Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 224

An interesting observation pointed out in Chapter 3 is that at the phylotypic stage,
the embryo has an operative CNS, which is the first organ system to develop in all
metazoan embryos. The development of embryonic structures proceeds from signal
cascades or elements of the CNS, suggesting that the CNS develops before any other
organ system and that this development is not accidental. All of the above clearly
support the evo-devo concept of the developmental rather than the genetic origin of
evolutionary change.
Changes in developmental pathways do occur, but they are neither frequent nor
predictable, so that the appearance of the evolutionary change in embryos can-
not be examined. However, direct examination of the mechanism of the evolution-
ary change may not be as secretive as it appears. Another approach seems to offer
biologists relevant clues, if not the real developmental mechanism of evolutionary
change. The evolutionary change is a new/modified trait transmitted to the offspring
and maintained for more than one generation in the absence of the agent that initially
induced the change. If this is true, then the described cases of the TDP fall in the cat-
egory of evolutionary changes. Their study may provide in vivo models of the mech-
anisms of evolutionary change or, at least, provide important clues of its mechanism.
Here are a few examples of TDP.
The water flea, Daphnia pulex , in response to detecting kairomones (chemi-
cal cues) released by the predatory phantom midge Chaoborus flavicans larvae in
the environment, develops an outgrowth called neckspine with a varying number
of teeth, delays its reproductive maturity to increase its body size, and may change
its life history. The formation of the neckteeth diminishes the danger of juve-
nile Daphniae being ingested by the predator larvae. The water flea is more sensi-
tive to the kairomone during embryonic development. At this stage, the embryonic
crest epithelium thickens and continues developing neckteeth until the third instar
stage. The embryo can form neckteeth via maternal signals deposited in the eggs of
mothers that have perceived the presence of the kairomone in the environment or
embryos can themselves respond to the kairomone by developing neckteeth ( Imai
et al., 2009 ).
Experimental studies have outlined the mechanism of the development of neck-
teeth in D. pulex ( Figure 4.14 ).
Antennae of the D. pulex carry asthetascs (olfactory receptors), which con-
tain, among others, chemoreceptors for kairomones. The information they trans-
mit is integrated in the deuterocerebrum (midbrain) ( Hanazato and Dodson, 1992 ),
which innervates the antennulae. This is the region of the brain where kairomone
is perceived ( Hallberg et al., 1992 ). Neural signals secreted in the Daphnia's brain
stimulate expression of the DD1 gene and secretion of endocrine hormones, includ-
ing insulin and JH. The latter, downstream, induces the expression of morphoge-
netic genes ( Hox3 , DD2 , DD3 , etc.) determining formation of neckteeth structure
( Miyakawa et al., 2010 ).
Below the site of the neckteeth development, a high concentration of polyploid
cells is observed, but the role of these cells in neckteeth formation is not established
( Weiss et al., 2012 ).