BIOLOGY RESEARCHER ADVANCE UNDERSTANDING OF SEXUAL EVOLUTION

Roanoke College Assistant Professor of Biology, Dr. Marilee Ramesh, along with Dr. John Logsdon and Banoo Malik from the University of Iowa, 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 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). While Giardia intestinalis is more commonly known by the intestinal discomfort it causes to those people unfortunate enough to have come in contact with contaminated water, this parasite has been a favorite to evolutionary biologists because it is thought to represent a modern example of very ancient eukaryotes.

By looking for genes necessary for sexual reproduction in Giardia, the researchers uncovered evidence that eukaryotic cells have been capable of sex for a very long time. An analysis of the Giardia genome has revealed the presence of numerous genes implicated in meiosis — the cellular division process that results in gametes (haploid reproductive cells).

“We know that plants, animals and fungi have sexual cycles and bacteria do not,” Ramesh says. “However, defining sexual cycles in protists has not been so straightforward. Sexual reproduction has never been observed in Giardia and the prevailing idea was that this species was asexual. A major question in biology has been how important sexual reproduction is to the evolution of the eukaryotic cell. Are there intermediate eukaryotic species that represent the presexual (premeiotic) stage or is sexual reproduction so important that meiosis is a universal process among all eukaryotes? ”

In their new work, the researchers 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, Ramesh and colleagues have 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.