Edmunds et al. report that a single exposure to o,p'-DDT can lead to a complete, permanent and functional sex reversal in this species of fish. They showed this experimentally by microinjecting small quantities of DDT directly into the egg yolks of medaka a few hours after fertilization, i.e., very early in embryonic development. Fish that under normal circumstances would have matured as males instead matured as females. Not only did these sex-reversed fish have a full complement of female reproductive characteristics, they were able to bear young at a breeding success rate comparable to normal females.

The researchers used microinjection into embryonic yolk as their experimental approach because they believed this route of exposure would be parallel to the actual maternal transfer of persistent lipophilic contaminants to the eggs. Other research has shown that o,p'-DDT concentrates highly in fish ovaries, and within the ovaries, almost all the DDT concentrates in oil droplets within the oocytes.

In medaka, sex is determined chromosomally. Normal males are XY while females are XX. Edmunds et al. examined the chromosomal sex and the morphological/functional sex of individuals at the age of 10 weeks, after injection. They found that at a DDT dose of 227 nanograms per egg (less than one half of the lethal dose, 511 ng/egg) 86% of the genetic males were sex-reversed. Genetic females showed no effects of treatment. None of the control group were sex reversed. They also exposed another treatment group to 2 ng/egg of 17ß-estradiol and found that 1 in 5 genetic males were sex reversed.

Edmunds et al. also tested whether treated, sex reversed males were fertile. They paired sex-reversed males with normal males and found that 50% (3 of 6 pairs) were fertile.

The doses needed to induce complete sex reversal (227 ng/egg or 227 ppm) were high relative to concentrations reported for background field levels. On the other hand, the lethal dose they observed was substantially higher than that reported for wild lake trout (10 ppm). The reasons for this discrepancy are unknown. They may involve species differences or differences in the toxicity of the form of DDT used, or differences in the impact of delivery via microinjection vs. maternal transfer.

Talk to us: email