Yolk protein is expressed in the insect testis and interacts with sperm

We collected seminal fluid proteins from males before and after daily sperm release. These samples were separated by 2-D gel electrophoresis, and gels were treated with a glycoprotein-detecting probe. We observed a group of abundant glycoproteins in the sample collected after sperm release, which was absent in the sample collected before sperm release. Sequencing of these glycoproteins by mass spectroscopy revealed peptides bearing homology with components of yolk, which is known to accumulate in developing oocytes. This unexpected result was confirmed by Western blotting demonstrating that seminal fluid contains protein immunoreactive to antibody against yolk protein YP2 produced in the follicle cells surrounding developing oocytes. We cloned the fragment of yp2 cDNA from S. littoralis and determined that it is expressed in both ovaries and testes. yp2 mRNA and YP2 protein were detected in the somatic cyst cells enveloping sperm inside the testis. During the period of sperm release, YP2 protein appears in the seminal fluid and forms an external coat on spermatozoa.One of the yolk protein precursors YP2, which in females accumulate in the oocytes to provision developing embryos, appears to have a second male-specific role. It is produced in the testes and released into the seminal fluid where it interacts with sperm. These data reveal unexpected common factor in the maturation of insect eggs and sperm.Germ-line cells of male and female undergo a complicated process of maturation before they become fertilization-competent spermatozoa or oocytes, respectively. At the earliest stages of their development, germ-line stem cells become enveloped by somatic cells; the two cell types interact extensively throughout gamete maturation. In insects, cells surrounding developing oocyte are called the follicle cells, while cells enveloping clones of differentiating spermatozoa are called the cyst cells.Follicle cells have multiple functions; they protect and nourish the growing o