A new study on the sex life of molds is raising startling new questions about gene silencing, speciation and perhaps some facets of human reproduction.
The study, featured in the journal Cell, focuses on the mating habits of Neurospora crassa, commonly called pink bread mold - a fungus that has been a useful genetic model organism for more than half a century. Neurospora became famous when George Beadle and Edward Tatum used it at Stanford in 1941 for the first experiments in biochemical genetics - an achievement that won them the Nobel Prize.
"Fungus is very easy to manipulate," said Patrick K. T. Shiu, a postdoctoral fellow in the Stanford Department of Biological Sciences and lead author of the Cell paper. "It only takes two weeks for a genetic cross to mature, and you can insert or delete any gene you want."
When it comes to sex, molds and humans share at least one fundamental principle: In both species, the parents must donate a copy of their DNA to the offspring in order to successfully reproduce.
In most human cells, DNA resides in 23 pairs of chromosomes. One chromosome is inherited from the father, one from the mother.
Neurospora, on the other hand, contains only seven different chromosomes, and - during most of its life cycle - only one copy of each. During the sexual phase, one set from each parent briefly forms a cell with 14 chromosomes, each chromosome containing a grab bag of genetic information from one parent or the other. The corresponding chromosomes from each parent pair up and then separate to form progeny, which again have only seven chromosomes.
Silence of the genes
This complex cellular process - in which parental chromosomes pair up and split apart to form offspring or sex cells (sperm and eggs) - is called meiosis and occurs in all organisms that reproduce sexually, from people to plants to fungi.
In their recent Cell study, Shiu and his colleagues took a closer look at meiosis in mold and made a surprising discovery: Each cell with 14 chromosomes has some kind of internal mechanism that scans the paired chromosomes before they split apart. The researchers determined that, if one chromosome in a pair carries an extra copy of a gene not found in its partner chromosome, the fungus will turn off all copies of that gene in the cell.
Because the genes are turned off in the early stages of meiosis before the two parental chromosomes separate and still have the chance to check for mismatched (unpaired) genes, Shiu and his co-workers have dubbed the process MSUD - "meiotic silencing by unpaired DNA."
The results of MSUD are devastating. Instead of turning out healthy black spores capable of reproduction, silencing of essential genes by MSUD produces white spores that are dead - or no spores at all.
"In meiosis, normal chromosomes pair with one another perfectly," noted Stanford Research Professor Robert L. Metzenberg, co-author of the Cell study. "We discovered that, when chromosomes pair, there`s a built-in checking system we didn`t expect to find that checks if the pairing is correct. It does not detect tiny differences in the two DNA sequences, but any deviation the size of a gene or larger triggers the checking system."
Three copies distributed between two parents is sure to make trouble, Metzenberg said, because one copy is likely to be unpaired, but four genes are not necessarily bad because they can pair normally and do not trigger the MSUD checking mechanism.
"If there`s a gene missing or appears in one chromosome but not in its mating partner, the cell says, `Something is wrong. There`s something from one of the parents that doesn`t belong there," he added.
The extra gene may be from a virus that jumped into the chromosome or from an insertion sequence - a mobile segment of DNA that can interfere with normal genetic function.
"Organisms are constantly under siege by viruses and insertion sequences," Metzenberg observed. "Most of them are bad. They make you carry something you shouldn`t, or they may disrupt a gene you need. They would like to hitch a ride into the future by jumping into the progeny - the children, grandchildren and great-grandchildren."
With MSUD, organisms can prevent unwanted viral genes and insertion sequences from spreading.
"It`s as if the organism says, `No thanks, I don`t want that. I`m going to activate my cellular machinery to turn off genes that are not paired properly at the 14-chromosome stage," Metzenberg explained.
"It`s a meiotic defense system that defends the fungus against invasion at a time when chromosomes are especially vulnerable to the spread of viruses and insertion sequences," Shiu added.
Humans and speciation
In addition to eliminating deleterious genes in mold, Metzenberg suggested that MSUD could be involved in screening out genetic parasites in other organisms that reproduce sexually - plants, insects and even people. One example is oogenesis in women - a biological process in the ovary that results in the formation of eggs.
"Human oogenesis is, at first glance, a bizarre process," Metzenberg wrote in Cell, noting that, at birth, a girl already will have developed some seven million egg cells that are "frozen" in an early stage of meiosis during which all 23 chromosomes sets are paired. Remarkably, the chromosomes remain in this frozen state until menstruation begins some 12 years later. Of the original seven million cells, only 400 or 500 will be made available for reproduction during a woman`s lifetime.
"We speculate that this is not a random process," Metzenberg observed. "It`s a perfect situation for weeding out extra genes or seeing if there`s a bad match or too many bad matches in the chromosomes. We suspect that there is a system in humans that causes gene silencing, but we don`t know the mechanism yet."
The researchers made another surprising discovery with evolutionary implications. Animals, plants and fungi are divided into species based, in part, on their ability or inability to interbreed. Redwoods and Douglas firs have some physical similarities, but it`s unlikely that they will be able to mate to give hybrids, especially fertile ones. Clearly, they are different species of trees.
Likewise several species of Neurospora - N. crassa, N. sitophila and N. tetrasperma - are normally infertile when crossed in the laboratory. Yet, by including a dominant mutant gene called Sad-1 in the DNA of the three mold species, Shiu and his colleagues were able to produce viable spores through cross-breeding of species that normally are sterile with one another.
The ability of Sad-1 to breach interspecies sexual barriers apparently works by preventing meiotic silencing from occurring, according to Metzenberg.
"To our knowledge, this is the first case where the barrier between interspecies crosses has been observed to break down as a result of mutation in a single gene," Shiu added, "but since gene silencing is universal, it could occur in other kingdoms, including plants and animals."
The Cell study is the latest in a series of discoveries in gene silencing - one of the most explosive fields in biology in the past decade.
According to Metzenberg, MSUD silences genes by destroying messenger RNA (mRNA) - molecules that carry specific instructions ("transcripts") from DNA telling the cell which proteins to build. In gene silencing, mRNA molecules are destroyed after they are transcribed - a method known as "post-transcriptional gene silencing" (PTGS).
Researchers in a number of industries - including pharmaceuticals and agriculture - are particularly interested in using PTGS to screen for disease resistance, flavor enhancement and other commercially valuable traits by turning off several genes simultaneously.
"MSUD could provide a quick and dirty way of testing how genes function in meiosis," Shiu concluded. "We can silence a gene simply by inserting an extra copy, without interfering with the growth of the fungus before meiosis. We`re still not clear whether extra copies of a gene can trigger meiotic silencing in plants and animals - or other fungi, such as penicillium. That would be interesting for scientists to study in the future."
Other co-authors of the Cell study are Stanford senior research scientist Namboori B. Raju and Professor Denise Zickler of the Institut de Genetique et Microbiologie at the Universite Paris-Sud. The research was supported by grants from the U.S. Public Health Service and the National Science Foundation.
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