A friend and colleague asked me whether all the media excitement about transgenic mosquitoes represents real accomplishments that beat non-transgenic sterile insect technique (SIT). Good question. In the first of a two-part blog, I’ll tell you where I think things stand, first in SIT. In the second part, I’ll look at population replacement.
This blog is about genetic control which I define as “Dissemination of factors that reduce damage using mating and inheritance.” The “factor” in this blog is sexual sterility primarily. I’m going to stick to two SIT(-like) technologies that are in field trials. I want to focus particularly on classical sterile insect technique in which radiation is used to sexually sterilize males and Release of Insects Carrying Dominant Lethals (RIDL,TM, Oxitec). The latter transgenic technology confers lethality to progeny in the field, thus theoretically increasing the effect by allowing them to compete with wild larvae before death.
By going to the field first, RIDL is taking it on the chin as the game to beat. Oxitec is the unquestionable leader in implementation of transgenic suppression of Aedes aegypti. They have been active in releases for control in both Grand Cayman and Brazil, and from what we know about the former case, their mosquitoes showed good target population suppression. Fortunately, lack of a specific female-elimination system is mitigated by the fact that Ae. aegypti females can be removed by their greater pupa size, albeit one must culture all those needless females first, thus reducing efficiency. (Transferring this technology to Ae. albopictus won’t be quite as easy as male and female pupa sizes overlap significantly. This makes one wonder what happened to the original system of RIDL in Drosophila in which sex-specific tetracycline-repressible female lethality was accomplished. Be nice to have that working in mosquitoes now.)
How does RIDL potential and realization stack up against Aedes classical SIT right now? On paper, RIDL wins. But on the ground, unfortunately we don’t know much. Romeo Bellini is quietly reporting suppression of Ae. albopictus in Italy using a traditional irradiation approach that does not appear to reduce competitiveness greatly, also using pupa size selection for males. While models suggest that RIDL technology will be more effective because of superior mating competitiveness due to lack of irradiation and (also according to models) greater effectiveness in suppression due to larval competition, if the latter has been demonstrated experimentally, I’ve missed it.
The team of scientists led by Jeremie Gilles in Vienna is also busily developing classical SIT, mainly against Anopheles arabiensis, but Aedes are in their sights and in their labs, so it will be an interesting competition to watch as greater activity occurs in the field matching classical SIT against RIDL.
Now, both the Vienna and Italy groups have enough experience that if they were asked to set up an aegypti SIT program in Grand Cayman or Brazil using their diets, irradiation and equipment, they could pull it off using locally collected mosquitoes. Would they be as effective as a RIDL release? That would be an interesting experiment that I suspect will never be conducted, but Bellini might welcome the opportunity to get away given the disruptions to their activities caused by the recent earthquakes!
Oxitec has taken the lead and put feet on the ground, and RIDL aegypti look promising, but don’t write off classical SIT. Unfortunately, there is not enough experience with either the transgenic or classical approaches to conclude that in the field – which is the only place it matters - either has great technical advantages. Models are great. Experiments are better.
Oxitec is publishing results to allow an assessment – but in isolation. Unfortunately, the classical SIT efforts have not resulted in publication of trial results that permit a fair comparison. This is unfortunate since the obstacles to trials of classical SIT are much lower than those for transgenics.
So in my opinion, there is still great potential for classical SIT. Oxitec is in the spotlight because of their leadership, but it’s too early to say whether their technology will better classical SIT.