Biology Reference
In-Depth Information
Embryonic stage
Postembryonic juvenile
Kairomone
Kairomone
reception
(Morphogenisis for
Neckteeth formation)
Hox3, exd, esg,
DD2, DD3
Developmental
mechanisms
DD1
Insulin signalling
JH signalling
Developmental-fate
determination
Endocrine
pathways
Figure 4.14 Schematic diagram showing the process of defense morph formation with
the putative involved genes and biological pathways suggested by the present study. DD1 is
thought to be involved in kairomone reception and/or fate determination during the embryonic
stage. The other genes are considered to be involved in the morphogenesis of postembryonic
juveniles.
Source : From Miyakawa et al. (2010) .
It is worthwhile noting that the full expression of transgenerational predator-
induced defenses in some Daphnia species requires more than a generation of expo-
sure to predator cues. For instance, Daphnia mendotae forms the biggest size of
its round helmet in the third generation of exposure to chemical cues of the largest
planktonic cladoceran, Leptodora kindtii ( Tanner and Branstrator, 2006 ).
Populations of Daphnia magna , a native Daphnia species of north-west America
switch to one of the reproductive modes to better adapt themselves according to con-
ditions in the environment. Under favorable conditions, they expand rapidly as dip-
loid all-female populations that reproduce parthenogenetically, but when conditions
deteriorate (crowding, depletion of food resources, deterioration of the food qual-
ity, etc.) or presage deterioration (i.e., shortening photoperiod), they enter the sex-
ual reproduction phase by giving birth to male and female individuals ( Figure 4.15 ).
This is very adaptive: by mating with males, females produce fertile diapausing eggs
resistant to freezing and desiccation, that can survive for decades before hatching
into male individuals. Producing male individuals during this phase is correlated
with an increased level of methyl farnesoate (MF), a crustacean JH ( Olmstead and
LeBlanc, 2007 ).
All stressful stimuli are perceived in the brain where they activate the cholinergic
system, which inhibits the synthesis and secretion by neurons of the X-organ-sinus
gland complex of mandibular organ-inhibiting hormone 1 (MO-IH-1) and mandibu-
lar organ-inhibiting hormone 2 (MO-IH-2). These neurohormones derepress the gene
for MF in the mandibular organ. The synthesis and secretion of MF, which binds its
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