evolution of disparate developmental patterns

From: Dr. David Campbell <amblema@bama.ua.edu>
Date: Fri Nov 18 2005 - 14:17:53 EST

Here's a news summary that addresses how disparate developmental
patterns may arise in closely related species:

Science 18 November 2005:
Vol. 310. no. 5751, pp. 1109 - 1110

News Focus

Development Out of Sync
Elizabeth Pennisi

In development, timing is everything. Get it wrong, and organs fail to
grow or wind up in the wrong place. But for some animals, altering the
normal sequence of organ formation can be key to survival--and, some
evolutionary biologists argue, a driver of evolution. Take the case of
the spadefoot toad.

These amphibians often lie buried in desert dirt for months, emerging
within minutes of a rainstorm to hop to the nearest water to mate.
Their tadpoles must then race through development before the pond dries
up--in some cases, in little more than a week. Some species beat the
clock not by speeding up growth overall but by accelerating changes in
body parts critical to escaping before their watery world disappears,
says Daniel Buchholz, a comparative endocrinologist at the National
Institute of Child Health and Human Development in Bethesda, Maryland.
In these toads, limbs and many organs develop faster than normal, but
development of the gonads--which aren't needed until later in life and
require develop-mental energy--isn't speeded up.

Some spadefoot toad tadpoles speed up metamorphosis, but gonad
differentiation lags behind.The key to this selective acceleration may
be thyroid hormone, Buchholz reported at the meeting. Thyroid hormone
is known to prompt spadefoot tadpoles to lose their tails and gills and
punch out legs as they metamorphose into adults. Working with Tyrone
Hayes, a comparative endocrinologist at the University of California,
Berkeley, Buchholz measured thyroid hormone concentrations in the tail
and liver of tadpoles from two spadefoot species, one that takes 33
days to metamorphose and another that needs only about 14 days. The
tissues of the earlier-metamorphosing species were two- to fivefold
more sensitive to the hormone, he reported at the meeting. But there
was no difference between the species in the rate of growth of the
gonads, which do not respond to thyroid hormone. As a result, the
33-day tadpoles develop gonads before they metamorphose, whereas the
14-day tadpoles metamorphose and leave the pond before their gonads

Christopher Rose, a developmental biologist at James Mason University
in Harrisonburg, Virginia, says Buchholz hasn't completely nailed down
the role of thyroid hormone. But he is impressed by the work. "He has
[addressed] a developmental problem that is important not only to
evolutionary biologists but ecologists as well," Rose says.

Spadefoot toads are not alone in speeding up the development of some
body parts to gain an ecological advantage. Susan Hill, a developmental
biologist at Michigan State University in East Lansing, reported at the
meeting that at least one species of Capitell a, a marine polychaete,
develops adult musculature prematurely. Polychaete eggs typically hatch
as larvae that swim and feed with the aid of bands of cilia. But a few
species start out segmented like adults, with fewer bands of cilia and
adult muscles throughout their bodies, says Hill. By staining and
tagging proteins key to cilia or muscle development in one Capitella
species, Hill and Barbara Boyer, a developmental biologist at Union
College in Schenectady, New York, found that the cilia and muscles
appear simultaneously rather than sequentially, as is usual for
polychaetes. This accelerated development allows juveniles to put down
roots fast when they come upon the right habitat, says Hill.

Both studies point to the value of asynchronous development in
conferring survival advantages, says Rose. The phenomenon, he
speculates, "has played a major role in the evolution of life history
traits, morphological innovations, and possibly, body plans."

Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA
Received on Fri Nov 18 14:19:58 2005

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