These minute jewel wasps, seen here 

 against grains of sand, are parasites that 

 are themselves victims of parasitic 

 bacteria and parasitic DMA. 



Ed Bridges 



ganism's eggs (see "Feminist Bacteria of 

 Ladybird Beetles," page 32). Others are 

 actual pieces of DNA that reside in the 

 host organism's chromosomes. For ex- 

 ample, in most organisms small, mobile 

 pieces of DNA called transposons make 

 and insert extra copies of themselves in the 

 chromosomes of their hosts. Humans have 

 hundreds of thousands of copies of a trans- 

 posonlike element called Alu that makes 

 up more than 5 percent of our DNA. This 

 parasite is relatively benign, although 

 every once in a while it causes a harmful 

 mutation by inserting itself in the wrong 

 place. Other organisms, such as mosqui- 

 toes, mice, and fruit flies, have parasitic 



chromosomes that are able to insure that 

 they end up in all the host's reproductive 

 cells, rather than just half, as would nor- 

 mally occur during meiosis. 



What makes the jewel wasp unusual is 

 the variety of genetic parasites it harbors 

 and the severity of their effects. Not all in- 

 dividuals are infected with all these para- 

 sites at any given time, but among those 

 commonly found are bacteria that kill 

 male embyros; a second element (which 

 we have not identified yet) that is transmit- 

 ted only through eggs and that causes the 

 wasp to produce nearly 100 percent 

 daughters; and a bacterium called Wol- 

 bachia that prevents the development of 



hybrid offspring engendered by the mat- 

 ing of jewel wasps with wasps of closely 

 related species. But the most remarkable 

 piece of parasitic DNA found in the jewel 

 wasp is the paternal sex ratio chromosome, 

 PSR for short. 



PSR is a killer chromosome. Diminu- 

 tive — about one-fifth the size of a regular 

 chromosome — it is found only in some 

 males of the species. PSR hitches a ride in 

 the spermatozoon along with the other 

 chromosomes. Just as picking up human 

 hitchhikers can sometimes be dangerous, 

 sharing a sperm with PSR is fatal for its 

 fellow travelers. 



After an egg is fertihzed, PSR destroys 

 all the other paternal chromosomes, caus- 

 ing them to condense into a mass, which is 

 eventually lost during development. PSR 

 alone survives to join the maternal chro- 

 mosomes within the egg. Without the frat- 

 ricidal action of PSR, the egg would have 

 been diploid, and the fertile embryo would 

 have developed into a female. With PSR 

 on board, the fertihzed egg will remain 

 haploid and produce a male. This sex 

 change is advantageous for the parasite 

 because PSR in male wasps is transmitted 

 to 100 percent of the spermatozoa (and 

 thus to the next generation). But PSR 

 stuck in a female tends to get lost during 

 meiosis and reaches significantly less than 

 50 percent of her eggs. 



PSR is not only a killer of chromo- 

 somes; it is also a serial killer In each gen- 

 eration, it becomes associated with and de- 

 stroys a new set of chromosomes, 

 converting females into males. Because 

 this chromosome is so deadly, inevitably 

 eliminating all the chromosomes with 

 which it is associated, generation after 

 generation, it is considered the most ex- 

 treme example of parasitic DNA so far 

 identified from any species. 



Genetic parasites such as PSR chal- 

 lenge our basic concept of what an organ- 

 ism is. For example, PSR is pan of the 

 jewel wasp's DNA, but it is harmful to the 

 rest of the genetic material. We now know 

 that most organisms contain a variety of 

 parasitic DNAs. Certainly an organism's 

 genome is not a completely cooperative 



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