December 31, 2013
Researchers Discover Molecular Causes For Sex Determination In Honey Bees
redOrbit Staff & Wire Reports – Your Universe Online
New research appearing in the December issue of the journal Cell Biology puts together the final pieces in a puzzle 200 years in the making – determining the molecular evolution in the genes that separate male honey bees from female ones.
Lead author Martin Beye, a professor with the Institute of Evolutionary Genetics at the University of Düsseldorf, and his colleagues studied 14 natural sequence variants of the complementary sex determining switch gene (Csd) for 76 genotypes of honey bees.
They found that there are at least five amino acid differences that can control allelic differences to create femaleness through the Csd gene, essentially serving as the control switch for gender. They estimated that there was an average of six pairwise amino acid differences that evolved under positive selection.
“We also identified a natural evolutionary intermediate that showed only three amino acid length differences in the PSD relative to its parental allele,” the study authors wrote. “This genotype showed an intermediate fitness because it implemented lethality regularly and induced femaleness infrequently. We suggest incomplete penetrance as a mechanism through which new molecular switches can gradually and adaptively evolve.”
The first genetic mechanism for sex determination in honey bees was proposed by a Silesian monk named Johann Dzierson in the mid-1800s, according to the researchers. While Dzierson knew that queen bees and female worker bees in colonies emerged from different quantity and quality of food, he wondered how males came about.
Dzierson suggested that male honey bees were haploid (possessing one set of chromosomes). This was confirmed following the invention of the microscope in the 1900s, which allowed researchers to determine that eggs that gave rise to drones were not penetrated by sperm. However, developmental geneticists still were unclear exactly how this haplodiploid sex determination system evolved at a molecular level.
Arizona State University Provost Robert E. Page Jr. and former ASU graduate student Greg Hunt identified genetic markers close to the complementary sex-determining locus, which allowed for gene mapping. They also found that the recombination rate of honey bees (that is, the process through which DNA is mixed during sexual reproduction) is higher than any other known animal.
That knowledge allowed Beye to isolate, sequence and characterize the complementary sex-determining locus, the researchers said. He and Page were also able to knock out an allele, demonstrating how it was possible to get a male from a diploid genotype in research published 10 years ago in the journal Cell.
Even then, the issue of which alleles were essential to the process, exactly how they worked with one another, and why this particular system evolved remained unclear – although the researcher team said that the answers were “tantalizing close.” That prompted Page and his associates to look further at what actually constitutes an allele.
“There has to be some segment of that gene that is responsible in this allelic series, where if you have two different coding sequences in that part of the gene you end up producing a female,” he said, adding that they set out to determine what degree of difference was required in two alleles, and whether or not these sets of sequences could be off by one or two pairs, or if they always had to be exactly the same.
Page explained that they developed a plan of attack, during which they would look at 18 to 20 alleles and determine which genetic regions were responsible among these variants. They also had to establish whether or not there were intermediate types of alleles and, if so, to learn how they might have evolved.
“We discovered that different amounts of arginine, serine and proline affect protein binding sites on the csd gene, which in turn lead to different conformational states, which then lead to functional changes in the bees – the switch that determines the shift from female to not female,” he concluded.