SUPERNUMERARY SEGMENTS INDUCED BY DNA SYNTHES工S 工N!
l:IBITORS AND
主ACT 工N THE HORSESHOETHE CANCELLING EFFECT OF EGG EXTITACHYPLEUS TRIDENTATUS(CHELICERATA:MEROSTOMATA:

CRAB，XIPHOSURIDA)

Tomio 工七 ow，l:liromi Inoue，Emiyo l:lirai and Takao Taguchi
Department of Bio1ogy，Facu1ty of Education，Shizuoka University，
Ohy品 836，Shizuok，
昌 422 Jap呂ロ

Why handhas 5 fingers? Why insect h丑 s 6 1egs? ¥vhydoes thehorseshoe
crab have 10 abdomina1 segments?

We can not reply th日 se questions as yet.

The mechanism of determination of number and differentiation of homologou日
structures in an individua1 is an exciting prob1em in d母 velopmenta1 bio1ogy.
申r and differentiation
We can fee1 that the mechanism of determination of numbof 日egments wi11 becom白 c1ear in n日担 r futur自 (Lewis，1978; Wareing & Phillips，1981; Beachy et al.
， 1985).
Hydroxyurea，an inhibitor of DNA synth日 sis，ch呂 nges 七he different工昌七l.onof abdomina1 segments of the hor呂 田 shoe crab，Tachvp1eus trident昌 tus and in叩creases the number of 日 egments in 90‑100も of the surviving embryos (工 tow et.
， 1984). We examined these mons七巳 rs from that year on，and obtain巴 d som日
alinstructive resu工ts.

This report de丑 ls with th邑 se resu1ts.

(1) Ce11 pro1iferation of treated embryos.

treated embryos wa日 目 topped.

The c日 11 pro1iferation ofTh工 s fact suggests that hydroxyurea stops

ce11 cycl邑 s at S period in horseshoe cr呂 b 日mbryos.
(2) Effects of the other DNA synthesis 工nhibitors.

Th告 inhibitors of DNA

日ynthesis other than hydroxyurea， such as aphidicolin， fluorodeoxyuridin，日
fluorouraci1 or thymidine，also induced the embryos with sup日 rnumerary
segments at a high r呂 te. The type of the monsters was same 昌 s th品七 inducedwith hydroxyurea.
(3)

The effect of M‑period blockers.

Treatment with 10 to 100 ug/m1 colchi
司cine stopped cell cycles at M‑p由 riod.
27

Besides，it stopped embryonic deve申

28lopment.

However，the tr日ated embryos never developed into 日 mbryos withsupernumerary segments，and 3 (1‑anilinoethylidin巴)ー 5‑benzylpyrrolidine‑
同4‑dione (TN‑16) did not induce supernumerary segments，n日ither.
2，That is，M‑period blockers did not induce supernumerary s邑 gments.
Low temperature (50 C) stopped ce工1 prolife主 ation and embryonicdevelopm位 nt，and the treatment at 5 C never induced supernumerary segments.
In summary，only inhibitors of DNA synthe日 is (blockers of S‑period ofcell cycl日) induced sup巴 rnumerary segments.
(4) Electrocauterization of the posterior end of embryonic area.

The cell‑mass under the posterior end of embryonic 丑rea was.electric品 lly c丑uterized，and then the cauterized embryos 官 ere 七re品 ted with hydroxyurea. The s呈 g‑ments of these embryos did not increase in number.

工七 means that thenormal segments and supernumerary ones are formed from the cell‑mass ofposterior end of embryonic area.
(5) The cancelling effect of the embryo extract. The egg 巴 xtract count巴 racted
the effects of hydroxyure丑. When the extract was applicat自 d immediately
昌

fter th告 七 reatmemt of hydroxyurea，the embryos developed norm丑工工 y. 工七means that th告 effect of hydroxyure丑 appears aft邑 r the treatment. Of cours邑
when the 日xtract was stimulately treated with hydroxyurea，th日 exp巴 r工mental
embryos developed normally. But in these cases the extract did not cancel
the delay of development and th巴 stop of cell cycle by hydroxyurea. The
extract could not cancel the effect of fluorodeoxyuridine to form super‑
num邑 E昌 ry segments. The 昌 ctive substances of the 告 xtr昌 ct seemed to be aminoacids.

References
Beachy，P. A.，S. L. Helfand 在 nd D. S. Hogn巴 ss (1985) Nature，313:545‑551.
工tow，T.，E. Iizuk昌 昌 ndH. 工noue (1空84) Proc. Arthropod Embryo1.

Soc. Jpn.，1ヲ84.
Lewis，E. B. (1978) Nature，276:565 570.
Wareing，P.F. andI. D.J.Phillips(1981) Growth and differentiation inplants. pergamon Press，Oxford.

Proc.Arthr
口p口a.Embrl
リol.Soc.Jpn.，
1985