Biology Reference
In-Depth Information
Environmental stimuli
(photoperiod, temperature)
Reception by
sensory neurons
Stimulus
electrical
concerted into
signals
Processing in the
neural circuit
Chemical
the neural
output of
processing
X-organ/
sinus gland
Neuropeptides
(MO-IHs + CHH)
Mandibular
organ
Methyl farnesoate
Mandibular
organ
No methyl farnesoate
secreted
Methyl farnesoate
receptor
Suppression of genes
for mictic offspring
Activation of genes for
mictic offspring
Sexual
generation
Parthenogenetic
generation
Adaptive phenotypic
result
Figure 5.19 Illustration of the recruitment of genes in two different pathways leading
to birth of asexual (parthenogenetic) and sexual generations in D. magna . The point of
divergence between the two pathways starts in the crustacean's brain with the failure of the
X-organ/sinus gland to synthesize and secrete neuropeptides MO-IH1, MO-IH2, and CHH
(left cascade) and recruitment of the methyl farnesoate in the case of sexual generation-
producing Daphnias and recruitment of the above genes and prevention of the synthesis of
methyl farnesoate in the case of production of the asexual generation. Most insects have
photoperiodic clocks and calendars that allow them to measure the length of the day and to
count days. Based on this information, they decide whether to produce diapausing eggs or
nondiapausing eggs, in order to rear their offspring under favorable climatic conditions. When
these insects perceive the shortening of the day and the drop in environmental temperatures,
they produce diapausing eggs, in which the embryo holds off the development until the spring.
In a simplified form, the mechanism of diapausing-egg production is this: when the mother's
photoperiodic clock and calendar has counted a specific number of short days, the mother
stimulates a specific firing pattern of specific neurons, which inactivates in the GABAergic
inhibitory mechanism brain, thus enabling the recruitment of the diapause hormone (DH) in
the signal cascade that induces the formation of diapausing eggs.
Source : Cabej (2011) .
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