Engineered underdominance as a method of insect population replacement and reproductive isolation

Insect vector-borne diseases, such as malaria and dengue fever (both spread by mosquito vectors), continue to significantly impact health worldwide, despite the efforts put forth to eradicate them. Suppression strategies utilizing genetically modified disease-refractory insects have surfaced as an attractive means of disease control, and progress has been made on engineering disease-resistant insect vectors. However, laboratory-engineered disease refractory genes would probably not spread in the wild, and would most likely need to be linked to a gene drive system in order to proliferate in native insect populations. Underdominant systems like translocations and engineered underdominance have been proposed as potential mechanisms for spreading disease refractory genes. Not only do these threshold-dependent systems have certain advantages over other potential gene drive mechanisms, such as localization of gene drive and removability, extreme engineered underdominance can also be used to bring about reproductive isolation, which may be of interest in controlling the spread of GMO crops. Proof-of-principle establishment of such drive mechanisms in a well-understood and studied insect, such as Drosophila melanogaster, is essential before more applied systems can be developed for the less characterized vector species of interest, such as mosquitoes. This work details the development of several distinct types of engineered underdominance and of translocations in Drosophila, including ones capable of bringing about reproductive isolation and population replacement, as a proof of concept study that can inform efforts to construct such systems in insect disease vectors.

Taxonomic problems in the Drosophila melanica species group (Diptera : Drosophilidae) from southern China, with special reference to karyotypes and reproductive isolation

Karyotypes and reproductive isolation were studied in two allopatric populations of Drosophila tsigana, one from Guizhou Province in southern China and the other from Hokkaido in northern Japan, and in one population of a closely related species, D. longi

A widespread chromosomal inversion polymorphism contributes to a major life-history transition, local adaptation, and reproductive isolation.

The role of chromosomal inversions in adaptation and speciation is controversial. Historically, inversions were thought to contribute to these processes either by directly causing hybrid sterility or by facilitating the maintenance of co-adapted gene complexes. Because inversions suppress recombination when heterozygous, a recently proposed local adaptation mechanism predicts that they will spread if they capture alleles at multiple loci involved in divergent adaptation to contrasting environments. Many empirical studies have found inversion polymorphisms linked to putatively adaptive phenotypes or distributed along environmental clines. However, direct involvement of an inversion in local adaptation and consequent ecological reproductive isolation has not to our knowledge been demonstrated in nature. In this study, we discovered that a chromosomal inversion polymorphism is geographically widespread, and we test the extent to which it contributes to adaptation and reproductive isolation under natural field conditions. Replicated crosses between the prezygotically reproductively isolated annual and perennial ecotypes of the yellow monkeyflower, Mimulus guttatus, revealed that alternative chromosomal inversion arrangements are associated with life-history divergence over thousands of kilometers across North America. The inversion polymorphism affected adaptive flowering time divergence and other morphological traits in all replicated crosses between four pairs of annual and perennial populations. To determine if the inversion contributes to adaptation and reproductive isolation in natural populations, we conducted a novel reciprocal transplant experiment involving outbred lines, where alternative arrangements of the inversion were reciprocally introgressed into the genetic backgrounds of each ecotype. Our results demonstrate for the first time in nature the contribution of an inversion to adaptation, an annual/perennial life-history shift, and multiple reproductive isolating barriers. These results are consistent with the local adaptation mechanism being responsible for the distribution of the two inversion arrangements across the geographic range of M. guttatus and that locally adaptive inversion effects contribute directly to reproductive isolation. Such a mechanism may be partially responsible for the observation that closely related species often differ by multiple chromosomal rearrangements.

Reproductive isolation and genetic differentiation of ferox trout from sympatric brown trout in Loch Aweand Loch Laggan, Scotland

Molecular marker studies reported here, involving allozymes, mitochondrial DNA and microsatellites, demonstrate that ferox brown trout Salmo trutta in Lochs Awe and Laggan, Scotland, are reproductively isolated and genetically distinct from co-occurring brown trout. Ferox were shown to spawn primarily, and possibly solely, in a single large river in each lake system making them particularly vulnerable to environmental changes. Although a low level of introgression seems to have occurred with sympatric brown trout, possibly as a result of human-induced habitat alterations and stocking, ferox trout in these two lakes meet the requirements for classification as a distinct biological, phylogenetic and morphological species. It is proposed that the scientific name Salmo ferox Jardine, 1835, as already applied to Lough Melvin (Ireland) ferox, should be extended to Awe and Laggan ferox.

Contact zone dynamics and the evolution of reproductive isolation in a North American treefrog, the spring peeper (Pseudacris crucifer)

Despite over seven decades of speciation research and 25 years of phylogeographic studies, a comprehensive understanding of mechanisms that generate biological species remains elusive. In temperate zones, the pervasiveness of range fragmentation and subsequent range expansions suggests that secondary contact between diverging lineages may be important in the evolution of species. Thus, such contact zones provide compelling opportunities to investigate evolutionary processes, particularly the roles of geographical isolation in initiating, and indirect selection against hybrids in completing (reinforcement), the evolution of reproductive isolation and speciation. The spring peeper (Pseudacris crucifer) has six well-supported mitochondrial lineages many of which are now in secondary contact. Here I investigate the evolutionary consequences of secondary contact of two such lineages (Eastern and Interior) in Southwestern Ontario using genetic, morphological, acoustical, experimental, and behavioural evidence to show accentuated divergence of the mate recognition system in sympatry. Mitochondrial and microsatellite data distinguish these two lineages but also show ongoing hybridization. Bayesian assignment tests and cline analysis imply asymmetrical introgression of Eastern lineage nuclear markers into Interior populations. Male calls are divergent between Eastern and Interior allopatric populations and show asymmetrical reproductive character displacement in sympatry. Female preference of pure lineage individuals is also exaggerated in sympatry, with hybrids showing intermediate traits and preference. I suggest that these patterns are most consistent with secondary reinforcement. I assessed levels of post-zygotic isolation between the Eastern and Interior lineages using a laboratory hybridization experiment. Hybrid tadpoles showed equal to or greater fitness than their pure lineage counterparts, but this may be countered through competition. More deformities and developmental anomalies in hybrid tadpoles further suggest post-zygotic isolation. Despite evidence for pre-mating isolation between the two lineages, isolation appears incomplete (i.e. hybridization is ongoing). I hypothesize that potentially less attractive hybrids may circumvent female choice by adopting satellite behaviour. Although mating tactics are related to body size, genetic status may play a role. I show that pure Eastern males almost always engage in calling, while hybrids adopt a satellite tactic. An absence of assortative mating, despite evidence of female preference, suggests successful satellite interception possibly facilitating introgression.

Where the Lake Meets the Sea: Strong Reproductive Isolation Is Associated with Adaptive Divergence between Lake Resident and Anadromous Three-Spined Sticklebacks

Contact zones between divergent forms of the same species are often characterised by high levels of phenotypic diversity over small geographic distances. What processes are involved in generating such high phenotypic diversity? One possibility is that introgression and recombination between divergent forms in contact zones results in greater phenotypic and genetic polymorphism. Alternatively, strong reproductive isolation between forms may maintain distinct phenotypes, preventing homogenisation by gene flow. Contact zones between divergent freshwater-resident and anadromous stickleback (Gasterosteus aculeatus L.) forms are numerous and common throughout the species distribution, offering an opportunity to examine these contrasting hypotheses in greater detail. This study reports on an interesting new contact zone located in a tidally influenced lake catchment in western Ireland, characterised by high polymorphism for lateral plate phenotypes. Using neutral and QTL-linked microsatellite markers, we tested whether the high diversity observed in this contact zone arose as a result of introgression or reproductive isolation between divergent forms: we found strong support for the latter hypothesis. Three phenotypic and genetic clusters were identified, consistent with two divergent resident forms and a distinct anadromous completely plated population that migrates in and out of the system. Given the strong neutral differentiation detected between all three morphotypes (mean FST = 0.12), we hypothesised that divergent selection between forms maintains reproductive isolation. We found a correlation between neutral genetic and adaptive genetic differentiation that support this. While strong associations between QTL linked markers and phenotypes were also observed in this wild population, our results support the suggestion that such associations may be more complex in some Atlantic populations compared to those in the Pacific. These findings provide an important foundation for future work investigating the dynamics of gene flow and adaptive divergence in this newly discovered stickleback contact zone.

DIFFERENTIATION OF DROSOPHILA-SERIDO (ISOFEMALE LINE A95F3) AND D-KOEPFERAE (ISOFEMALE LINE B20D2) - REPRODUCTIVE ISOLATION, DEVELOPMENT TIME AND POLYTENE CHROMOSOME-BANDING PATTERNS

Drosophila serido is considered to be a superspecies consisting of two species: D. serido, from Brazil and D. koepferae from Argentina and Bolivia. However this probably does not express the entire evolutionary complexity of its populations. Isofemale lines A95F3 (from Brazil) and B20D2 (from Argentina), at present representing, respectively, the first and second species, were analyzed for fertility and fecundity in pair-mating intracrosses and intercrosses, as well as for development time, banding patterns and asynapsis of polytene chromosomes in the isofemale lines and their hybrids.Although variations in experimental conditions resulted in some variability in the results, in general A95F3 fertility and fecundity were lower than in B20D2. Intercrosses of A95F3 females and B20D2 males showed lower fertility and fecundity than the reciprocal crosses, following more closely characteristics of the mother strains. This is in contrast to the results obtained by Fontdevilla et al. (An. Entomol. Soc. Amer. 81: 380-385, 1988) and may be due to the different geographic origin of D. serido strains they used in crosses to B20D2. This difference and others cited in the literature relative to aedeagus morphology, karyotype characteristics, inversion polymorphisms and reproductive isolation strongly indicate that A95F3 and D. serido from the State of Bahia, Brazil are not a single evolutionary entity, reinforcing the idea of greater complexity of the superspecies D. serido than is known today.The reproductive isolation mechanisms found operating between A95F3 and B20D2 were prezygotic and postzygotic, the latter included mortality at the larvae stage in both directions of crosses and sterility of male hybrids in intercrosses involving B20D2 females and A95F3 males. The two isofemale lines differed in egg-adult development time, which was also differently affected by culture medium composition.A95F3 and B20D2 also showed differences in the banding patterns of proximal regions of polytene chromosomes 2, 3 and X, a fixed inversion in chromosome 3 (here named 3t), apparently not described previously, and a high degree of asynapsis in hybrids.These observations, especially those related to reproductive isolation and chromosomal differentiation (including the karyotype, previously described, and the differentiation of banding patterns, described in this paper), as well as the extensive asynapsis observed in hybrids reinforces the distinct species status of A95F3 and B20D2 isofemale lines.

Phylogeographic Structure And Outbreeding Depression Reveal Early Stages Of Reproductive Isolation In The Neotropical Orchid Epidendrum Denticulatum

Phylogeographic studies provide an important framework for investigating the mechanisms operating during the earliest stages of speciation, as reproductive barriers can be examined among divergent lineages in a geographic context. We investigated the evolution of early stages of intrinsic postmating isolation among different populations and lineages of Epidendrum denticulatum, a Neotropical orchid distributed across different biomes in South America. We estimated genetic diversity and structure for both nuclear and plastid markers, using a haplotype network, differentiation tests, Bayesian assignment analysis, and divergence time estimates of the main lineages. Reproductive barriers among divergent lineages were examined by analyzing seed viability following reciprocal crossing experiments. Strong plastid phylogeographic structure was found, indicating that E. denticulatum was restricted to multiple refuges during South American forest expansion events. In contrast, significant phylogeographic structure was not found for nuclear markers, suggesting higher gene flow by pollen than by seeds. Large asymmetries in seed set were observed among different plastid genetic groups, suggesting the presence of polymorphic genic incompatibilities associated with cytonuclear interactions. Our results confirm the importance of phylogeographic studies associated with reproductive isolation experiments and suggest an important role for outbreeding depression during the early stages of lineage diversification. © 2013 The Society for the Study of Evolution.

The role of chromosome variation in the speciation of the red brocket deer complex: the study of reproductive isolation in females

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A Social Parasite Evolved Reproductive Isolation from Its Fungus-Growing Ant Host in Sympatry

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Sex-pairing pheromones and reproductive isolation in three sympatric Cornitermes species (Isoptera, Termitidae, Syntermitinae)

The species-specificity of pairing has been studied in three sympatric Neotropical termites: Cornitermes bequaerti, Cornitermes cumulans and Cornitermes silvestrii (Termitidae, Syntermitinae). Bioassays showed that sex attraction was highly species-specific between C. bequaerti and C cumulans but not between C. cumulans and C. silvestrii. The sex-pairing pheromone of the three species is secreted by the tergal glands of female alates. It consists of a common compound (3Z,6Z,8E)-dodeca-3,6,8-trien-1-ol. In C. bequaerti, this polyunsaturated alcohol is the only compound of the sex-pairing pheromone, whereas it is associated with the oxygenated sesquiterpene (E)-nerolidol in C. cumulans, and with (E)-nerolidol and (Z)-dodec-3-en-1-ol in C silvestrii. (3Z,6Z,8E)-Dodeca-3,6,8-trien-1-ol is responsible for sexual attraction, whereas (E)-nerolidol, which is inactive in eliciting attraction of male alates, is responsible for the species-specificity of the attraction. This is the first time that a multicomponent sex-pairing pheromone has been identified in termites. The role of (Z)-dodec-3-en-1-ol present on the surface of the tergal glands of the female alates of C. silvestrii could not be definitively determined, but it is suggested that this compound could be involved in the species-specificity of sex attraction with other sympatric species of Cornitermes. Our study shows that the reproductive isolation in termites is due to a succession of factors, as the chronology of dispersal flights, the species-specificity of sex-pairing pheromones and the species-specific recognition. (C) 2011 Elsevier Ltd. All rights reserved.