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Tweedledum and Tweedledee Help in the Search for a Novel Gene Function

Tweedledum and Tweedledee Help in the Search for a Novel Gene Function

Neurospora tetrasperma. Image: Namboori B. Raju, Stanford University.

In most species, including humans, an individual begins life as a single cell with a single nucleus. This is also true for the fungus Neurospora crassa. But its close relative N. tetrasperma is a rare exception: an individual begins life as a single cell with two non-identical nuclei.

One nucleus specifies mating type A information (or just mat A) and the other specifies mat a. Having both mating types gives N. tetrasperma the ability to mate with itself and produce progeny. In contrast, it takes two N. crassa individuals, or strains – one mat A, and the other mat a – to come together, mate and make progeny.

Neurospora hyphae, or cell bodies, are narrow tubes that enclose multiple nuclei. All the nuclei in a hypha are descended from those of the initial cell. The hyphae grow by elongation and branch out, and their inter-connections form a web-like mycelium. They bud off hundreds of thousands of powdery orange-pink spores, each with around three to eight nuclei. Wind-borne spores alight on a new food source, germinate, put out new hyphae and initiate a secondary mycelium of the individual.

A subset of N. tetrasperma spores can, by chance, package all nuclei of the same mating type. Secondary mycelia from them are self-sterile because they lack the other mat nucleus. Also, occasionally, N. tetrasperma makes two uninucleate initial cells, one mat A, and the other mat a, instead of the usual binucleate one. These also develop into self-sterile mycelia. Self-sterile mycelia can mate with like-mycelia of the opposite type, as in N. crassa.

Thus, N. tetrasperma can self-mate or out-cross. (“A genetic cross is the purposeful mating of two individuals resulting in the combination of genetic material in the offspring”: source.) In fact, its out-crossing ability allows geneticists to transfer genes from N. crassa into N. tetrasperma, or vice-versa. The cross-species transfer of genes and DNA segments is called introgression.

Scientists at the Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad, introgressed chromosomes from N. crassa into N. tetrasperma. (This author was part of the study team.) Chromosome rearrangements called translocations transfer a stretch of one chromosome into another.

Consider a mating between an N. crassa strain that contains normal sequence chromosomes (N) and one that contains a translocation (T) that transfers a 100-gene stretch of chromosome 1 to chromosome 2.  All progeny inherit one or the other copy of each parental chromosome pair.  Those that receive both chromosomes 1 and 2 of the N parent resemble the N parent, and those that receive both of the T parent resemble the T parent.

Progeny receiving chromosome 1 of the N parent and chromosome 2 of the T parent have two copies of the 100-gene stretch – i.e. they have duplicated DNA and are said to be of Dp type. Those with chromosome 1 of the T parent and chromosome 2 of N lack the 100-gene stretch, and are said to be Df type. Df progeny are non-viable because at least a few of the missing genes are required for viability. But the N, T and Dp progeny are viable.

By introgressing translocations from N. crassa into N. tetrasperma, Dev Ashish Giri, a PhD student, obtained two kinds of progeny that, similar to human identical twins, shared all of the same genes. But they differed in an unusual way – somewhat like the Lewis Carroll characters Tweedledum and Tweedledee.

Tweedledum had one nucleus with normal chromosomes (N), and the other with translocation chromosomes (T). That is, it was [N + T]. Tweedledee had the duplication in one nucleus and the deficiency in the other. So it was [Dp + Df].  Selvam Rekha, a technical assistant, showed that when either Tweedle was self-mated, it generated both kinds of Tweedle progeny. Note that in Tweedledums, every nucleus has one copy of each gene, whereas in Tweedledees, some genes are present in two copies in the Dp nuclei and no copies in the Df nuclei.

The Tweedles differed with respect to the mutational process called repeat-induced point mutation (RIP).  RIP specifically alters duplicated DNA segments, and it occurred in self-crosses of the Dees ([Dp + Df]) but not the Dums ([N + T]). If any additional differences are uncovered between the Tweedles then it can potentially flag the fact that the Df nuclei are missing a gene whose function is not compensated by the Dp nuclei in the same cell – presumably because the missing function is restricted to the vicinity of the nucleus expressing the gene.

Thus, the Tweedles, for the first time, enabled us to ask whether any genes have a “tethered-effect”. That is, a function restricted only to the expressing nuclei.  Although no such gene functions are as yet known, the evidence suggests they might exist.

Most animal and plant cell types contain only a single nucleus. But some of our muscle, bone, and liver cells share with fungi the property of multiple nuclei. Might some gene functions in them be tethered only to nuclei expressing the gene?  The CDFD study represented a first systematic search for such a gene function in any organism.

D.P. Kasbekar is a retired scientist.

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