other limb causes

maa32 · 20 Mar 2002, 02:44 · from urth-v0203.mbox · original

This is the message as the crawl holds it, before anything here touched it. What the conversation shows has been through ingest: transfer encodings decoded, attachments dropped, quote markers repaired, and the archiver’s ? resolved into quotation marks and dashes. Those repairs are judgements and some of them are wrong, which is what this page is for. Email addresses are obscured here as they are everywhere else on the site, so it is before our repairs rather than byte for byte.

From: maa32 <maa32 at dana.ucc.nau.edu>
Subject: (urth) other limb causes
Date: Tue, 19 Mar 2002 19:44:21 -0700
Message-ID: <urth-v0203-386 at digest.urth.net>
X-Urthmirror-Source: http://www.urth.net/urth/archives/v0203/386.txt.shtml
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: quoted-printable

Hey, Jerry.  Thanks for looking into that stuff more thouroughly.  Here is=20
something else: trematodes that cause extra limbs in X Leavis.  (Anyone not=20
interested in this should definitely skip this - it just involves extra lim=
bs=20
in frogs)

Of especial note is the statement:

"The predominant observed limb deformity in both amphibian species was=20
mirror-image
duplication"

=20
(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 at Hartwick.edu

The occasional occurrence of high frequencies of limb abnormalities, includ=
ing=20
missing limbs
and extra limbs, in natural populations of amphibians has long been a puzzl=
e.=20
Recent reports
suggest an "epidemic" of such anomalies in natural populations of amphibian=
s=20
from the
central United States. Here I report the discovery of a population from=20
northern California
in which such limb abnormalities appear to be caused by a parasitic flatwor=
m=20
(trematode:
Manodistomum syntomentera, family Plagiorchiidae) that uses amphibians as=20
second
intermediate hosts in a complex life cycle (Fig. 1).

The primary host is a vertebrate carnivore (especially garter snakes, genus=20
Thamnophis)
which releases trematode eggs in its feces. The eggs are eaten by pond snai=
ls=20
(e.g. Physa
sp.), which serves as the first intermediate host. Within the pond snail, e=
ach=20
egg undergoes
at least two rounds of "polyembryonic amplification", generating hundreds o=
r=20
thousands of
free-swimming cercarial trematode larvae. The cercariae attack larval=20
amphibians (in this
case, Pacific treefrogs, Hyla regilla, and Long-toed Salamanders, Ambystoma
macrodactylum), aggressively boring into the skin to form cysts=20
(metacercariae, (Fig. 2)),
but are apparently unable to penetrate the skin of adult metamorphosed Hyla=
.=20
The cysts are
nonrandomly localized around the mouth, the cloaca, and the exposed develop=
ing=20
hind limb
buds of the amphibian larvae (Fig. 3).

Approximately 72% of a total of 280 tadpoles and young froglets, presented =
a=20
wide range
of limb, including missing limbs or parts of limbs, fused skin, ectopic lim=
b=20
structures, and
duplicated limb structures ranging from extra digits to several extra whole=20
limbs (Fig. 4),
Table 1). A similar range of deformities was observed in approximately 40% =
of=20
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 salamande=
rs=20
(Tables
3,4,&5).

The predominant observed limb deformity in both amphibian species was=20
mirror-image
duplication (Fig. 5), suggesting massive disruption of the positional=20
relationships of cells in
the developing limb bud (Fig. 6). I hypothesized that these limb abnormalit=
ies=20
result from
localized regulatory responses of developing and regenerating limb tissues =
to=20
mechanical
disruption of normal limb pattern forming mechanisms caused by the trematod=
e=20
cysts. This
idea was experimentally confirmed by microsurgically implanting inert resin=20
beads into
developing limb buds of lab-raised frogs (Xenopus laevis) and salamanders=20
(Ambystoma
mexicanum), a treatment that causes the development of supernumerary limb=20
structures
and other deformities (Fig. 7).

I conclude that trematode cyst infestation is sufficient to cause the major=
ity=20
of observed
deformities in natural populations of amphibians. I think that the occurren=
ce=20
of such
phenotypic anomalies in natural populations of amphibians probably reflects=
 a=20
sporadic and
localized natural phenomenon, especially fluctuations in pond snail=20
populations (Fig. 8).
Frogs and trematodes have probably been co-evolving through shifts in the=20
timing of their
developmental stages, an ancient phenomenon dating back millions of years.=20
These
possibilities should be explored in all reported but unexplained occurrence=
s=20
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

--=20

An unofficial mirror of a public mailing list · about & removal requests · download the archive · before the list · original archive at urth.net