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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.

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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