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Abstract The effects of heavy metals on survival of Biomphalaria alexandrina the intermediate host of Schistosoma mansoni under natural conditions has important consequences for predicting and assessing the outcome of their use in bio-monitoring programs and their role in S. mansoni transmission. In the present study we tested the tolerance to Cd-toxicity in Biomphalaria alexandrina snails through generations produced by self- fertilization from the most tolerant parents. Cd-tolerant of the third generation (selected) have been tested and compared with the (non-selected) laboratory breeding and field collected snails for their tolerance to other heavy metals zinc (Zn), lead (Pb) and copper (Cu) and molluscicides (bayluscide as chemical one and Anagallis arvensis as plant one). Also, their alteration in the susceptibility to S. mansoni infection, physiological responses (haematological criteria and biochemical criteria), protein pattern (SDS-PAGE), and the genetic structure (PCR-ISSR) were tested and compared. Results of toxicity assessment showed a gradual increase in the LC50 and LC90 values through successive generations than the parents indicating Cd tolerance upgrading phenotype. Also, the selected Cd-tolerant snail group showed higher tolerance to copper (Cu) and A. arvensis than field collected and laboratory breeding snail groups suggesting correlation between Cd tolerance and tolerance to Cu, and A. arvensis plant compounds. Cd-tolerant snails of the 3rd generation showed refractory tendency to infection, higher pre-patent period over 72 days and higher survival percentage of 35%. On the other hand, non-selected laboratory snails showed moderate susceptibility of 33.3% infection percentage, prepatent period of 26 days, number of cercariae /snail of 1790 and survival percentage of 15%. The haematological criteria (total and differential blood cell counts) and biochemical parameters, liver enzymes [Aspartate aminotransferase (AST), Alanine aminotransferase (ALT), Alkaline phosphatase (ALP)], protein contents [Total protein (TP), Albumin (ALB)], and kidney functions (Urea and Creatinine), were used to estimate the toxic effect of 24 hrs. exposure to Cd among the three different snail groups under study, selected Cd-tolerant third generation, adult negative field collected snails and laboratory breeding (non-selected) in comparison to non-exposed groups. Results indicated that the selected snails showed a significant increase in haemocytes; granulocytes; which indicate that their defense mechanism was stronger than non-selected ones. Also, results indicated that the selected Cd-tolerant group had stable liver enzymes and protein levels while had increase in the levels of urea (significant) and creatinine (non-significant) indicating that the liver cells were still intact while renal cells were affected. On the other hand, field snail group was the most affected followed by laboratory one. Protein electrophoretic patterns as revealed by SDS-PAGE and ISSR analysis were used to differentiate between five snail groups; three were under the selective pressure for Cd tolerance through successive three generation; G1, G2 and G3; and two non-selected snail parent field, and lab B. alexandrina. Results of SDS-PAGE revealed that G3 snail group was the most unique by excess four bands (fractions 1, 7, 16 and 17) and the absence of two common bands (fractions 2 and 3). Dendrogram showed that the lab breed snail sample was the most similar to parent snail samples and the 3rd generation of Cd-tolerant snails was the most diverge forming a distinct group. Also, the dendrogram based on the similarity matrices of ISSR-PCR banding patterns resulted with ten random primers, showed that the selected groups of the G1 and G2 were the most similar and clustered together forming a distinct group that in turn cluster with laboratory group that in turn cluster with parent group, while G3 was the most divergent group, indicating its genetic drift. |