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Abstract
BackgroundMale and female gametes follow diverse developmental pathways dictated by their distinct roles in fertilization. While oocytes of oviparous animals accumulate yolk in the cytoplasm, spermatozoa slough off most of their cytoplasm in the process of individualization. Mammalian spermatozoa released from the testis undergo extensive modifications in the seminal ducts involving a variety of glycoproteins. Ultrastructural studies suggest that glycoproteins are involved in sperm maturation in insects; however, their characterization at the molecular level is lacking. We reported previously that the circadian clock controls sperm release and maturation in several insect species. In the moth, Spodoptera littoralis, the secretion of glycoproteins into the seminal fluid occurs in a daily rhythmic pattern. The purpose of this study was to characterize seminal fluid glycoproteins in this species and elucidate their role in the process of sperm maturation.ResultsWe 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.ConclusionOne 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.
Highlights
Male and female gametes follow diverse developmental pathways dictated by their distinct roles in fertilization
We cloned the fragment of yp2 cDNA from S. littoralis and determined that it is expressed in both ovaries and testes
We reported previously that – coincident with sperm release – several glycoproteins are secreted into the upper vas deferens (UVD) seminal fluid of the moth S. littoralis [17]
Summary
Male and female gametes follow diverse developmental pathways dictated by their distinct roles in fertilization. Mammalian spermatozoa released from the testis undergo extensive modifications in the seminal ducts involving a variety of glycoproteins. The purpose of this study was to characterize seminal fluid glycoproteins in this species and elucidate their role in the process of sperm maturation. Germ-line cells of male and female undergo a complicated process of maturation before they become fertilization-competent spermatozoa or oocytes, respectively. Cells surrounding developing oocyte are called the follicle cells, while cells enveloping clones of differentiating spermatozoa are called the cyst cells. The antibody against yolk protein YP2, which is produced in the female follicle cells [18,19] immuno-detected a single protein in the male seminal fluid. We present evidence that YP2 protein is produced in the male cyst cells and interacts with spermatozoa released into the UVD lumen
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