Biomarkers provide early warning signals of the health status of organisms inhabiting the contaminated environment. Gamma radiation induced specific biomarkers of exposure in fish exposed to low doses are not available. So far, studies on ionizing radiation induced genetic damage in fish were carried out between 0.16 Gy/min to 3.2 Gy/min under laboratory conditions. To our knowledge, present study is the first of its kind in studying the effects of gamma radiation and its associated genetic biomarkers at very low protracted doses i.e. 0.002 Gy/min in the fish O. mossambicus under controlled laboratory conditions.
Increased levels of MN were appeared in peripheral blood of ...view middle of the document...
Interestingly, a matched frequency of MN formation on day 3 at a dose of 5Gy was observed in the fish O. mossambicus with that of earlier studies conducted at the author’s laboratory (Anbumani and Mohankumar, 2012) in the fish C. catla at the same dose rate (0.002Gy/min) of gamma radiation under controlled conditions. But the time response in MN induction varied greatly between these two species.
Ionizing radiation induced micronuclei is a well understood phenomenon. Micronuclei (MN) are thought to originate from chromosome fragments or whole chromosomes that lag behind at anaphase during nuclear division. Some molecular mechanisms proposed for increased expression of MN include the hypomethylation of satellite DNA, increased hypermethylation due to cell cycle check points and DNA repair during genomic damage (Fenech, 2000, 2011). In the present study, a statistically significant increase in the MN frequency was observed until 5 Gy of post exposure thereafter started declining with increased frequency of apoptotic cells at 10 Gy. This could be attributed due to the phenomenon of cell death or apoptosis that hasten at higher doses which is experimentally proven (Fig. 3) and it is well known that under xenobiotic exposure damaged cells tend to remove faster than the undamaged ones (de Flora et al. 1993). It could also be due to the active involvement of spleen in removal of damaged cells from the circulation in O. mossambicus.
A possible reason behind the increased frequency of MN at different duration could be due to the aneugenic and / or clastogenic potential of gamma radiation in the erythropoietic organ (sensitive) during the genesis of cell differentiation and the time taken to deliver the nascent cells that experienced DNA damage from the cephalic kidney or the spleen to the peripheral circulation. Another possible reason could be the low level of apoptosis at 2.5 and 5 Gy leading to increased MN frequencies than 10 Gy exposures. This is in good agreement with the findings of Decordier et al. (2002) and Polard et al. (2011).
The appearance of MN in the blood stream even after chronic intervals of post exposure reflects the phenomenon of chromosomal instability and impetus us to explore certain biomarkers of radiation exposures in Pisces.
4.2. Nucleoplasmic bridges and tailed nuclei as specific biomarkers of radiation exposure in fish
In the present study, low doses of protracted exposures induced elevated frequencies of nucleoplasmic bridges in binucleated cells and tailed nuclei. Nucleoplasmic bridges occur when centromeres of dicentric chromosomes are pulled to opposite poles of the cell at anaphase. Various mechanisms could lead to NPB formation following DNA misrepair of strand breaks in DNA. Typically, a dicentric chromosome and an acentric chromosome fragment are formed after ionizing radiation exposure that result in the formation of an NPB and an MN, respectively (Hamza and Mohankumar, 2008). This study is the first of its...