In wine-making circles, ‘noble rot’ is an exalted title for the botrytis fungus (Botrytis cinerea). It infects grapes, penetrates the pores and skin, causes the berries to lose water by evaporation and shrivel up, and thus concentrates the sugars and flavours in them. Since solely a small share of grapes in a winery are contaminated, they should be picked by hand.
This makes the selecting course of labour-intensive and drives up the value. The crushed grape juice from rotted grapes is used to make high-quality candy wines like the Sauternes of Bordeaux, the Trockenbeerenauslese of Germany and Austria, and the Tokaji Aszús of Hungary. They are additionally very costly.
Befitting its exalted standing, the botrytis fungus was additionally discovered lately to exhibit an uncommon idiosyncrasy. In all animals, crops, and fungi, the nucleus of a cell incorporates a number of units of all of the chromosomes of the organism. This property of nuclei permits us to clone animals. Scientists can switch such a nucleus, which incorporates all the DNA directions, into an egg cell whose personal nucleus has been eliminated and, in the proper situations, immediate it to become a brand new organism.
But as a result of of the idiosyncrasy, botrytis fungus can’t be cloned — nor can one other fungus referred to as Sclerotinia sclerotiorum.
A workforce of researchers from Sichuan University in China and the University of British Columbia in Canada have made a startling discovery: in these fungi, no single nucleus incorporates an entire set of chromosomes. Instead, the chromosome set is distributed throughout two or extra nuclei, and anyone nucleus incorporates solely a subset.
These surprising findings have been reported in Science.
Ascomycetes, asci, ascospores
Botrytis and Sclerotinia are ascomycetes fungi. The first cell of a child fungus born following a mating between two ascomycetes fungi is referred to as the ascospore. All the subsequent different cells of the particular person are derived from it. This is the defining characteristic of ascomycetes fungi. The ascospores are produced in a sac-like cell referred to as the ascus (plural asci). An ascus, produced when two parental strains mate, incorporates two full units of chromosomes.
In many well-studied ascomycetes fungi, eight ascospores are made in every ascus. All the nuclei of a person ascospore are genetically similar. That is, all of them have the identical set of chromosomes. B. cinerea and S. sclerotiorum additionally make asci with eight spores. The researchers had no motive to suspect them to be any completely different.
How are discoveries made?
People are sometimes curious to understand how scientists make their discoveries. Most discoveries originate in experiments that didn’t work in the means they have been meant to. Sadly, the converse is not true.
The commonest clarification for experiments that don’t work the means have been meant to is some variety of ‘operator error’ — i.e. a foolish mistake of some variety: a development medium was not correctly made, the incubator was not set to the proper temperature, the flawed pressure was used, and so on. Silly errors are extra frequent than serendipitous leads.
Not surprisingly, scientists get mad with experiments that don’t work. But every now and then, this sort of experiment is a harbinger of an surprising discovery. This is the scientist’s dilemma.
Improbable versus true
The analysis workforce got down to receive mutants of S. sclerotiorum. For this they uncovered the ascospores to ultraviolet gentle. Each S. sclerotiorum ascospore incorporates two nuclei. Both nuclei have been assumed to hold the identical set of chromosomes. UV-induced mutations happen at random. Therefore, it was extremely unlikely the identical gene would turn out to be inactivated in each nuclei of an ascospore.
Consequently, a colony containing mutant cells was additionally anticipated to incorporate a sector with non-mutant cells. The non-mutant cells would have nuclei descended from the ascospore nucleus with the non-mutant gene.
But in the experiment, of the greater than 100 mutant colonies the researchers examined, all contained solely mutant cells. None of them had a non-mutant sector. This was most surprising. Why weren’t any non-mutant cells seen in these colonies?
This statement set the researchers up for his or her Sherlock Holmes second: “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”
Could the two nuclei between them include just one set of chromosomes?
Closer examination
The researchers wrote of their paper: “Because this prediction challenges established principles of chromosome biology, we conducted a closer examination of the ascospores’ nuclei and chromosomes.”
They used molecular probes that bind particularly to particular person chromosomes, permitting them to say whether or not or not a nucleus incorporates the chromosome. When the probes have been used individually, they lit up solely one nucleus per ascospore. The probe by no means lit up each nuclei.
This meant the two nuclei harboured distinct chromosome units. When each probes have been used collectively, in some ascospores the indicators confirmed up in just one nucleus and in different ascospores the indicators have been seen in each nuclei. This meant the distribution of chromosomes in the nuclei differed between ascospores.
Further exams revealed that every nucleus of a S. sclerotiorum or B. cinerea ascospore contained solely three to eight chromosomes.
New questions
The findings have already spawned many questions in the analysis neighborhood. What is the mechanism by which chromosomes are allotted to the completely different nuclei? How is genetic integrity preserved throughout cell division? What restores an entire set of chromosomes when the fungus mates, and with its mating companion types new asci? Which genes and mechanisms are concerned in chromosome sorting and regulation? What benefit does chromosome distribution confer to Botrytis and Sclerotinia?
The questions have generated a brand new buzz in fungal biology. Right now, scientists doing analysis with fruit flies, nematodes, zebrafish, mice, and different mannequin organisms could be envying these working with rot fungi — noble or in any other case.
D.P. Kasbekar is a retired scientist.
Published – June 30, 2025 05:30 am IST
