A Comparative Study On The Biology Of Spermatozoa From Diploid, Triploid And Tetraploid Rainbow Trout Oncorhynchus Mykiss

The cultivation of Rainbow trout Oncorhychus mykiss (Walbaum) have received considerable attention due to the fish’s flesh meat quality, rapid somatic growth and artificial breeding are simple. In fish, the energy consumption for sexual maturation may generate lower growth and reproduction. With the development of polyploid induction, the cultivation of triploid fish has received considerable attention. The triploids have three sets of chromosomes and are generally sterile as result of irregular meiotic division of chromosomes resulting in reduced gonadal development and aneuploid gametes. However, tripoid male develope to a spawning state and produce functional spermatozoa as has been demonstrated in rainbow trout. Thus the biology of spermatozoa from triploid male rainbow trout has become an interesting issue. Induction of tetraploidy has been achieved in many species of fish as the parent for triploid production.The present study examined the biology of rainbow trout spermatozoa from diploid, triploid and tetraploid parents through studying sperm volume, ploidy level, motility, viability, density, morphological characteristics and the ploidy and development of hybrid embryos (2n(?)×3n(?)) by flow-cytometric, light microscope, scanning electron microscope, fluorescent microscope. The mechanism of spermatogenesis and the relationship between sperm density and the time elapse of rapid movement, viability in tetraplids and triploids, and the survival of hybrid embryos was predicted and analyzed. The results were showed as follows:1. Contrast with that of diploids, the erythrocytes in triploid and tetraploid rainbow trout was larger. The volume of the erythrocyte nuclei in triploid and tetraploid were1.65and1.87times larger than that of diploids which were corresponded to the expectant theoretical value of1.5and1.74times.2. The relative DNA content of sperm and hybrid embryos (2n(?)×3n(?)) was determinrd by the flow cytometer showed that majority of spermatozoa produced by triploid rainbow trout contained1.5n, while a small portion displayed3n. The spermatozoa produced by tetraploid were2n. The result was supported by the head volume measured under light microscope. The observation suggests that the tetraploid rainbow trout allows normal meiosis, produced2n spermatozoa. The triploid rainbow trout produced two types of spermatozoa. The rainbow trout,2n=60, is a "new tetraploid" species, therefore the triploid is someway a "hexaploidy". During meiosis, pairing between homologous chromosomes can occur, the triploid males can undergo meiosis and produce 1.5n spermatozoa. The "new tetraploid" has existed as a diploid for a long time, the bivalent with a probable extensive nonhomologous synapsis in the third set of chromosomes, and produce a small portion of spermatozoa of different ploidy, such as3n.3. Diploid produced more semen than tetraploid and triploid (p<0.05). Tetraploid produced more semen than triploid (p<0.05).4. The non-activated spermatozoa had straight tail, and stayed in rest. When spermatozoa were activated with volume of distill water, spermatozoa immediately started vigorous movement. The intensity of movement decreased as the time went on. The three types of spermatozoa had similar movement pattern:initial rapid movement—circle around—vibration—stop.5. Spermatozoa from triploids and tetraploids rainbow trout displayed average time of rapid movement and viability were significantly higher than those spermatozoa In from diploid fish (p<0.05). The spermatozoa from triploids and tetraploids had comparable the time of rapid movement period and viability (p>0.05). The triploid and tetraploid rainbow trout males produced spermatozoa with characteristics like linearity coefficient in time elapse of rapid movement and viability during different sperm density and percentage of viability similar to those recorded in diploid males. However, there was a significant variation in time elapse of rapid movement and viability during sperm density different in diploids rainbow trout, but it was no significant variation in triploids and tetraploids. The observation shows that due to the larger head in triploid and tetraploid spermatozoa had more mitochondria in comparison with diploid, permitting more stored and obtained ATP. Moreover, the volume of diploid rainbow trout producrd spermatozoa is more consistent than the spermatozoa from triploids and tetraploids.6. The sperm density of diploid fish was significantly higher than in triploids and tetraploids. But the sperm viability in three ploidys rainbow trout is similar. The results indicate that In spermatozoa are more densely packed in the semen and produced more CO2may lead to decrease environmental pH, inhibit sperm movement.7. Larvae from the diploid×triploid crosses are mostly deformity, and all of them are died in the floating period. The fertilization, eyed and hatching rate of triploid rainbow trout spermatozoa was depressed as compared to diploids.The results indicate that triploid rainbow trout semen is capable of fertilizing the eggs obtained from diploid females, and stimulate embryonic development. But genetic or physiological differences in parental combinations result in the lower fertilization rate. In addition, the lower sperm density of triploid rainbow trout, and the same volume of semen from diploid with an equal number of In eggs fertilized, may lead to lower fertilization rate. Moreover, the head volume of sperm from triploid is greater than In micropyle block fertilization. In addition, it was discovered that the color of fertilized egg of hybrid group were lighter. The larvae had smaller body and yolk ares, and the yolk utilization was slower. And therefore, it infers that gene expression messy due to nuclear of eggs are incompatible with the sperm result in egg yolk supply of nutrients can not be used or the use of slow and the larvae were gradually death.