Brachymeiosis was a hypothesized irregularity in the sexual reproduction of ascomycete fungi, a variant of meiosis following an "extra" karyogamy (nuclear fusion) step. The hypothesized process would have transformed four

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

nuclei into eight

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

ones. The current

scientific consensus Scientific consensus is the generally held judgment, position, and opinion of the majority or the supermajority of scientists in a particular field of study at any particular time. Consensus is achieved through scholarly communication at confe ...

is that brachymeiosis does not occur in any fungi. According to the current understanding,

ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

reproduce by forming male and female organs ( antheridia/spermatia and ascogonia), transferring

nuclei from the antheridium to the ascogonium, and growing a dikaryotic

ascus An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...

containing both nuclei. Karyogamy then occurs in the ascus to form a diploid nucleus, followed by meiosis and

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

to form eight haploid nuclei in the ascospores. In 1895, the botanist R.A. Harper reported the observation of a second karyogamy event in the ascogonium prior to ascogeny. This would imply the creation of a tetraploid nucleus in the ascus, rather than a diploid one; in order to produce the observed haploid ascospores, a second meiotic reduction in chromosome count would then be necessary. The second reduction was hypothesized to occur during the second or third mitotic division in the ascus, even though chromosome reduction does not typically occur during mitosis. This supposed form of meiosis was termed “brachymeiosis” in 1908 by H. C. I. Fraser. The existence of brachymeiosis was controversial throughout the first half of the twentieth century, with many conflicting results published. Then, research with improved staining techniques established clearly that only one reductive division occurs in the asci of all examined species, including some which had been believed to undergo brachymeiosis. As a result of these studies, the theories of double fusion and subsequent brachymeiosis were discarded around 1950.

References

Mycology Cell cycle Reproduction {{cell-cycle-stub