Genetic 

 Invasion of 

 the Insect 

 a Body 

 S Snatchers 



o 

 > 



By controlling sex and survival, 



some parasites can turn their 



hosts into new species 



by Jack Werren 



My first encounter with a jewel wasp in 

 the wild occurred along a roadside in the 

 mountains of Utah. I had stopped to inves- 

 tigate a porcupine that had been run over 

 several weeks previously. Flies had long 

 since arrived and done their handiwork. 

 All that remained of the original animal 

 was skin, bones, and quills. Beneath the 

 skin, thousands of fly larvae had pupated 

 and were metamorphosing into adults. But 

 another organism was doing to the flies 

 what they had done to the porcupine. This 

 was the jewel wasp, Nasonia vitripennis. 



SmaU (about 3 mm long) and gnatlike, 

 the jewel wasp is unremarkable to the 

 naked eye, but seen through a microscope, 

 it is a beauty. Its finely faceted body shim- 

 mers with iridescent colors that change 

 with die angle of light. A female jewel 

 wasp seeks out fly pupae and kills them by 

 injecting them with venom. She then lays 

 twenty to forty eggs in each fly puparium. 

 The eggs hatch into larvae one to two days 

 later and begin to devour the meal pro- 

 vided by their mother. In about two more 

 weeks, the adult wasps emerge. Tlie short- 

 winged, flightless males mate and die in 

 the patch of fly pupae they were bom in. 

 The newly emerged winged females fly 

 off immediately after mating in search of 

 fresh fly pupae in which to lay their eggs. 



What originally attracted me to these 

 creatures was the female's ability to con- 

 trol the sex of her offspring. In wasps, 

 bees, and ants, males develop from unfer- 

 tilized eggs and are haploid (that is, fliey 

 have just one set of chromosomes, inher- 

 ited from die mother), whereas females 

 develop from fertilized eggs and are 



diploid (with two sets of chromosomes, 

 one from each parent). After mating, the 

 female jewel wasp stores sperm in a spe- 

 cial organ called a spermatheca. This 

 organ resembles a balloon with a strawlike 

 tube at one end; attached to the nibe is a 

 muscle that can either straighten out and 

 allow sperm to pass to the egg (resulting in 

 a daughter) or can crimp the tube and 

 block the sperm (resulting in a son). How 

 many daughters a female produces de- 

 pends on a number of factors, including 

 whether she is the first wasp to lay eggs in 

 a fly pupa (in which case shell lay mostly 

 daughters) or the second (in which case 

 she will lay more sons). 



Despite the female's impressive ability 

 to influence the sexual identity of her 

 progeny, her control is far from complete. 

 The jewel wasps, like the porcupine and 

 the fly larvae before them, are fliemselves 

 victims of parasites. They harbor an as- 

 semblage of genetic parasites that can alter 

 an insect's reproductive system for their 

 own advantage. 



The jewel wasp is not alone in this. As 

 scientists have discovered over the last 

 decade, virtually all organisms carry ge- 

 netic parasites that perpetuate fliemselves 

 at the expense of flieir host. Some of these 

 parasites are bacteria "inherited" from one 

 generation to the next through die host or- 



36 Natural History 6/94 



