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Interakce spermií a jiker u sladkovodních ryb: vliv prostředí na fertilizační proces
KHOLODNYY, Vitaliy
Fertilization of fresh water fish occurs in an environment which may affect negatively the gametes, therefore the fish male gametes should reach their counterpart, the female gamete, as soon as possible because spermatozoa become damaged within minutes or less due to osmotic shock. Existence of specific mechanism triggering, supporting and guiding the encounter of gametes would be highly expedient in these conditions. The eggs of many externally fertilizing freshwater fish species are released into the external milieu surrounded by a coat of ovarian fluid (OF) with a composition ideal for supporting and protecting eggs and sperm against the deleterious effect of freshwater. The existing data support the idea that the properties of OF and/or the specific compounds contained in it or released by the eggs could significantly affect the behavior of male gametes and consequently influence the outcome of fertilization in terms of the number of fertilized oocytes. The mechanisms which facilitate and trigger gametes' encounter are also highly demanded in terms of natural selection. It was found that presence of OF affected significantly the behavior of rainbow trout spermatozoa, in particular, their motility traits: higher velocity was supported for longer time and trajectories were straightened, comparing to those observed in water. In the microcapillary spermatozoon accumulation test (test of chemotactic response) the rainbow trout OF showed a trapping effect on activated male gametes which depended on osmotic properties of the activating media. Different molecular weight fractions from OF affected the tactic behavior of the cells in a various way. The most significant trapping effect was rendered by low molecular fraction and the possible chemotactic agent was found to be thermostable. The trapped cells showed specific turn-and-run behavior accompanied by asymmetric bending of flagella and burst-like increase of calcium concentration in the bended area. The in vitro fertilization test revealed the enhancement of spermatozoa performance, especially in the samples from the related individuals, which led to the higher embryo development rate. Presence of particular concentration of ovarian fluid (30% solution in water) had inhibiting effect on sterlet spermatozoa motility initiation. Lower concentrations of the ovarian fluid improved the longevity of spermatozoa and did not affect their trajectories. Test of chemotactic response showed no effect of ovarian fluid on spermatozoa behavior, while at the same time the attracting effect of egg conditioned medium was evident (i.e. due to some substances released from the eggs during their contact with fresh water). The results of in vitro fertilization test showed that presence of ovarian fluid prevented the eggs from losing the fertilizing ability due to the contact with water, as well as promoted the spermatozoa to fertilize the eggs during longer period of time. Presence of common carp ovarian fluid in the activation medium caused the decrease of the velocity of spermatozoa comparing to the OF-free medium and significantly altered the motility pattern from straightforward motility observed in the water to the tumbling in the medium with high OF content (50%). Introduction of OF (in the sperm accumulation test) entailed immediate and prominent chemotactic-like reaction of spermatozoa. The environmental conditions which accompany the encounter of gametes, in particular presence of OF, calcium ion content and osmolarity, significantly affect the performance of male gametes in spermatozoa in rainbow trout, sterlet and common carp in terms of changes in velocity, path linearity and ability to response to the external signals, e.g. attractants. The conducted study allowed to conclude that way how the ovarian fluid affects the behavior of spermatozoa in these species may be associated with their reproduction (spawning) strategy.
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