. L'hérédité de la cellule. La cytogénétique. La recombinaison DANS LES VIRUS ET BACTÉRIES 135 où ils peuvent entrer en contact. Bien sûr, il pourrait être suggéré qu'une substance filtrable est toujours l'agent d'échange de gènes, mais qu'il est ad- sorbées sur le filtre, ou a une demi-vie très courte. L'assemblage de faits d'être présenté s'oppose à cette interprétation. En fait, la bactérie peut être observé à rejoindre l'un avec l'autre par paires. Dans ce comportement il y a un système de polarité ; pas chaque souche de bactéries s'accouplent avec chaque autre souche. Ceux qui sont appelés F ne s'assemblent pas avec eux, mais t

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. Cell heredity. Cytogenetics. RECOMBINATION IN VIRUSES AND BACTERIA 135 where they can come in contact. Of course, it could be suggested that a filterable substance is still the agent of gene exchange but that it is ad- sorbed on the filter, or has a very short half-life. The assemblage of facts to be presented precludes this interpretation. Actually, the bacteria can be observed to join with one another in pairs. In this behavior there is a system of polarity; not every strain of bacteria will mate with every other strain. Those that are referred to as F~ will not mate with each other, but they will mate with certain other strains called F"*" and Hfr. The Hfr (high frequency of recombination) strain is by far the most active. When Hfr bacteria are genetically marked by factors for motility and cell shape, they can be distinguished micro- scopically in the same suspension from F~ bacteria possessing the opposite characters. In such mixed cultures it is observed that members of the two strains quickly pair, and join together across a narrow cyto- plasmic bridge (Figure 5.14). This attachment is temporary, lasting about an hour. The paired members then separate and each exconjugant divides to form a clone. But onlv the F~ clone gives recombinant progeny; the Hfr parent continues to produce its original type ex- clusively. This suggests first that there has been a one-way transfer of genetic material from Hfr to F and, second, that since the Hfr parent survives, the transfer does not include all of its genes. But we know that the cells of E. coli usually have more than one set of genes, in the form of several haploid DNA-containing bodies, so it might be asked whether a whole or an incomplete set is transferred across the cytoplasmic bridge. Lederberg, in his original work on this subject, found that certain genes of the Hfr strain were very frequently absent from the recom- binants; in the strain he was using these included the loci for strepto- mycin resis

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