UCSB researchers recently discovered the mechanism behind sex-selection in the unicellular freshwater organism Tetrahymena thermophila, a free-living ciliate species that has seven sexes.
Their results, published in the paper “Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila,” explains how the Tetrahymena develops into one of seven possible mating types, each of which are expressed as proteins on the surface of the cell. The biologists collaborated with researchers at the Institute of Hydrobiology of the Chinese Academy of Sciences and the J. Craig Venter Institute to study the process of DNA recombination in the organism.
Dr. Eduardo Orias, a Research Professor and Emeritus in UCSB’s Molecular, Cellular and Developmental Biology department, said that the organism can develop into any one of the seven sexes, but cannot be the same sex as either one the parents. Similar to humans, the parent cells must be different sexes and offspring sex determination is randomly selected through site-specific DNA recombination.
“What we started out with was this very interesting observation over 50 years ago that the Tetrahymena has seven mating types which is roughly equivalent to genders or sexes,” Orias said. “There are chemical differences that cause the cell to recognize different genders. When two cells mate the progeny cells end up with a mating type that is different from the parents.”
The organism, which has two separate nuclei within its single cell, stores its genetic information in the germline nucleus, analogous to ovaries and testes in humans. Its somatic ‘working’ nucleus expresses the phenotype of the cell. The germline genes contain two alleles that code for sex-determination, with one allele containing five of the possible seven sex genes and another allele containing six. The researchers worked only with the allele capable of expressing one of six sexes.
Postdoctoral biological engineer Michael Lawson said that the germline genetic material contains discreet allele variations for gene expression of mating types.
“The germline is sort of like an egg and the somatic nuclei is something like tissue. The somatic nucleus keeps the cell alive but the germline stores the information,” Lawson said. “What we have found in the germline cell is that each of the mating type genes are next to each other and each has a middle region that is variable to that mating type and there is a left side and right side for each mating type that is specific for each one.”
According to MCDB postdoctoral researcher Marcella Cervantes, the sexes are distinguished not through anatomical variance, but rather through the expression of different proteins on the surface of the cell.
“The different proteins determine the specificity of the mating type,” Cervantes said. “We are pretty sure that the proteins go to the surface of the cell and so the particular mating type is on the surface pointing out. Each mating type has different proteins on the surface and presumably they are in a signaling pathway.”
After fertilization, the new somatic nucleus directs DNA segments from opposite ends of a tandem array of incomplete gene pairs, each with specific sequences different for each mating type, to fuse into a complete pair, deleting the remaining mating type gene pairs and leaving the new cell with only one of the six sexes.
Cervantes said that her team hopes that their research may lead to advancements in cancer therapy, tissue transplants, and treating infections and illnesses.
“We are trying to figure out how things work, so we do the basic research and other people may apply it to whatever it may be useful for,” Cervantes said. “Hopefully cancer or cell recognition or other applications relevant to life on the planet.”
PHOTO COURTESY OF George Foulsham
A version of this article appeared on page 4 of April 2nd, 2013′s print edition of the Nexus.