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Selection of Spermatozoids
Testes
outside of the body - are
they a “refrigerator”
or a “thermometer”?
“The original reason that a scrotum evolved has long been a
subject of debate
among reproductive biologists and still has no universally
accepted answer.”1
 With
development of internal fertilization the necessity in a
great number of gametes disappears. But only a female line
is subjected to reduction, passing from millions of ova in
fishes to several ones in mammals. The number of
spermatozoids remained on the level of hundred millions, in
spite of evident possibilities for reduction and a certain
trend of evolution to lose everything useless. The
maintenance of the "population" system in spermatogenesis
and the ecological specialization of males make one to
assume the existence of the natural selection of
spermatozoids. For the animals with a little posterity and a
seldom change of generations it should be advantageous.
It is necessary for the realization of
such a selection that selected genes should be represented
in the phenotype of the spermatozoids. It can be assumed,
that these are primarily the genes of stability for the most
fundamental factors of the environment (temperature and
others). If this is true, Mendel's distribution of
phenotypes must be disturbed by the crossing of a
heterozygous male with a homozygous female. Such
disturbances were found in mice by the transmission of the
taillessness gene from a heterozygous male to his
offspring. In a reciprocal crossing this phenomenon was
absent. The length of the tail is connected with an ambient
temperature. According to Allan's rule, the more northern
is the habitat of animals, the shorter their tails. In
rodents kept under extreme temperatures their tail length
varied accordingly.
The second necessary condition for the
realization of an adequate selection of spermatozoids is the
availability of their information contact with environment.
In case of external fertilization the spermatozoa were in
direct temperature and chemical contact with the
environment. After the transition to the internal
fertilization the chemical contact with the environment was
lost. Among warm-blooded animals only surface parts
of the body, keep a temperature contact with the
environment. Perhaps this compelled evolution to bring
testis of mammals out of the abdominal cavity, while the
ovaries remain inside the body.
In very large underground and water
animals, not subjected to great temperature fluctuations, as
well as in birds, having homogametic males, the testis are
in the abdominal cavity. It is considered, that the outside
testes are a “refrigerator” [The blood that reaches the
testes is 1.5o to 2.5oC cooler than
the core body temperature1]. Aren’t they also a
“sensor” of environmental temperature (a
thermometer)?
If the outer testes served only as a
refrigerator it seems that testes of birds should first of
all be brought out of the abdominal cavity, as the body
temperature of birds is several degrees higher than that of
mammals. However, the testes of birds are located in the
abdominal cavity. It makes us believe that selection of
spermatozoa is possible only in the heterogametic sex and is
realized only in spermatozoa carrying Y-chromosome.
First, the Y-chromosome is the “ecological”
chromosome, which realizes contact with the environment.
Second, appearance of the male with a required genotype is
more efficient for adaptive transformation of the population
than of the female, since the male produces more offspring.
One more indirect argument in favor of the fact that
selection proceeds among Y-carrying spermatozoa is
the increased sexual activity of males of rare genotypes.
And it is known that sexual activity of males is regulated
namely by the Y-chromosome.
1. K. S. Saladin “Anatomy and physiology”, Third Ed. McGraw
Hill, 2004 p. 1024-1027.
More about
sperm selection:
Natural
selection of spermatozoids. Geodakyan V. A. Proc.
Symp. Natur. Select. Liblice, CSAV, Praha, 1978, p.
707–713.
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