Imagine, a snail. Kind of cute right? A little, squishy bugger living inside a curly-cued shell. Now how do you tell a male snail from a female snail? Okay, male snails will have male reproductive organs (penis and testes) while females will have female reproductive organs (ovaries and an egg gland). But wait, I just found a snail that looks like a female (ovaries are present) but there's also a small penis?? Am I crazy? Actually not on this point; some marine snails undergo imposex development, where females grow male accessory sex organs. Talk about W-I-L-D!
Alright, so how do these female snails start growing penises?
It turns out that imposex development has been linked to the chemical family of organotins, including tributyltin. Tributyltin (TBT) was popular in the second half of the 20th century as an anti-fouling substance. Boat owners painted the bottom of their boats with TBT to prevent colonization and growth of aquatic creatures (fouling), like barnacles and algae. A boat with lots of fouling is undesired because the fouling will degrade the boats' material and keep it from going very fast. TBT was extremely effective at preventing fouling. Too effective in fact and it has since been banned due to its correlation with a wide array of funky developments in marine organisms, including imposex in snails.
How do you tell a female, male and imposex females apart?
The easiest way to determine the sex of snails is to break open their shell and pull them out (tragic I know, but this is done in the spirit of science). Then you can pull out the snail and identify sex organs. A female will have cream-colored ovaries at the narrowest tip of her body (the part that was highest up in the shell) and a white, oblong gland (called the egg gland) in the wider, middle part of her body. A male will have orange/red testes at the tip of his body (the same part where a female would have ovaries), and a opaque hockey stick shaped penis just under the mantel skirt (pull back the mantel skirt to find it). An imposex female will retain her female organs but also develop a penis under the mantel skirt (length and size of penis will depend on how advanced into imposex development the female is).
|Ilyanassa obsoleta male; all rights reserved to Richard Fox|
|Ilyanassa obsoleta female; all rights reserved to Richard Fox|
|Ilyanassa obsoleta imposex female; all rights reserved to Dr. McClellan-Green|
How does TBT cause imposex development?
Well we don't have an answer for this one yet, partly because we're still trying to figure out how regular male development occurs in snails and other gastropods. We know a signal, named the Penis Morphogenic Factor (PMG), is necessary to induce the development of male accessory sex organs. We assume that TBT can activate, probably directly, this signal in females where it is usually inactive. However, the structure and specific function of the PMG has not been identified. In the fall of 2010, I worked with Dr. Patricia McClellan-Green of NC State University to try to figure out if the PMG was a neuropeptide called APGWamide (named after the amino acids present: alanine-proline-guanine-tryptophan amide). Dr. McClellan-Green and her lab previously observed that application of APGWamide increased penis elongation of normal males. They then hypothesized that APGWamide was the PMF necessary for the development of male accessory sex organs. I tested this hypothesis by quantifying levels of APGWamide in males, females and imposex females at various time points of exposure to TBT. I was looking for increased APGWamide in males and imposex females as compared to normal females. Unfortunately, my results were inconclusive so I don't have an answer for you today. But I am sure toxicologists won't rest until this mystery is solved so keep your ears and eyes open!
So that's the story on female snails growing penises. Unless we want more sex organs growing in the the wrong spot we should try to be a bit more cautious about what we put in that big, blue watery road!