A paper that appeared in Scientific Reports by Evans et al. (1)asked whether changes in the genotypes of target mosquito populations would occur due to introgression of released transgenic mosquito genomes. The paper in fact describes the introgression of released mosquito strain genome into wild populations of Aedes aegypti after releases of the OX513A strain developed by Oxitec and the paper has generated a lot of controversy. Is this a surprising finding or an entirely predictable outcome? Is it cause for alarm?
When transgenic or radiation-sterilized insects are released, the genetic barrier between the released and wild-type target populations is seldom, if ever, perfect, and in some cases could be non-existent. Even in the ‘Sterile’ Insect Technique (SIT), sterility is usually incomplete – often by design. This is often the chosen approach for radiation-sterilized insects because males become less competitive for mates as the radiation dose increases; the suppressive effect may increase as the individual sterility decreases.
This is because sterility causes a population effect, not simply an individual one. It’s more effective to release more-competitive but partially sterile males than uncompetitive and fully sterile ones. While developers may refer to a ‘sterile’ insect, this is often shorthand for the overall effect rather than an absolute description of the attribute.
In contrast, transgenic mosquitoes might be created that result in almost full sterility (i.e. fully penetrant lethality in this case) with little loss of competitiveness, an approach that was considered by Oxitec (2). The OX513A strain of Aedes aegypti was not one of these. It resulted in 3-4% of F1 adult progeny. To my knowledge, Oxitec did not claim that the OX513A mosquitoes were fully sterile nor that the lethal phenotype was fully penetrant, but a casual observer might take that understanding away.
Evans et al. observed that after releases of millions– repeat millions- of transgenic males into the city of Jacobina, Brazil that the frequency of exotic single-nucleotide polymorphisms (SNPs) from the release strain genome increased so that 10-60% of the individuals contained detectable release-strain SNPs. This demonstrated that release strain genotypes were being introduced into the population. Small, but detectable levels of the OX513A genomes were also observed in nearby populations, presumably due to migration.
This ‘introgression’ effect was detectable because the release strain had exotic genotypes, a unique fingerprint if you will. If the transgene had been introgressed into the Jacobina genetic background prior to releases, the effect not only could not have been detected, but it should not have resulted in an effect that might be of concern.
To be clear, the authors did not describe the spread or persistence of the transgene itself, only of the genome of the release strain.
This is likely not an isolated case and has nothing to do with transgenic or Oxitec technology per se. A similar effect is certainly occurring wherever a strain of transgenic or irradiated insects is released when sterility is not absolute and the release and target genomes are different. The salient question is, does harm result?
One possible avenue of harm that might result would be if the release strain conferred an ability to transmit a pathogen at higher rates than the target population. In this case, the study determined that as far as this was examined, there was no evidence to indicate this (a result puzzlingly presented in the Evans et al. Discussion section). Another possibility, greater insecticide resistance, had been dismissed previously by studies conducted at LSTM (3).
The Evans et al. paper includes another possible harm. They speculate in the Discussion section that hybrid vigor would result in a ‘more robust population than the pre-release population’. Ok. That’s commonly observed in many biological systems and is not a controversial statement though less certainty might be prudent.
In my estimation, less well-supported is their attribution of diminished effectiveness of releases at one point to ‘discrimination against OX513A males, a phenomenon known to occur in sterile male release programs.’ for which they cite a Concept Paper monograph by the senior author(4), which in turn cites a single MedFly example(5). I don’t feel the generalization is merited but the reviewers disagreed I suppose. I also cannot wrap my head around why if the target population was becoming more similar to the release strain that assortative mating would arise late in the program. In the same context, the statement that ‘This observation also implies that introgressed individuals may be at a selective disadvantage…’ seems at odds with the hybrid vigor hypothesis elsewhere.
The authors do not discuss other possible well-established causes of reduced suppression including changes in mass-rearing, holding and release methods, all of which are clearly causes of loss of effect, but these are the kinds of suggestions that reviewers should have made.
Much of the ‘surprise’ surrounding this paper implies that the introgression of the release genome was unexpected. Given what has been published, there was no reason to believe that introgression would not occur. The question was how much. In fact, developers of the Oxitec technology have previously detailed the possibility of introgressing beneficial alleles into target populations using a variation of their transgenic technology (6). Was anybody listening?
The developers of the OX513A strain had reported that adults were observed among progeny of transgenic females and males at rates of 4.2 and 2.6% respectively (Table 1 of (2)and possibly elsewhere) and repeatedly acknowledge incomplete penetrance, so if there is surprise surrounding the existence of F1 adults and the possibility of matings, it’s not a result of misrepresentation by the developers as has been implied in the press.
Sterile Insect Technique and similar methods inundate target populations so there are far more released males than those in the target population – 100 times as many is not uncommon – so if introgression had NOT been observed it would have been surprising. Millions of transgenic males were released over several months (Table 2, panel B of (3)), and the developers acknowledged survival to adulthood, so some degree of transgenic strain introgression was inevitable. Evans et al. have served the field well to describe an instance of this and their speculation regarding changes in the population should stimulate follow-up observations.
If, as Evans et al. suggest, a more robust population has resulted, this is of concern.
In my estimation, further research into whether the Jacobina population is indeed more robust and now mates assortatively seems not only feasible but extremely enlightening. One good experiment stops endless arguments and the authors are well-suited to pursue this.These populations offer an ideal opportunity to test some of the generalizations that are stated. The results would determine whether the controversy surrounding this paper is a tempest in a teapot or indicate that a significant concern has been realized.
The take-home lesson is that considerable numbers of low frequency events will occur when the magnitude of all events is great. That’s neither surprising nor alarming.
(I welcome comments. Please stay on topic, polite and reference specific language above if you'd like to make a suggestion for improvement of the blog. Thanks for reading. The opinions above are mine alone.)
1. Evans BR, Kotsakiozi P, Costa-da-Silva AL, Ioshino RS, Garziera L, Pedrosa MC, et al. Transgenic Aedes aegyptiMosquitoes Transfer Genes into a Natural Population. Sci Rep. Springer US; 2019 Sep 2;:1–6.
2. Phuc H, Andreasen MH, Burton RS, Vass C, Epton MJ, Pape G, et al. Late-acting dominant lethal genetic systems and mosquito control. BMC Biol. 2007;5(1):11.
3. Carvalho DO, McKemey AR, Garziera L, Lacroix R, Donnelly CA, Alphey L, et al. Suppression of a Field Population of Aedes aegypti in Brazil by Sustained Release of Transgenic Male Mosquitoes. Olson KE, editor. PLoS Neglected Tropical Diseases. 2015 Jul 2;9(7):e0003864.
4. Powell J. Genetic Variation in Insect Vectors: Death of Typology? Insects. 2018 Dec;9(4):139–15.
5. McInnis DO, the DLAO, 1996. Behavioral resistance to the sterile insect technique by Mediterranean fruit fly (Diptera: Tephritidae) in Hawaii. annals of the Entomological Society of America.
6. Alphey N, Coleman PG, Donnelly CA, Alphey L. Managing insecticide resistance by mass release of engineered insects. Journal of Economic Entomology. 2007 Oct;100(5):1642–9.