Merodiploids (Molecular Biology)

A merodiploid is an essentially haploid organism that carries a second copy of a part of its genome . The term is derived from the Greek, meros = part, and was originally used to describe both unstable partial diploidy, such as that which occurs briefly in recipients after mating with an Hfr strain (1), and the stable state, exemplified by F-prime strains (see Hfr’S And F-Primes). Over time, usage has tended to confine the term to descriptions of stable genetic states.

The inability of bacteria to maintain more than a single copy of their chromosome makes it easier to isolate mutations. Most of the mutations useful to a geneticist are recessive, and their effects would be hidden if a second unmutated gene copy were present to provide wild-type function, as in eukaryotes. Because bacteria are haploid, mutations are expressed directly as mutant phenotypes, and mutants are readily isolated by straightforward selection and screening procedures, vastly expanding the range of genes attainable by classical genetic methods. Beyond this point, however, the haploid state is inconvenient, because analyzing the function of a gene identified by a mutation usually involves determining how the mutated function interacts with the wild-type one or with related mutants, for which diploidy is needed. Merodiploidy provides a solution to this problem.

The discovery of natural merodiploidy arrived happily at a moment when it would make one of its most notable contributions. Hfr strains in which the integrated F factor, or F plasmid, is situated near the lac genes were found to give rise to F ~lac+ plasmids, by excision of the lac region along with the F DNA (2, 3). When an F’lac transferred itself into a new cell, it rendered the lac gene region of the chromosome diploid. At this time, Francis Jacob and his colleagues had proposed that lac gene expression is regulated by the action of a repressor at a control locus, the operator, situated next to the lac genes. Seeking evidence for the operator, they used an F’lac/lac+ strain to isolate mutants that expressed the lac genes even in the presence of the repressor. The mutations caused unregulated expression only of the lac genes to which they were linked (cis- acting) and not of lac genes in trans, just the property expected of the hypothetical cis-acting control locus (4).

The advantages offered by merodiploidy were well-illustrated by an analysis of E. coli mutants unable to make flagella (5). The fla mutants were first mapped to three clusters by complementation with F-primes carrying defined regions of the chromosome. The mutations were then crossed onto the F-primes by recombination, and the resulting F-primes transferred into each mutant to determine which pairs of mutations complemented to allow synthesis of flagella. These tests provided an estimate of the minimum number of fla genes in each region. The literature of bacterial genetics is replete with similar examples.

The uses of merodiploidy are limited to neither E. coli nor plasmids. The genes for catechol metabolism in Pseudomonas aeruginosa were mapped by first allowing recombinational transfer of transposon insertion mutations from R-prime plasmids to the chromosome and then carrying out complementation analysis using overlapping cosmid clones (essentially, smaller prime plasmids) (6). Potassium transport mutants of E. coli were analyzed by infection with lambda phage for transduction and plating on potassium-deficient media: Formation of turbid plaques containing lysogens occurred only for complementing mutants (7).

The existence of natural stable merodiploid states is one reason for raising the question, Why are bacteria haploid? There are others. Partial diploidy of the F-prime type is usually well-tolerated, suggesting that doubling the gene dosage is not generally deleterious. In several bacteria, the genome is distributed over more than one DNA molecule, implying the ability to coordinate replication and gene expression of physically distinct chromosomes. Bacteria have no trouble maintaining oligo- and multicopy plasmids in a polyploid state and, when growing fast, maintain their chromosomes in a state of pseudopolyploidy owing to the formation of multiforked chromosomes. J.-P. Bouche and his colleagues (8) produced a conditionally diploid E. coli by reducing synthesis of the essential cell division protein FtsZ, therby moving the replication cycle forward relative to division.

Nevertheless, haploidy is the invariable ground state. Could it be that the advantage that haploidy confers on the bacterium is the same as that offered to the geneticist? Whereas multicellular eukaryotes require the phenotypic stability of diploidy on which to build a developmental program and to prevent unregulated growth, bacteria have a different imperative: to adapt rapidly to diverse environmental and chemical challenges by generating from within their enormous populations mutations that are expressed immediately.

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