Re: Epigenetic control of gametogenesis

On Feb 10, 1:11 pm, CNCa...@xxxxxxx wrote:
If epigenetic information is essential to evolution of
metazoans, then, given that evolutionary changes
appear first in the course of individual development,
one has to expect that epigenetic information has
to play an essential role in the individual development.
Let's see whether this prediction of the epigenetic theory
of evolution is validated.
Commonly, the individual development is considered to
begin with the formation of the zygote. However,
parthenogenetic metazoans develop from eggs and both
the egg cell and the sperm cell are parentally provided with
epigenetic information in the form of specific spatial patterns
of cytoplasmic factors and centrioles. We know that this
epigenetic information is responsible for all the entire embryonic
development. Hence, it makes sense to consider the individual
development from the process of the formation of gametes in
parental organisms.

Epigenetic control spermatogenesis

For at least two decades it is acknowledged that
spermatogenesis "is ultimately controlled by neurons in
the CNS" (Sharp PJ, and Gow CB 1983. Poultry Science
62: 1671-1675) and, that the CNS in vertebrates exerts
that control via the GnRH pulse generator and the
hypothalamic-pituitary-testicular axis (A. Vander, J.
Sherman and D. Luciano. 2001. Human Physiology. 8th
edition. McGraw Hill, p. 646). Environmental and
somatic stimuli are integrated, processed in the nonhypothalamic
brain and a signal output is sent to the hypothalamus
where it triggers production of GnRH (gonadotropin-releasing
hormone), which stimulates production of gonadotropins
by the pituitary, which are essential for the function of
gonads and formation of sperm cells from spermatogonia.
Additionally, a direct hypothalamic-testicular (pituitary-
independent) neural pathway has also been demonstrated
to be operational in mammals.

The synthesis of testosterone by Leydig cells in the testicular
interstitium is cerebrally regulated by the brain-hypothalamus-
pituitary axis with the pituitary LH as proximate cause,
but recently it has been suggested the existence of a LH-
independent neural "brain-testicular circuit". It is also demonstrated
that "splanchnic innervation regulates testicular LH receptors
and blood flow" (Lee, S. et al. 2002. Endocrinology 143: 4447-4454).

Some fish species regulate gametogenesis according to lunar
periodicity. Siganus guttatus, e.g., uses as a cue the first
quarter moon, whereas the rabbitfish, S. argenteus -
the last quarter moon (btw, the information for starting
gametogenesis is not in the exterenal stimulus, as it is
often claimed, but is adaptively generated by processing
the environmental data). The lunar cue in S. argenteus does
not act directly on the gonadal hormonal system but via the CNS:

"The effects of lunar factors in the rabbitfish would occur in
higher parts of the hypothalamus-pituitary-gonadal axis"
(Rahman et al., 2003. General and Comparative Endocrinology
131: 302-309).

Epigenetic control of oogenesis in insects and gastropod molluscs

Signal cascades for oogenesis in invertebrates originate in
the CNS. In the female fire ant, Solenopsis invicta, e.g.,
electrical activation of the dopamine system, resulting from
the processing in the brain of an external stimulus
(queen pheromone), controls oogenesis and oviposition
(Boulay R, et al. 2001. Physiological Entomology 26: 294-299).
The uptake of a blood meal by the predatory bug of the
Reduviidae family, Panstrongylus megistus, acts as a stimulus
to which certain median neurosecretory cells of the pars
intercerebralis (cells A) respond by releasing a neurohormone
that stimulates proliferation of oogonia and prefollicular cells
in the fifth instar up until day 6, and a neuron of another type
(cells A'), which responds by stimulating secretion of ecdysone
by prothoracic glands, thus determining the differentiation
of ovarioles between days 16 and 24 (Heming, 2003).

Oogenesis in Drosophila is proximately regulated by
ecdysteroid hormone and JH (sesquiterpenoid juvenile hormone)
(Carney GE and Bender M. 2000. Genetics 154: 1203-1211).
Both hormones, ecdysone and JH, are cerebrally regulated
by brain signals, PTTH (prothoracicotropic hormone)
(Zitnan D, et al. 1993. Developmental Biology 156:
117-135) and allatotropins/allatostatins respectively.
There are indications that upstream these brain signals are
other brain signals (neurotransmitters/neuromodulators)
released as a result of the processing of internal/external
stimuli in neural circuits. So, e.g. a queen pheromone in
colonies of Solenopsis invicta ants prevents the reproductive
activity in virgin females via a neural pathway (olfactory
signals are transmitted from the antennal lobe for processing
in the brain).

In molluscs reproduction is seasonally regulated in response
to environmental stimuli, with the photoperiod being the
most important cue. The neuronal structures for processing
these reproductive stimuli in mollusks

"are diffusely located in the brain and the neuroendocrine
circuitry is highly complex." (Wayne, N.L. 2001. Journal
of Biological Rhythms 16: 391-402)

The central nervous system in molluscs comprises varied
numbers of ganglia, each of them composed of thousands
of neurons.

When continually kept under laboratory short day conditions,
males and females of the slug Limax maximus remain
sexually immature, but they develop the mature
reproductive tract and produce mature gametes soon
after they are transferred under long day conditions.
Transplantation of the ganglia of long day-stimulated slugs
into the immature short day-inhibited slugs induced
development of the mature reproductive tract whereas
transplantation of short day ganglia did not induce the
development of the reproductive tract (Wayne, 2001).
Under the influence of stimulating long days, the slug
brain secretes a maturation-inducing hormone (MH),
which induces the synthesis of one or more hormones
by the gonads . Implantation of the brains of long
day-stimulated slugs into brains of short day-inhibited
slugs, even under short day conditions, stimulates development
of gonads and accessory sex organs in the latter (McCrone,
E.J. and Sokolove, P.G. 1986. Journal of Comparative
Physiology 158: 151-158).

Biologists have known for some time that in many invertebrates
hormones, as mediators of neural signals, regulate the ovulation
and the number of eggs to be deposited, but recently it has
been demonstrated that in the snail, Helix aspersa, a branch
of the intestinal nerve is immediately responsible for that
regulation (Antkowiak, T. and Chase,R. 2003. The Journal of
Experimental Biology 206: 3913-3921).

Oogenesis and ovulation in both ovulation-inducible vertebrates,
such as rabbits, and cyclic ovulators as humans (Villa-Diaz, L.G.
and Barrell, G.K. 1999. Reproduction, Fertility and Development
11: 95-103; Johnson and Crowley, 1986) are under neurogenic
control (Adler BA, and Crowley WR, 1984. Neuroendocrinology
38: 248-253; Kalra PS et al. 1987. Endocrinology 120: 178-185;
Gore, A.C. and Terasawa, E. 2001. Journal of Neuroendocrinology
13: 239-248).
Taking as an example the production of the egg cell by a female
insect, Bruce S. Heming described the essence of oogenesis as
follows:

"She can do this because she is sensitive to diverse cues
from the environment (photoperiod, temperature, relative
humidity, substrate odor, food, proximity and state of mates,
etc.), is able to integrate this information in her nervous
system, and via her endocrine system can use it to develop
and deposit her eggs at the right time and place." (Heming,
B.S. 2003. Insect Development and Evolution. Cornell
University Press, p. 58).

Deposition of epigenetic information in the egg cell

What makes the egg cell uniquely capable of developing
into an adult metazoan is not the genetic information, which
is identical in all cells of the body, but the presence in its
cytoplasm of epigenetic information, in the form of thousands
of types of orderly arranged maternal cytoplasmic factors
(mRNAs, secreted proteins, growth factors, hormones,
neurotransmitters, nutrients, etc.).

The vast majority of those factors in the insect oocytes come
from nurse cells, in the form of mRNAs. The transport of
maternal cytoplasmic factors from nurse cells into the oocyte
occurs along a common microtubule complex extending from
the posterior part of the oocyte to nurse cells. An ecdysone pulse
(result of a cerebral pulse of the neuropeptide PTTH
(prothoracicotropic hormone) induces apoptosis of the nurse cell,
which is characterized by contraction of the actin cytoskeleton,
which forces the nurse cell cytoplasm into the egg cell.

It has also been demonstrated that brain signals, a "cephalic event"
(Handler, A.M. and Postlethwait J.H. 1977. The Journal of
Experimental Zoology 202: 389-402; Sorge D, et al. 2000.
Journal of Insect Physiology 46: 969-976) via ecdysone,
control the synthesis and uptake of yolk protein (Richard DS,
et al. 2000. International conference in honour of Professor
David Saunders. Edinburgh, March 20-24, (Abstract)) as well
as other maternal cytoplasmic factors from the hemolymph
(Chapman, R.F. 1998. The Insects - Structure and Function,
4th edition. Cambridge: Cambridge University Press, p. 306)
by the egg cell.

Although scarcer, evidence on the neural regulation of
maternal cytoplasmic factors in vertebrates is not lacking.
In many birds, e.g., the yolk concentration of testosterone
is regulated by the pituitary prolactin (Sockman KW, et al.
2001. Hormones and Behavior 40: 462-471). The processing
of internal and external stimuli in the birds' CNS activates
the following signal cascade:
prolactin-releasing peptide (PrRP) secreted by neurons
of the nucleus of the solitary tract ? hypothalamic secretion
of PRF (prolactin-releasing factor)/PRIF (prolactin-releasing
inhibiting factor) ? ? pituitary prolactin.

In the canary (Serinus canaria), as well, seasonal changes
of photoperiod via brain-hypothalamic-pituitary-ovarian axis
determine the concentration of maternal testosterone in
the egg, which is considered to be a communication of
environmental conditions "from the mother to the offspring."
Schwabl H, 1993. Proceedings of the National Academy
of Sciences USA 90: 11446-11450).
In viviparous animals, such as mammals, cerebrally regulated
maternal signals (hormones) pass through placenta and
essentially influence the course of early development
(Das, SK et al. 1994. Endocrinology 134: 971-98).

For additional information on the neural control of oogenesis
and deposition of parental cytoplasmic factors read chapter 4
of Epigenetic Principles of Evolution, also accessible through
my website
nelsoncabej.com or http://www.epigeneticscomesofage.com

There is so much wrong here it is hard to know where to begin. So I
will just comment on the neural control of oogenesis, related to the
neural control of pretty much anything else except thought and
behavior where the brain really does control things.

Think of a big city mayor at a large ceremony sticking a golden shovel
into the ground. A number of years later, there appears at that very
site a tall skyscraper. It is very obvious that the mayor's shovelful
'initiated' or 'triggered' or 'set into motion' the process. However
it is equally obvious that neither the act of shoveling nor the shovel
itself had any notion of what a 'skyscraper' is. One can quite easily
imagine that the mayor, although understanding fully what was about to
ensue, had absolutely no idea just how to go about building a
skyscraper or what might be required even to consider the process.

Animals tend to synchronize their reprodutive activities with each
other, with the seasons, or with both. Animals tend to use their
nervous systems to detect the activities of other conspecifics and the
environmental cues that indicate the season. As a result, animals
tend to use their nervous systems to 'initiate' or 'trigger' or 'set
into the motion' the process of reproduction which usually includes
gametogenesis. There is absolutely no reason to believe that the few
thousand (or hundred thousand) molecules of neurotransmitter or
neurohormone that a neurone drops into the blood stream or onto
another cell has any knowledge or information about gametogenesis nor
about producing another generation nor about the information to be
transmitted to the next generation at all. There is absolutely no
reason to believe that the nervous system, taken as a whole, has any
knowledge or information about these things, either. It is just a
process that, once triggered, runs to completion on its own rules.

Sponges have gametogenesis with no form of a nervous system
whatsoever. I am not familiar with the full range of animals and
their reproductive patterns, but I would be very surprised if all or
even most animals require the nervous system for gametogenesis. I
know for a fact that isolated gonads with no nervous innervation
whatsoever, can easily produce gametes under hormonal control. Yes,
hormones can be and often are (but not always) controlled by the
nervous system, but just about everybody in the world thinks in terms
of the hormones controlling gametogenesis, not the nervous system, at
least for vertebrates. Insects have their own world of hormones and
neurohormones but the system is quite similar; the nervous system
triggering a downstream process that can be triggered by other means
and that, in any event, runs to completion without further nervous
guidance.

Plants produce complex bodies with well differentiated cells and
tissues and organs, too. Plants also tend to synchronize their
reproductive activities with each other, with the seasons, or with
both. Plants also have gametogenesis, both spermatogenesis and
oogenesis, although highly modified in form in the case of seed
plants. Plants have no nervous system whatsoever, also there do exist
specialized cells in highly specialized individual species, that can
make action potentials and use them as intra- and intercellular
signals. So there is nothing about development nor about
gametogenesis that requires nervous systems.

The effect of maternal hormones or other chemical signals acting on
development is another issue that I may take up separately, although I
would rather let John or Howard take care of that issue.


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