SUMMARY: Cultures of non-motile Salmonella strains treated with phage lysates of motile strains produce in semi-solid agar: (i) swarms, each consisting of a clone of motile bacteria, attributed to complete transduction of motility; (ii) trails, i.e. unbranched linear groups of microcolonies stretching some millimetres from the site of inoculation, attributed to abortive transduction of motility; the latter was thought to result from the importation by phage of a gene conferring motility, which was not replicated but was transmitted, as a supernumerary gene, down one line of descent (unilinearly) to but one of the descendants of the organism which received it, the trail marking the path of the gene-bearing descendants of successive generations.

The present work has led to a modified hypothesis: that the phage-imported non-replicated gene confers on the organism the ability to synthesize motility conferring (MC) particles, which are distributed amongst its non-gene-bearing progeny, which cannot make new ones; that each particle may be unilinearly transmitted for many generations; and that while one MC particle confers motility in broth, several are needed to enable a bacterium to travel in a semi-solid medium.

The main evidence for this hypothesis is provided by pedigrees of bacteria made motile by abortive transduction, isolated by micromanipulation from treated suspensions. All such bacteria produce mainly non-motile offspring. A minority (exceptional or bacteria) produce clones which after 10-15 generations include 20 to 100 motile descendants; the rest give clones containing only a few (0-12) motile organisms. bacteria are identified as ones containing the supernumerary gene, the others as ones motile through possession of a few MC particles only.

In each of six extensive pedigrees a single bacterium was isolated from amongst the 9th-21st generation progeny of the original parent. None of a large number of other motile bacteria isolated in collateral sublines (and so not gene-bearing) produced more than 12 motile descendants. These data show that the character (ability to produce 15 or more motile descendants) is unilinearly transmitted.

Any motile bacteria found amongst the progeny of a non- bacterium after six or more generations were inferred to be motile through possession of one MC particle each; of many such which were isolated none produced more than one motile descendant; when this in turn was isolated, a single motile organism was sometimes again detected in the clone produced. This shows that there is unilinear transmission of motility (in broth).

Samples of populations of motile bacteria isolated by micro-manipulation from treated suspensions were transferred to semisolid agar, or to individual droplets; the proportion of bacteria which generated trails was about equal to the proportion found to be This was to be expected if both characters reflect the presence of the non-replicated gene.

The MC particle is probably a flagellum, or a granule which determines the production of one. The particle presumed to account for the unilinear transmission of the and trail-forming characters is most economically interpreted as a phage-imported fragment of genetic material which has failed to replace its homologue in the genetic organelle of the recipient organism.


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