With no conclusive studies, a number of factors-mostly including thermal stress-have been suggested as the etiology for the sperm defect known as diadem/crater (Larsen nd Chenoweth, 1990;Vogler et al., 1993). A 1-year study conducted on 16 Bos taurus and Bos indicus bulls showed a consistently high percentage (up to 95.5%) of spermatozoa bearing a ''diadem/crater'' (D/C) defect in the semen of a 6-year-old Grey Brahman bull during the hotter months at an artificial insemination center in western Venezuela, an area representing the dry tropics. This sperm pattern reverted to essential normality during the cooler periods.In affected samples, phase contrast and transmission electron microscopy (TEM) both revealed typical diadem patterns of vacuoles in the sperm equatorial region (Fig. 1A,B) as well as vacuoles occurring in other regions of the sperm head. These were shown by TEM as deep invaginations within the nuclear chromatin (Fig. 1C,D). In affected samples, dead sperm ranged from 27.5% to 54% and sperm progressive motility varied between 45% to 60%. Associated sperm defects included deformed and detached heads, bent tails, proximal droplets, and damaged acrosomes. Semen with !15% D/C had depressed sperm mass activity (P 0.5%) compared with semen containing 15% D/C.Although a previous in vitro fertilization (IVF) study (Thundathil et al., 1998) determined defective zona binding and penetration only, we observed low fertilization rates. Here, IVF rates (fertilization, cleavage rates, and formation of both morulae and blastocysts on d7 and d8) were compared using semen samples containing high (95%), medium (52%), and negligible (0%) D/C levels. Both high and medium levels of sperm D/C were associated with low fertilization rates and poor development of morulae and blastocysts in comparison to the results of IVF using control semen.In conclusion, this condition evidently reflects an interaction between genetic predisposition and environmental effects, as this bull was the only 1 of 16 housed and monitored under identical circumstances to demonstrate elevated D/C levels during the hotter period of the year. To our knowledge, no previous report has shown the possibility of genetic  environmental interaction as a likely etiology for the D/C defect.
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