o 



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unit, as we used to think. There is conflict 

 within the genome. 



How does such genetic conflict begin? 

 For instance, where did PSR come from? 

 Trying to answer such questions brought 

 me back, somewhat circuitously, to Wol- 

 bachia. Ail the jewel wasps I have col- 

 lected from the wild carry these intracellu- 

 lar bacteria, which reside in cells of the 

 male and female reproductive tract. The 

 bacteria can be quite numerous: jewel 

 wasps typically harbor one to two thou- 

 sand in every egg. 



At first glance, Wolbachia would appear 

 to be simply going along for the ride. 

 However, when we cure insects of the 

 bacteria by treating them with antibiotics 

 or arrange matings between insects carry- 

 ing different strains of Wolbachia, we find 

 that the bacteria exercise considerable 

 control over the insects' reproduction. 



In these developing jewel wasp sperm 

 cells, the stubby, darkly stained PSR 

 chromosomes (see arrows) lie next to the 

 larger, lighter chromosomes that 

 they will ultimately destroy. 



Wolbachia bacteria in a male's testes can- 

 not be transmitted via the sperm, but they 

 do modify his chromosomes, probably by 

 producing proteins that bind to the sperm's 

 DNA. Unless bacteria of the same strain 

 are also present in the egg to undo this 

 modification, the sperm-delivered chro- 

 mosomes will fragment and be destroyed 

 in the fertilized egg. For most insects, this 

 results in the death of the embryo. In the 

 jewel wasp, the outcome is less than lethal: 

 it results in (haploid) males. 



The bacteria benefit indirectly because 

 ehminating the daughters of females who 

 do not have the same bacterial strain actu- 

 ally increases the frequency of that strain 

 in the population. By this mechanism, in- 

 fected females can eventually predomi- 

 nate, as is seen in populations of jewel 

 wasps and many other insects. 



Some scientists have speculated that 

 control by Wolbachia over the insects' re- 

 production may be important in the evolu- 

 tion of new species. A key step in specia- 

 tion is reproductive isolation of 

 populations, which allows them to evolve 

 in divergent directions. If bacteria cause 

 reproductive incompatibility between 

 populations that once interbred, bacteria 

 may also promote speciation. 



The situation in jewel wasps suggests 

 this may indeed happen. Nasonia vitripen- 

 nis, which lives throughout the world, has 

 two close relatives in North America: N. 

 longicomis, in the west, and N. giraulti, in 

 the east. The cosmopolitan N. vitripennis 

 overlaps with the two others in some 

 places, making hybridization between 

 them a real possibility. In our lab, we have 

 found that while the three different species 

 of jewel wasp wiU mate with one another, 

 no hybrid progeny result. Closer examina- 

 tion reveals that chromosomes from sperm 

 are chopped up into httle pieces in the fer- 

 tiUzed egg. However, when we cured the 

 wasps of their Wolbachia infections and 

 repeated the crosses, true hybrid progeny 

 developed. In other words, reproductive 

 isolation is "curable." 



What does all this have to do with PSR? 

 Occasionally in incompatible crosses a 

 piece of chromosome survives the frag- 

 mentation process and is passed on to the 

 next generation. Bryant McAllister, a 

 graduate student in my laboratory, has 

 found that DNA sequences on PSR are 

 much more similar to DNA from A^. longi- 

 comis than to DNA of the jewel wasp, in- 

 dicating that PSR is an "ahen chromo- 

 some" that came from the former species 

 during an incompatible cross. One of the 

 pieces of PSR DNA that McAUister has 

 studied is itself a transposon, which makes 

 it a piece of parasitic DNA on a piece of 

 parasitic DNA, generated by a parasitic 

 bacterium within a parasitic wasp. PSR 

 may owe yet another debt to the Wol- 

 bachia — its ability to destroy chromo- 

 somes. We are now testing the possibility 

 that PSR acquired the relevant genetic ma- 

 terial from Wolbachia by genetic ex- 

 change during formation of the chromo- 

 some. 



Wolbachia are turning out to be quite 

 common in insects. During one trip to the 

 rain forests of Panama, for example, I col- 

 lected and examined more than a hundred 

 species and found that more than 5 percent 

 were infected with Wolbachia. Extrapolat- 

 ing to the global insect fauna, which is cur- 

 rently estimated to be at least five million 

 species, an amazing 250,000 species may 

 be infected with Wolbachia. Only time 

 will tell whether these reproductive para- 

 sites are important in the evolution of new 

 species, but the possibility is tantalizing. 

 At any rate, I have had to give up my con- 

 ception of an organism as a strictly coop- 

 erative unit. When I peer through a micro- 

 scope these days, I am no longer even 

 certain where one organism ends and an- 

 other begins. 



38 Natural History 6/94 



