Hidden sex life of an early eukaryote revealed

By looking for genes necessary for sexual reproduction, researchers have uncovered evidence that eukaryotic cells have been capable of sex for a very long time. Recent evolutionary analyses of the genome of Giardia intestinalis, a unicellular protist parasite that represents an ancient, early-branching lineage of eukaryotes, has revealed the presence of numerous genes implicated in meiosis, the cellular division process that results in gametes.

Most eukaryotes are known to exhibit sexual reproduction and meiosis, but such processes are unknown in some single-celled protists (protozoa). Despite more than a century of study, Giardia intestinalis was not known to have a sex life. Because Giardia is thought to be a modern representative of one of the earliest diverging eukaryotic lineages, it was simply suspected to have never acquired meiosis. However, the inability to observe processes does not necessarily mean that they are not present.

In this new work, John Logsdon of the University of Iowa and colleagues Marilee Ramesh and Banoo Malik (all previously at Emory University) surveyed the genome sequence of Giardia. In this genome, Logsdon and colleagues found clear evidence for meiosis in Giardia: five genes that encode meiosis-specific proteins broadly in other eukaryotes are also present in Giardia. These data suggest that Giardia is capable of sex and that the earliest eukaryotes diverged after the advent of this key biological process. This report provides the first clear evidence that meiosis arose very early in eukaryotic evolution, bringing science one step closer to understanding the mystery of sexual evolution.