. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 318 N. TAKEDA. VP B Figure 2. (A) General appearance of the vestigial penis in a female apple snail (Pomacea canaliculata: control) reared by artificial mass cul- ture. Scale bar represents 1 mm. (B) Histological appearance of the vestigial penis in A. The regions containing a vestigial penis were fixed in Bouin's fluid and embedded in paraffin wax by the standard method. Sections were stained with hematoxylin and eosin. Scale bar represents 200 ;im. A. anus; C, ctenidium; OD, oviduct; VP. vestigial penis. within it (24). Th
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 318 N. TAKEDA. VP B Figure 2. (A) General appearance of the vestigial penis in a female apple snail (Pomacea canaliculata: control) reared by artificial mass cul- ture. Scale bar represents 1 mm. (B) Histological appearance of the vestigial penis in A. The regions containing a vestigial penis were fixed in Bouin's fluid and embedded in paraffin wax by the standard method. Sections were stained with hematoxylin and eosin. Scale bar represents 200 ;im. A. anus; C, ctenidium; OD, oviduct; VP. vestigial penis. within it (24). The penis and the penis sheath are located together to the left of the extreme right margin of the mantle cavity. In treated females, the arrangement of these male copulatory organs was similar but differed in the distance between the penis sheath and the penis: the penis sheath in females was located at the edge of the ctenidium at the mantle skirt, at a distance from the penis. It has been suggested that tributyltin inhibits cytochrome P450 aromatase, which converts testosterone to estradiol in females (25, 26). Inhibition of aromatase activity thus in- creases levels of testosterone which induces imposex, with the development of male copulatory organs. Development of the imposex phenomenon in P. canaliculata was also confirmed by direct treatment with testosterone. Female snails reared in water that contained 500 ng/1 testosterone exhibited changes similar to those induced by tributyltin, including the development of a penis sheath and a penis. Therefore, these observations support the proposed mecha- nism of action of tributyltin. It is difficult to explain the unusual phenomenon of a rudimentary penis in females; however. I propose the fol- lowing hypothesis. In the early stages of development, both sexual rudiments develop as an undifferentiated tissue mass. Once the sex of the snail is determined genetically (27), however, these rudiments differentiate in response to the s
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
