Probiotics are live microorganisms that can have benefits on to the host, probiotics are used as feed additives and have been shown to have beneficial effects on immune system modulation, stress tolerance and growth rate of farmed fish.
One of the most serious bottlenecks in the development of commercial aquaculture is the control reproduction; this is because many suffer from reproductive dysfunctions. Therefore understanding the effects of probiotics on the reproduction and gamete quality is extremely important.
The Zebrafish (Danio rerio) is widely used as a model in studying genetics and development, and more recently it has been used in screening of therapeutic drugs.
In fish ovarian development is usually divided into two major phases, growth and maturation. During the growth phase estradiol promotes the production of vitellogenin in the liver, when taken up by developing oocytes vitellogenin is cleaved into yolk protein to provide a nutritional reserve.
Oocyte maturation in teleosts is stimulated by the release of luteinizing hormone (LH) from the pituitary, and is controlled by Maturation Inducing Hormone (MIH). MIH activates the cytoplasmic maturation promoting factor (MPF), which is made up of two subunits: cyclin B and cdc2.In D.rerio MIH stimulates the synthesis of cyclin B, the cyclin B proteins then bind to cdc2 to form MPF. MPF is responsible for stimulating all the changes associated with oocyte maturation, for example germinal vesicle break down, spindle formation and chromosome condensation.
The aim of the study was to assess the effects of Lactobacillus rhamnosus (probiotic), administered through food on ovarian development and reproductive performance of female D. rerio.
Several methods were used to determine the effect of L rhamnosus on reproduction of D.rerio, the first of these was assessing the effects on folic growth. In oviparous vertebrates ovary growth induces the vitellogenic process, this is where vtg is synthesised, and once in the blood stream is selectively incorporated into growing oocytes. Probiotic admiration is Zebrafish induced a significant increase in the estradiol receptor (era) and vtg gene expression. The cyp19a gene codifies for the enzyme responsible for converting androgen into estrogen. The higher level of this gene indicates a possible increase in plasma estradiol and, in turn, in VTG. The increase of these two genes was also correlated with the significant increase of the cyp19a expression at an ovarian level. The higher level of cyp19a indicates a possible increase in plasma estradiol, and in turn, in VTG. Histological results also indicated that probiotics increased follicle growth, as in the probiotic fed fish there was an increase in vitellogenic follicles compared with the control.
After ten days of exposure the overall wet weight and ovary weight of the fish was taken. The gonadosomatic index (GSI) was calculates, this is an expression of the gonad weight as a percentage of the total body weight. It was found that in the probiotic treatment there was a higher GSI that in the control, this relates to an increase in follicle growth, showing the involvement of L. rhamnosus in follicle growth.
The effect of probiotics on oocyte maturation and ovulation was also determined in this study, cyclin B, activinβA1 and smad2 gene expression code for important signals involved in the induction of oocyte maturation; it was found that these increased significantly upon probiotic administration.
The final endpoint measured to assess the effects on reproduction was the total number of ovulated eggs; it was found that this increased significantly in response to probiotic administration. The significant increase in ovulated eggs occurred simultaneously with an elevation in cox2a mRNA in the ovaries, cox2a is the enzymes in charge of prostaglandin biosynthesis, which is the hormone responsible for ovulation.
The gut microbiota of the zebrafish was also assessed, it found that in the control group the levels of L. rhamnosus were too low to enumerate, while in the probiotic fed group the levels were 1.89x105 ± 0.7x105 CFUg-1, this suggest that L. rhamnosus can populate the GI tract of D. rerio.
This study suggests that the fecundity of female D.rerio can be improved upon addition of a probiotic to the feed, these findings are important in terms of aquaculture as an increase in fecundity would lead to a higher yield of fish. Although these findings may be important, further experiments would be needed to see if the same effect is experienced in fish important to aquaculture eg: Salmon. Furthermore these effects were experienced in a controlled laboratory experiment; the same effects may not be seen if administered in fish farms.
Overall this paper is well written and the theory and importance of findings is well explained, the methodology was explained well and it is easy to follow. I think the fact that in the results they explain the relevance of different genes is helpful instead of saying these genes were up regulated, and having to go back to the introduction or to the discussion to find out what those genes do.
A review of Gioacchini, G., Lombardo, F., Merrifield, D.L.,Silvi, S., Cresci, A., Avella, M. A.M, Carnevali, O. 2011. Effects of Probiotics on Zebrafish Reproduction. J Aquac Res Development.
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