Author(s): Matos, M, Bárbaro M, Mira M, Fragata I, Simões P, Lima M, Lopes-Cunha M, Kellen B, Santos J, Magalhães S
Species with wide distributions may be highly differentiated across contrasting environments. While gene flow may help maintain similarities among populations, local adaptation may lead to their divergence, as well as promote reproductive isolation, further fostering evolutionary diversification. The interplay between these two processes is of major importance for Conservation, as it will determine if populations differentiate or become more similar in the long run, as a result of environmental changes such as those imposed by man. In Drosophila subobscura, populations are differentiated along a latitudinal gradient, but recent evidence indicates that northern populations are becoming more similar to southern ones. An important issue is how much populations from the extremes of the cline differ in mating preferences, as this might contribute to reduce genetic introgression when populations meet. To address this, we analyzed the evolutionary dynamics of reproductive isolation of D. subobscura populations derived from the extremes of the European cline, while adapting to a common, laboratorial environment. We show that mating performance increased during laboratory adaptation. In general northern populations had a better performance than southern ones, and this difference was sustained across generations. Moreover northern females preferred mates from their own populations while southern females preferred males from the north. The assortative mating of the northern populations was stable through time, while disassortative mating of the southern populations faded away during laboratory evolution. Overall this study suggests that reproductive barriers may slow down the genetic introgression due to migration to the north, an important finding in evolutionary and conservation terms.
Author(s): Martins, NE, Faria VG, Nolte V, Schlötterer C, Teixeira L, Sucena É, Magalhães S
Because hosts and parasites exert strong selection pressure on each other, it is particularly relevant to study their interaction in an evolutionary context. Experimental Evolution permits the establishment of causality between evolutionary processes and adaptation patterns. Here we use experimental evolution of Drosophila melanogaster exposed to Drosophila C virus (DCV) to address the phenotypic and genotypic changes of hosts evolving in presence of parasites.
Upon exposure to the virus, Drosophila survival increased from 33% to almost 90% after 35 generations of selection. This response carried no detectable costs in fitness in the absence of infection, and was not lost after 10 generations in the absence of selection. Cross-resistance was found for other viruses, such as CrPV and FHV, but not to bacterial pathogens.
Whole genome sequencing of pooled samples of virus-selected populations and their matching controls at generation 20 uncovered two regions of significant differentiation between these groups of populations. The first corresponded to a region of 4 megabases(Mb) in the 3L chromosomal arm. This region’s peak of differentiation corresponded to a polymorphism in pastrel (pst), a gene recently associated with increased DCV resistance. The second was a pair of significantly differentiated SNPs in the X chromosome, in genes not previously associated with virus resistance. Results with a panel of deficiencies in the 3L chromosome confirmed that deficiencies which encompass pst are the ones with more influence on survival after DCV infection, in a region of approximately 2 Mb. There is ongoing work to confirm the involvement of other candidate genes in this region and of the genes in the X chromosome in resistance to DCV infection.
Overall we show that selection for increased virus resistance I) is stable and bears little costs, II) is advantageous in the defense against other viral pathogens, and III) has a simple genetic basis.
Horizontally-transmitted pathogens can infect their hosts through different routes. Yet, the physiological and evolutionary consequences to the host of distinct modes of pathogen access are virtually unknown. To tackle this question, we used Experimental Evolution of Drosophila melanogaster infected with Pseudomonas entomophila by two different routes (oral and systemic). We found that adaptation to both routes relied on resistance. Moreover, adaptation to infection through one route did not protect from infection through the alternate route, indicating distinct genetic bases. Also, the two selection regimes led to markedly different evolutionary trajectories. Finally, relatively to the control population, evolved flies were not more resistant to bacteria other than Pseudomonas and showed higher susceptibility to viral infections. These specificities and trade-offs may contribute to the maintenance of genetic variation for resistance in natural populations. Moreover, our data shows that pathogen infection route affects host evolution. Therefore, the study of host-pathogen interactions should account not only on host and pathogen evolution, but also on the ecology of the infection, when interpreting patterns of variation in natural populations.
Author(s): Rodrigues, L, Clemente S, Carvalho J, Duarte F, Ponce R, Varela S, Olivieri I, Magalhães S
In Tetranychus urticae, only the first mating is effective, except if the interval between first and second copulations is shorter than 24 hours or if the first mating is interrupted. However, males often attempt to copulate with mated females. Here, we address this paradox.
We first tested whether males prefer to mate with females that have mated within the preceding 24 hours or with those that have mated before that period. We found that males show no preference between these two types of females. Moreover, the time to mating was longer and the mating duration shorter when males mated with mated females, relative to virgins, irrespective of their timing of mating. This confirms a lack of distinction between different types of mated females and suggests that males are either less motivated to mate with mated females or that the second mating occurs for a different reason than siring offspring.
Subsequently, we investigated the consequences of polyandry for the reproductive fitness of females, depending on the frequency and timing of the mating events. We predicted that if females benefit from polyandry, fecundity and survival of multiply mated females would be higher than those of once mated females. Indeed, multiply mated females have higher fecundity than once mated females, suggesting that females potentially benefit from mating multiply. No difference in survival and sex ratio was found between these females.
Our data shows that females benefit from multiple matings, hence this behaviour is probably under female control. This result has implications for our understanding of mating behaviour in spider mites and other organisms.
Incomplete specific recognition can lead to the occurrence of reproductive interference (RI) - reproductive interactions between two species resulting in fitness loss for at least one of them. RI can play an important role in the coexistence of species, being especially important in the fate of introduced exotic species and in pest management.Tetranychus urticae and Tetranychus evansi are two closely related haplodiploid spider mite species that often coexist in solenaceous crops. Incomplete specific recognition occurs among these species: heterospecific matings were observed, although no hybrid progeny has been found. We tested two possible RI mechanisms: (1) the effect of mating with heterospecifics on virgin (haploid) offspring and (2), the consequences of heterospecific crosses for the offspring of females that have or will mate with conspecifics. Behavioural assays showed that (1) only T. urticae females and T. evansi males prefer to mate with conspecifics; (2) regarding latency to copulation individuals behave as virgins after mating heterospecifically, (3) T. urticae females copulate for a shorter period with heterospecifics than with conspecifcs. Results for fecundity and sex-ratio revealed that (1) for both species, fecundity of females mated with heterospecifics are similar to that of virgins - heterospecific crosses do not affect egg viability; (2) T. evansi females that mate with both conspecific and heterospecific males had higher fecundity than females that mated with a conspecific male only; this was not observed in T. urticae. (3) T. urticae females that mate with a heterospecific male after a conspecific mating had a lower percentage of female offspring. The results obtained point to the occurrence of asymmetric RI, in which T. evansi females benefits from mating with heterospecifics, whereas T. urticae pay a cost of such matings. These results may affect the coexistence of these species, a hypothesis requiring further testing.