Section 12 — Plasmatic Inheritance 



stances of presumed cytoplasmic inheritance in 

 Drosophila are the consequences of hereditary 

 infection raises the question of the nature of 

 all non-nuclear inheritance in that genus. 



The maternally transmitted condition referred 

 to as "sex-ratio", SR, in D. willistoni and D. 

 nebulosa was shown to be the result of the pre- 

 sence in the particular strains of hereditarily 

 transmitted spirochetes closely resembling those 

 of the genus Treponema. This was subsequently 

 demonstrated to be true in the cases of SR in 

 other related species, equinoxialis (Malogolow- 

 kin) and paulistorum (Malogolowkin and Poul- 

 son), of the neotropical willistoni group. On the 

 other hand, no direct evidences of spirochetes 

 could be found in the SR strain of the palearctic 

 species bifasciata. Success in transfer of the SR 

 condition from D. willistoni into D. melanogaster 

 and D. pseudoobscura provided unequivocal 

 proof that the spirochetes are the etiological 

 agents of SR. Expression and stability of the 

 transferred SR have been shown to be depen- 

 dent on a number of factors of which host geno- 

 type, spirochete strain, level of infection, and 

 environmental conditions, such as temperature, 

 are important. 



Introduction of SR spirochetes from D. wil- 

 listoni into D. bifasciata has been carried out 

 with the result that a "sex-ratio" condition has 

 been artificially established showing many of 

 the features associated with naturally occurring 

 "sex-ratio" in D. bifasciata. The data suggest that 

 the SR condition in that species, although of 

 such long standing as to have lost its infective 

 character, had its origin in a similar way from 

 an infection which has now become wholly 

 stabilized. Other evidence indicates that here- 

 ditary infections are widespread in nature and 

 are the probable sources of most cases of plas- 

 matic inheritance. Our data will appear in a 

 series of papers being publishedin Geneti cs. 



The relationship between both mechanisms 

 has been studied by means of ovary transplanta- 

 tions and of injection of cell-free extracts. Es- 

 pecially the last technique proved to be useful, 

 consisting in injecting extract of tumourous 

 flies tu A2 (recessive abdominal and thoracic 

 tumours) into tumourless ones. The results point 

 to a transmission of a factor which — originally 

 located in the cytoplasm and capable to migrate 

 from cell to cell — becomes eventually integrated 

 in the germ-plasm, and is transmitted with equal 

 efficiency by the egg and by the sperm. 



Using the dominant mutant Freckled, lethal 

 in double dose, and apparently located in the 

 2nd chromosome, it proved possible to obtain 

 again a transmission to the progeny of non 

 Freckled individuals, injected with Freckled 

 extract. Transmission through the sperm has 

 been also demonstrated. 



Moreover, Freckled can be transmitted also in 

 a way which recalls paramutation in maize, 

 since from heterozygotes Freckled, one recovers 

 a proportion of cychromosomes "infected" with 

 Freckled. 



The results obtained so far show a mode of 

 transmisson which can be understandable in the 

 best way interpreting the involved factor as a 

 unit similar to an episome, the integration of 

 which within the nucleus and the cytoplasm is 

 demonstrated, while the integration within the 

 chromosome still requires further work to be 

 elucidated. 



12.20. Action of FUDR on the Multiplication of 

 Virus of Drosophila. N. Plus (Gif sur Yvette, 

 France). 



12.19. A Case of Episome-like Transmission in Dro- 

 sophila melanogaster (Melanotic Tumouis). 

 C. Barigozzi, A. M. Kravina and M. Sari 

 Gorla (Milan, Italy). 



Melanotic tumours in Drosophila prove to be 

 a chromosomally inherited trait, when tested 

 with mendelian crosses; nonetheless, there are 

 different reasons to believe that an additional 

 extrachromosomal mechanism is at work, which 

 collaborates with the former one. The chromo- 

 somal mechanisms seems to be the most efficient, 

 while the extrachromosomal one seems to be 

 rather weak. 



FUDR delays the multiplication of o virus, 

 when injected in the flies together with the virus. 

 This inhibition is not reversed by thymidine. 

 This nucleoside, by itself, acts as an inhibitor 

 of cr multiplication and its action is synergic 

 of FUDR action. The inhibition of virus produc- 

 tion by either FUDR or by thymidine is reversed 

 by addition of an equal quantity of deoxycytidi- 

 ne. FUDR seems, with respect to the virus multi- 

 plication, to act as an analogue of thymidine, 

 blocking deoxycytidine formation. These facts 

 seem to indicate that a virus contains DNA. This 

 conclusion is enhanced by the finding that both 

 FU and RNAase are ineffective on a virus multi- 

 plication. It has been so far impossible to ana- 

 lyse the virus nucleic acid directly. 



207 



