Biology in Parasites and Microbes

**135**

**Chapter 8**

**Abstract**

generally.

genetic variation

**1. Introduction**

Eukaryotes

*Harris Bernstein and Carol Bernstein*

Sexual Processes in Microbial

Two principal ideas have been proposed to explain the primary adaptive function of the sexual process of meiosis: (1) meiosis, and particularly meiotic recombination, is a process for repairing DNA and (2) meiosis, by means of meiotic recombination, is a process for generating beneficial genetic variation among progeny. We review the sexual processes of a number of well-studied microbial eukaryotes: *Saccharomyces cerevisiae*, *Saccharomyces paradoxus*, *Schizosaccharomyces pombe*, *Candida albicans*, *Ustilago maydis*, *Paramecium tetraurelia*, *Volvox carteri*, *Trypanosoma brucei*, *Neurospora crassa*, and *Amoebozoa.* We indicate aspects of the sexual processes of these microbial eukaryotes, where they have been established, that support the idea that meiosis is primarily a process for repairing DNA. In addition, we review the likely origin of meiotic sex among the microbial eukaryotes. A prokaryotic archaeon is the likely ancestor of eukaryotes. Extant archaea are capable of a sexual process involving syngamy and recombinational repair of genome damage, suggesting that the precursor of eukaryotic meiotic sex may already have been present in the archaeal ancestor of eukaryotes. We believe that attainment of an understanding of the adaptive function of meiotic sex in microbial eukaryotes is of considerable importance since it will likely apply to meiotic sex in eukaryotes

**Keywords:** meiosis, adaptive benefit, DNA repair, homologous recombination,

discuss, below, *Candida albicans,* uses parasexual meiosis.

Different microbial eukaryotic species are capable of a variety of sexual processes. Basically, however, the different sexual processes have, as a central element, syngamy and meiosis. Syngamy is the fusion of two cells or two nuclei. Meiosis is ordinarily initiated in a diploid cell that contains a pair of homologs, which is two copies of each chromosome. In meiosis, generally, first the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Next, homologous chromosomes undergo intimate pairing with each other and exchange genetic information by homologous recombination. Recombination is succeeded by two cycles of cell division to yield four haploid daughter cells each having half the number of chromosomes as the original diploid cell. Some microbial eukaryotes, however, use a similar process, parasexual meiosis. This is where ploidy (the number of complete sets of chromosomes in a cell) is determined, both before and after homologous recombination, by processes other than those in standard meiosis. One of the microbial eukaryotes we
