corolla length as reproductive barriers in the pollinator-driven divergence of Erica shannonea and Erica ampullacea.

A variety of reproductive barriers can enable reproductive isolation and stable coexistence of plant species. Differing floral traits might play an important role in reproductive isolation imposed by pollinators. Such shifts in pollinator use have been hypothesized to contribute to the radiation of Erica (Ericaceae) in the Cape Floristic Region, South Africa. The sister species Erica shannonea and Erica ampullacea co-occur and overlap in flowering phenology. Both have unscented long-tubed flowers consistent with adaptations for pollination by long-proboscid flies (LPFs), but differences in flower orientation and corolla tube length are indicative of a shift in pollinator species. We conducted controlled pollination experiments and pollinator observations to determine the breeding system and pollinators of the two species. Both species are self-incompatible and require pollinator visits for seed production, suggesting that pollinators could strongly influence flower evolution. The horizontally orientated flowers of E. shannonea were found to be pollinated by Philoliche rostrata (Tabanidae), which has a long, fixed forward-pointing proboscis, while the vertically upright orientated flowers of E. ampullacea were found to be pollinated by Prosoeca westermanni (Nemestrinidae), which has a shorter proboscis that can swivel downwards. The nemestrinid fly's proboscis is too short to access the nectar in the relative long-tubed flowers of E. shannonea and the tabanid fly's proboscis cannot swivel down to access the upright flowers of E. ampullacea. Consequently, these traits are likely to act as reproductive barriers between the two Erica species and thereby might have contributed to speciation and enable stable coexistence.

The reproductive isolation mechanism of allopatric sister species Iris dichotoma and I. domestica (Iridaceae) under the sympatric condition

In allopatric species, the geographical isolation is generally disregarded as a legitimate cause of isolation. Beside geographical isolation, the other prezygotic isolating mechanisms in allopatric species have not been well proved to cause isolation. In order to explore if two allopatric sister species Iris dichotoma and I. domestica (Iridaceae) have other prezygotic isolating mechanisms, the natural hybrids, the positions of pollen deposited on the honeybee's body, the insect visiting period and frequency, and the daily dynamic changes of pollen quantity were detected; the daily flowering times were compared; the interspecific crossing and the pollen transfer were tested; the flower-visiting insects were categorized and identified. No natural hybrids of the two species were detected under the sympatric condition. Their daily flowering times were significantly different but had 2.6 h overlap, during which they had good cross compatibility. They had common legitimate pollinators, but the visiting time did not overlap and I. domestica’s pollen quantity was down to 0 before I. dichotoma open. The same honeybee could visit both of them as I. domestica had pollen in the net house, but their pollens were deposited on different parts of the honeybee’s body. The temporal isolation caused complete reproductive isolation, and the difference in flower structure also played an important role in reproductive isolation.

Seasonal stability and species specificity of environmentally acquired chemical mating signals in orchid bees.

Traits that mediate reproductive isolation between species, such as those involved in mate choice and/or recognition, are predicted to experience stabilizing selection towards the species mean. Male orchid bees collect chemical compounds from many sources, such as plants and fungi, which they use as a perfume signal (pheromone) during courtship display, and are suggested to contribute to reproductive isolation between species. Environmentally acquired signals are more prone to variation as source availability can vary through space and time. If orchid bee perfumes are important for reproductive isolation between species, we expect them to exhibit stable species-specific differences in time and space. Here, we describe phenotypic patterns of inter- and intraspecific variation in the male perfumes of three sympatric species of Euglossa orchid bees across an entire year, investigating both their seasonality and species specificity. Our analysis revealed considerable within-species variation in perfumes. However, species specificity was maintained consistently throughout the year, supporting the idea that these perfumes could play an important role in reproductive isolation and are experiencing stabilizing selection towards a species mean. Our analysis also identified strong correlations in the abundance of some compounds, possibly due to shared collection sources between species. Our study suggests that orchid bee perfumes are robust in the face of environmental changes in resource availability and thus can maintain reproductive isolation between species.

A Brief Review of Meiotic Chromosomes in Early Spermatogenesis and Oogenesis and Mitotic Chromosomes in the Viviparous Lizard Zootoca vivipara (Squamata: Lacertidae) with Multiple Sex Chromosomes

This brief review is focused on the viviparous lizard Zootoca vivipara (Lichtenstein, 1823), of the family Lacertidae, which possesses female heterogamety and multiple sex chromosomes (male 2n = 36, Z1Z1Z2Z2/Z1Z2W, female 2n = 35, with variable W sex chromosome). Multiple sex chromosomes and their changes may influence meiosis and the female meiotic drive, and they may play a role in reproductive isolation. In two cryptic taxa of Z. vivipara with different W sex chromosomes, meiosis during early spermatogenesis and oogenesis proceeds normally, without any disturbances, with the formation of haploid spermatocytes, and in female meiosis with the formation of synaptonemal complexes (SCs) and the lampbrush chromosomes. In females, the SC number was constantly equal to 19 (according to the SC length, 16 SC autosomal bivalents plus three presumed SC sex chromosome elements). No variability in the chromosomes at the early stages of meiotic prophase I, and no significant disturbances in the chromosome segregation at the anaphase-telophase I stage, have been discovered, and haploid oocytes (n = 17) at the metaphase II stage have been revealed. There should be a factor/factors that maintain the multiple sex chromosomes, their equal transmission, and the course of meiosis in these cryptic forms of Z. vivipara.

Between‐partner concordance of vertically transmitted gut microbiota diminishes reproductive output in the dung beetle Onthophagus taurus

AbstractIn this study, we explored the potential contribution of the gut microbiome to reproductive isolation in tunnelling dung beetles, using Onthophagus taurus (Schreber, 1759) and its sister species O. illyricus (Scopoli, 1763) as a model system (Coleoptera: Scarabaeidae: Scarabaeinae: Onthophagini). Gut microbiota play critical roles in normative development of these beetles, and are vertically inherited via a maternally derived faecal pellet called the pedestal. We first compared the developmental outcomes of individuals reared with pedestals derived from either the same or the sister species (Self and Cross inoculation treatments, respectively). We then crossed the resulting adult O. taurus in three combinations (Self female X Self male; Self female X Cross male; Cross female X Self male). We predicted that if the vertically transmitted gut microbiome plays a role in reproductive isolation by facilitating species recognition, the Self X Self line would have improved reproductive outcomes compared to the lines in which partners had mismatched gut microbiomes. Instead, we found that between‐partner concordance of maternally transmitted gut microbiota resulted in fewer offspring, and that this reduction was due to partial pre‐copulatory isolation as evidenced by reduced sperm transfer in the Self X Self line. This pattern is consistent either with microbiome‐mediated familiarity/kin recognition, or with absence of mate choice in crosses with mismatched microbiomes. We discuss our results in the light of recent research on the influence of extracellular microbial symbionts over insects' mating preferences.

Exploitation of an ancestral pheromone biosynthetic pathway contributes to diversification in Heliconius butterflies.

During courtship, male butterflies of many species produce androconial secretions containing male sex pheromones (MSPs) that communicate species identity and affect female choice. MSPs are thus likely candidates as reproductive barriers, yet their role in speciation remains poorly studied. Although Heliconius butterflies are a model system in speciation, their MSPs have not been investigated from a macroevolutionary perspective. We use GC/MS to characterize male androconial secretions in 33 of the 69 species in the Heliconiini tribe. We found these blends to be species-specific, consistent with a role in reproductive isolation. We detected a burst in blend diversification rate at the most speciose genus, Heliconius; a consequence of Heliconius and Eueides species using a fatty acid (FA) metabolic pathway to unlock more complex blends than basal Heliconiini species, whose secretions are dominated by plant-like metabolites. A comparison of 10 sister species pairs demonstrates a striking positive correlation between blend dissimilarity and range overlap, consistent with character displacement or reinforcement in sympatry. These results demonstrate for the first time that MSP diversification can promote reproductive isolation across this group of butterflies, showcasing how implementation of an ancestral trait, the co-option of the FA metabolic pathway for pheromone production, can facilitate rapid speciation.

P-elements strengthen reproductive isolation within the Drosophila simulans species complex.

Determining mechanisms that underlie reproductive isolation (RI) is key to understanding how species boundaries are maintained in nature. Transposable elements (TEs) are ubiquitous across eukaryotic genomes. However, the role of TEs in modulating the strength of RI between species is poorly understood. Several species of Drosophila have been found to harbor P-elements (PEs), yet only D. simulans is known to be currently polymorphic for their presence in wild populations. PEs can cause RI between PE-containing (P) and PE-lacking (M) lineages of the same species. However, it is unclear whether they also contribute to the magnitude of RI between species. Here, we use the simulans species complex to assess whether differences in PE status between D. simulans and its sister species, which do not harbor PEs, contribute to multiple barriers to gene flow between species. We show that crosses involving a P D. simulans father and an M mother from a sister species exhibit lower F1 female fecundity than crosses involving an M D. simulans father and an M sister-species mother. We also find that another TE, I-element, might play a minor role in determining the frequency of dysgenesis between species. Our results suggest that the presence of PEs in a species can strengthen isolation from its sister species, providing evidence that TEs can play a role in RI.

The natural hybridization between species Ligularia nelumbifolia and Cremanthodium stenoglossum (Senecioneae, Asteraceae) suggests underdeveloped reproductive isolation and ambiguous intergeneric boundary.

Natural hybridization is frequent in plants and is considered an important factor facilitating speciation. The natural intergeneric hybridization between Ligularia and Cremanthodium was previously confirmed using a couple of DNA markers. However, the mechanism of this intergeneric hybridization and the role of reproductive isolation in the process of hybridization remain unclear. Here we used double-digest restriction site-associated DNA sequencing (ddRAD-seq) to further quantify the occurrence of hybridization, the genetic structure of the hybrid population and the role of reproductive isolation between Ligularia nelumbifolia and Cremanthodium stenoglossum. The results based on the ddRAD-seq SNP data sets indicated that hybridization between L. nelumbifolia and C. stenoglossum was restricted to F1s, and no gene introgression was identified between these two species. STRUCTURE analysis and maximum likelihood (ML) tree results showed a slightly larger genetic contribution of L. nelumbifolia to putative hybrid F1s. We deduced that the reproductive isolation between these two parent species is not well-developed but still strong enough to maintain the genetic integrity of the species, and that their F1s are sterile or with low fertility. Given the poorly resolved phylogenetic relationship between Ligularia and Cremanthodium, the occurrence of natural hybridization between L. nelumbifolia and C. stenoglossum may provide new insights into the re-circumscription and re-delimitation of these two genera.

Cryptic speciation in the Warbling Vireo (Vireo gilvus)

AbstractEastern (Vireo gilvus gilvus) and western (V. g. swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada. In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species. However, to date, Barlow’s suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology. We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species. Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two well-defined, genetically differentiated, and monophyletic clades corresponding to previously recognized taxonomic groups. The two taxa come into contact in a narrow (~85 km) zone in Barrhead County, northwest of Edmonton, Alberta. They show evidence of limited hybridization. The distinct genetic differences are maintained in the contact zone, where individuals of the two taxa may occupy neighboring territories. Differences in spring arrival dates, molt schedules, and migration routes indicate that a migratory divide may play an important role in reproductive isolation. We suggest that the two taxa are distinct cryptic species: an eastern form, Vireo gilvus, and a western form, Vireo swainsoni.

The rates of introgression and barriers to genetic exchange between hybridizing species: sex chromosomes vs autosomes.

Interspecific crossing experiments have shown that sex chromosomes play a major role in reproductive isolation between many pairs of species. However, their ability to act as reproductive barriers, which hamper interspecific genetic exchange, has rarely been evaluated quantitatively compared to Autosomes. This genome-wide limitation of gene flow is essential for understanding the complete separation of species, and thus speciation. Here, we develop a mainland-island model of secondary contact between hybridizing species of an XY (or ZW) sexual system. We obtain theoretical predictions for the frequency of introgressed alleles, and the strength of the barrier to neutral gene flow for the two types of chromosomes carrying multiple interspecific barrier loci. Theoretical predictions are obtained for scenarios where introgressed alleles are rare. We show that the same analytical expressions apply for sex chromosomes and autosomes, but with different sex-averaged effective parameters. The specific features of sex chromosomes (hemizygosity and absence of recombination in the heterogametic sex) lead to reduced levels of introgression on the X (or Z) compared to autosomes. This effect can be enhanced by certain types of sex-biased forces, but it remains overall small (except when alleles causing incompatibilities are recessive). We discuss these predictions in the light of empirical data comprising model-based tests of introgression and cline surveys in various biological systems.

Clustering of loci controlling species differences in male chemical bouquets of sympatric Heliconius butterflies.

The degree to which loci promoting reproductive isolation cluster in the genome—that is, the genetic architecture of reproductive isolation—can influence the tempo and mode of speciation. Tight linkage between these loci can facilitate speciation in the face of gene flow. Pheromones play a role in reproductive isolation in many Lepidoptera species, and the role of endogenously produced compounds as secondary metabolites decreases the likelihood of pleiotropy associated with many barrier loci. Heliconius butterflies use male sex pheromones to both court females (aphrodisiac wing pheromones) and ward off male courtship (male‐transferred antiaphrodisiac genital pheromones), and it is likely that these compounds play a role in reproductive isolation between Heliconius species. Using a set of backcross hybrids between H. melpomene and H. cydno, we investigated the genetic architecture of putative male pheromone compound production. We found a set of 40 significant quantitative trait loci (QTL) representing 33 potential pheromone compounds. QTL clustered significantly on two chromosomes, chromosome 8 for genital compounds and chromosome 20 for wing compounds, and chromosome 20 was enriched for potential pheromone biosynthesis genes. There was minimal overlap between pheromone QTL and known QTL for mate choice and color pattern. Nonetheless, we did detect linkage between a QTL for wing androconial area and optix, a color pattern locus known to play a role in reproductive isolation in these species. This tight clustering of putative pheromone loci might contribute to coincident reproductive isolating barriers, facilitating speciation despite ongoing gene flow.

Identification of a Male-Produced Volatile Pheromone for Phymatodes dimidiatus (Coleoptera: Cerambycidae) and Seasonal Flight Phenology of Four Phymatodes Species Endemic to the North American Intermountain West.

Research over the last 15 yr has shown widespread pheromone parsimony within the coleopteran family Cerambycidae, with a number of highly conserved pheromone motifs, often shared within and across subfamilies, tribes, and genera. Our goals were to increase our understanding of the evolution of volatile pheromones within the Cerambycidae, their role in reproductive isolation and to identify pheromones for use in the development of lures for monitoring cerambycids. Over 3 yr, we tested 12 compounds known to be cerambycid pheromones as possible attractants at sites across Idaho. This study focused on species within the cerambycine genus Phymatodes (Tribe: Callidiini). We also collected and analyzed headspace volatiles of captured Phymatodes dimidiatus (Kirby). Our results demonstrate that (R)-2-methylbutan-1-ol is a male-produced volatile pheromone for P. dimidiatus. These results are consistent with prior research suggesting that (R)-2-methylbutan-1-ol and (R)-3-hydroxyhexan-2-one, individually or in a blend of both compounds, commonly serve as pheromones for Phymatodes spp. We captured Phymatodes starting in mid-May, continuing through mid-August. Our data indicate that flight periods of Phymatodes spp. in Idaho overlap. These species may be utilizing various mechanisms to ensure reproductive isolation, such as the production of different volatile pheromones, minor components, and/or proportions of components, utilizing different host species and/or host volatiles, differing daily activity periods, and/or occupying different heights in the tree canopy. Our results contribute to the basic understanding of the chemical and behavioral ecology of the Cerambycidae and can be applied to the development of pheromone lures for monitoring of economically important or endangered species.

Evolution of multiple sex-chromosomes associated with dynamic genome reshuffling in Leptidea wood-white butterflies

Sex-chromosome systems tend to be highly conserved and knowledge about their evolution typically comes from macroevolutionary inference. Rapidly evolving complex sex-chromosome systems represent a rare opportunity to study the mechanisms of sex-chromosome evolution at unprecedented resolution. Three cryptic species of wood-white butterflies—Leptidea juvernica, L. sinapis and L. reali—have each a unique set of multiple sex-chromosomes with 3–4 W and 3–4 Z chromosomes. Using a transcriptome-based microarray for comparative genomic hybridisation (CGH) and a library of bacterial artificial chromosome (BAC) clones, both developed in L. juvernica, we identified Z-linked Leptidea orthologs of Bombyx mori genes and mapped them by fluorescence in situ hybridisation (FISH) with BAC probes on multiple Z chromosomes. In all three species, we determined synteny blocks of autosomal origin and reconstructed the evolution of multiple sex-chromosomes. In addition, we identified W homologues of Z-linked orthologs and characterised their molecular differentiation. Our results suggest that the multiple sex-chromosome system evolved in a common ancestor as a result of dynamic genome reshuffling through repeated rearrangements between the sex chromosomes and autosomes, including translocations, fusions and fissions. Thus, the initial formation of neo-sex chromosomes could not have played a role in reproductive isolation between these Leptidea species. However, the subsequent species-specific fissions of several neo-sex chromosomes could have contributed to their reproductive isolation. Then, significantly increased numbers of Z-linked genes and independent neo-W chromosome degeneration could accelerate the accumulation of genetic incompatibilities between populations and promote their divergence resulting in speciation.

Pre-Mating Reproductive Isolation in Tiger Beetles (Carabidae: Cicindelinae): an Examination of the Role of Visual and Morphological Feedback

The biological species concept depends on mechanisms isolating reproduction to taxonomically define a species and to explain the disadvantages, and subsequent lack, of hybridization. In co-occurring tiger beetles (Carabidae: Cicindelinae), these between-species barriers are not readily apparent. To understand the characteristics potentially responsible for this isolation, we conducted behavioral and morphological experiments with two pairs of tiger beetle species. Choice trials showed males use visual cues to trigger the pursuit of potential mates and prefer females who appear similar in color. Geometric morphometric techniques allowed us to quantify shape differences in the mandibles of all four species. Previously, it was proposed that the mandible-coupling sulcus pairing acted as a lock-and-key mechanism to provide feedback necessary for species recognition. Our results, however, suggest that although there are significant differences in shape, these differences do not provide any feedback to the male or enhance his ability to retain his grasp on the female. Taken together, these findings suggest that vision, rather than the traditional mandibular lock-and-key hypothesis, may play a key role in reproductive isolation for co-occurring tiger beetle species.

Prezygotic reproductive isolation of two sympatric species of Ligularia weeds in eastern Qinghai-Tibet Plateau, China

Reproductive isolation, the key process to prevent interspecific hybridization and keep the completeness and independence of species, is crucial to the formation and maintenance of biodiversity. The forming steps, methods and intensities of reproductive isolation between different species are not completely the same. Literatures on reproductive isolation between species provide valuable evidence for speciation and its maintenance. Flowering phenology, flowering-visiting insects, and pollen-stigma compatibility were observed for Ligularia virgaurea and L. sagitta, two species of weeds sympatrically distributed in eastern Qinghai-Tibet Plateau. The characteristics of reproductive isolation mechanism of both species and its role in reproductive isolation were also studied. The results showed that the flowering period of L. virgaurea was 20 d earlier than that of L. sagitta. Although the full-blooming period separated, there was a roughly 10 d overlap between the two species. Meanwhile, the pollens and stigmas of the two species were both viability in the overlapped 10 d. L. virgaurea and L. sagitta shared the same types of pollinating insects, which was a generalized pollination type. Besides, the flower-visiting behavior towards the two species from the pollinators was similar, along with the interleaved access between the two species. There was no complete time isolation and no complete pollinator isolation between the two species, but the stigma had poor compatibility towards heterogenic pollen. The germination of pollen on stigma and growth of the pollen tube was hampered. Furthermore, the rejection of pollen-stigma was likely to be an important prezygotic reproductive isolation between L. virgaurea and L. sagitta, thus avoiding the natural hybridization and maintaining the diversity and genetic stability of the two species.

Sex Attractant Pheromones of Virgin Queens of Sympatric Slave-Making Ant Species in the Genus Polyergus, and their Possible Roles in Reproductive Isolation.

Species of the ant genus Polyergus are social parasites that steal brood from colonies of their hosts in the closely related genus Formica. Upon emergence as adults in a mixed population, host Formica workers carry out all the normal worker functions within the Polyergus colony, including foraging, feeding, grooming, and rearing brood of the parasitic Polyergus ants. Some unmated Polyergus gynes (queens) run in the raiding columns of their colonies and attract males by releasing a pheromone from their mandibular glands. There are two Polyergus species groups in North America: an eastern P. lucidus group and a western P. breviceps group. One species of each of these groups, P. lucidus Mayr and P. mexicanus Emery, are sympatric in Missouri. In this study, we characterized the sex pheromones of virgin queens of two species of the P. lucidus group (P. lucidus sensu stricto and P. sanwaldi) and one species of the P. breviceps group (P. mexicanus), and compared these with the previously identified sex pheromone of P. topoffi of the P. breviceps group. We then used sex pheromone blends reconstructed from synthesized components of the two groups to test their efficacy at reproductively isolating these species. We found that methyl 6-methylsalicylate is conserved as the major component of the pheromone blends for both Polyergus species groups; however, methyl (R)-3-ethyl-4-methylpentanoate is the species-specific minor component produced by P. lucidus group queens, and (R)-3-ethyl-4-methylpentan-1-ol is the crucial minor component for P. breviceps group queens. The optimal ratio of the major and minor components for P. lucidus group queens was about 100:1 salicylate to ester. In concurrent field trials in Missouri, males of P. lucidus sensu stricto and P. mexicanus (a member of the P. breviceps group) were attracted almost exclusively to their particular blends of sex pheromone components. To our knowledge, this is the first example of a possible sex-pheromone-based reproductive isolating mechanism in ants.

Analysis of natural female post-mating responses of Anopheles gambiae and Anopheles coluzzii unravels similarities and differences in their reproductive ecology

Anopheles gambiae and An. coluzzii, the two most important malaria vectors in sub-Saharan Africa, are recently radiated sibling species that are reproductively isolated even in areas of sympatry. In females from these species, sexual transfer of male accessory gland products, including the steroid hormone 20-hydroxyecdysone (20E), induces vast behavioral, physiological, and transcriptional changes that profoundly shape their post-mating ecology, and that may have contributed to the insurgence of post-mating, prezygotic reproductive barriers. As these barriers can be detected by studying transcriptional changes induced by mating, we set out to analyze the post-mating response of An. gambiae and An. coluzzii females captured in natural mating swarms in Burkina Faso. While the molecular pathways shaping short- and long-term mating-induced changes are largely conserved in females from the two species, we unravel significant inter-specific differences that suggest divergent regulation of key reproductive processes such as egg development, processing of seminal secretion, and mating behavior, that may have played a role in reproductive isolation. Interestingly, a number of these changes occur in genes previously shown to be regulated by the sexual transfer of 20E and may be due to divergent utilization of this steroid hormone in the two species.

Spiny mice of the Zambezian bioregion – phylogeny, biogeography and ecological differentiation within the Acomys spinosissimus complex

Despite the high degree of biological diversity in the Zambezian region, little information is available regarding the evolutionary history of its biota, and this is especially true for the fauna and flora of non-forested habitats. Here we use the most comprehensive multi-locus genetic dataset available to (1) reconstruct the phylogeny of spiny mice of the Acomys spinosissimus complex and to describe the spatial distribution of its genetic diversity; (2) to assess the level of reproductive isolation between genetic clades; (3) to analyse differences in climatic niche between potential species; and (4) to model their current and past distribution. The complex comprises four main genetic clades that correspond to four nominal species living in the east of the Zambezian region. Their distribution is delimited by important geographical divides including the Eastern Arc Mountains, Lake Malawi and the Zambezi River. Two species considered Tanzanian endemics, A. ngurui and A. muzei, are now known to be more widespread than previously thought and they have their centres of genetic diversity just north of the Zambezi. The first split between the extant members of the A. spinosissimus complex is estimated at 3 Mya. Most intraspecific diversification events are thought to have occurred in the Pleistocene, suggesting that climatic oscillations played an important role in their diversification. The climatic niches of parapatric taxa differ significantly; hence, local adaptations have likely played an important role in reproductive isolation. Species distribution modelling predicts a dramatic reduction of occurrence probability across the area during the last interglacial period, facilitating allopatric diversification of fragmented populations. Our results strongly suggest the combined role of Pleistocene climatic change, biogeographical barriers and local adaptation in forming current taxa diversity in east African savannah-like habitats. While such processes have frequently been observed in forest-living organisms (theory of allopatric diversification in forest refugia), similar mechanisms were likely also working in open ecosystems ("savannah refugia").