Abstract
The lipid composition of sperm membranes is crucial for fertilization and differs among species. As the evolution of internal fertilization modes in fishes is not understood, a comparative study of the sperm lipid composition in freshwater representatives of externally and internally fertilizing fishes is needed for a better understanding of taxa-specific relationships between the lipid composition of the sperm membrane and the sperm physiology. The lipidomes of spermatozoa from stingray, a representative of cartilaginous fishes possessing internal fertilization, and sterlet, a representative of chondrostean fishes with external fertilization, have been studied by means of nuclear magnetic resonance (NMR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), electrospray MS, gas chromatography-(GC) MS, and thin-layer chromatography (TLC). NMR experiments revealed higher cholesterol content and the presence of phosphatidylserine in stingray compared to sterlet sperm. Unknown MS signals could be assigned to different glycosphingolipids in sterlet (neutral glycosphingolipid Gal-Cer(d18:1/16:0)) and stingray (acidic glycosphingolipid sulpho-Gal-Cer(d18:1/16:0)). Free fatty acids in sterlet sperm indicate internal energy storage. GC-MS experiments indicated a significant amount of adrenic acid, but only a low amount of docosahexaenoic acid in stingray sperm. In a nutshell, this study provides novel data on sperm lipid composition for freshwater stingray and sterlet possessing different modes of fertilization.
Highlights
Fishes, representing a taxonomically not ranked group of animals, possess different modes of fertilization—internal and external
Sperm cells are the most diverse cell type on earth, regarding their cell morphology [26], and taxa-specific physiological properties associated with their lipid composition [27]
This study characterized sperm lipid compositions of the two particular freshwater fish species P. motoro and A. ruthenus. These species were selected for the following reasons: (I) while in most cartilaginous fishes the osmolality of the internal body fluids is close to the high osmolality of the marine environment, potamotrygonids as obligate freshwater species [29] are characterized by an osmolality of body fluids typical for bony fishes known to be osmoregulators, regardless of the environmental osmolality [9]. (II) Stingrays possess an ancient mode of internal fertilization, while sturgeons developed an external fertilization mode, but preserved some features in the spermatozoon structure characteristic for internal fertilization in non-fish animals
Summary
Fishes, representing a taxonomically not ranked group of animals, possess different modes of fertilization—internal and external. Internal fertilization in modern chondrichthyan fishes, which evolved from other groups of vertebrates [1] some 500 million years ago, was preserved from an ancient ancestor [2] of fishes, reptiles, birds, and mammals. In modern actinopterygian fishes, which evolved from Osteichthyes [3] about 350 million years ago, the ancient internal mode of fertilization was substituted by an external one. The most ancient groups of Actinopteryhii (Cladistia and Chondrostei) have preserved some features of the evolutionary old internal mode of fertilization, e.g., the presence of the acrosome in spermatozoa. All these facts make fishes an attractive model for understanding the evolution of internal fertilization in vertebrates
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