University of Iowa researchers have uncovered evidence of sexual reproduction in a single-celled organism long thought to reproduce asexually, according to a paper published in the January 26, 2005 issue of the journal Current Biology.
The finding by John M. Logsdon, Jr., assistant professor in the UI College of Liberal Arts and Sciences Department of Biological Sciences and the Roy J. Carver Center for Comparative Genomics, and colleagues from the UI and Roanoke College (Virginia), 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.
The paper, "A Phylogenomic Inventory of Meiotic Genes: Evidence for Sex in Giardia and an Early Eukaryotic Origin of Meiosis," describes their work studying eukaryotes (cells having nuclei, including plants, animals and fungi). By looking for genes necessary for sexual reproduction, the researchers 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 (microbial eukaryote) parasite that represents an early-branching lineage of eukaryotes, has revealed the presence of numerous genes implicated in meiosis -- the cellular division process that results in gametes (haploid reproductive cells).
Logsdon says: "Most eukaryotes are known to have sexual cycles and meiosis, but such processes are unknown in some single-celled protists (protozoa). Despite over a century of study, Giardia intestinalis was not known to have a sex life. And since Giardia is thought to be a modern representative of one of the earliest diverging eukaryotic lineages, it was suspected to simply have never acquired meiosis. But the inability to observe processes does not necessarily mean that they are not present."
In their new work, Logsdon and colleagues Marilee Ramesh and Banoo Malik (all previously at Emory University) surveyed the genome sequence of Giardia. (The Giardia genome is being completed by Mitch Sogin and colleagues at the Marine Biological Laboratory, Wood's Hole, Mass.) In this genome, Logsdon and colleagues found clear evidence for meiosis in Giardia; in particular, five genes that encode meiosis-specific proteins are present in Giardia and broadly in other eukaryotes. These data suggest that Giardia is capable of sex and that the earliest eukaryotes diverged after the advent of this key biological process.
Logsdon and his colleagues plan to continue their research by investigating the function of these genes in Giardia. He says: "Now that we have found these genes, we want to know what they are doing and when they are doing it." Logsdon's group is also extending their approach to other species in which sex has not been documented. Logsdon says, "This will help us to understand if meiosis is a requirement of all eukaryotes."