Circular genetic maps in viruses
Viruses can undergo recombination in host cells. For instance if two strains of a virus having genotypes a+b- and a-b+ are allowed to infect the host cells in such a manner that both strains infect the same host cells (this can be achieved by increasing the population of virus particle so that the probability of infection of a host cell by both virus strains increases), the resulting progeny of virus (virus particles are liberated due to lysis) will also have recombinants a+b+ and a-b-, in addition to parental combinations a+b- and a-b+. The phenomenon of recombination in viruses was described for the first time by A.D. Hershey and R. Rotman in 1949 and has been utilized for preparing linkage maps of viruses. Linkage maps inT2andT4 phages have been found to be circular, as is the case in Escherichia coli chromosome.
The viruses, being very small particles, are often considered not suitable for inheritance studies due to non-availability of scorable traits. However, plaque morphology (large vs. small, fuzzy vs. sharp), host range and virulence are characteristic features of bacteriophages (plaque = clear area produced on opaque lawn of bacteria on the surface of a dish of solid medium), which have been used for inheritance and recombination studies. Inheritance and recombination in viruses can be illustrated by a cross in T2 phage, attempted by Alfred Hershey.
This cross (h-r+
involved (i) host range (h+
can infect only strain 1 and h-
can infect both strains 1 and 2); (ii) plaque morphology (r+
lyses slowly producing small plaques and r-
lyses rapidly producing large plaques). The strain 1 was infected by both phage genotypes (mixed infection
or double infection)
and the lysate was analysed by spreading it onto a bacterial lawn composed of a mixture of strains 1 and 2 (h-
will produce clear and h+
will produce cloudy plaques). Four plaque types were distinguishable (Fig. 12.23) :
(i) clear and small (h-r+),
(ii) cloudy and large (h+r-),
(iii) cloudy and small (h+r+)
and (iv) clear and large (h-r-).
The first two of these four are parental phenotypes and the last two are recombinants, so that the recombination frequency (RF) can be calculated as follows:
It has been shown, that the above relationship holds good, despite the fact that recombinants are a consequence of population of events, as suggested by the following characteristic features of the phage : (i) Each parental phage particle can be duplicated many times, and hundreds of phages can be released from a single infected cell. Therefore, a recombinant produced shortly after infection may undergo further exchanges at later times, so that several rounds of exchange are possible within the host before lysis. (ii) Recombination can occur between genetically similar phages or between dissimilar phages, e.g. P1
|Fig. 12.23. Phage phenotypes produced by progeny of a cross h-r+ x h+r- (see text for details; redrawn from Stcnt, 1963).