SAVE THE MITOCHONDRIA

Jim Cummins cummins at possum.murdoch.edu.au
Thu Mar 9 20:13:36 EST 1995


MASSES of stuff deleted.

>  : Richard Lee <rrcolley at uclink2.berkeley.edu> writes:

>  : Mitochondria is [sic] indeed passed solely through the line of 
matriarchy,
>   Yep, but the reason for this is _spooky!_ The mitochondrial DNA from 
the 
>  male is contained in the tail of the sperm, which breaks off before 
it 
>  can contribute its mitochondrial DNA. We are _truly_ our mothers' 
>  children, i.e., moms contribute _slightly_ more than 50% to the 
child's 
>  makeup.

Wrong, sorry.  The sperm *midpiece* contains about 30-50 mitochondria 
and in most mammals is indeed incorporated at fertilization.  In fact 
the sperm tail centrosomal material is the template (probably not 
absolutely essential) for the spindle apparatus of the first mitotic 
division.  However, the egg contains redundant mechanisms that can form 
a spindle in the absence of sperm cytoplasm, as seen in parthenogenetic 
organisms (many lizards) and in experimental situations where embryos 
and live young have been produced by fusion of isolated roudn spermatid 
(haploid) nuclei with eggs.  The first pregnancies have been announced 
in humans by this technique (references supplied if anyone's 
interested).

The jury is out as to whether maternal inheritance is due to swamping 
(1000 fold ratio of maternal to paternal mtDNA), degradation of sperm 
mtDNA (several weeks of independent activity may have worn it out), or 
selective destruction or inactivation.  Some evidence supports the 
latter, as there is ultrastructural evidence of breakdown of the sperm 
mitochondria.  

A wide survey of organisms reveals that maternal inheritance, while 
widespread, is not universal.  It's paternal in conifers, for example.  
Biparental inheritance is extremely rare (some mussels).   This has led 
Hurst to suggest that, in multicellular organisms with multiple genome 
inheritance, natural selection has produced a situation wherein one 
parent's cytoplasmic genes are invariably suppressed to avoid lethal 
"genomic conflict" (ie the cytoplasmic genes are subservient to the 
greater interest of the somatic genes and the survival of the germ cell 
lineage).

Hurst, L.D. (1992). Intragenomic conflict as an evolutionary force. Proc 
Roy Soc Lond B., 248, 135-140.
Ogura, A., Matsuda, J., and Yanagimachi, R. (1994). Birth of normal 
young after electrofusion of mouse oocytes with round spermatids. Proc 
Natl Acad Sci U S A., 91, 7460-7462.
Ogura, A., Yanagimachi, R., and Usui, N. (1993). Behaviour of hamster 
and mouse round spermatid nuclei incorporated into mature oocytes by 
electrofusion. Zygote., 1, 1-8.
Schatten, G. (1994). The centrosome and its mode of inheritance - the 
reduction of the centrosome during gametogenesis and its restoration 
during fertilization. Dev Biol., 163, 534.
Sofikitis, N., Miyagawa, I., Sharlip, I., Hellstrom, W., Mekras, G., and 
Mastelou, E. (1995). Human pregnancies achieved by intra-ooplasmic 
injections of round spermatid (RS) nuclei isolated from testicular 
tissue of azoospermic men. Las Vegas, Nevada: AUA Meeting 
Abstracts/PRISM Productions, 0616.
Yanagida, K., Bedford, J.M., and Yanagimachi, R. (1991). Cleavage of 
rabbit eggs after microsurgical injection of testicular spermatozoa. Hum 
Reprod., 6, 277-279.






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