Noble rot, the alchemist of wines, is setting fungal biology abuzz

In wine-making circles, ‘noble rot’ is an exalted identify 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 choosing course of labour-intensive and drives up the fee. The crushed grape juice from rotted grapes is used to make high-quality candy wines just like the Sauternes of Bordeaux, the Trockenbeerenauslese of Germany and Austria, and the Tokaji Aszús of Hungary. They’re additionally very costly.

Befitting its exalted standing, the botrytis fungus was additionally discovered just lately to exhibit an uncommon idiosyncrasy. In all animals, vegetation, and fungi, the nucleus of a cell accommodates a number of units of all the chromosomes of the organism. This property of nuclei permits us to clone animals. Scientists can switch such a nucleus, which accommodates all of the DNA directions, into an egg cell whose personal nucleus has been eliminated and, in the best circumstances, immediate it to turn into a brand new organism.

However due to the idiosyncrasy, botrytis fungus can’t be cloned — nor can one other fungus referred to as Sclerotinia sclerotiorum.

A crew of researchers from Sichuan College in China and the College of British Columbia in Canada have made a startling discovery: in these fungi, no single nucleus accommodates an entire set of chromosomes. As an alternative, the chromosome set is distributed throughout two or extra nuclei, and anybody nucleus accommodates solely a subset.

These surprising findings have been reported in Science.

Ascomycetes, asci, ascospores

Botrytis and Sclerotinia are ascomycetes fungi. The primary cell of a child fungus born following a mating between two ascomycetes fungi is named the ascospore. All the next different cells of the person are derived from it. That 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, accommodates two full units of chromosomes.

In lots of well-studied ascomycetes fungi, eight ascospores are made in every ascus. All of the nuclei of a person ascospore are genetically equivalent. That’s, 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?

Persons are typically curious to know the way scientists make their discoveries.  Most discoveries originate in experiments that didn’t work in the best way they have been meant to. Sadly, the converse isn’t true.

The commonest rationalization for experiments that don’t work the best way have been meant to is a few sort of ‘operator error’ — i.e. a foolish mistake of some sort: a progress medium was not correctly made, the incubator was not set to the best temperature, the fallacious pressure was used, and so forth. Foolish errors are extra frequent than serendipitous leads.

Not surprisingly, scientists get mad with experiments that don’t work. However infrequently, one of these experiment is a harbinger of an surprising discovery. That is the scientist’s dilemma.

Inconceivable versus true

The analysis crew got down to acquire mutants of S. sclerotiorum.  For this they uncovered the ascospores to ultraviolet mild. Every S. sclerotiorum ascospore accommodates two nuclei. Each nuclei have been assumed to hold the identical set of chromosomes. UV-induced mutations happen at random.  Subsequently, it was extremely unlikely the identical gene would change into 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.

However within 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’ve got eradicated all which is not possible, then no matter stays, nevertheless inconceivable, should be the reality.”

Might the 2 nuclei between them comprise just one set of chromosomes?

Nearer examination

The researchers wrote of their paper: “As a result of this prediction challenges established rules of chromosome biology, we carried out a more in-depth 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 accommodates 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 2 nuclei harboured distinct chromosome units. When each probes have been used collectively, in some ascospores the alerts confirmed up in just one nucleus and in different ascospores the alerts have been seen in each nuclei. This meant the distribution of chromosomes within the nuclei differed between ascospores.

Additional 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 within the analysis neighborhood. What’s 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 associate kinds 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. Proper now, scientists doing analysis with fruit flies, nematodes, zebrafish, mice, and different mannequin organisms may be envying these working with rot fungi — noble or in any other case.

D.P. Kasbekar is a retired scientist.

Printed – June 30, 2025 05:30 am IST