Desiccation resistance in interspecific Drosophila crosses. Genetic interactions and trait correlations

High altitude favours long-chained cuticular hydrocarbons in Drosophila

Cuticular hydrocarbons (CHCs) are key components of the insect cuticle and contribute to the wide geographical distribution of this taxon. Many studies have investigated sex and population differences in CHC profiles, with these investigations mostly focusing on latitudinal CHC variation, whereas CHC variation across altitudinal transects is less well-studied. Here, we tested whether CHC profiles vary along an altitudinal gradient in the cosmopolitan vinegar fly Drosophila melanogaster. We collected from three populations of D. melanogaster in the Western Himalayas at altitudes ranging from 760 to 2,592 m above sea level and tested their CHC profiles for standing and plastic variation. We found quantitative differences in 25 CHCs across populations, and at higher elevations, males and females expressed higher amounts of particular long-chained hydrocarbons. We also found plastic shifts in CHC profiles in all three populations when flies were exposed to desiccating conditions. Overall, our findings suggest that there is an altitudinal cline in CHCs. However, this does not mirror the well-established latitudinal clines in fly hydrocarbons.

Using body size as an indicator for age structure in field populations of Aedes aegypti (Diptera: Culicidae)

BACKGROUND

The Aedes aegypti mosquito is a vector of several viruses including dengue, chikungunya, zika, and yellow fever. Vector surveillance and control are the primary methods used for the control and prevention of disease transmission; however, public health institutions largely rely on measures of population abundance as a trigger for initiating control activities. Previous research found evidence that at the northern edge of Ae. aegypti's geographic range, survival, rather than abundance, is likely to be the factor limiting disease transmission. In this study, we sought to test the utility of using body size as an entomological index to surveil changes in the age structure of field-collected female Aedes aegypti.

METHODS

We collected female Ae. aegypti mosquitoes using BG sentinel traps in three cities at the northern edge of their geographic range. Collections took place during their active season over the course of 3 years. Female wing size was measured as an estimate of body size, and reproductive status was characterized by examining ovary tracheation. Chronological age was determined by measuring transcript abundance of an age-dependent gene. These data were then tested with female abundance at each site and weather data from the estimated larval development period and adulthood (1 week prior to capture). Two sources of weather data were tested to determine which was more appropriate for evaluating impacts on mosquito physiology. All variables were then used to parameterize structural equation models to predict age.

RESULTS

In comparing city-specific NOAA weather data and site-specific data from HOBO remote temperature and humidity loggers, we found that HOBO data were more tightly associated with body size. This information is useful for justifying the cost of more precise weather monitoring when studying intra-population heterogeneity of eco-physiological factors. We found that body size itself was not significantly associated with age. Of all the variables measured, we found that best fitting model for age included temperature during development, body size, female abundance, and relative humidity in the 1 week prior to capture . The strength of models improved drastically when testing one city at a time, with Hermosillo (the only study city with seasonal dengue transmission) having the best fitting model for age. Despite our finding that there was a bias in the body size of mosquitoes collected alive from the BG sentinel traps that favored large females, there was still sufficient variation in the size of females collected alive to show that inclusion of this entomological indicator improved the predictive capacity of our models.

CONCLUSIONS

Inclusion of body size data increased the strength of weather-based models for age. Importantly, we found that variation in age was greater within cities than between cities, suggesting that modeling of age must be made on a city-by-city basis. These results contribute to efforts to use weather forecasts to predict changes in the probability of disease transmission by mosquito vectors.

Fitness consequences of targeted gene flow to counter impacts of drying climates on terrestrial-breeding frogs

Targeted gene flow (TGF) could bolster the adaptive potential of isolated populations threatened by climate change, but could also lead to outbreeding depression. Here, we explore these possibilities by creating mixed- and within-population crosses in a terrestrial-breeding frog species threatened by a drying climate. We reared embryos of the crawling frog (Pseudophryne guentheri) on wet and dry soils and quantified fitness-related traits upon hatching. TGF produced mixed outcomes in hybrids, which depended on crossing direction (origin of gametes from each sex). North-south crosses led to low embryonic survival if eggs were of a southern origin, and high malformation rates when eggs were from a northern population. Conversely, east-west crosses led to one instance of hybrid vigour, evident by increased fitness and desiccation tolerance of hybrid offspring relative to offspring produced from within-population crosses. These contrasting results highlight the need to experimentally evaluate the outcomes of TGF for focal species across generations prior to implementing management actions.

Pure species discriminate against hybrids in the Drosophila melanogaster species subgroup

Introgression, the exchange of alleles between species, is a common event in nature. This transfer of alleles between species must happen through fertile hybrids. Characterizing the traits that cause defects in hybrids illuminates how and when gene flow is expected to occur. Inviability and sterility are extreme examples of fitness reductions but are not the only type of defects in hybrids. Some traits specific to hybrids are more subtle but are important to determine their fitness. In this report, we study whether F1 hybrids between two species pairs of Drosophila are as attractive as the parental species. We find that in both species pairs, the sexual attractiveness of the F1 hybrids is reduced and that pure species discriminate strongly against them. We also find that the cuticular hydrocarbon (CHC) profile of the female hybrids is intermediate between the parental species. Perfuming experiments show that modifying the CHC profile of the female hybrids to resemble pure species improves their chances of mating. Our results show that behavioral discrimination against hybrids might be an important component of the persistence of species that can hybridize.

Biological Adaptations Associated with Dehydration in Mosquitoes

Diseases that are transmitted by mosquitoes are a tremendous health and socioeconomic burden with hundreds of millions of people being impacted by mosquito-borne illnesses annually. Many factors have been implicated and extensively studied in disease transmission dynamics, but knowledge regarding how dehydration impacts mosquito physiology, behavior, and resulting mosquito-borne disease transmission remain underdeveloped. The lapse in understanding on how mosquitoes respond to dehydration stress likely obscures our ability to effectively study mosquito physiology, behavior, and vectorial capabilities. The goal of this review is to develop a profile of factors underlying mosquito biology that are altered by dehydration and the implications that are related to disease transmission.

Wax, sex and the origin of species: Dual roles of insect cuticular hydrocarbons in adaptation and mating

Evolutionary changes in traits that affect both ecological divergence and mating signals could lead to reproductive isolation and the formation of new species. Insect cuticular hydrocarbons (CHCs) are potential examples of such dual traits. They form a waxy layer on the cuticle of the insect to maintain water balance and prevent desiccation, while also acting as signaling molecules in mate recognition and chemical communication. Because the synthesis of these hydrocarbons in insect oenocytes occurs through a common biochemical pathway, natural or sexual selection on one role may affect the other. In this review, we explore how ecological divergence in insect CHCs can lead to divergence in mating signals and reproductive isolation. We suggest that the evolution of insect CHCs may be ripe models for understanding ecological speciation.

Effects of size, sex and teneral resources on the resistance to hydric stress in the tephritid fruit fly Anastrepha ludens

Water availability is recognized as one of the most important factors in the distribution and activity of terrestrial organisms. In the case of insects, hydric stress imposes a major challenge for survival because of the small surface-area-to-volume ratio they exhibit. In general, stress resistance is expected to co-vary positively with size; however, this pattern can become obscured in insects that exhibit sexual size dimorphism, as sexes differ in size and/or shape and have dissimilar resource allocations. In the present study, we use an allometric-based approach to (i) assess the desiccation and starvation stress resistance of teneral Anastrepha ludens flies, (ii) disentangle the relationships between resistance, size and sex and (iii) examine the adult fly body differences in water and lipid contents before and after exposure to stress. After controlling for sexual size dimorphism, an allometric increase of resistance with overall size was observed for all stress-based treatments. The scaling exponents that define the proportion of increase resistance varied with size traits and with type and degree of hydric stress. In this allometric relationship, and also in the relationships between mass and wing length and between size and teneral resources, the sexes maintained similar scaling exponents but differed in the intercepts. Males were more resistant to stress than females; this competitive advantage is probably linked to greater amounts of teneral lipids and more water use during stress.

A single gene affects both ecological divergence and mate choice in Drosophila

Evolutionary changes in traits involved in both ecological divergence and mate choice may produce reproductive isolation and speciation. However, there are few examples of such dual traits, and the genetic and molecular bases of their evolution have not been identified. We show that methyl-branched cuticular hydrocarbons (mbCHCs) are a dual trait that affects both desiccation resistance and mate choice in Drosophila serrata. We identify a fatty acid synthase mFAS (CG3524) responsible for mbCHC production in Drosophila and find that expression of mFAS is undetectable in oenocytes (cells that produce CHCs) of a closely related, desiccation-sensitive species, D. birchii, due in part to multiple changes in cis-regulatory sequences of mFAS. We suggest that ecologically influenced changes in the production of mbCHCs have contributed to reproductive isolation between the two species.

Premature attraction of pollinators to inaccessible figs of Ficus altissima: a search for ecological and evolutionary consequences

Adult life spans of only one or two days characterise life cycles of the fig wasps (Agaonidae) that pollinate fig trees (Ficus spp., Moraceae). Selection is expected to favour traits that maximise the value of the timing of encounters between such mutualistic partners, and fig wasps are usually only attracted to their hosts by species- and developmental-stage specific volatiles released from figs at the time when they are ready to be entered, oviposited in and pollinated. We found that Ficus altissima is exceptional, because it has persistent tight-fitting bud covers that prevent its Eupristina altissima pollinator (and a second species of ‘cheater’ agaonid) from entering its figs for several days after they start to be attracted. We examined the consequences of delayed entry for the figs and fig wasps and tested whether delayed entry has been selected to increase adult longevity. We found that older pollinators produced fewer and smaller offspring, but seed production was more efficient. Pollinator offspring ratios also varied depending on the age of figs they entered. The two agaonids from F. altissima lived slightly longer than six congeners associated with typical figs, but this was explainable by their larger body sizes. Delayed entry generates reproductive costs, especially for the pollinator. This opens an interesting perspective on the coevolution of figs and their pollinators and on the nature of mutualistic interactions in general.

A re-examination of Wolbachia-induced cytoplasmic incompatibility in California Drosophila simulans

BACKGROUND

In California Drosophila simulans, the maternally inherited Riverside strain Wolbachia infection (wRi) provides a paradigm for rapid spread of Wolbachia in nature and rapid evolutionary change. wRi induces cytoplasmic incompatibility (CI), where crosses between infected males and uninfected females produce reduced egg-hatch. The three parameters governing wRi infection-frequency dynamics quantify: the fidelity of maternal transmission, the level of cytoplasmic incompatibility, and the relative fecundity of infected females. We last estimated these parameters in nature in 1993. Here we provide new estimates, under both field and laboratory conditions. Five years ago, we found that wRi had apparently evolved over 15 years to enhance the fecundity of infected females; here we examine whether CI intensity has also evolved.

METHODOLOGY/PRINCIPAL FINDINGS

New estimates using wild-caught flies indicate that the three key parameters have remained relatively stable since the early 1990s. As predicted by our three-parameter model using field-estimated parameter values, population infection frequencies remain about 93%. Despite this relative stability, laboratory data based on reciprocal crosses and introgression suggest that wRi may have evolved to produce less intense CI (i.e., higher egg hatch from incompatible crosses). In contrast, we find no evidence that D. simulans has evolved to lower the susceptibility of uninfected females to CI.

CONCLUSIONS/SIGNIFICANCE

Evolution of wRi that reduces CI is consistent with counterintuitive theoretical predictions that within-population selection on CI-causing Wolbachia does not act to increase CI. Within taxa, CI is likely to evolve mainly via pleiotropic effects associated with the primary targets of selection on Wolbachia, i.e., host fecundity and transmission fidelity. Despite continuous, strong selection, D. simulans has not evolved appreciably to suppress CI. Our data demonstrate a lack of standing genetic variation for CI resistance in the host.

Hydric stress-dependent effects of Plasmodium falciparum infection on the survival of wild-caught Anopheles gambiae female mosquitoes

BACKGROUND

Whether Plasmodium falciparum, the agent of human malaria responsible for over a million deaths per year, causes fitness costs in its mosquito vectors is a burning question that has not yet been adequately resolved. Understanding the evolutionary forces responsible for the maintenance of susceptibility and refractory alleles in natural mosquito populations is critical for understanding malaria transmission dynamics.

METHODS

In natural mosquito populations, Plasmodium fitness costs may only be expressed in combination with other environmental stress factors hence this hypothesis was tested experimentally. Wild-caught blood-fed Anopheles gambiae s.s. females of the M and S molecular form from an area endemic for malaria in Mali, West Africa, were brought to the laboratory and submitted to a 7-day period of mild hydric stress or kept with water ad-libitum. At the end of this experiment all females were submitted to intense desiccation until death. The survival of all females throughout both stress episodes, as well as their body size and infection status was recorded. The importance of stress, body size and molecular form on infection prevalence and female survival was investigated using Logistic Regression and Proportional-Hazard analysis.

RESULTS

Females subjected to mild stress exhibited patterns of survival and prevalence of infection compatible with increased parasite-induced mortality compared to non-stressed females. Fitness costs seemed to be linked to ookinetes and early oocyst development but not the presence of sporozoites. In addition, when females were subjected to intense desiccation stress, those carrying oocysts exhibited drastically reduced survival but those carrying sporozoites were unaffected. No significant differences in prevalence of infection and infection-induced mortality were found between the M and S molecular forms of Anopheles gambiae.

CONCLUSIONS

Because these results suggest that infected mosquitoes may incur fitness costs under natural-like conditions, they are particularly relevant to vector control strategies aiming at boosting naturally occurring refractoriness or spreading natural or foreign genes for refractoriness using genetic drive systems in vector populations.

Effects of larval growth condition and water availability on desiccation resistance and its physiological basis in adult Anopheles gambiae sensu stricto

Background

Natural populations of the malaria mosquito Anopheles gambiae s.s. are exposed to large seasonal and daily fluctuations in relative humidity and temperature, which makes coping with drought a crucial aspect of their ecology.

Methods

To better understand natural variation in desiccation resistance in this species, the effects of variation in larval food availability and access to water as an adult on subsequent phenotypic quality and desiccation resistance of adult females of the Mopti chromosomal form were tested experimentally.

Results

It was found that, under normal conditions, larval food availability and adult access to water had only small direct effects on female wet mass, dry mass, and water, glycogen and body lipid contents corrected for body size. In contrast, when females subsequently faced a strong desiccation challenge, larval food availability and adult access to water had strong carry-over effects on most measured physiological and metabolic parameters, and affected female survival. Glycogen and water content were the most used physiological reserves in relative terms, but their usage significantly depended on female phenotypic quality. Adult access to water significantly influenced the use of water and body lipid reserves, which subsequently affected desiccation resistance.

Conclusions

These results demonstrate the importance of growth conditions and water availability on adult physiological status and subsequent resistance to desiccation.

Do Species Converge during Adaptation? A Case Study inDrosophila

Adaptation to novel environments is a crucial theme in evolutionary biology, particularly because ex situ conservation forces populations to adapt to captivity. Here we analyze the evolution of life-history traits in two closely related species, Drosophila subobscura Collin and Drosophila madeirensis Monclus, during adaptation to the laboratory. Drosophila madeirensis, an endemic species from Madeira, is here shown to have less ability to adapt to the laboratory. Early fecundity was the only trait where this species showed a significant improvement with time. By comparison, D. subobscura improved in most traits, and its early fecundity increased faster than that of D. madeirensis. Our findings suggest that different species, even closely related ones, may adapt at different rates to the same environment.

Evolutionary domestication in Drosophila subobscura

The domestication of plants and animals is historically one of the most important topics in evolutionary biology. The evolutionary genetic changes arising from human cultivation are complex because of the effects of such varied processes as continuing natural selection, artificial selection, deliberate inbreeding, genetic drift and hybridization of different lineages. Despite the interest of domestication as an evolutionary process, few studies of multicellular sexual species have approached this topic using well-replicated experiments. Here we present a comprehensive study in which replicated evolutionary trajectories from several Drosophila subobscura populations provide a detailed view of the evolutionary dynamics of domestication in an outbreeding animal species. Our results show a clear evolutionary response in fecundity traits, but no clear pattern for adult starvation resistance and juvenile traits such as development time and viability. These results supply new perspectives on the confounding of adaptation with other evolutionary mechanisms in the process of domestication.

Between a rock and a hard place: evaluating the relative risks of inbreeding and outbreeding for conservation and management

As populations become increasingly fragmented, managers are often faced with the dilemma that intentional hybridization might save a population from inbreeding depression but it might also induce outbreeding depression. While empirical evidence for inbreeding depression is vastly greater than that for outbreeding depression, the available data suggest that risks of outbreeding, particularly in the second generation, are on par with the risks of inbreeding. Predicting the relative risks in any particular situation is complicated by variation among taxa, characters being measured, level of divergence between hybridizing populations, mating history, environmental conditions and the potential for inbreeding and outbreeding effects to be occurring simultaneously. Further work on consequences of interpopulation hybridization is sorely needed with particular emphasis on the taxonomic scope, the duration of fitness problems and the joint effects of inbreeding and outbreeding. Meanwhile, managers can minimize the risks of both inbreeding and outbreeding by using intentional hybridization only for populations clearly suffering from inbreeding depression, maximizing the genetic and adaptive similarity between populations, and testing the effects of hybridization for at least two generations whenever possible.

Differences in egg thermotolerance between tropical and temperate populations of the migratory locust Locusta migratoria (Orthoptera: Acridiidae)

The migratory locust Locusta migratoria L., which is widely distributed throughout the world, exhibits within- and between-population variation in cold tolerance. To understand physiological adaptation in populations, we studied the genetic basis of thermotolerance in Hainan (tropical) and Liaoning (temperate) populations and measured expression of Hsp70 and Hsp90 mRNA in both populations at low (0 degrees C) and high temperatures (40 degrees C). Phenotypic variation of thermotolerance is heritable. Heritable characteristics differed among different stages of locust egg development, as well as among different measures of thermotolerance. Nuclear genetic factors, rather than cytoplasmic factors, contribute to differences in cold tolerance between the tropical and temperate populations of the migratory locust; for heat tolerance, maternal effects were involved in three stages of egg development. Expression of Hsp90 mRNA was induced in temperate population after heat shock (40 degrees C x 12h), whereas expression of Hsp70 and 90 was induced in tropical population after cold shock (0 degrees C x 12h). We suggest that thermotolerance of locust eggs has a complex genetic basis and heat shock proteins may be involved in differences of thermotolerance between locust populations.

Molecular, morphological and behavioural data reveal the presence of a cryptic species in the widely studied Drosophila serrata species complex

The Drosophila serrata species complex from Australia and New Guinea has been widely used in evolutionary studies of speciation and climatic adaptation. It is believed to consist of D. serrata, D. birchii and D. dominicana, although knowledge of the latter is limited. Here we present evidence for a previously undescribed cryptic member of the D. serrata species complex. This new cryptic species is widespread in far north Queensland, Australia and is likely to have been previously mistaken for D. serrata. It shows complete reproductive isolation when crossed with both D. serrata and D. birchii. The cryptic species can be easily distinguished from D. serrata and D. birchii using either microsatellite loci or visual techniques. Although it occurs sympatrically with both D. serrata and D. birchii, it differs from these species in development time, viability, wing size and wing morphology. Its discovery explains patterns of recently described mitochondrial DNA divergence within D. serrata, and may also help to clarify some ambiguities evident in early evolutionary literature on reproductive incompatibility within the D. serrata species complex.

Low potential for climatic stress adaptation in a rainforest Drosophila species

The ability of sensitive rainforest species to evolve in response to climate change is largely unknown. We show that the Australian tropical rainforest fly Drosophila birchii exhibits clinal variation in desiccation resistance, but the most resistant population lacks the ability to evolve further resistance even after intense selection for over 30 generations. Parent-offspring comparisons indicate low heritable variation for this trait but high levels of genetic variation for morphology. D. birchii also exhibits abundant genetic variation at microsatellite loci. The low potential for resistance evolution highlights the importance of assessing evolutionary potential in targeted ecological traits and species from threatened habitats.

The genetic bases of divergence in desiccation and starvation resistance among tropical and temperate populations of Drosophila melanogaster

Desiccation and starvation resistance are two stress-related traits which vary geographically with climate in Drosophila melanogaster. To investigate the contribution of epistasis to population divergence for these traits, we crossed tropical and temperate populations from two different geographical regions to produce F1, F2 and first backcross generations. Line-cross analysis of generation means revealed that genetic bases of divergence for both traits were complex and remarkably similar in a number of respects. Strong additive and dominance effects were present in most of the models, whereas epistatic and maternal effects were less common. The presence of epistatic effects in approximately half of the models presented in this study is consistent with line-cross studies of diverged traits in other animals, and does not support the view that epistasis is the predominant means by which populations diverge. In addition, evidence of maternal effects in both traits adds to a growing body of recent evidence that suggests maternal contributions to population differentiation are more widespread than previously thought. This finding undermines the accuracy of studies inferring epistasis directly from the magnitude of F2 breakdown. More line-cross analysis studies of naturally diverged populations that take into account maternal effects will shed further light on the true incidence of epistasis and its importance in the evolutionary process.

Levels of variation in stress resistance in drosophila among strains, local populations, and geographic regions: patterns for desiccation, starvation, cold resistance, and associated traits

Stress resistance traits in Drosophila often show clinal variation. Although these patterns suggest selection, there is generally no attempt to test how large differences at the geographical level are relative to levels of variation within and between local populations. Here we compare these levels in D. melanogaster from temperate Tasmania versus tropical northern Queensland by focusing on adult resistance to desiccation, cold and starvation stress, as well as associated traits (size, lipid content). For starvation and desiccation resistance, levels of variation were highest among strains from the same population. whereas there was little differentiation among local populations and a low level of differentiation at the geographic level. For adult cold resistance, there was local differentiation and strain variation but no geographic variation. For size (thorax length), geographic differentiation was higher despite some overlap among strains from the tropical and temperate locations. Finally, for lipid levels there was only evidence for variation among strains. The low level of differentiation among geographic locations for stress resistance was further verified with the characterization of isofemale strains from 18 locations along a coastal transect extending from Tasmania to northern Queensland. Crosses among some of the isofemale strains showed that results were not confounded by inbreeding effects. Strains derived from a cross between a tropical and temperate strain differed for all traits, and variation among strains for body size was higher than strain variation within the geographic regions. Unlike in previous studies, lipid content and starvation resistance were not correlated in any set of strains, but there was a correlation between cold resistance and lipid content. There was also a correlation between desiccation resistance and size but only in the geographic cross strains. These findings suggest a large amount of variation in stress resistance at the population level and inconsistent correlation patterns across experimental approaches.

Evolution of water balance in the genusDrosophila

Fruit flies of the genus Drosophila have independently invaded deserts around the world on numerous occasions. To understand the physiological mechanisms allowing these small organisms to survive and thrive in arid environments, we performed a phylogenetic analysis of water balance in Drosophila species from different habitats. Desert (cactophilic) species were more resistant to desiccation than mesic ones. This resistance could be accomplished in three ways: by increasing the amount of water in the body, by reducing rates of water loss or by tolerating the loss of a greater percentage of body water (dehydration tolerance). Cactophilic Drosophila lost water less rapidly and appeared to be more tolerant of low water content, although males actually contained less water than their mesic congeners. However, when the phylogenetic relationships between the species were taken into account, greater dehydration tolerance was not correlated with increased desiccation resistance. Therefore, only one of the three expected adaptive mechanisms, lower rates of water loss, has actually evolved in desert Drosophila, and the other apparently adaptive difference between arid and mesic species (increased dehydration tolerance) instead reflects phylogenetic history.

Desiccation and starvation resistance in Drosophila: patterns of variation at the species, population and intrapopulation levels

A substantial number of Drosophila studies have investigated variation in desiccation and starvation resistance, providing an opportunity to test for consistent patterns of direct and correlated responses across studies and across the species and population levels. In general, responses to laboratory selection for these traits in D. melanogaster are rapid and indicate abundant genetic variation in populations. However, slower responses to selection for desiccation resistance occur in other species including D. simulans. Clines suggest adaptive divergence although specific selection pressures have not been documented empirically. Drosophila species differ markedly in desiccation and starvation resistance and there is also marked variation within species for desiccation resistance that may be linked to local climatic conditions. Laboratory selection experiments on starvation resistance in D. melanogaster suggest that changes in lipid content are largely responsible for resistance variation but this factor may be less important in explaining variation among species. For desiccation, lines with increased resistance show reduced rates of water loss but no changes in the minimum water content that flies can tolerate. Changes in life history traits are sometimes associated with altered levels of stress resistance. Increased starvation resistance is associated with longer development time and reduced early age reproduction in different studies. However, other associations are inconsistent between studies as in the case of stress resistance changing following selection for longevity. Multiple mechanisms may underlie genetic variation in stress resistance and future studies should address the evolutionary importance of the different mechanisms at the population and species levels.

Does interspecific hybridization influence evolutionary rates? An experimental study of laboratory adaptation in hybrids between Drosophila serrata and Drosophila birchii

The low initial fitness of progeny from interspecific crosses in animals and the rarity of interspecific hybridization in natural environments have led to a debate about the evolutionary importance of this phenomenon. Here we directly assess the effects of hybridization between Drosophila serrata and Drosophila birchii on evolutionary rates. We looked at the effects on laboratory adaptation over 30 generations in two laboratory environments, one of which involved nutrition and temperature stress. Laboratory adaptation occurred over time in both environments as reflected by a marked change in viability. However, whilst hybrid lines at no stage performed poorly relative to parental lines, their rate of adaptation never exceeded that of the parentals. Thus, there was no evidence that hybridization increased evolutionary rates. Instead, hybrid lines converged phenotypically with one of the parental species.