Philip T. Leftwich

Evolutionary biology and genetic techniques for insect control

The requirement to develop new techniques for insect control that minimize negative environmental impacts has never been more pressing. Here we discuss population suppression and population replacement technologies. These include sterile insect technique, genetic elimination methods such as the release of insects carrying a dominant lethal (RIDL), and gene driving mechanisms offered by intracellular bacteria and homing endonucleases. We also review the potential of newer or underutilized methods such as reproductive interference, CRISPR technology, RNA interference (RNAi), and genetic underdominance. We focus on understanding principles and potential effectiveness from the perspective of evolutionary biology. This offers useful insights into mechanisms through which potential problems may be minimized, in much the same way that an understanding of how resistance evolves is key to slowing the spread of antibiotic and insecticide resistance. We conclude that there is much to gain from applying principles from the study of resistance in these other scenarios – specifically, the adoption of combinatorial approaches to minimize the spread of resistance evolution. We conclude by discussing the focused use of GM for insect pest control in the context of modern conservation planning under land‐sparing scenarios.

Genetic elimination of field-cage populations of Mediterranean fruit flies

The Mediterranean fruit fly (medfly, Ceratitis capitata Wiedemann) is a pest of over 300 fruits, vegetables and nuts. The sterile insect technique (SIT) is a control measure used to reduce the reproductive potential of populations through the mass release of sterilized male insects that mate with wild females. However, SIT flies can display poor field performance, due to the effects of mass-rearing and of the irradiation process used for sterilization. The development of female-lethal RIDL (release of insects carrying a dominant lethal) strains for medfly can overcome many of the problems of SIT associated with irradiation. Here, we present life-history characterizations for two medfly RIDL strains, OX3864A and OX3647Q. Our results show (i) full functionality of RIDL, (ii) equivalency of RIDL and wild-type strains for life-history characteristics, and (iii) a high level of sexual competitiveness against both wild-type and wild-derived males. We also present the first proof-of-principle experiment on the use of RIDL to eliminate medfly populations. Weekly releases of OX3864A males into stable populations of wild-type medfly caused a successive decline in numbers, leading to eradication. The results show that genetic control can provide an effective alternative to SIT for the control of pest insects.

Variation in adult sex ratio alters the association between courtship, mating frequency and paternity in the lek‐forming fruitfly Ceratitis capitata

The intensity with which males deliver courtship and the frequency with which they mate are key components of male reproductive success. However, we expect the strength of the relationship between these traits and a male’s overall paternity to be strongly context dependent, for example to be altered significantly by the extent of post‐mating competition. We tested this prediction in a lekking insect, Ceratitis capitata (medfly). We examined the effect of manipulating the sex ratio from male‐ to female‐biased (high and low male competition, respectively) on courtship behaviour, mating frequency and paternity of focal males. Under high male competition, focal males delivered significantly more courtship but gained lower paternity than under lower competition. Paternity was positively associated with mating frequency and small residual testes size. However, the association between mating frequency and paternity was significantly stronger under low competition. We conclude that manipulation of sex ratio significantly altered the predictors of mating success and paternity. The relationship between pre‐ and post‐mating success is therefore plastic and alters according to the prevailing level of competition. The results highlight the importance of post‐copulatory processes in lekking species and illuminate selection pressures placed on insects such as medflies that are mass reared for pest control.

Gut microbiomes and reproductive isolation in Drosophila

Significance The evolutionary significance of assortative mating by diet, mediated by gut bacteria is a puzzle, but it has had a huge impact and has provided a keystone to support increasing interest in the “holobiome.” However, in species such as Drosophila melanogaster that have flexible gut microbiomes, any reproductive isolation mediated by gut bacteria specific to host diets can only be transient. Here, we replicated and extended tests of this idea. Despite differences in gut microbiomes, we failed to recover previously observed patterns of nonrandom mating and found no evidence that mating preferences were associated with diet or gut bacteria. This suggests that the evolutionary importance of gut microbiomes in host divergence needs careful consideration on a case-by-case basis. Experimental studies of the evolution of reproductive isolation (RI) in real time are a powerful way in which to reveal fundamental, early processes that initiate divergence. In a classic speciation experiment, populations of Drosophila pseudoobscura were subjected to divergent dietary selection and evolved significant positive assortative mating by diet. More recently, a direct role for the gut microbiome in determining this type of RI in Drosophila melanogaster has been proposed. Manipulation of the diet, and hence the gut microbiome, was reported to result in immediate assortative mating by diet, which could be eliminated by reducing gut microbes using antibiotics and recreated by adding back Lactobacillus plantarum. We suggest that the evolutionary significance of this result is unclear. For example, in D. melanogaster, the microbiome is reported as flexible and largely environmentally determined. Therefore, microbiome-mediated RI would be transient and would break down under dietary variation. In the absence of evolutionary coassociation or recurrent exposure between host and microbiome, there are no advantages for the gut bacteria or host in effecting RI. To explore these puzzling effects and their mechanisms further, we repeated the tests for RI associated with diet-specific gut microbiomes in D. melanogaster. Despite observing replicable differences in the gut microbiomes of flies maintained on different diets, we found no evidence for diet-associated RI, for any role of gut bacteria, or for L. plantarum specifically. The results suggest that there is no general role for gut bacteria in driving the evolution of RI in this species and resolve an evolutionary riddle.

Vertically transmitted rhabdoviruses are found across three insect families and have dynamic interactions with their hosts

A small number of free-living viruses have been found to be obligately vertically transmitted, but it remains uncertain how widespread vertically transmitted viruses are and how quickly they can spread through host populations. Recent metagenomic studies have found several insects to be infected with sigma viruses (Rhabdoviridae). Here, we report that sigma viruses that infect Mediterranean fruit flies (Ceratitis capitata), Drosophila immigrans, and speckled wood butterflies (Pararge aegeria) are all vertically transmitted. We find patterns of vertical transmission that are consistent with those seen in Drosophila sigma viruses, with high rates of maternal transmission, and lower rates of paternal transmission. This mode of transmission allows them to spread rapidly in populations, and using viral sequence data we found the viruses in D. immigrans and C. capitata had both recently swept through host populations. The viruses were common in nature, with mean prevalences of 12% in C. capitata, 38% in D. immigrans and 74% in P. aegeria. We conclude that vertically transmitted rhabdoviruses may be widespread in a broad range of insect taxa, and that these viruses can have dynamic interactions with their hosts.

Reply to Rosenberg et al: Diet, gut bacteria and assortative mating in Drosophila melanogaster

Rosenberg et al. (1) suggest that the lack of evidence for assortative mating we reported (2) in comparison with a previous study (3) could be due to gut microbiomes in Drosophila melanogaster being affected by differing dietary history before culturing on the CMY (0.65% agar, 7.6% cornmeal, 7.6% molasses, 5% inactivated brewer’s yeast, 0.1% methyl-4-hydroxybenzoate, 0.76% ethanol, and 0.4% propionic acid) and starch (3% starch, 5% inactivated brewer’s yeast, 1% agar, 0.5% propionic acid) diets used by both studies (2, 3). Variation in methyl paraben (mp; also known as methyl 4-hydroxybenzoate or Nipagin) concentration is highlighted. Before initiating our experiments (2), flies were maintained on a diet containing 0.3% mp and then, as in … [↵][1]1To whom correspondence should be addressed. Email: tracey.chapman{at}uea.ac.uk. [1]: #xref-corresp-1-1

Adaptation to divergent larval diets in the medfly, Ceratitis capitata

Variation in diet can influence the timing of major life‐history events and can drive population diversification and ultimately speciation. Proximate responses of life histories to diet have been well studied. However, there are scant experimental data on how organisms adapt to divergent diets over the longer term. We focused on this omission by testing the responses of a global pest, the Mediterranean fruitfly, to divergent selection on larval diets of different nutritional profiles. Tests conducted before and after 30 generations of nutritional selection revealed a complex interplay between the effects of novel larval dietary conditions on both plastic and evolved responses. There were proximate‐only responses to the larval diet in adult male courtship and the frequency of copulation. Males on higher calorie larval diets consistently engaged in more bouts of energetic courtship. In contrast, following selection, larval development time, and egg to adult survival showed evidence of evolved divergence between diet regimes. Adult body size showed evidence for adaptation, with flies being significantly heavier when reared on their “own” diet. The results show the multifaceted responses of individuals to dietary selection and are important in understanding the extreme generalism exhibited by the medfly.

Reply to Obadia et al.: Effect of methyl paraben on host–microbiota interactions in Drosophila melanogaster

Obadia et al. (1) suggest that variation in the concentration of the fly media fungicide methyl paraben (mp) (also known as Tegosept or Nipagin) can restrict gut microbial growth and diversity (2, 3), and that this could provide insight into conflicting data on the role of the gut microbiome in generating positive assortative mating by diet in Drosophila melanogaster (4⇓–6). mp is reported to reduce the culturable bacterial diversity of wild Drosophila to a level comparable with established laboratory populations (2), although the effect of mp on the microbial diversity of long-established laboratory populations is not yet clear. This is consistent with … [↵][1]1To whom correspondence should be addressed. Email: tracey.chapman{at}uea.ac.uk. [1]: #xref-corresp-1-1

Diet, Gut Microbes and Host Mate Choice: Understanding the significance of microbiome effects on host mate choice requires a case by case evaluation

All organisms live in close association with microbes. However, not all such associations are meaningful in an evolutionary context. Current debate concerns whether hosts and microbes are best described as communities of individuals or as holobionts (selective units of hosts plus their microbes). Recent reports that assortative mating of hosts by diet can be mediated by commensal gut microbes have attracted interest as a potential route to host reproductive isolation (RI). Here, the authors discuss logical problems with this line of argument. The authors briefly review how microbes can affect host mating preferences and evaluate recent findings from fruitflies. Endosymbionts can potentially influence host RI given stable and recurrent co‐association of hosts and microbes over evolutionary time. However, observations of co‐occurrence of microbes and hosts are ripe for misinterpretation and such associations will rarely represent a meaningful holobiont. A framework in which hosts and their microbes are independent evolutionary units provides the only satisfactory explanation for the observed range of effects and associations.