Recombination 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.

Circular genetic maps in viruses
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⁺ x 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. P₁x P₁, P₂ x P₂ or P₁ x P₂.
In order to prepare the linkage map, several rapid-lysis genotypes (r1, r2, r3, in order of discovery) were available and the corresponding strains were called r_a, r_b and r_c. Utilizing these three strains with h⁺ and h^- genotypes, following crosses were made : (i) r^-_a h⁺ x r⁺_a hr; (ii) r^-_b h⁺ x r⁺h^-, (iii) r^-h⁺ x r⁺_c h^-, (iv) r^-_cr⁺_c x r⁺_c r^-_b. The recombination frequencies were calculated and following two equally probable jlinear orders were available : r_a-r_c-h-r_b and r_c-h-r_b-r_a. This can be explained only if the circular map shown in Figure 12.24 is accepted. This gives evidence for a circular genetic map for a T-even (T2or T4) phage.

Recombination in T4 phage has also been studied by famous scientist S. Benzer, utilizing rll locus responsible for giving rise to rough morphology to the plaque (space cleared of bacteria due to phage infection). These studies enabled Benzer to work out the fine structure of gene at rll locus. A detailed discussion on this subject is presented in Fine Structure of Gene-at the Genetic Level (A New Concept of Allelomorphism).