Date: Tue, 19 Mar 2002 19:44:21 -0700
From: maa32 
Subject: (urth) other limb causes

Hey, Jerry.  Thanks for looking into that stuff more thouroughly.  Here is 
something else: trematodes that cause extra limbs in X Leavis.  (Anyone not 
interested in this should definitely skip this - it just involves extra limbs 
in frogs)

Of especial note is the statement:

"The predominant observed limb deformity in both amphibian species was 
mirror-image
duplication"

 
(skip if not interested)
Evidence that Trematodes Cause
  Deformities, including Extra Limbs, in
                        Amphibians

                           Stanley K. Sessions

                           Department of Biology
                             Hartwick College
                            Oneonta, NY 13820
                          SessionsS@Hartwick.edu


The occasional occurrence of high frequencies of limb abnormalities, including 
missing limbs
and extra limbs, in natural populations of amphibians has long been a puzzle. 
Recent reports
suggest an "epidemic" of such anomalies in natural populations of amphibians 
from the
central United States. Here I report the discovery of a population from 
northern California
in which such limb abnormalities appear to be caused by a parasitic flatworm 
(trematode:
Manodistomum syntomentera, family Plagiorchiidae) that uses amphibians as 
second
intermediate hosts in a complex life cycle (Fig. 1).

The primary host is a vertebrate carnivore (especially garter snakes, genus 
Thamnophis)
which releases trematode eggs in its feces. The eggs are eaten by pond snails 
(e.g. Physa
sp.), which serves as the first intermediate host. Within the pond snail, each 
egg undergoes
at least two rounds of "polyembryonic amplification", generating hundreds or 
thousands of
free-swimming cercarial trematode larvae. The cercariae attack larval 
amphibians (in this
case, Pacific treefrogs, Hyla regilla, and Long-toed Salamanders, Ambystoma
macrodactylum), aggressively boring into the skin to form cysts 
(metacercariae, (Fig. 2)),
but are apparently unable to penetrate the skin of adult metamorphosed Hyla. 
The cysts are
nonrandomly localized around the mouth, the cloaca, and the exposed developing 
hind limb
buds of the amphibian larvae (Fig. 3).

Approximately 72% of a total of 280 tadpoles and young froglets, presented a 
wide range
of limb, including missing limbs or parts of limbs, fused skin, ectopic limb 
structures, and
duplicated limb structures ranging from extra digits to several extra whole 
limbs (Fig. 4),
Table 1). A similar range of deformities was observed in approximately 40% of 
a sample of
4,148 captured and released larval and newly metamorphosed salamanders, and
approximately 5% of a sample of 1,778 captured and released adult salamanders 
(Tables
3,4,&5).

The predominant observed limb deformity in both amphibian species was 
mirror-image
duplication (Fig. 5), suggesting massive disruption of the positional 
relationships of cells in
the developing limb bud (Fig. 6). I hypothesized that these limb abnormalities 
result from
localized regulatory responses of developing and regenerating limb tissues to 
mechanical
disruption of normal limb pattern forming mechanisms caused by the trematode 
cysts. This
idea was experimentally confirmed by microsurgically implanting inert resin 
beads into
developing limb buds of lab-raised frogs (Xenopus laevis) and salamanders 
(Ambystoma
mexicanum), a treatment that causes the development of supernumerary limb 
structures
and other deformities (Fig. 7).

I conclude that trematode cyst infestation is sufficient to cause the majority 
of observed
deformities in natural populations of amphibians. I think that the occurrence 
of such
phenotypic anomalies in natural populations of amphibians probably reflects a 
sporadic and
localized natural phenomenon, especially fluctuations in pond snail 
populations (Fig. 8).
Frogs and trematodes have probably been co-evolving through shifts in the 
timing of their
developmental stages, an ancient phenomenon dating back millions of years. 
These
possibilities should be explored in all reported but unexplained occurrences 
of a high
frequency of limb deformities in natural populations of amphibians.

http://www.mp1-pwrc.usgs.gov/naamp3/papers/54/54df.html

Hopefully he won't mind me ripping off his paper ...

Marc



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